Thursday, October 31, 2013

40 Meters Delta Loop Mechanical Design

Finally the weather broke and I had a few hours to construct and raise the delta loop for 40 meters. Cutting to the chase, it works. However it does need some work, both mechanical and electrical. I'll describe the mechanical design in this article and follow up with the electrical design in a subsequent article.

A delta loop for 40 meters is big. It is in fact much taller than the small tower (Site C) that supports it. This requires a mast to support the apex of the loop which needs to be at least 14 meters above grade so that it doesn't grab anyone's neck as they walk under it. The height constraint is tight since it must also be no higher than 15 meters in order to avoid the regulatory process of both Industry Canada and the city.

Since the tower is 8.8 meters high, the support mast must extend 6 meters above the top of the tower. This is not something that is undertaken without due regard to the wind and bending forces on both mast and tower. The entire system must survive year-round weather events from thunderstorm-caused wind squalls to freezing rain.

The adjacent picture shows the tower, mast, modified TH1vn and delta loop. This is not the final configuration since more work on the mast guying was done.

At full height the TH1vn is up just over 11 meters. While this is not clear in the picture the 4-band dipole runs inside the vertical legs of the delta loop. This is deliberate. I needed that orientation and EZNEC modelling judged the arrangement acceptable. That is, very little interaction between the dipole and loop on all bands of interest. This was confirmed by measurement once it was all in the air.

When I was adding 17 meters to the TH1vn I also took down the mast so that I could prepare the fittings for the delta loop mast. There are two parts to this:
  • A second clamp was placed below the TH1vn element/mast clamp for added support and redundancy. Although the dipole doesn't need it (it's very light), the antenna clamp now also supports the weight of the extended mast and the downward force due to the guys pulling on the corners of the loop. It is made of same 1.5" Schedule 40 ABS pipe that couples the TH1vn clamp to the steel (bottom) mast. It is cut on one side and then compressed with a stainless hose clamp.
  • Three of the 4' sections of fibreglass mast that I previously used for the experimental 20 meters delta loop form the extended mast. They nest together and slip onto 4" of exposed steel mast above the TH1vn clamp. A shim made of aluminum flashing was cut and formed to allow a snug coupling of the two masts. This is needed since the steel mast is 1.5" O.D. and the fibreglass mast is about 1.6" I.D. The shim is held in place by the pipe clamp that secures the TH1vn to the steel mast.
The snugness of the fit between the masts is important. It must be tight enough to avoid stress points when the delta loop is tensioned but also be easy to drop into place when strapped at the top of the tower and holding vertical the 12' fibreglass mast plus guys and antenna wire above my head to drop it into place. This is the the most dangerous operation in the entire process. I did a test with a single section of fibreglass mast to confirm that I built it properly. Only then did I assemble the extended mast on the ground and lift it into place. It all went more smoothly than I expected, so that was a relief.

Since the fibreglass mast sections are held together only by gravity I had to temporarily secure them in another way during lifting. The rope stay and the vertical legs of the loop were attached to the guy ring, slipped over the mast then pulled tight and secured with a hose clamp to the bottom of the 3 sections. The lift rope was friction fit to both the top and bottom section, with the top one knotted to allow it to be released with one hand.

As I pulled it up to me I slipped the top knot, continued raising it over my head and (holding it very still) dropped it into place. With that done I removed the lift rope and temporary hose clamp and tied the rope guy to one of the tower guys cables. The loop wire had to be cut at the centre of the bottom (horizontal) leg so that one side could be reeled up and dropped over the other side of the TH1vn. All I then needed to do was loosen the mast clamp, raise it to its full height, swing the TH1vn to its final orientation and clamp the mast.

The reason I've gone to the trouble of described this process in detail is to highlight an important requirement of all tower and antenna work: plan everything in advance (and I mean everything!) and test every crucial step on the ground. Too many hams injure or kill themselves by over-confidence, imagining they can resolve any difficulty they run into, then find themselves in poor position or insufficient main strength at a critical step.

Can you really hold that load above your head without it tipping and crashing to the ground and onto you someone else? Try it on the ground, and do it without moving your feet. If you can't or it's iffy you should resolve the difficulty and not take a chance. Trust me, it isn't worth it. Really.

The following are additional notes on the mechanical design. These can apply to any wire antenna.
  • Shear pin -- The delta loop is a big antenna that covers a lot of ground. It spans 14 meters over the ground, plus the length of ropes to tie down the corners. As the pictures show there are a number of neighbouring old trees which branches that arch over my property, and the delta loop. A branch that breaks in a storm could fall onto the antenna. The south corner tie-down rope includes a length of synthetic twine to act as a shear pin (a mechanical fuse). Its breaking strength is about 40 lbs (18 kg). The idea is that the twine will break when a tree hits, thus preventing more extensive destruction of the mast and tower.
  • Wire selection -- A 40 meters 1λ loop (naturally) requires about 40 meters of wire. In addition to supporting its own weight it must also support the tension needed to reasonably ensure its delta shape. The wire must be up to the mechanical forces. For this antenna I am using 12 AWG insulated soft-drawn stranded copper wire. I would not recommend anything less.
  • Coax deflection -- The weight of the coax distorts the shape of the delta loop, as the adjacent picture demonstrates. This can be reduced with higher tension on the loop's tie-down ropes but at the cost of greater stress on the wire and extended mast. The deflection in my case is large since I tried to avoid excess coax droop which would increase coupling of antenna currents onto the coax, and because I used RG-11/U (¼-wave transformer). RG-59 would cause less deflection, if QRO is avoided.
  • Mast support -- As mentioned earlier, the extended mast must be guyed. Three-point guying is used. Two of those are the vertical legs plus tie-down ropes, and the third is a rope orthogonal to the loop plane that is tied to the tower. Some care must be taken when setting the tensions of the loop and third rope so that the mast is under sufficient tension, does not deflect from vertical by more than a few inches, and the loop itself is secure. I will need to add a horizontal crossbar to the tower to extend outward the tie point for the rope guy. For the present the rope is tied to the top egg insulator of the most suitable tower guy.
In the next article I'll talk about the antenna itself. As I said, it does work but it requires tuning. Putting it through a tuner to get the SWR down in its present state I was able to work 40 meters DX in Europe and Central America even under poor conditions (October 30 evening) and only the 10 watts from my KX3.

Wednesday, October 23, 2013

Tuning the TH1vn + 17 Meters Hybrid Trap Dipole

I have gone as far as I will go for the present with multi-band tuning of the TH1vn trap dipole with its 17 meters addition. First I will finish up the discussion of its mechanical design since I have made changes.

The feed point was a week point since the original TH6DXX driven element has nothing more than clamps to attach wires to each half-element. This is suitable for the Hy-Gain BN86 voltage-balun plus beta match on the yagi, but not so much when used standalone as a trap dipole. I replaced my rat's nest of wires with a more structured and robust rat's nest, as shown in the adjacent picture.

As you can see I continue to use pieces of the PVC pipe in additional projects since it is cheap and strong, and sitting in my basement ready for use.

The wires to each half-element clamp run through the pipe to the SO-239 UHF connector. The connector nestles inside the cut-out I made in the pipe centre. The centre pin of the connector fits through a drilled hole in the back of the pipe (not visible in the picture). The connector is screwed onto the pipe, with one screw doing double duty as a post to connect a wire and round lug. It sits diagonally across the pipe since there is no practical way to mount it to the pipe otherwise since the pipe is about the same width as the connector.

The pipe is slightly bent due to the screw tension. While there is no danger of breakage I plan to add a spacer to eliminate any risk of a wire touching the large steel clamp that holds the element halves.

Wires run from the same feed point to hose clamps on the 17 meters element. The stainless steel hose clamps are rated down to ¼-inch pipe (or aluminum rod in this case) but would not work properly without the aluminum shims I added between the clamps and wires. After the picture was taken and initial tuning the solid 12 AWG wires to the 17 meters element were replaced by 14 AWG stranded wire. There was some wire motion in high winds and I want to avoid a mid-winter failure due to metal fatigue.

Here is a somewhat-blurry picture of one half-element of the completed antenna on the tower, sitting 9 meters above ground. You can see how the PVC spacers and plastic ties hold the 17 meters element a constant distance from the trap dipole.

The spacers are secure enough with the antenna mounted and the wind blowing but are prone to rotation when the antenna is moved around during lifting, mounting and tuning procedures. Keeping the aluminum rod directly below the trap dipole is more for aesthetic appearance rather than functional performance. I may eventually snip the ends of the ties.

Tuning the antenna is tricky. The TH6DXX was not designed to accommodate additional parallel dipole. In particular the narrow spacing between the traps for 10 and 15 meters allows little tuning room on 15 meters. To make gross tuning adjustment meant lowering the antenna to the ground. This is easy enough since the tower is short and the antenna lightweight, but would push the spacers out of position as it dragged on the ground or bumped into the tower and guys.

Smaller adjustment I made on the tower by tying the antenna to the side of the tower and accessing the parts of the antenna to be adjusted. This consisted of either the end tip of the trap dipole element or the lateral position of the 17 meters rod.

During this procedure I left the 10 meters tuning alone. It still resonates up in the SSB segment the SWR is a bit higher at the bottom of the band where I primarily operate. Now at 1.6 to 1.7 SWR at 28.0 MHz it work just fine. The other reason I left it alone is that lengthening the inner section of the antenna would lower the resonance on 15 and 20 meters. As I said above, there is little room between the traps to shorten the antenna for operation on 15 meters. This matters since as is now stands the hybrid antenna resonates a little below the band bottom, at about 20.950 MHz. This is fine for CW but the SWR is between 2 and 3 across the SSB segment.

Tuning on 20 meters should be straight-forward since the ends of the element are easily telescoped and have almost no effect on 10 or 15 meters. It did not turn out to be quite that easy.

As I mentioned in the previous article, after adding the 17 meters element the minimum SWR point dropped from 14.0 MHz to 13.7 MHz. Due to the intervening traps this results in a narrow-band SWR response and and SWR above 2 even at 14.0 MHz. The reason for the major impact is circled below.

The end of a dipole is particularly sensitive to the presence of metal, in this case the 17 meters aluminum rod. The same occurs with tuning on 17 but this can be independently adjusted with the wires between the coax and rod. This is why, in general, antennas are shortened with capacity hats and linear loading section connected towards the ends of the dipole elements.

My "toy model" of the hybrid antenna in EZNEC captured the mutual sensitivity at the tips but not the magnitude of the interaction. The model told me to reduce the length of the 20 meters element by 20 cm at each end. I did this and ended up with a resonance at 14.6 MHz! With some quick mental math I decided to try again with 8 cm shortening and the result was right where I wanted it, near 14.1 MHz.

On 17 meters the SWR dipped up around 18.5 MHz but was still broadband enough to work well at 18.068 MHz. Even so, when I spliced in the stranded wire (as described above) I took the opportunity to tune the antenna. It now resonates around 18.2 MHz with an SWR that is low across 17 meters.

In summary, the SWR minimums on 20, 15 and 10 are not far different from what they were before. The main difference is that the SWR bandwidths are somewhat narrower, especially on 20 meters. Plus 17 meters, of course.

Even a dummy load has a 1.0 SWR so this is not an indication of performance. However I can report that it works quite well. Not better than a dipole, but it loads, hears and transmits just fine. It was usable during the tuning -- which was done intermittently over the past week -- and was available for use during the fantastic recent band conditions. I have now rocketed past 150 DXCC countries this year with my QRP plus wires station. The most recent was VU2 over-the-pole on 10 meters, using this antenna.

Since I already have antennas for 10, 15, 17 and 20 meters was this exercise worthwhile? Yes, it was. Apart from the challenge of doing it I do have additional reasons. More on these in a future article.

Tuesday, October 15, 2013

Mechanical Design to Add 17 Meters to the TH1vn Tri-band Dipole

My feeble modelling attempts to add a parallel 17 meters dipole onto the TH1vn tri-band dipole were not as conclusive as I'd have liked. As I said in that article, one problem is the difficulty of modelling traps, especially those in this Hy-Gain element since they are physically long and I do not know the design parameters. I instead opted to model the element as an inductor-loaded tubing element that would resonate at 14.1 MHz at the measured length of the element.

I did not expect to get the model right but to hopefully get it close enough to identify the key sensitivity parameters, if any, to simply tuning and adjusting. I chose to jump into the actual construction and just see what would happen.

You can see the original antenna feed system I came up with in my first article on adapting the TH6DXX driven element into a simple tri-band dipole. Since the antenna is similar to Hy-Gain's TH1 commercial product I chose to call it the TH1vn.

The physical design and construction started before the modelling work so I was able to get the modified antenna back in the air only one day after my previous article on modelling the design. Before I come to how that went I will take the opportunity in this article to show how I went about building the antenna.

The first thing I needed was a set of spacers (or hangers) to suspend the parallel 17 meters element from the TH1vn. I needed something fast, cheap and reasonably reliable. This is an experiment which does not, at this time, require the effort to make it last for years.

With the ample quantity of PVC pipe left over from construction of the spreaders for the multi-band inverted vee antenna I made the spacers in the same fashion.

As can be seen in the picture there are 8 spacers, 4 for each half of the antenna. Ultraviolet-resistant cable ties thread through the drilled holes to bind the spacers to the TH1vn and 17 meters element. The holes for the TH1vn (on the right) are placed to accommodate the tubing diameters at various points along the antenna. Later I enlarged the holes for the 6 inner spacers to fit longer and wider cable ties.

Notice that the holes for the bottom of the TH1vn are aligned so that the elements are parallel along their lengths. Perfection isn't required for this operation so don't look for precision cutting and drilling!

I then made an error in the material choice for the 17 meters element. Solid copper 12 AWG wire may seem rigid but it isn't. This is amply demonstrated in my first attempt to hang a 4 meters long half-element from the TH1vn.

This is obviously not going to work. Wire in this application can only work if it can be placed under tension. In this application that is difficult to achieve. Note how the element extends beyond the tip of the antenna.

Not having a suitable alternative in my junk box I made a trip to a commercial metals supplier and purchased two 12' lengths of ¼-inch diameter 6061-T6 solid aluminum alloy rod. These are cheap, lightweight and don't dangle too far out the passenger window of my car.

As you can see this worked out far better. However it is not perfect. When the assembled antenna touches an object during the lifting process -- which happens a lot -- the spacers are easily pushed out of position. Fortunately the antenna is very light, which allowed me to move the antenna hand-over-hand at the top of the tower and manually reposition the rods and spacers. As I said, the mechanical design is not perfect.

With a cobbled together feed system -- alligator clip leads, bits of plastic, wire ties, etc. -- I managed to load it up before dark. There is now only a brief window in late afternoon before sunset arrives.

It works, after a fashion. With the same wire lengths connecting the coax to the tri-band element the same as before the SWR minimums moved on each of 20, 15 and 10. What it didn't do is shift in the way I expected.

In the typical fan dipole the antenna for the lowest frequency is relatively unaffected by additional of dipoles for the other bands. That is not what happened. The SWR shifted from the lower end of 20 meters to 13.7 MHz. This is not acceptable, and its under-performance shows. The shift on 15 meters was more modest, having been lowered by about 100 kHz to 21.0 MHz. On 10 meters it shifted upward to 28.6 MHz.

The resonance on 17 meters is off by a little, but will work fine once the other kinks are worked out. Tuning the antenna for this band is relatively easy since it is just the one band. For the tri-band element the tuning will have to be done in stages since there are 3 lengths to be adjusted:
  • The section inside of the first trap, including the wire to the coax, affects all 3 bands.
  • The section between the traps primarily affects 15 and 20 meters.
  • The section beyond the last trap primarily affects 20 meters.
It is that last bit that will need the most adjustment to shift resonance back toward 14.1 MHz.

There is rain forecast for the next few days, which will reduce the time when I can complete this task. Once it's done I'll write a follow-up article on the details of the feed system and how the antenna performs. In this case the performance is all about bandwidth and SWR (impedance) since none of this work changes the far-field pattern. It's still just a multi-band dipole on a short tower.

Monday, October 14, 2013

Modelling Parallel (Fan) Dipoles

I've been remiss on blogging recently due to a combination of travel and other matters. As mentioned in my previous article I had to take close to 2 weeks away from antenna work. As of this long weekend I have restarted antenna work.

I was however able to get in several hours of operating during the great conditions the last few days. The conditions were good enough to put several new countries in the log, even those with large pile-ups which are the bugaboo of QRP operating with zero-gain wire antennas. Examples include: TN, TO2TT (Mayotte), FR, TX5D (Austral I.), ZM90DX (Campbell & Auckland Is.)[no, it's ZL]. As always there are the many that got away, that are just very difficult with my small station.

Now back to the present subject. In an earlier article I showed how fan dipoles are easier to model and reliably build when the antenna wires are parallel rather than radiating outward from the centre. Having established that I went on to build a fan dipole for 30, 20, 17 and 15 (plus 10 and 6) meters. That antenna remains up and continues to work well. It just needs some maintenance to prepare it to survive the winter.

Using EZNEC I modelled the basics of the antenna -- though not all of it -- to test its performance with respect to tuning sensitivity, wire placement, etc. Below is the EZNEC view of the feed point of a parallel fan dipole for 30 (top) and 20 meters, which appeared in that article. Wires #4 and #5 connect the antennas, and the source is at the centre of the 30 meters dipole.

This is not the best model for the feed. The reason is that there are a variety of hidden mathematical quirks in NEC2 that afflict short wires, sharp angles between wires and parallel wires. I did not run into those problems in that particular model but I did in a similar model.

This weekend I took down the TH1vn trap dipole and mast to work on them. The mechanical work to add an extended mast for a 40 meters delta loop was straight-forward. Not so for my other, related project to add 17 meters to the this trap dipole.

It is not easy to model a trap dipole with EZNEC (or other NEC2-based software). The model is nonetheless important since there is a potential for differing interactions on each of 20, 15, and 10 meters. There are both mechanical and electrical challenges which I am currently dealing with. When something comes of this experiment I will report back.

The driven element of the trap dipole (the driven element of my old TH6DXX) is quite short. Each ½-element is 3.8 meters long. This is well short of ¼-wavelength on 20 meters (~5.1 meters) due to the load added by the traps for 10 and 15 meters. Of particular concern to me is that this is shorter than a full-length ½-element for 17 meters (~4 meters).

While this is a rat's nest of issues I chose to concentrate on just one for now, since I expect that to be most messy. This is the coupling of the 17 meters parallel dipole to the full TH1vn on 20 meters. It is generally true that the antenna at the lowest frequency is mostly unaffected by parallel wires for higher frequencies, while the reverse is definitely not true. Right now I need to get the 17 meters design figured out.

I added inductors to the centre of each 20 meters ½-element so that it resonated at 14.1 MHz while at the correct physical length of 3.8 meters. I then added 17 meters wires using the feed as used before (and shown above). EZNEC choked on it, unable to calculate feed point impedance.

After some experimentation I isolated the problem to those short vertical wires. Changing the segment length changed how the problem was manifest but did not make things better.

When you see 2 closely-spaced wires connecting sources and loads in an antenna system, what does that make you think of? It is a transmission line. Transmission lines are much easier to model in EZNEC than all those problematic small wires, so that's what I did.

In the above EZNEC view of the modified feed, the source is placed at the centre of the 20 meters element (wire #1) and a transmission line (red square) connects the centres of both elements. This is certainly a less complex model.

Transmission lines in EZNEC have their limitation since, unlike wires, they do not interact with the rest of the model. They are pure and perfect transmission lines that neither radiate nor absorb RF. For such a short section of line (10 cm in this model) the impact should be negligible. It isn't even necessary to get the nominal impedance of the line exact since it's so short relative to wavelength. I got nearly identical results with impedances from 300 to 600Ω. If you like you can always calculate the actual impedance from the wire diameter and separation.

That's one (big) problem solved. If you model feed systems such as this you may want to do the same. You can find similar parallel dipole feeds in a number of commercial antennas such as the Spiderbeam and HEXbeam.

The other problem is the choice of segment length. While it may be difficult to pick out both of the above pictures, there are green dots showing segment boundaries. In the bottom picture this is ~12 cm for both the 20 and 17 meters elements. This is the important point: the segment lengths should be matched as closely as possible for close spaced wires. Do this for any NEC2-based engine.

Even seemingly small differences can, in the right (or wrong) circumstances, cause significant errors in the modelled results. In some of my models the impact was small and in others it was large. If in doubt, modify the segment count in one of the wires and check the SWR, current and far-field patterns. Or do what I do and always strive to adjust the segment count so that the segment lengths match.

Choosing the transmission line feed model makes this task easier since it is nearly impossible in most cases to also get the segment lengths matched as well with the separate wires in the wired feed model. Sometimes that matters, too, but not always.

To summarize, remember these points in the modelling of parallel dipoles.
  • Connect the dipoles with transmission line equivalents of the physical wires. If there more than two dipoles add more transmission lines from each dipole feed point to the next.
  • Match the segment lengths in each parallel dipole. Closer matches are possible as you increase the segment count.
  • Place the source and transmission line terminations in the centre of each dipole. Do this by selecting an odd number for the segment count and connecting  to the middle segment. When you then place the connection (source or transmission line) 50% from one end of the dipole it will be properly centred.
Once I resolve the mechanical issues with adding that 17 meters element and attempt to tune it to operate on all four bands I will be back.

Wednesday, October 2, 2013

Overnight DX

This is the season for colds and other virus infections. Such is my plight. One of the downsides is disturbed sleep cycles. But, as they say, when you are given lemons you should make lemonade. So when I found myself wide awake and feeling miserable at 3 AM last night I decided to head to the shack and check out the bands.

There have been some interesting openings this past week due to the equinox and quiet geomagnetic conditions. Much of this passed me by since the bulk of my operating must be in the evening. The polar paths in particular have been intriguing due to the northern route coinciding with the gray line (path along the sunrise or sunset terminator).

When I turned on the radio the very first thing I noticed was the immense quiet. No computers, no lighting and no appliances to spray their debris across the HF bands. It was wonderful. Even the weakest signals were a pleasure to copy.

Since the MUF seemed to fall below 18 MHz the only bands available to me were 20 and 30 meters. I expect the lower bands were also quite good but without antennas for them I did not go there. On these two bands the open propagation paths favoured east and west.

Europeans were scattered here and there. When they did transmit they were quite strong. These were for the most part DXers, not casual operators. That is, they were most likely scanning the bands for attractive DX in the brief period between waking up and heading off to work.

On the other side of the world it was the end of the work day and early evening. VK7CW on Tasmania has a nice strong signal. After losing out to a few Europeans I had a short pleasant QSO with my 10 watts. On this path the inverted vee outperformed the dipole -- likely due to its greater height. While this may sound surprising this was my first VK using my new station. QRP to the other side of the planet requires low competition (from North America at least) and good propagation. This time both were in my favour.

On 30 meters the story was very similar. There were eager Europeans snapping up the available DX in their short morning operating period. As can be seen above, this was shortly after sunrise in western Europe when the low bands experience an enhancement. On 30 meters this can last an hour or two, and gets progressively shorter as one descends in frequency.

After working CY0P the previous evening on 30 meters I was eager for the challenge. The only station that interested me was another VK, this time VK4DX, with a strong signal. He was perfectly workable with my QRP but he was focused on working Europe. It was disappointing but I did not call out of turn. I am sure that if I'd found a frequency and called CQ I could have worked a number of European stations. Except by then I was ready to give sleep another chance and I shut down the station.

In addition to CY0P, the previous evening displayed some good conditions to select areas of southeast Asia. This is evident in the following picture which shows the terminator around the time I was operating.

In particular 9M6XRO had a nice strong signal on 17 meters, even with my simple inverted vee. Notice that 9M is right on the sunrise terminator.

He would have been easy to work if only I had more power. Even so I appreciate the lengthy effort he made to try and pull me through. That's ok, it's simply a challenge to anticipate for another day. I had more success working D2EB, so the evening session was still worthwhile.

Pay attention to the terminator and the path of propagation through areas of light and dark. If you have a small signal similar to you can log some great DX by target times when propagation is most favourable, and when your domestic competition is literally asleep. From time to time step outside your habitual operating pattern. While big stations can "make" DX propagation, little ones should intelligently target advantageous times and frequencies.