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 I was given the original parts where those have been replaced by something better.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.
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
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.