Thursday, July 30, 2015

6 Meter E-season Wrap-up

Over the past couple of weeks the number and quality of sporadic-E openings has drastically declined. While there may still be a few good ones, for me this marks the end of the 6 meter season. The temporary small yagi I built and installed to get back on 6 has done what I intended.

In this article I will recap my brief return to the "magic band", now that it is coming to a close.


First, the numbers.
  • Contacts: At least 100, but not counted. Most came in the ARRL VHF contest and the Es peak in late June.
  • Grid squares: 75+ worked, with over half already confirmed on LoTW.
  • DXCC countries: 11 worked, including Canada and the US. The other 9 ranged from XE to the southwest, several in the Carribean, and several more across the Atlantic Ocean. More countries were heard but not worked.
  • Continents: North America, Europe and Asia. South America heard but not worked.
I as satisfied with these numbers even though they are not impressively large. Some have reported the season to be below average. That may be true. Although I have lots of experience on 6 meters from years ago my recollection is fuzzy and my poor antenna tended to make every opening this season a poor one.

It was well worth the effort of putting up an antenna and wasting some nice summer weather closeted in the basement shack.

Antenna performance

Running 150 watts I am pretty well able to work what I can hear. I only wish I heard as much as others. Many DX and marginal openings allowed others in and near my own grid (FN25) to work stations I could not hear at all.

The poor performance I'm experiencing is a combination of a compromised antenna -- nestled close to the tri-band yagi -- living in a river valley, and noise level. There is no easy way to disentangle their respective effects, except to not they all limited my results.

I seem to do well on aurora scatter, mostly done aiming across and along the wide Ottawa River. The yagi is small and so has a wide beam width for scattering off aurora well above the horizon. I only suffered on the longer auroral-E paths to VE6, VE7 and KL7, which is more due to poor antenna gain. Stations in all these call areas were heard during one excellent aurora opening.

The same appears to be true for Europe since the northeast direction is unobstructed. It comes down to a matter of antenna height and gain, of which I have little. I was very happy to work the few Europeans that I could. There were others I either could not hear or could not work.

The big problem is south, looking into the hill I mentioned in an earlier article. XE was not a problem, which skirts the hill by aiming southwest. Caribbean and South America were the toughest DX paths. I was happy to work what I could, and dream of what I might have worked with a better antenna.


With a proper antenna I am able to hear noise much better than before. This is clearly not good. Getting the noise source off the side of the yagi helps, but that is unfortunately in directions with little to no activity. The is typically in the range of S3 to S7 at SSB bandwidths, and occasionally even louder.

I mostly kept to CW where a narrow filter usually cuts the noise to a managable level. The high CW activity on 6, more so than I remember back in the 1980s, made this strategy a successful one for me.


I use the Elecraft KX3 on 6 meters since the FT-1000MP is HF only. An outboard amplifier raises my signal from QRP to 150 watts. It can now update my earlier opinions of the KX3, which had focused on HF contests and DXing.

In sum, I am not too impressed with the KX3 on 6 meters. There must be some aspects of the DDS and receiver that are different than on HF. I did not delved deeper to discover the reason for what I observed.
  • Tuning artifacts: When the VFO dial is spun there is often a loud ratcheting sound as the DDS makes its frequency steps. It can hide the very signals you wish to hear. Sometimes I had to tune more slowly than I'd like The manual talks about this and suggests how to reduce the effect, but not eliminate it entirely. There are trade-offs. Maybe I'll try it someday as an experiment.
  • Single signal reception: In what is largely another tuning artifact, when tuning through the opposite side of zero beat, signals bleed through to the AGC. The effect seems less severe when not turning the VFO. The AGC pumping is worse than I've encountered on any of the HF bands.
  • Noise artifacts: When tuned to a reasonably loud signal it can often be heard to crackle (sizzle?). It is more apparent on a continuous tone (CW/carrier) than SSB, but it's there nonetheless.
  • Spurious signals: Disconnect the antenna and tune the band and I discovered perhaps a half-dozen spurious signals of significant amplitude between 50.0 and 50.2 MHz. Considering the receiver technology (direct conversion) these are not "birdies", so I label them as spurious. When I connect the antenna there are lots more to be heard, but those are not the fault of the receiver.
  • Noise blanker: The dreadful noise I am dealing with can be significantly attenuated by the noise blanker (NB). Unfortunately the NB also reduces signal amplitude and adds substantial distortion. Careful adjustment of the NB level can help, though in most cases I get better results with the NB off.
Despite all the negativity in the above list, the KX3 did the job and I still like it just fine. It isn't the greatest rig around, but then that is not its purpose. For a small, portable, high-performance QRP transceiver it does very well indeed.

Hurry up and wait

Years ago it was tedious work to watch for openings. Often when I was home I would leave the receiver tuned to 50.125 MHz (the domestic calling frequency) with the volume set low. If I heard something I might check it out for a potential opening. There was also WWV for the geomagnetic indices that could herald aurora openings. At the height of the solar cycle high solar flux readings promised real DX.

It's much easier today. DX spotting networks are available with any internet connection, including my smart phone. All I need to do is look for spots on 6 meters and judge whether it's worth going down to the shack.

Even so there is time and effort required for success on 6. Most openings on 6 are marginal: signals are weak and fleeting. If you hear something enticing you have to jump. Wait a few minutes and the opening or wanted station can be gone, and may not come back until next year, or longer. Listening isn't enough: someone has to transmit. Many times I would go down below 50.1 MHz and loop CW CQs with the antenna pointed in a likely direction. Someone will answer, eventually, if sporadic-E has been reported by others in my vicinity.

Other times it's tedious tuning of the VFO, with longer stops at every station or beacon spotted. This is not welcomed by those with busy lives, hams who can only operate when life allows and not when the propagation dictates. Be prepared for that if you venture onto 6 meters. The rewards are many, but then so are the sacrifices.

Next up

Temporary means temporary, so the 6 meter yagi's presence will last only a little longer. I expect that by mid-August the yagi will be taken down and stored in the garage until next year, or perhaps even later. My plans for 2016 are unclear.

The feed line I co-opted has kept me off 80 since early June. This was little sacrifice during the summer's high noise level and low activity on the low bands. The coax will likely be reconnected to the 80 meters antenna for at least a short time. I have plans for improved low band antennas before the contest season arrives. Those plans will be the subject of a future post.

Sunday, July 19, 2015

Closer Look at the XM240 LCA

For my recently-purchased Cushcraft XM240 40 meter yagi I speculated on modifying it for improved performance. The primary options are to convert it into a W6NL Moxon or to replace the low-Q coils with high-Q coils. I built an approximate model of the antenna in EZNEC to evaluate the second option.

The coil is part of the LCA (loading coil assembly). The coil is close-wound on a fibreglass form, an attached to short aluminum tubes at each end. The LCA with the protective coating removed can be see on VE6WZ's web site. You may want to keep that page open while you read the rest of this article since I frequently refer to it.

Calculating the Q of a coil is difficult due to the many factors at play. It is typically easier and more accurate to measure it. Most hams, including me, do not own suitable test equipment. VE6WZ used software to estimate the coil Q and ESR (equivalent series resistance). As I said in my previous article that, if correct, the loss is as much as -3 db and coil heating is excessive. As I said there, I doubted these figures. But how to proceed? It is important to know the loss since it will be a key factor in whether or how I modify the antenna before its expected use in 2016.

The inside story

A close-up of one end of the LCA is shown at right. The ¾" fibreglass form fits snugly inside the ⅞" aluminum tubes, attached by what appear to be rivets. A screw electrically bonds the coil wire to the tube.

Some internet searching told me that the loss tangent of fibreglass is heavily dependent on the formulation. At the high end of the range I calculated an ESR of 8 Ω for the coil at 7.1 MHz, which is what VE6WZ calculated. Presumably that is the loss tangent for fibreglass used in K6STI's software calculator. The calculated coil Q is very low. However at the other end of the loss tangent range the ESR would be a very good 1.5 Ω. Calculation alone is clearly inadequate to gain the required insight.

Puzzled by this difficulty I did continued searching. A passing remark on a ham forum gave me the hint I needed. That person called the form a fibreglass tube. A tube is hollow, not solid. K6STI's calculator appear to assume that the form is a solid rod. I picked up an  LCA, pointed it at an open window and held the other end close to my eye. I saw daylight. Now the trail was hot.

It was difficult to be certain how the interior was structured because the light coming from the other end cast dark shadows from several dark protuberances from the tube walls. But the small diameter tube does not easily allow illumination from the same end I'm looking into with my eye. Fortunately my smart phone camera has a small lens and an adjacent LED "flashbulb" . With some care I was able to get both positioned within the tube opening. I pointed the far end  at daylight and took the following unusual photograph.

The flash was so bright within the LCA's confined space that the metal reflections stopped down the automatic exposure to reduce the far-end daylight to black! Despite this the interior structure is clear.

The rivets are obviously metal (presumably aluminum), as evidenced by their reflectivity. The fibreglass form is definitely a tube and (not clear in the picture) has a narrow wall thickness. The self-tapping screw that bonds the coil to the aluminum is very evident. As an aside, this reminded me that I need to replace the screw with a bolt that goes through the other side of tube, which is a well-known design flaw Cushcraft has persistently failed to fix. Due to the perspective of the camera and hardware alignment the rivets and screw at the far end of the tube are hidden behind those in front.

The upshot of this exercise is that the coil does indeed have an air core. The note by VE6WZ that some have measured the coil Q to be 200 is now entirely credible. Since fibreglass fills only a small fraction of the coil's interior volume its contribution to inductance and Q is small. This is good news.

Inductance, Q and loss

Since those unnamed sources and VE6WZ's calculations for an air-core coil of the LCA's dimensions agree on a Q of 200, let's proceed on that assumption. We can now compare an ideal, zero-loss coil (Q=∞), the stock LCA (Q=200) and VE6WZ's high-Q coil (Q=767).

First we need to agree on the inductance value. VE6WZ's measurement is ~15 μH. An air-core coil of the LCA's dimensions give an inductance of 11 μH. This is a difference that must be explained. I think it is fair to conclude that, per his pictured test apparatus, there is ample stray reactance due the long test leads and the attached tubes to account for the difference. There is no need here to speculate on the accuracy of the pictured test device.

I am also swayed by my EZNEC model which only works well when the load is set to about 11 μH. While I cannot use the Leeson correction, I did use an average element diameter that is roughly consistent with a stepped-diameter correction for an XM240 element. Whereas an inductance of 15 μH in the model is wide of the mark, requiring unrealistic element truncation.

For a Q of 200 and X of 500 (11 μH at 7.1 MHz) the ESR is 2.5 Ω (R=X/Q). I then chose a test frequency where the radiation resistance is a little above its minimum near the frequency of maximum gain. Recall that in a 2-element yagi with a reflector parasite the maximum gain ought to be placed at the bottom of the target band for optimum performance across the band. Since I am targetting the CW and DX SSB segments I chose 7.05 MHz.

The resulting gain figures versus ESR for the several LCA options are as follows:
  • 0 Ω (ideal, zero loss): 5.71 dbi
  • 0.9 Ω (VE6WZ high-Q coils): 5.43 dbi
  • 2.5 Ω (estimated stock LCA): 4.94 dbi
  • 8 Ω (now-invalidated estimate of stock LCA): 3.43 dbi
F/B differences are negligible. Feed point impedance and SWR curve are only modestly different across the first 3 selections. Relative to 7.05 MHz the gain differences (coild loss) will tend higher toward the lower band edge, and gradually decline with increasing frequency. This is due to the radiation resistance change with frequency.

The high-Q coils do very well, being about -0.3 db from the ideal. Compared to these coils the stock LCA is down a further -0.5 db. That's also very good, and better than I expected.


I am now inclined to stick with the stock LCA. In my judgment an additional 0.5 db is not worth the effort involved nor the damage risk due to the exposed and more fragile high-Q coils. The better alternative is to forgo the coils and do the W6NL Moxon conversion which does away with the coils and coil loss while also improving SWR and F/B performance.

However I still need to replace those self-tapping screws on the LCA with through-tube stainless steel bolts. I don't want a coil failure to occur in the midst of a contest during a frigid northern winter.

Tuesday, July 14, 2015

For Future Consideration: 40 Meter Yagi

In the description of my roughly laid out long term ham radio plans I did say I would like a significantly larger antenna farm. That is a task that takes time, including both planning and action. In particular, unless one intends to buy everything new and hire people to install the lot (very, very expensive) it is necessary to do some scrounging and learn some new skills. No matter the level of ambition (or zealotry) this takes time.

To that end I have been planning and searching for various elements of a bigger station. Some of this should be obvious from certain topics in this blog, such as models of large antennas, stacking, relative advantages of gain versus directivity, and more. To get beyond plans to real results there is a need for action. In particular, I will need stuff. Lots of stuff.

Some things I must postpone for practical purposes. If I were able to acquire a large used tower there is no place I can conveniently store it, nor am I in a position to transport it. Other things I can deal with today, purchasing items and setting them aside for future use. I recently acquired one such item.

If you don't recognize it this is a Cushcraft XM240. It is a short (inductor loaded) 2-element yagi for 40 meters on a 6.7 meter boom. If you been a regular reader you may have noticed my interest in rotatable and fixed 40 meter yagis.

When this item appeared on the used market I was quick to make a deal. Several attributes attracted me to making the purchase:
  • It's used, but new. It has never been on a tower. The aluminum positively glows.
  • The previous owner strengthened this antenna to address its well-known mechanical weaknesses. I was given the original parts where those have been replaced by something better.
  • There are tried-and-tested modifications to this antenna to improve its performance. That is, it makes a great base from which to build a yagi that is competitive with full-size yagis. This gives me a few options to choose from.
  • The seller is someone I have met before, and is someone I can trust.
I was able to combine a family visit into a longer trip to pick up the antenna. Several hours of driving saved shipping fees, risk of shipping damage and allowed me to inspect the antenna before taking possession. Considering the identity of the seller I expected no problems and there were none. This is how a ham-to-ham transaction ought to proceed, not what unfortunately sometimes happens.

This antenna will go into storage for the present, waiting for a suitable location and tower. Not only can my present tower not handle the load, the elements would tangle with the upper branches of adjacent trees.

This gives me time to consider which, if any, modification to apply to this antenna. It is also possible that I would install it as-is if time is pressing and other necessary tasks take priority.


To compensate for shortened elements it is necessary to have loads to compensate for the reactance due to the unloaded element's higher resonant frequency. Loads are never perfect: they are lossy. Sometimes the loss is small and sometimes it is large. This is true whether the load is a coil, a capacitor, transmission line stub, capacity hat, linear loading, something else or a combination of these. The stock XM240 uses a coil and a small capacity hat in each element half (4 in total).

The loss of a coil can be substantial if its shape, wire gauge and core material are not ideal. Most commercial designs have lossy coils and traps since there is a greater emphasis on material cost, size and robustness. Ironically, poor (lossy) coils are low-Q (where Q=X/R) resulting in improved SWR bandwidth and simpler matching networks. Few hams are in a position to measure loss, and many seem not to care. Manufacturers cater to the desires of their market.

Most of these short, lossy yagis are used and enjoyed in their stock configuration. That does not mean those hams are not experiencing losses. Often it is that the yagi replaces a single-element wire antenna or some variety of urban-friendly vertical. Even with the loss the short yagi can substantially outperform what it replaces. If optimum performance is your objective it is often possible to do better, with some extra effort.

I built a simple EZNEC model of the XM240 so that I could estimate the loss in the loads. The critical data required, the coil ESR (equivalent series resistance), is not easy to measure. Like most hams I do not have the means to do so. A calculation can be done, though that too is subject to error due to the need for assumption about coil and environmental properties. VE6WZ used K6STI's coil calculator and got an ESR estimate of 8 Ω for the stock XM240 coils.

Plugging the 8 Ω value into my model predicts a loss of up to -3 db. At least ⅔ of the loss is in the driven element because the current is higher than the reflector, as is typical for any yagi. If his coil ESR estimate is correct the loss in each driven element coil could be as high as 170 watts when the antenna is fed with 1,000 watts. That's a lot!

Even at a typical 50% duty cycle for CW and SSB this is 85 watts of heating per coil. I am dubious about the 8 Ω ESR estimate since that would cause instances of coil damage that I have not seen reported for this antenna. Unfortunately I can do no better estimation. Even so I would strongly consider implementing VE6WZ's modification to reduce the coil loss to a negligible amount. Even -1 or -2 db of loss for such an antenna is undesirable.

Perhaps the only good thing about lossy coils is that the antenna is a pretty good match to 50 Ω coax (the antenna comes with a 1:1 balun). With loss-less coils the radiation resistance would be below 50 Ω and require a matching network such as a hairpin or gamma match. Since the loss resistance is in series with the radiation resistance the 50 Ω match can be good. But at a cost.

Gain and F/B

Perhaps the better way to avoid loss is to replace the coils with a large capacity hat. Unfortunately the size of the capacity hat would have to be large. This adds substantial weight to the elements at a point where it can reduce survivability and cause a lot of sag. It is a project that requires a careful eye to design and implementation.

Luckily for me the design work has already been done by W6NL. He transferred his 2-element Moxon design onto the XM240 antenna, and provided detailed instructions to do the conversion. I am intrigued by the advantages of this design, which inspired me to consider how to deal with its potential drawbacks in an earlier article.

Since my antenna's previous owner has already taken care of strengthening the antenna, it is only required to take the additional measures W6NL specifies for conversion to a Moxon. To recap, the advantages of the converted XM240 include:
  • Low SWR across the 40 meter band
  • Improved F/B
  • Negligible loss
What you don't get is improved gain. Gain and gain bandwidth are comparable to an XM240 modified with VE6WZ's coil substitution. To be more precise, the equalization of element current the Moxon modification provides contributes to improving F/B and match, but do not optimize forward gain. I believe this is a reasonable trade-off with respect to elimination of loss in the stock coils or the fragility of a large, low-loss coil.

Deferred decision

I do not need to decide on what direction to take with this antenna until at least 2016. This is plenty of time to consider the alternatives. I fully expect to implement one of the VE6WZ or W6NL modifications. For the present it will share storage space with a TH6, A50-6 and several rolls of Andrews Heliax. More items may join these over the coming year.