Wednesday, February 22, 2017

Scaling a Yagi: NW3Z 20-15 Meters Interlaced Yagi

There are many excellent yagi designs in the amateur literature. With modern modelling and optimization software tools it is possible to design yagis that perform exactly to specified performance metrics. This was not always the case. In decades past yagi design was often hit or miss, relying on laborious trial and error measurements and adjustments in the field.

The difficulty of the task is due to the complex non-linear relationship of mutual coupling between close-spaced elements, defying efforts to finding analytical solutions. Numerical solutions only became effective in the 1980s with the evolution of computing technology and the concurrent evolution of algorithms and their software implementations.

My first exposure to the problem of yagi design and optimization was in the late 1980s. I and several other hams interested in building more competitive contest stations were unhappy with many of the commercial antennas we were using. Some with downright dreadful, relying more on myth and reputation than on measured performance. Yet the tools available to design something better were lacking. Then along came John Lawson W2PV and his excellent book Yagi Antenna Design (out of print), formalizing his analytical and experimental work on yagi design over many years.

With that book in hand I implemented several of his algorithms and designed realistic yagis for several of my friends. Aside from performance was the difficult problem of scaling the design to the mechanical specification of telescoped tube elements and the effects of clamps and boom. W2PV's algorithms provided an excellent analytical approach to solve those difficulties.

Stepped Diameter Correction (SDC)

There has been progress in the past 30 years. MiniNEC and then NEC2 engines arrived and were incorporated into many commercial and non-commercial antenna modelling applications. However neither engine correctly models tapered elements using telescoping tubes. NEC4 does handle it but it is an expensive solution for most hams. I have not seen fit to pay up for it.

It was long ago determined that NEC2 can be manipulated to give correct results for tapered elements. W2PV did it long ago (I don't know if he was the first) and then W6NL codified it for NEC2. EZNEC and others incorporate W6NL's SDC algorithms. They have been amply verified in the field so that we can confidently employ those algorithms and know that the results will closely match NEC4 and real antennas. The algorithms do have limitations on their application (e.g. loading coils). EZNEC 6, for one, has improvements in this area but I do not know how reliable those are.

Problem statement

Someone approached me with a request to model an optimized dual-band yagi for their custom tubing schedule and element-to-boom clamps. The antenna is a long boom, wide band interlaced yagi for 15 and 20 meters with 6 active elements on each band designed by NW3Z. The antenna is an intriguing one. It is in the class of OWA yagis that achieves excellent performance along with exceptional low SWR across both bands.

There were ample design challenges which you can read about in that document. In fact you must read the details of that antenna there since I will not repeat any of it. Otherwise the discussion that follows could be confusing in several important aspects. You can read some commentary about this type of antenna by Cebik. There is another discussion of the design principles starting on page 18-22 of this book extract.

The antenna has separate feeds for each band. Simultaneous operation on both bands absolutely requires very good high power filters. Otherwise you can use a remote switch to use one transmission line for both bands. No matching is needed for 50 Ω coax.

Certainly an antenna of this type would be best modelled with NEC4, and that is what NW3Z did. NEC2 with SDC can give accurate results although not without additional effort and some residual uncertainty. I use EZNEC+ version 5 (NEC2 engine) with the standard W6NL SDC algorithm. There were many challenges to overcome yet in the end the result matches the performance quoted by NW3Z.

This article is about how I did it. There is nothing here that is novel -- it's all been done before. The point is to guide readers along should they wish to do something similar. Being hams we are often likely to want to build yagis based on a proven design while using hardware that is locally available or cheaper than that specified in the design. Once you know how to scale the design to the chosen hardware you can proceed with construction confident that your actual performance will be a close match.

For me the interest was the interlacing of yagis. You cannot do this with tools such as YW, YO and some others, and is still quite challenging with a comprehensive modelling tool such as EZNEC. There was more to my initiative than doing someone a favour: I wanted to learn something about a subject I care about.

The antenna is already optimized

One important thing to state is that the NW3Z design is already thoroughly optimized. That is, it is optimized to its performance objectives. The antenna is not a maximum gain design, giving up around 0.5 db on 20 meters and perhaps 1.0 db on 15 meters. Its F/B is similar or better than mono-band designs of similar size. The SWR is exceptionally low by design, and is in part responsible for the lower gain. Despite giving up some gain it is a great antenna and will fare better than a multi-band antenna using traps or other element loading techniques.

Model view and currents when excited at 14.100 MHz
While an exceptionally low SWR -- well under 1.5 -- is not needed for most operators it is of great benefit to contesters. High power broadband amplifiers are not tolerant of even moderately high mismatches. For the competitive contester who must quickly and repeatedly change frequency and band during a contest the time avoided adjusting an amplifier's output network can give an operator a winning edge.

Other operators, even competitive DXers, can get by without this feature. For them a slightly higher SWR at the band edge (say, 2 to 2.5, or even 3) should be an acceptable trade off for an additional decibel of gain. If that's you there are other designs from which you can choose. Do not try to "optimize" this antenna since you are almost certain to make it worse. Small departures from the published dimensions will do just that. I experienced this when the ham asking me to do the model made a small calculation error on one of those 12 elements.

In accord with these points I am accepting the NW3Z antenna as is; I am scaling the antenna, not optimizing it or changing it into an antenna with different performance metrics.

What needs to be scaled

In this article I will not publish dimensions of the scaled antenna. That would be pointless since every ham is likely to use their own set of materials, in each case producing unique scaling results. It's the scaling procedure that is at issue here. If you want this antenna and do not want to bother with scaling you are best advised to adhere to the exact dimensions provided by NW3Z.
  • The tubing sections must decrease in diameter toward the element ends, the element halves must be identical and no loading elements. This is easy to achieve in a mono-band yagi, and EZNEC will warn you when you make a mistake. Clamps at the tube boundaries can be ignored at HF.
  • Element-to-boom clamps must be converted to an equivalent diameter which is then specified in the model. 
  • Depending on the clamp style the effect of the boom may need to be included. For example, in Hy-gain yagis where the element effectively pierces the boom. In homemade yagis with the more typical rectangular plates and u-bolts (with or without a saddle) the boom effect can be ignored.
Effective diameter of plate style element-to-boom clamps

The yagi I modelled uses plate style element-to-boom clamps. W2PV in his book presents equations for this style of clamp and for those where the element pierces the boom. I will only cover the first. However the boom effect for the latter style is small, being the equivalent of electrically shortening the element by about 10% the boom diameter.

I am using W2PV's equations despite more accurate models that are more recent. For example, there is the improved model promoted by W6NL. Unfortunately I don't his book Physical Design of Yagi Antennas (out of print) where this is discussed. The difference from what I can tell from my limited ability to compare results is within ±2% for HF size material, which is not significant. Note that the error is in the effective diameter of the clamp, not the element length.

There is lots of software around that will calculate the effective diameter of several common element-to-boom clamps if you insist on that degree of accuracy. One I feel confident recommending even though I don't own a copy (or at least not yet) is AutoEZ by AC6LA, which does this for you and much more when used in combination with EZNEC.
Example clamp here

However the W6NL and related models don't appear to model clamps where a u-bolt saddle is placed between the tube and plate, or at least not that I know of. W2PV's equation does, and that factor is not insignificant. So I put his model into a spreadsheet and used it in this antenna design, with the resulting effective diameter placed into the taper schedule in EZNEC.

The screen capture of the spreadsheet with my implementation of the W2PV equation for a tube over plate style clamp includes the specs of the antenna I am scaling. Although in this case the units are centimeters any units can be used provided it is used consistently throughout. The cell with the W2PV equation is shown so that you can replicate it.


The saddle height is equal to the distance between the tube and plate. The height is zero when there is no saddle. The calculated values are: a1, radius of the tube; S1, circumference of the tube; a2, effective radius of the plate (width / 4); S2, perimeter of the plate cross-section; d, centre-to-centre distance between tube and plate. The effective diameter is twice the calculated effective radius. This is the number to use in the EZNEC wires table. The wire length is simply the length of the plate.

In all case the effective diameter should be intermediate between the tube diameter and plate width. If it isn't you've made a mistake.

The effective diameter calculation does not work for elements clamps that electrically isolate the element. This is common in driven elements in many antenna, including the NW3Z design. Although the calculated effective diameter, or no correction at all, will be in error it is not of serious consequence. The reason is that tuning of the driven element(s) in a yagi does not affect gain and F/B performance. Once constructed the driven elements can be adjusted, if needed, to get the desired match. By using the actual tube diameter rather than the effective diameter the required adjustment should be less.

Segmentation and tubing schedule

Nearly end-on view showing the segment and tube alignment
For yagis with such close spaced element, even though resonant on different bands, the segments must be equal length and ends aligned with respect to the boom (line orthogonal to the elements) for best accuracy using NEC2. You can play with this in a model and you'll quickly see why.

I used a segment length of 10 cm (4"). This adds up to over 1,000 segments in the model and so can be quite slow to calculate on older computers. The length works well for 20 and 15 meters and being a round number it is relatively easy to align tube junctions, which is also desirable for model accuracy. Luckily the builder provided a detailed tube schedule with all this taken care of. I believe his intent was cost and convenience, yet it also helped make for a good model.

The element-to-boom clamps must also follow the plan since it is in effect the centre tube. Happily the clamps in this instance are 20 cm long -- two segments. However I used one 20 cm segment for the driven elements in order to avoid using a split source, which in my experience can introduce errors.

To build the model I worked up the wires for one element on each band and copied it until I had a full complement of elements, moving each into position. Then it is a matter of adjusting the tip lengths on the elements to match the spec. It is necessary to be inspect the segment length of the tip so that each is as close as possible to, in my model, 10 cm. The work is bothersome yet necessary, and must be repeated several times during the scaling procedure.

Even with the extensive segmentation work there was still a residual error in the model. This shows up using the average gain test that W7EL describes in the EZNEC manual. Since all other potential error sources were covered to the best of my knowledge the solution is to adjust the gain figures by the average gain. That is, if the average gain is -0.32 db you subtract this value from the calculated gain. For example if the calculated gain in a particular direction is 3.79 dbi the true gain should be 4.11 dbi.

Making this adjustment brought my model's gain almost precisely equal to what NW3Z got with NEC4. That's a good indication that my model is correct. Unfortunately the average gain adjustment is a function of frequency so the true gain must be uniquely adjusted at several points on each band. The F/B does not require this adjustment, so you can read F/B directly from the pattern plot. The reason should be clear when you realize the average gain error affects every point on the far field plot.

Scaling the element lengths

In the discussion of segmentation I said that it is only the element tip lengths that are adjusted during the scaling procedure. I took the element length spec from the ham I did this for and simply ran the model once I had everything else taken care of. I compared the SWR, gain and F/B curves with those published by NW3Z to see by how much the antenna's frequency range.

However it wasn't quite that easy. I eventually discovered why on one band the performance was unexpectedly poor: the length of one element was miscalculated. I adjust this to conform with NW3Z's spec and the expected performance immediately emerged. Just at the wrong frequency range.

When scaling a yagi all lengths are geometrically adjusted, not arithmetically. This means all elements for one band are multiplied by a constant. The constant is determined by the ratio of the calculated frequency to the desired frequency. Never adjust elements by adding or subtracting. The geometrical adjustment ensures that the resonant frequency ratios of any two elements is unaffected by scaling. That relationship must be preserved to maintain the performance metrics.

It can be argued that even this scaling factor includes an inaccuracy since we are only scaling the tips of the elements and not each tube in the schedule. This is a quibble since the introduced error is very small for the degree of scaling we are doing, which is only 1% to 2%. The error can be very significant should you attempt to scale the antenna to a different HF band.

Now we can proceed. Assume, for example, the calculated frequency of maximum F/B is 14.250 MHz. Yet it ought to be 14.100 MHz. To pull the yagi's frequency range down by 150 kHz all six 20 meter elements must be lengthened. The scaling constant is 14250 / 14100 = 1.01064. You can round this off to 1.01. After every scaling operation remember to adjust the segment count of the tip sections to keep it close to the selected value, and then confirm that the gain, F/B and SWR are where they should be. If not, repeat.

It is important that in a multi-band antenna like this that you first scale the elements for the lowest frequency band (the longest elements) and then do each next higher band until you're done. Do it the other way and the higher band will be incorrect after scaling the lower one. In this antenna that means you first scale the 20 meter elements. Even so, check 20 meters again after scaling the 15 meter elements since there is a possibility that another adjustment is necessary. If it is you will of course also have to redo the 15 meter elements.

SDC works on only one band at a time

The W6NL SDC algorithm only works within 15% of the resonant frequency. EZNEC will perform SDC on the 20 meter elements or the 15 meters elements, but not both at the same time. Resonant frequency is the frequency you select in the main EZNEC window. Check in the wires window that SDC is being applied as it should.

To measure the antenna you must set that frequency for the band you are calculating. This is in addition to moving the source to the corresponding driven element. It's a bother but you must set the frequency for the correct band when you do an SWR plot since SDC is not selected per the frequency range of the plot. Otherwise some error will be introduced into the impedance calculations.

The failure to perform SDC on both bands simultaneously affects the higher band of a two band antenna more than the lower band. Therefore the 20 meter results are very reliable while there may be some error on 15 meters. As far as I can tell for an antenna of this type the error ought to be very small, and so I ignored this limitation of the SDC algorithm. That isn't always advisable since every antenna is a unique case.

If you have NEC4...

SDC corrections are not needed in NEC4. That eliminates many of the modelling precautions I've described above. But not all. You must still calculate the effective diameter of the mast clamp and, if a concern, adjust for the boom.

NEC4 is not perfect, nothing is. Its usable domain is greater than NEC2, which is very helpful provided you keep in mind its limitations and constraints. For straightforward mono-band yagis it is certainly easier than NEC2 with the supplementary extensions in EZNEC and some other software tools.

Should you happen to have a friend with a NEC4 license by all means ask them run the model for you. It is a good way to see how well you've scaled the antenna and the accuracy of NEC2 plus SDC and other modelling precautions.

Comparing results

NW3Z modelled his antennas using NEC4. How close can we come to his results using EZNEC? Very close indeed as it turns out. It is so close that I was hard pressed to find any differences after completing the scaled model and tuning it so that the frequency ranges matched the curves in his document.

Here are a couple of examples. The first is an azimuth plot at the 20 meter frequency where F/B is greatest. Due to the average gain issue described earlier it is necessary to subtract -0.31 db from the gain in this particular plot. This brings the actual gain to 10.3 dbi. The F/B is correct as is.


The second example is the SWR curve across 15 meters. The impedances are a good match. This is telling since even small deviations from equal segmentation and total element length and position can cause significant miscalculation of impedance by NEC2.

For an antenna of this type I would aim to have the measurement correct to less than 1 cm (½"), which works out to 0.1% on 20 meters and 0.13% on 15 meters. In many cases that may be more accuracy than needed since the presence of cables, guy wires and other antennas even some distance away will introduce errors of at least this amount.

Beyond this modelling you can only build the antenna and do a field strength test to measure the performance. Since few hams will undertake that amount of work it is important that the scaling be done correctly and that the antenna is built as exactly as possible. However there is the alternative of finding the frequency of maximum F/B with the assistance of a friend within ground range. Don't try this with stations via ionospheric propagation since signal strengths change faster than you can rotate the antenna.

Conclusion

For my primary interest of HF contesting this type of antenna is a poor choice. Using one effectively would require a tri-plexer, just as one would use with a tri-bander shared among two or more operating positions. I prefer to aim for more separation and directional diversity which requires independent mono-banders.

Where it does enter my plans is for the WARC bands: 12, 17 and 30 meters. These bands are not used for contests but fit well into my DXing activity, either for country chasing or casual operation at any time. I will use my experience with scaling the NW3Z to play with some configurations that provide up to 3 elements on 30 and 17 meters on a single boom.

The importance of this is that I can get good performance on these bands while occupying the minimum amount of tower space, space that is a priority for the HF bands used for contests. Maybe not this year, yet I will have to do something eventually.

Sunday, February 12, 2017

New Rig in the New Shack

A good antenna needs a good receiver to get the most out of it.. Not only good on dynamic range and IMD but also audio quality, ease of use, DSP, antenna options, and much more. Buying a top end rig was an item on my list for 2017. With all the money I'm spending on antennas and towers I decided to buy on the used market. I casually kept my eye on the market for just the right rig at the right price. One appeared and I jumped.


In the photo above you can see a Yaesu FTdx5000MP all set up in my new shack. It works well. The room itself is a combination shack and home office. The other furniture and office equipment populating the room is out of frame but I can assure you it is a very comfortable space. I expect to spend a lot of time in this room. Even the espresso machine is only a few steps away.

Compare this photo with that of the shack in my former Ottawa home when I first set it up after returning to air after a lengthy absence. There are some surprising commonalities. I am even using the same ancient (and slow) laptop as the contest/CAT computer. Three years ago I was still firmly committed to "low impact" amateur radio, making the desk seem empty from its original use in 1980s when I have multiple rigs and antennas, and a kilowatt amplifier.

The homemade desk is modular and for the present I have left off the upper shelf unit. I find it gets in the way of the computer screen.

About the FTdx5000MP choice

I am not one of those hams who always buys from the same manufacturer or one who must always have the best of everything. Apart from several features and performance figures I believe in flexibility. When I returned to the hobby I sprung for a brand new Elecraft KX3. I still have it, like it and I am not planning on selling it soon. Since the move it has been hibernating in its packing box.

Despite its limitations it was more than enough of a radio to work lots of DX and place at or near the top of several DX contests in the QRP category. Its receiver is top notch, though not without its irritating aspects. Since I've moved it is packed away in its box. It will see use again, I am sure.

I returned to Yaesu when I decided it was time to move beyond QRP once more. Remaining economical I chose an FT1000MP Mark V Field. It was a well regarded rig when it came out. Several generations of technology later it is now dated, with receiver performance that is relatively poor. It can still be found in the shacks of many top DXers and contesters, though usually as a second or backup rig.

Looking for a rig with DSP to achieve narrow bandwidth rather than expensive crystal filters and 6 meters I switched to the FT950. The receiver is better than the FT1000MP in several respects, and worse in others. Its DSP rings at the narrowest bandwidths and in the presence of noise becomes garbled. It places well in the Sherwood Engineering rankings. I've used it in many contests and it does just fine.

Apart from the DSP the FT950 has no receive antenna capability, has only the one receiver and can be fatiguing to listen to for hours on end. It will be kept around for awhile as a second radio, possibly for SO2R. Ultimately I expect to replace it.

When I shopped around for a primary rig that meet my new and extensive objectives there were several options I considered:
  • Used Elecraft K3/100, and upgrade the synthesizer to the K3S level of performance
  • New Yaesu K3S
  • Used Yaesu FTdx5000MP
  • Used Kenwood TS590SG
All of the above rigs have their good and bad points, and none to me stands well apart from the rest. It comes down to personal preference and the utility of certain features. It is also a matter of cost, which removed a few better rigs from the mix.

Here are a few rig options I rejected along with my reasoning. I expect some readers will be appalled by what I say, and that's fair. We all have our individual tastes and needs.
  • Flex: I do not like the user interface on these and other SDR based rigs. In the future when they integrate well with touch screens I will reconsider. But I will say that I really like what they're doing.
  • Icom: I do not like what I've seen and heard about computer interfacing. Otherwise their rigs seem quite good.
  • Kenwood TS990, Hiberling, Yaesu FTdx9000 and similar brutally expensive high-end rigs: Some of these rigs perform very well indeed. However there are severe diminishing returns with increasing price. I can do as well or better at a fraction of the price.
While I could go on bashing rigs that many others are in love with I'll stop there and merely repeat that I do not expect everyone (anyone?) to wholeheartedly agree with me. Should you feel compelled to comment do not expect a response from me.

The final faceoff

In the end it came down to the K3/K3S and the FTdx5000MP. Both have aspects that attract and irritate me. Neither is perfect. I have spoken to a number of hams I respect who will speak well of one or the other, and often they have owned both rigs in succession.

For example, there seems to be a consensus that the K3 receiver suffers in comparison to the FTdx5000 due to the synthesizer noise. There is a similar consensus that this deficiency is fully corrected in the K3S. That is why if I were to buy a K3 I would upgrade the synthesizer. Typically the K3/100 sells on the used market for CDN$2,000 in its basic form, and incrementally higher for those with a second receiver and optional roofing filters.

The used price of the FTdx5000MP has dropped to where it is competitive with a K3/100 with two receivers, ATU and roofing filters. This equivalence makes the choice more interesting. My ultimate decision comes down to the following:
  • Availability: Buying used means waiting for the right rig at the right price and in verifiable condition. I prefer to buy locally if at all possible to avoid shipping damage and to see it in person.
  • FTdx5000MP negatives: Weight, OLEDs, software and manufacturer support. This is a very heavy rig! It isn't one you travel with, the very opposite of the K3. The OLEDs are a known problem that Yaesu has never properly fixed and is almost certain to show up at some point. You can read the reviews to learn more. Yaesu is slow to push firmware updates and they are not very easy to load.
  • K3 negatives: Monochrome display, narrow front panel with overloaded controls. Even aficionados of the K3 will readily admit the display is small and difficult to read due to the lack of colour. The rig's smallness which makes it superior for travelling makes it cumbersome to operate. It is not as bad as the KX3 though still irksome in my experience. With practice I am sure it gets easier, and I have been assured that it does. Yet I remain unconvinced.
Then an FTdx5000MP from a local ham appeared on the market. It was local, at a fair price and lightly used by the original owner. I warmed up to the deal quickly.

Living with it

I have only had several days use of the rig. It sat unused for a while until the shack was complete and the cables routed. One warms to a piece of equipment only gradually even if it's a great rig. Similarly one will became increasingly annoyed when it falls short of expectations.

I would not hesitate to replace this rig if it disappoints. Whether a car, a rig or even a kitchen appliance, I never fall in love with a machine. It meets my needs and expectations or it is cast aside for something better.

The rig will soon be put to the test in contests and pursuing DXpeditions. By the spring I'll know whether this is the rig for me. From only a short period of use I have no hesitation saying that the FTdx5000MP has a far superior receiver compared to the FT950. That is no surprise. I cannot say how it compares to the K3S since I'd have to be able to use them side by side. However apart from the points I mention above I'd expect them to be similar.

I don't have a good reason to use the FT950. That's fortunate since I have the FTdx5000MP doing the antenna switching. I'd have to use manual switching to be able to use both rigs. So no SO2R for now, although the FT950 is sitting on the desk in perfect position for that style of contest operation. Another relative advantage is that the ATU in the FTdx5000MP has a wider range than the FT950. It easily tunes my 80 meter inverted vee on 30 and 160 meters, something the FT950 could not do. That's convenient for now when I have not raised antennas for those bands.

Shack evolution is ongoing

I moved more slowly than strictly necessary in constructing and moving into the new shack. There were important decisions to make beforehand if it was to fit well into my long term plans. This shack is for everyday use. For serious use, especially for contesting, a larger shack is planned for the basement level. I had to ensure that the locations, cable routing, switching systems and more were compatible between both shacks. It's worth the time to get it right.

Once I had the renovations done to the office space I had to decide on the location and detailed planning for the basement shack before choosing cable routes and, importantly, punching holes in walls and floors. It wasn't as easy as it might sound. There are the ordinary matters of furniture arrangement and placement of computers and rigs, and the technical matters of cable routing, switching system and location, basement shack outline, framing and wiring, and so forth.

Once I had all of that reasonably clear I punched a hole in the office floor. It is only 1" x 2", enough for several RG213 size coax cables and control cables.

I wanted the hole to be hidden while also not too ugly and accessible for maintenance. The rectangular hole is near an exterior wall in the corner adjacent to desks for the rigs and office use. It is lined by 2-sided tape and plastic floor edging. I cut through the laminate floor and plywood sub-floor with a drill and jigsaw. The internet Cat5e cable has its own pre-existing hole.

The exterior hole was a greater challenge. Its placement determines burial potential, grounding, accessibility, exterior cable termination and switching, and convenient cable routing within the house and shacks. It also has to look presentable and not be a safety hazard.

I chose a round conduit centred on a batten board that pierces the header sitting on the lower level frame. The house has a preserved wood basement, so there is no concrete to contend with. The ABS pipe serving as a conduit (Schedule 40, 1-½") is wide enough for multiple RG213 size coax cables and control cables. If necessary it can be later replaced by a larger conduit, though not much larger or the cut through the header will be too large. As you can see I've temporarily added protection for the cables from abuse by contractors until I can bury or elevate the permanent runs in the spring.

My intent is that eventually there will be either a flush mount box over the conduit to terminate all exterior cables and contain the switching systems, or a nearby ground mounted box to do the same. That way the number of cable coming into the house is kept to a minimum and the clutter of switching and cables is outside. Doing maintenance in the winter while not pleasant will be, in my opinion, better than larger conduits and interior clutter.

Switching between the office shack and basement shack can be placed in the basement. In most cases I expect that the switching can be manual since it is likely that equipment will need to be carried downstairs for major contests. As the technology evolves it is very possible that the rigs will be located downstairs and I'll operate as a remote from the main floor shack.

Time to relax and operate

In the few short days I've operated out of the new shack I can say with enthusiasm that it is very comfortable and convenient. I pushed aside some renovation and antenna farm tasks to do some operating. It's nice to sit in the shack and watch the snow falling outside while I tune the bands and work DX. I will return to the hard work of building my station soon enough.

Thursday, February 9, 2017

Fatal Attraction: Backhoes and Towers

Operating heavy equipment is a skill. The best that I've observed perform real artistry. Crane operators are among the best I've seen, and even worked with a few times on tower jobs. At the other end of the spectrum there are those who are not at all artful, and can even be a danger to life and property, even to themselves and the equipment.

Operators of small machinery cover a wide spectrum of ability and care. In particular I am thinking of backhoes and front-end loaders. Too often they are under tremendous time pressure by those hiring them and the equipment they're operating. Time is money after all. Slow careful work is appreciated though not with money when the job time stretches beyond budget. Operators are pressured by both the client and their employer to get the job done quickly and adequately, then get out of there and on to the next job site.

Standing amateur radio towers are given a wide berth just as for any permanent structure. Damage to a structure is always expensive to all concerned and likely a career limiting move for the backhoe operator. Tower sections stored on the ground are another matter. To the unschooled eye they can look like any other pile of refuse, just some scrap metal littering the job site. In the rush to get the job completed these stored towers are at considerable risk of an accidental contact. When that happens the tower always loses.

Far enough isn't far enough

Last week I had such an incident. This is especially galling for two reasons. One is that I am well aware of this attraction between towers and backhoes, and I have a lot of tower stored here. The other is that I spoke to the project manager beforehand, pointed at the stacked DMX tower sections and asked him to take care not to damage them. Although old and not in perfect condition I have plans for this tower.

He assured me all would be well. After all they were at least 6 meters from the excavations side, up against the stone wall and behind two fairly large spruce trees. It wasn't far enough. I should have known better. Luckily I had moved the more valuable LR20 tower sections much further away, so they were far more safe from an accident.

Promises are not promises

The project manager is not a fool nor is he careless. However he was in a rush to get the job done since his customer (me!) was getting increasingly irritated at the many delays getting the work underway. When the backhoe his company rented, and which he operated, wasn't large enough to easily penetrate the now frozen ground (because of the delay getting started) he called in a larger machine and operator to complete the excavation.

As you might guess that's when the accident occurred. In the fast two hours it took to complete the excavation some of the debris he was piling up tumbled down the far side, between the trees and into the stored tower. No one at the time noticed what had happened. Frozen soil broken up by the backhoe is really a bunch of large oblong boulders. If it was just granular dirt nothing untoward would have occurred. When a 50 to 100 kg boulder rolls down a slope into a pile of steel something is going to give.

Promises are not to be relied upon. No one was being careless. Everyone was simply trying to keep the cost down and make me happy. I was not happy when I discovered what had happened two days later.

Assessing the damage

These tower braces can be repaired
Once I realized what had occurred I made a phone call and at least got that complaint off my chest. Then I began the task of removing the partially buried tower sections. The ones at the bottom were frozen to the ground and had to be pried loose for removal and inspection.

I was lucky. Only one section was damaged and it is damage that I believe can be repaired. Although this is not expensive tower to replace it is inconvenient and time consuming to locate and acquire.

This is not the first time I've had tower damaged by a backhoe. When I had a house built for me many years ago I stored my 64' tower at the very back edge of my property where even grading work would not take place. Yet a backhoe found it.

My contractor apologized and took responsibility for the actions of his subcontractor. He did point out, however, I ought to have moved it off the site entirely. We agreed on a mutually acceptable settlement that didn't involve money changing hands. I really should have stored it on my neighbour's property.

About that Trylon

Not repairable: tower leg must be replaced
Last summer I acquired 6 sections of Trylon tower essentially for free. The reason was, as you might guess, backhoe damage. The ham selling it realized after close inspection he could not demand any price without extensive repairs.

These became the top two-thirds of the tower I recently put up. I took it off his hands as part of a larger deal and undertook repairs at my own expense and time. I knew what I was getting into since I've done this before.

Several components could not be saved and had to be replaced. Others I was able to repair with some careful assessment by me and, by photograph, with an engineer. Most of the damage was repairable by disassembly, bending steel and then reassembly and alignment.

Repair vs. replace

I have a future article planned on this very subject of tower damage and repair. It may even get written eventually when I find the enthusiasm to do so. However, unless you are up to it I recommend replacing not repairing damaged towers.

Considering the unavoidable attraction between towers and backhoes this is a not uncommon dilemma faced by many hams. In fact I suggest you closely inspect any used tower you are planning to buy for damage, not only backhoe damage.

Prevention still remains the best cure for this problem. If you must store a tower where heavy equipment is present you know what to do.

Tuesday, January 31, 2017

40 Meters 3-element Wide-band Wire Yagi

As I look forward to having a tall tower later this year I am talking to others while reviewing my objectives to make a final determination on what antennas to focus on first. This brought me back to 40 meters and wire yagis. I wrote several articles on this topic in 2015 and these remain among the most popular according to web site statistics and referrals from other sites.

This popularity is not too surprising to me since 40 meters is the first band for which an antenna with gain requires more effort than buying a small bundle of aluminum and tossing it up on a tower or roof. Whether you contest, DX and just want a reliable band for rag chewing through the downside of this solar cycle 40 meter antennas with gain are an asset.

Where I am and where I'm going

The XM240 currently up 21 meters atop the free-standing tower is adequate for short paths, including Europe and the US. The bandwidth is poor with respect to gain, F/B and match. Regarding the latter two the deficiency is readily apparent. Gain is not easy to assess without the ability to A-B test against another antenna of know characteristics. Nevertheless this is a perfectly fine antenna, even with its limitations, and I am happy with it.

A full size 3-element rotatable yagi up 43 meters, should the plan come to fruition, will do wonders for longer path DX and contesting, especially to snag rare multipliers and grab those rare and distant DXpeditions. For the high volume contest paths to the US and Europe it will help very little since those are well addressed by an antenna at half that height due to the typically higher radiation angle associated with those paths.

My tentative plan was to move the XM240 to the guyed tower when the tower is finally raised later this year, side mount it at a height of around 25 meters and make it fully rotatable (300°). However this introduces a problem. The US and Europe are in roughly opposite directions and the paths are frequently open at the same time. Rotation is impractical for switching between working those two paths during a contest.

This brought me back to the idea of a fixed, reversible yagi pointing northeast and southwest. If made of wire it can be built at low cost and would provide excellent diversity for contest and everyday operating. One disadvantage of that antenna is its narrow SWR bandwidth, although better than a short 2-element yagi like the XM240. If I were only interested in CW and digital modes that would be no problem. SSB coverage without matching and switching complications is desirable for contests.

The time had come to do some modelling.

Improving the 3-element wire yagi: coupled resonator

I will take the reversible 3-element yagi with inverted vee wire elements and try to improve it to meet my current needs. This is what I want to accomplish:
  • Keep the gain as high as possible, especially in the CW band segment
  • Low SWR across the band -- 7.0 to 7.3 MHz -- without switching or tuning
  • Instant switching between directions
  • Similar performance -- gain, F/B, match -- in both directions
This is a tall order. Per my earlier models on rotatable 40 meter yagis there are really only three methods of extending bandwidth:
  • Switched matching network, which adds complexity and operating inconvenience
  • Parasitically coupled resonator or a dual-driven element, both which add a fourth element
  • Spreading the resonance of the reflector and director, which lowers gain
I chose to go with the simplest solution: the parasitically coupled resonator. This is a popular technique for achieving a wideband 50 Ω match on multi-element HF yagis, such as the OWA series of designs by WA3FET. I previously used on to achieve a full band match on a 3-element yagi.

There is a substantial cost for the additional element -- money, wind load, weight -- on a 40 meter rotatable yagi, a cost that is almost entirely avoided with a fixed wire yagi. It was worth a shot. Let's review how I did.

Modelling procedure

To begin I used the exact dimensions of the director and reflector from earlier 3-element design. Recall that it is these elements, not the driven element, that determines gain and F/B and SWR bandwidth in a 3-element yagi. Eventually some change became necessary, which I will come to shortly.

The driven element and coupled resonator were initially separated by 1 meter per my modelling results from the full size yagi design with a coupled resonator. That antenna covered the entire 300 kHz of the 40 meter band with impressively low SWR. Rather than borrow one of WA3FET's designs I based mine on seeing them and combining that with what I know of yagis. I must have done something right!

I placed these two elements -- which together comprise the driven element system -- symmetrically around the centre of the boom to best preserve equal performance in the forward and reverse directions. True symmetry is however only possible if the driven element and coupled resonator are reversed along with the director and reflector, and I chose not to do this to keep the design relatively uncomplicated. There is therefore some asymmetry in performance.

Modelling a coupled resonator requires some care since it is only a short distance from the driven element with respected to wavelength: 1 meter = 0.024λ. Segment lengths in both elements must be made as equal as possible, even as the lengths are adjusted during modelling, or there will be significant errors. A short 1 segment centre 10 cm long wire is inserted in all elements to minimize NEC2 inaccuracy due to the angle between element legs and to facilitate insertion of source and loads.

When I started modelling this antenna I expected that it would be more difficult to achieve a broadband match as easily as with the full-size yagi. This is indeed what occurred. The reasons include:
  • Thinner conductors make for inherently higher Q antennas
  • Bending dipole elements into a vee lowers the radiation resistance, both increasing loss and constraining the current range in the driven element and couple resonator, which in turn affects the feed point impedance
Without a firm theoretical knowledge of what should happen I ran a series of modelling experiments to see how far I could push and prod the antenna towards my performance objectives. I used the current and net reactance to adjust each subsequent run. Ultimately I had a reasonable design with a great SWR and acceptable gain and F/B.

As it turned out that model was wrong. Checking the antenna with the average gain test in EZNEC I discovered that I needed to double the number of segments to coax NEC2 to generate more realistic results. The final model has about 40 segments per half leg, with some variation to equalize segment lengths in all elements.

The net gain is reduced by approximately -0.3 db due to copper conductor loss (AWG 12). The driven element and coupled resonator separation finally settled at 1 meter, exactly where I started. As far as I could discover this distance, while not overly critical, seemed to give the best results. Unfortunately this turned out to be large enough to make performance asymmetrical between forward and reverse directions.


Achieving nearly full band matching with an SWR below 2 required spreading the tuning of the director and reflector. To do this the director was shortened a small amount and the inductor load on the reflector was increased. The reduction in gain of about -0.2 to -0.3 db was, in my judgment, a good trade off. See the above EZNEC SWR chart of the yagi in the forward direction.

However performance is worse in the reverse direction so if you build this antenna you would have to pick your favourite direction and orient your driven element and coupled resonator accordingly. SWR bandwidth is reduced by ~30 to 50 kHz. A static or switchable L-network can help to improve the match, in both directions. I experimented with this but I am not prepared to make a firm recommendation on which is best. There is enough doubt in the quality of the modelled impedance that I would first build and tune the antenna, and only then measure the feed point impedance and build an L-network to transform it to 50 Ω with, hopefully, a far improved SWR.

Calibrating reality to the model

Wire gauge, insulation and other modelling inaccuracies make it vital to calibrate construction of the yagi. You cannot simply cut wires to the lengths found in the model and expect to achieve the modelled behaviour. For a simple dipole or vertical making an adjustment after construction is easy. Not so in this antenna unless you have a great deal of time on your hands.

Perhaps the simplest method of calibration is to build and tune one element so that its resonant frequency is identical to that in the model. In this wire yagi the best element for doing this is the director/reflector elements (excluding the reflector coil) since its tuning is critical to gain and F/B performance and any matching issues can be dealt with in the driven element and coupled resonator which are within reach of the tower.

To do that the modelled resonant frequency must be known. I deleted all of the wires from model other than the director and measured its impedance in free space and at several heights. In free space it is 56 Ω at 7.375 MHz. At the reference 25 meter height it is 48 Ω, also at 7.375 MHz. However at other heights the resonant frequency is different: 7.425 MHz at 20 meters and 7.350 MHz at 30 meters. For other heights you should run a model first. Remember to put the element under enough tension to remove sag that would reduce the interior angle and raise its resonant frequency.

Once the element is built and tuned to the corresponding modelled resonant frequency the other elements can be scaled accordingly. In my model the total element lengths are as follows:
  • Director and reflector: 19.56 meters, with a 1.95 μH centre coil switched in for the reflector
  • Driven element: 20.72 meters
  • Coupled resonator: 20.79 meters
The interior angle of the inverted vee elements is 120°, just as it was in all the other 40 meter wire yagis I've modelled on this blog.

The angle can be reduced if that's desirable or necessary in smaller areas. Should that be done the above measurements are no longer valid. The revised antenna would need to modelled, not simply scaled. It is not a trivial task to come up with a optimized design for these variations. So be prepared to do some work. Do not expect that you can go ahead and build the antenna per my measurements and then merely cut and trim to get similar performance.

Comparison to other yagis

I took several yagis from previous design articles to compare these wide band wire inverted vee yagis.
  • XM240 (proxy design since NEC2 cannot accurately model this antenna)
  • 3-element reversible inverted vee wire yagi
  • 3-element full-size yagi
  • 3-element full-size yagi with a coupled resonator
All antennas were modelled at an apex height of 25 meters over medium EZNEC ground [0.005, 13], including conductor loss. All the models were improved from their original appearance in this blog to ensure that gain errors due to segmentation problems are correct to ±0.2 db.
I plotted the actual maximum forward gain without regard to elevation angle rather than the 10° I used previously for general DX usage. From my QTH Europe and the US paths are typically associated with higher elevation angles. Since the antenna apex heights are the same (average height of inverted vee elements is lower) the maximum gain is within a narrow range of elevation angles -- 21° to 24° -- tending lower as frequency increases, as expected.

Bandwidth for 2:1 SWR ranges from approximately 150 kHz for the XM240 proxy, 200 kHz for the 3-element yagis, 250 kHz for the inverted vee yagi with a coupled resonator and more than 300 kHz for the full-size yagi with a coupled resonator.


There are a few things that jump out of these charts. One is the different behaviour of 2 and 3-element yagis. This was expected and is typical. The other is how the varieties of 3-element yagi behave at the upper end of their usable range. What is different is how their performance diverges.

Some of that performance can be recaptured by shifting the frequency range of the array upward, at the expense of some gain and F/B at the bottom of the band. Since I'm primarily a CW enthusiast I choose not to make that compromise. You might choose differently.

The wire yagis have lower gain, which is expected. This is due to conductor losses and the vee shape (typically -0.3 db) and the lower average height. Yet for most of the range the gain difference is quite small, coming in at around 0.5 db. The gain loss is greater at the top of the band. As already mentioned this can be corrected, with trade-off at the bottom of the band.

In every case the 2-element yagi is a poor performer in comparison to the simplest 3-element yagi. That does not make the XM240 a bad antenna, just one with the inherent limitations for any antenna of this type. It is still far superior to a rotatable dipole or fixed inverted vee, and without excess requirements for a rotator and tower.

Will it be built?

Short answer: I don't know. First the tower must be built and something put up at the top, 43 meters up and more. It is possible that if I feel the pressure of time I will instead do something different on 40 meters for at least the rest of 2017. In this same my fallback option for the top of the guyed tower is the XM240, if I run into difficulty putting up a full size yagi this year.

Even if I build a wire yagi it may be the 3-element design I based this one on, just for the symmetry and easier construction and tuning. Unlike in that article, since I will only have the one tall tower this wire antenna will require a more conventional, although it will not be a continuous conductor to avoid interactions with other yagis on the tower. There are a few ways to accomplish this feat despite its great length (48' or 14.5 meters).

Stacking with the top yagi is on the agenda. Again, that will depend on time. Once the top antenna is selected I will model the antennas to determine whether stacking can be beneficial. It often isn't for yagis this close together -- 18 meters or 0.4λ.

Not matter what I need diversity on 40 meters and that calls for more than one antenna.

Thursday, January 19, 2017

Winter 80 Meter Inverted Vee

I need an antenna for 80 meters. With the declining solar cycle the MUF easily falls below 7 MHz, reducing the opportunities for evening operation. There is also the matter of contests where one cannot possibly compete without 80 meters. With the North American QSO Party CW rapidly approaching I moved quickly. Yes, 160 matters as well, but one step at a time. I had a kludge prepared for 160 meters.

With the only the one tower as an easily available support my options were limited.The taller trees in the vicinity would certainly support a wire vertical or an inverted-L the time and effort required was not justified. Since radials are out of the question with this tower I decided to keep it simple and put up an inverted vee.

I toyed with the idea of modifying my multi-band inverted vee to extend the 40 meter element. That would remove interaction with the XM240, get me on 80 and on 30 and 17 meters with a resonant antenna. I stretched out the antenna to consider how to do it and decided that the mechanical changes would make the project difficult. Doable, certainly, just not quickly.

I instead decided to roll out some fresh wire and put up a monoband inverted vee for 80 meters. The antenna could be left where it is or deployed on another tower later in the year to complement the planned vertical array. A moderately low horizontal antenna for 80 meters usually does better for the short paths needed to draw in QSOs with the northeast US, and can sometimes outperform a vertical on select DX paths at sunrise and sunset.

I oriented the legs north-south to avoid the house. It is therefore primarily horizontally polarized in the east and west directions and vertically polarized to the north and south. The adjacent EZNEC view of the antenna gives an idea of how it's situated.

The apex of the vee is 2.5 meters below the XM240 boom, which is 18.5 meters high. Since an 80 meter λ/2 wire is not resonant on 40 meters I anticipated little interaction. I reasoned that even if there was some it would have to be tolerated for the time being.

A coax coil for a common mode choke is difficult on 80 meters. It takes a lot of cable wound as a single layer coil on a rigid form, all of exacting dimensions, to get a high impedance at 3.5 MHz. A scramble wound coil of indeterminate size will not work the way it can on higher bands. A few months ago I purchased several 1:1 toroidal baluns from another ham, and this was the first opportunity to put one of them to use. I tested them first on the work bench with an antenna analyzer and a few non-inductive resistors.


The construction of the feed point is mechanically robust, taking advantage of the Balun Designs backing plate and integrated hose clamps. They are designed for a boom mount. A 3' length of Schedule 40 ABS pipe serves as a boom proxy and as the centre insulator. The back end of the pipe (not visible) is secured to the tower with a u-bolt. Scrap dacron rope ties the forward end to the tower. The pipe level is just above the Tailtwister.

I installed the antenna on the same climb that I recalibrated the mast after a wind storm. I even dragged a 40 meter length of RG213 up with me. This was the old coax that previously showed a DC short. A new connector solved that problem. The repaired cable tested good with the analyzer. The vee legs and coax twirled together when hauled up by rope. I untangled what I could on the way down as I taped the coax to the tower, then puzzled out the rest when I back on the ground.

One nice thing about having a lot of land is you can do things with antennas you would never do on an urban lot. I paced out one side of the inverted vee and found myself in an empty expanse of grass. I marked the spot in the snow and went looking for a suitable rock: not too heavy to carry yet heavy enough to take the tension needed to remove wire sag. The other end was conveniently right where I stored the LR20 tower and could simply be tied to a section.

I connected the analyzer to the coax and found that resonance was around 3.650 MHz. The SWR at 3.5 MHz was too high. I moved to a wider set of anchor points (dragging the rock and tying the other end to a further tower section) to lower the resonant frequency. That pulled resonance down to just below 3.600 MHz which lowered the SWR at 3.5 MHz below 2. The larger interior angle increases risk of yagi interaction but that was easier than lengthening the antenna. Presumably the insulation had a higher velocity fact that I had included in the software model.

On the air

The wait to test the antenna was brief since I finished the work an hour before sunset. The band was good and soon after sunset the log filled with Europeans plus A45XR. The antenna clearly worked, possibly better than the tower vertical I had in Ottawa. However that's difficult to tell.

An inverted vee with an apex not quite λ/4 high is a poor radiator at low elevation angles. On the other hand a vertical with a poor radial system and surrounded by metal-rich suburban houses has higher ground loss and other environmental loss. A proper comparison is of course not possible so I can only speculate based on educated guesses of the prevailing factors in each case.

The antenna performed well in the NAQP contest. There was really nothing I could hear that I could not work. Calling CQ generated modest runs. The antenna meets my expectations for contest and DX operation. It'll do fine as an expedient solution for the duration of the winter.

Interaction

To my surprise and dismay the inverted vee interfered with the behaviour of the XM240. This is despite its non-resonance on 40 meters. The yagi's SWR would swing depending on direction (orientation to the fixed inverted vee) and the F/B declined on some paths. The F/B is already poor as expected for a 2-element yagi so the degradation was unwelcome. Looking west toward the Pacific the F/B was especially bad. It takes little disturbance to upset the fine balance of phase and amplitude between the driven element and reflector to get the best F/B.

I ran variations of the software model shown at the top of this article with different separations and orientations. The modelled performance was very good. There was only a slight distortion when the yagi's elements were parallel to the inverted vee. Measurement with an analyzer showed no resonance near 40 meters on the inverted vee.

Although the inverted vee alone is non-resonant I reran the model with a 40 meter long transmission line equivalent to RG213, with either a short or open at the shack end. Still nothing. Perhaps there is some effect due to the balun that is exciting a resonance that the analyzer does not see.

For now the interaction remains a mystery. Opening up the vee to tune it may have been partly responsible for a non-resonant coupling effect. Even so it is not evident in the model. Although he problem isn't so serious that I absolutely must track it down I want to understand what is happening. I may experiment with it a little to see what I can learn.

Looking ahead

After the contest I resumed DXing on 80 meters. This is when I was able to better discern the antenna's limitations. Operation on the low bands is primarily noise limited, both at your end and the other end. I find there are a large number of signals just riding within the receive noise level. Those I call tend to find the same with my signal. Only a few decibels would make a world of difference.It is particularly galling when a fairly rare DX station copies me fine while I struggle to pull him through.

While a higher horizontal antenna or a vertical with a large radial system will go some way in solving these problems that is only a partial solution. I need a directive, low noise receiving antenna for 80 and 160. That is in my plan for 2017, and perhaps soon. The other need is power.

Of course one can always watch and wait for a rare 3 minute opening to a far country that comes only once or twice a year and so make incremental progress toward 300 countries over the years. Many have done so with surprisingly modest suburban stations. That does not suit my particular objectives. When the DX is there I want to work it. When the contest is on there is no possibility of watchful waiting. Note how this is very different than how I operated for several years with QRP and small antennas. But then I always did poorly on 80 and 160, with some notable exceptions.

The next significant test for the inverted vee will be the ARRL DX contest coming up in February. I am keeping my expectations for 80 meters very modest. My expectations for 160 are worse: unlike CQ WW we cannot work Americans for points.

Third harmonic

I still have no antenna for 30 meters. I hoped that this antenna would serve as a stopgap until I could put up a proper antenna for that band. Much like a 40 meter dipole can perform well on 15 meter, at its third harmonic, the 80 meter inverted vee has a third harmonic near 30 meters. The model and the measurement agreed closely on this score, delivery a resonance at 10.8 MHz.

Unfortunately while that may seem close it really isn't. The SWR at 10.1 MHz is high. The rig's internal antenna tuner cannot deal with it. An external tuner is required. There is no convenient way of switching the tuners I own in and out of line. They lack that feature. I did experience one fortunate instance of freezing rain this week that lowered the resonant frequency enough for the rig's tuner to bite, allowing me to work a VK2.

I did a manual insertion of a tuner during NAQP to use this antenna on 160, with the coax shield disconnected. The tuner was adjusted prior to the contest, bringing the SWR within range of the rig's tuner. It did surprisingly well on 160 for what is in reality a dreadful antenna. But as in the parable of the dancing elephant you should not criticize its dancing but rather be amazed that it can dance at all.

Sunday, January 15, 2017

Hope and Wind Are 4-letter Words

Last week we had a wind storm that provided the first test of the survivability of the new tower and antennas. While everything did survive there was an effect that calls attention to the danger of assumptions and the best laid plans. Worse is relying on hope alone, which is all too common in the amateur ranks.

I didn't do this yet I did fall victim to my assumptions and a "sensible" risk assessment. As with hope, wind is also a 4-letter word. (For those outside the North American English language culture, many of the common obscene/swear words are 4-letters long.) There are no pictures of the antennas in this article. It would not be informative since a picture would only show the tower and antennas standing there as usual. A picture would not show what happened.

The wind

This was not a severe weather event. For many it was welcome since it brought a strong southerly air flow that quickly raised the temperature to a near record high. The winds were sustained at and above 50 to 60 kph for almost 12 hours with gusts up to and possibly exceeding 90 kph. A better estimate isn't possible since I am not near a weather station and the darkness did not allow for direct observation. Instead I slept, reasonably secure in the confidence of my planning.

The wind was within my survivability threshold even with the relatively large wind load I have temporarily placed atop the Trylon. Since the wind area is at its lowest with the yagis broadside to the wind I turned the yagis northwest at the start of the storm. The wind was from the southwest. In the morning I discovered that they were pointing northeast, with the booms aligned with the wind. That is, the mast slipped within the Tailtwister mast clamp until the maximum wind area was exposed to the storm wind.

Precautions taken

When I installed the mast and rotator I deliberately avoided pinning the mast to the rotator. I didn't even excessively torque the u-bolts of the clamp. In consideration of the antenna wind area my preference is that if the yagis want to move I'll let them. I'd rather that than have the peak torque transferred to the tower. Climbing the tower and turning the mast is preferable to risking the integrity of the tower.

Some hams don't like the idea of climbing, or depending on others to do it for them, and prefer to pin the mast. A safer approach is to wind balance the yagis, whether home brew or commercial product. There are software tools available to do this, or one can follow the construction details of yagis that have been wind balanced, such as those in the ARRL Antenna Book. My yagis are moderately well balanced, however I have not confirmed that.

Calculation of antenna wind area

Calculating the wind area of elements can be time consuming and prone to error. There are many factors: taper schedule, boom-to-element clamps, tubing clamps, coils, traps, capacity hats, linear loading, matching networks, etc. Most just go by what the manufacturer claims in the product specification. This can be misleading since some report the highest wind area (dependent on antenna orientation to the wind), lowest wind area or something in between.

Booms are more easily calculated. There's just the aluminum tubing, clamps and a small protrusions such as beta matches, baluns and trusses. However it is more typical that the elements have a greater area than the boom so we need to know that area. It is also helpful to know that the maximum wind area is when either the boom or the elements point directly into the wind, not at some point in between. I refer you to the excellent work of K5IU and the cross flow principle in Spring 1993 Communications Quarterly (I can't locate a link right now). Yagi design tools such as YagiStress now include this work.

I finally decided to buckle down and do the calculation with a calculator in one hand and the XM240 and Explorer 14 manuals in the other. I'll spare you the messy details. Even with all the calculation there will be some error in the true wind force since antenna elements with many diameter transitions are imperfectly modelled by the long cylinder wind load coefficient.
  • XM240 boom: 5.5 ft², including estimates for the balun, boom-to-mast clamp and boom truss.
  • XM240 elements: 3.7 ft², including an allowance for element-to-boom clamps, tubing clamps and the capacity hats.
  • Explorer 14 boom: 3.5 ft², including estimates for the boom-to-mast clamp, beta match and coaxial choke.
  • Explorer 14 elements: 10.2 ft², including an allowance for element-to-boom clamps and tubing clamps.
The wind area of the tri-bander elements is higher than I expected and is more than the 7.5 ft² I found in my copy of the Hy-gain manual. They appear to be using the average of the boom and elements. That is misleading and potentially dangerous. Cushcraft does better, specifying 5.5 ft² which agrees with the maximum value that I calculated.

Many hams who stack yagis are aware that tri-bander elements have a large wind area and choose to mount those close to the tower top plate and an XM240 or similar antenna above it. I know it as well but chose to do the opposite for expediency considering the weather difficulties of winter antenna work. Getting the heavier antenna higher and the even higher boom truss is not easy in our winter weather.

I performed my survivability calculations accordingly, but with the published wind load value. That was a mistake even though I know to turn the booms broadside to the wind. Recalculating for the yagis pointed into the wind the mast survivability dips to below 100 kph. That's quite poor, although I was conservative in choosing the steel strength estimate for the mast. In fact the range of wind survivability is 30+ kph, depending on the wind direction.

Shadowing

As wind speed increases there is increased shadowing for close spaced element. This is due to the time it takes for the airflow to recombine after being diverted around an antenna element. Laminar flow is not immediately reestablished, instead resulting in turbulence.

The only likely location of shadowing in my installation involved the so-called para-sleeve (coupled resonator) surrounding the Explorer 14 driven element. If there is shadowing the wind area reduction in high winds is probably no more than 1 ft². That isn't helpful.

Conclusion

The tower survivability remains sufficient even with the revised wind area calculation. The mast however is more of a concern than expected. All I can do for the time being is to lower the yagis a small amount and gain 10 kph of wind survivability. The XM240 can go down 6" so the Explorer 14 could go down a foot or more and only risk added 15 meter pattern distortion. With 15 meters so poor at this point in the solar cycle that may be the right thing to do. It's only until spring.

I could also increase the mast clamping force to reduce the chance of the mast turning in a high wind since weather vaning seems to be a problem. Or the clamp could have an added insert that bites into the metal rather than solely rely on friction. But pinning the mast is a step too far in my opinion. A few days after the storm I reoriented the yagis, put a little more torque on the rotator mast clamp bolts and tightened some other fasteners I had suspicions about.

While I am not terribly concerned about the mast it would be inconvenient if it were to bend. Hopefully statistics and bolt torque are enough protection. Otherwise I'll need to take action or accept a higher risk until spring.

Friday, January 13, 2017

2017: The Adventure Begins

What a year 2016 turned out to be at VE3VN. What was a strong possibility at the beginning of the year became a reality. I am now committed and facing 2017 with a mix of hope, trepidation and a sense of accomplishment. As seems to have become a tradition for me in this blog, every January I write a retrospective of the past year and look forward to the new year. In particular what plans I have and why.

Onward with the 2017 edition. You can read the 2016 edition to form your own impression of how well I did. I scored close to 100% by my estimate. That's a pretty good prediction. I doubt I'll score so high in 2017 but then what's the point of objectives that are not ambitious? So let's get into it.

Where I stand

The first tower is up and decorated with aluminum. My hope to at least get started on the 150' guyed tower got squashed by the weather. The concrete work ran into problems and I was only able to plant two guy anchors. The remaining anchor and the base were ready for concrete, and was in fact ordered, when things went south (as we say).

It could have turned into a financial disaster. Although that was averted the consequence was that work had to stop for the winter. I'll have more to say on the topic later. As of now the work is planned to resume in early spring when the weather warms up and the ground is still frozen. If nothing else the delay gives me time to better approach the work involved in a project of this size.

To get by for winter operating I put up an inverted vee for 80 meters -- more on this in a future article. Its apex is only 19 meters to keep it out of the way of the yagi rotation loops and reduce the potential for interaction (yagi pattern disturbance). Although not an ideal performer it does pretty well. What it does very well is to get me on the band. Without it my operating would be too limited due to the short days for high band operating and the early closing of 40 meters. Yes, the MUF easily falls below 7 MHz on many paths with the solar flux hovering in the 70s during the long winter nights.

I may yet do something for 30 and 17 meters, but I don't want to clutter the tower too much. The XM240 loads well enough on 17, but none of the antennas loads well on 30 meters.

For the present I have no shack. The coax and rotator cable route to the upstairs balcony door and into the master bedroom. There is no operating desk. It's uncomfortable. This arrangement is necessary until renovation on the house reaches the point that I can make better arrangements without risking contractor damage to the installation.

Riding out the winter

I had hoped to have the LR20 planted so that I could slowly raise the 150' tower over the winter as the weather allowed. Since that won't happen I have extra time on my hands. What it means is adjusting my priorities rather than doing nothing.

My objectives for the winter are modest:
  • Do something to get on 160 meters. This is needed in contests to add multipliers and whatever QSOs I can garner. Shunt feeding the tower is a possibility, however it was not designed to allow for radials: there just isn't the room. But then a few meandering radials is better than nothing. Other alternatives are more difficult or problematic for a short-term solution. For example, inverted L or planting a vertical in a field somewhere.
  • Try a directive receive antenna for the low bands. I have what I need to put up a Beverage antenna. All I need to get out there and run some wire towards Europe and run some small coax back to the shack. It can be removed in the spring before it becomes a navigation problem.
  • Build the first shack. I intend to build two shacks: one for everyday operation in my main floor office and another in the basement for multi-op contesting. The main floor shack is the priority. The downstairs shack needs to be located, framed and wired. That work will begin this winter. Finishing it may be put off to the autumn.
  • Plan out the important tasks of automating antenna switching and filtering necessary to SO2R and multi-op contests. I don't need to build much right now. The critical part is having a detailed plan to which I can gradually work towards. One decision I need to make is how much to buy and how much to build. The former can be expensive but is expedient.
  • Contest! There are contests to enter and I plan to get in there with what I have. Despite just having the one tower what I have is pretty good. It only seems poor in comparison to what I am working towards.
There is lots of ongoing house renovation work. Some I am doing myself. By the spring I expect the house to be far more livable. I also need things I didn't need in the city. For example just this week I acquired a garden tractor. In addition to its ordinary uses it will play a role in tower work.

With snow on the ground I am also able to explore my 50 acres better than before. With snowshoes I was able to get out into the swamp and cross overgrown and bush areas. I do this to become acquainted with what I own and to inspect potential sites for low band receive antennas.

Categorization of tasks for the year ahead

Before I delve into specifics I'll list the categories of tasks I've set for myself in 2017. I don't really expect that all my plans will come to fruition because, well, life happens. From the categories you will gain some insight into my approach to building out the station. In any large project it is advantageous to break it down into pieces, otherwise it may be too complex to attack.
  • Structures: this includes towers and other antenna supports
  • Mounts: mechanical design and construction to affix antennas and rotators to structures
  • Antenna design: software model evaluation for target usage, before committing to building
  • Antenna selection and construction
  • Interaction: necessary for contests and important for achieving modelled performance.
  • Switching systems for direction control: start with mechanical switching and gradually automate
  • Switching systems for antennas and operating positions/shacks
  • User interfaces (software and hardware) for selection of antennas, direction and operating position
  • Low band receive antennas: choose sites, designs and get them built
  • Transmission lines and control lines: selection, burial, connectors, etc.
  • Rigs: The FT-950 with be supplemented with a better transceiver suited to my new objectives
  • Amplifier: It's time to return to QRO operating, at least some of the time
Spring plans

I can't mention spring without talking about the Dayton Hamvention. While plans are not final it appears I will be attending this year for the first time since around 1991. For me it is an opportunity to become reacquainted with the broader contesting community and to do some shopping. Well, actually it'll be a lot of shopping. There is a great deal of small parts and equipment I need that are inconvenient to gather piecemeal by mail order.

The first order of business in station building is to finish concrete work on the guyed tower and get it raised. By the fall I intend to have several yagis on that tower including two 40 meter yagis. What I can't say for the moment is whether the high yagi at 43 meters will be 2 or 3 elements, at least this year. Joining it up top will be a tri-band yagi or a 20 meter monoband yagi. They will be turned with a prop pitch rotator. These high antennas are for the difficult long distance DX paths for general DXing and acquiring precious DX multipliers and contacts necessary to contest scores.

The antennas on the Trylon tower will all come down. The likely candidates to replace them will be a TH7 I have in storage and the redesigned A50-6. The old multi-band inverted vee may join them to get me on 40, 30, and 17 meters until I can do something better for the latter two bands and to allow 40 meter operation while the tower work is ongoing.

Transmission lines and control cables will be buried. The Heliax will finally be put to use. I will run LDF7 up the Trylon for 6 meters and perhaps other VHF bands with remote switching. The workhorse for the guyed tower(s) will be LDF5. I mayy need more to get through 2017 than what I current have on hand. LDF4 will be used for HF on the Trylon and for some low band antennas with long runs. This work will stretch well into the summer.

Summer

Once the basics are completed in spring I have some difficult decisions to make. My objective is a modestly competitive station for SO2R and small multi-op (multi-single or multi-two). The antennas must support that. The waning solar cycle is a helpful constraint since it allows me to defer major antenna effort for 10 and 15 meters.

I have 3 tri-banders in my stock: Explorer 14, TH6 and TH7. One of these may go on top of the big tower, as I mentioned. As already mentioned the Trylon probably gets the TH7. The remaining one will be side mounted on the guyed tower. Whether it is fixed, rotatable over 120° or 300° will depend on my progress on other projects.

A one acre plot will be selected for an 80 meter array. I am seriously considering the 3-element switchable vertical yagi I described previously. Of course a 4-square is a more typical choice but there are advantages of physical simplicity, cost and the opportunity to experiment. After all, my objectives are not limited to operating or having a big signal but to learn and try new things.

For 160 meters I will most likely choose a vertical integrated with the 80 meter array. Anything better will almost certainly be deferred to future years.

Once I decide on my tower configuration for the year I will build yagis to suit. I'd rather build than buy and yagis are really not difficult to construct. The two 32' booms I have in stock will either become 5-element yagis for 15 meters or 4-element yagis for 20 meters. Or perhaps one of each is more sensible. If I build one for 20 meters it may go on top of the 40 meter yagi, with tri-banders fixed or rotatable at a lower height or on a separate tower. When used for 20 meters the boom may require strengthening.

Autumn

A stretch objective for the autumn is a second guyed tower of similar height to be placed on the south side of the house. That tower will allow excellent separation and diversity for contests and also remove clutter on a single tall tower. Stacks for 20 and 15 meters are possible.

With that second tower I am giving serious thought to a 3-element wire yagi for the first guyed tower, switchable between Europe and USA, at an apex of around 30 meters. Any higher and there would be interactions with the 40 meter on top of the tower, be too close for future stacking and too high for those productive paths. This antenna becomes possible by reducing clutter and interactions by putting 15 and 20 meter antennas on the second guyed tower.

Next winter

I will probably back off the outdoors work since I've had enough of miserable winter tower and antenna work this year. Perhaps I will play with low band receive antennas, but little else. The shack is where I plan to spend most of my time, improving the operating positions for comfort and with increased automation. I may get far enough along that I can organize the first multi-op for my station.

Summary

My plan for 2017 is ambitious. This retrospective and looking forward article is like none of its predecessors in January of the past few years.

No matter how far I progress I intend to have a decent station for the fall contest season. With large and competitive antennas for 80, 40 and 20 meters there is every chance of doing well at this point in the solar cycle. For the remaining bands -- 160, 15 and 10 meters -- I can get by with merely adequate antennas. For general operating and DXing I will come up with something simple for 30 and 17 meters.

How much I can accomplish depends on many factors, including those outside of the hobby. My life is more than amateur radio, notwithstanding the intensity of activity these past several months.

One year hence I am looking forward to sitting in a new comfortable shack with the world at my fingertips with a better class of antennas and supporting hardware and software. You will hear me on the bands. Is this a great hobby or what?

Comments and contact address

Comments on articles are moderated. My intent isn't to prevent criticism or for other nefarious reasons. It's a matter of practicality. Spam is a problem. Google is pretty good at filtering out the majority yet a lot gets through. When those show up in the moderation queue they are summarily deleted. Everything else gets published.

The other problem is dealing with Google's identification system which makes commenting a hassle, often depending on your choice of browser. I get around this by allowing anonymous commenters; you'll even see comments signed by me that are identified as anonymous. By invoking moderation I make it easy for you to comment while also making it easy for me to kill spam. The downside is a delay until comments are published. Usually the delay is no more than a few hours.

To contact me by email you should address messages to my call sign in front of the rac.ca domain name. That will forward to my gmail account reserved for amateur radio use. If we've communicated by email last year or earlier you should not use any other email address since it may silently vanish into the aether. I will never see it. My previous ISP does not reliably issue bounce messages to senders.

If you want a direct reply about any article or to get more information, such as antenna models, use email. I treat blog comments as addressed to the public, not me, and often publish them without commenting in return.

I look forward to seeing everyone on the bands in 2017.