 |
Get a load of this…
18 July 2025 | by Ground-mounted verticals are all the rage these days in portable HF operations. This is particularly true in POTA activations. We all like to get extra mileage out of our verticals in terms of their frequency range and efficiency, no?
One method is to add an inductor to the radiating element to extend what its length looks like for RF resonance. Some versions of this are to bottom-load the vertical (Wolf River Coils does this with their Sporty Forty coil) as well as center-load it (as does Chelegance does with some of their JPC line of verticals). There are top-loaded designs, too.
An issue the portable HF operator might face if they create their own vertical antenna system is determining the value of the inductor coil. I’ll walk through this briefly to illustrate one problem that many vendors create for them in their product offerings.
Shown above is a center-loaded vertical that I’ve designed. It’s called the Eiffeltenna because of the similarity to the Eiffel Tower from the tripod legs. The details will be forthcoming once it is fully tested but the focus in this article is is that it is center-loaded as the inset photo illustrates.
What inductance value should I use? It all depends on the band, height before the coil’s insertion, and the total height of the vertical itself. Oh, and the ground and counterpoise element can play a role as well. Here, I’m using a 42″x35″ sheet of Faraday Cloth on a washed gravel driveway next to my garage. While this is far from good ground conditions, it functions very well as shown in an RF sweep below.
There are a number of calculators to help hams answer these questions. One is from 66pacific.com. I’ve placed a screenshot of the calculations for this test antenna below. The design goals are for the 40 meter band (7.0 MHz). But I also want to get 20 meters available, too. The total height of the antenna is specified as 16.75′. The coil is inserted at 7.5′ so what is the value of the required inductor to make a 20 meter vertical resonant on 40 meters here? According to this calculator, we need a coil that measures 12.1 micro-Henries.
One option is to simply build a fixed (non-adjustable) coil for this value. There are many online coil calculators for this. It is a desirable option unless there might be another band or the ground counterpoise system is very different or something else that changes things here. The other option is to purchase a coil from a number of vendors. One gotcha: very, very few actually tell the customer the inductance value for their coil (or the range if it’s an adjustable one)! They usually just say it’s “for 40M” referring to their own commercial antenna product for which it is an accessory.
Since I have several coils like this, I used one of my calibrated bench LCR meters (HP 4275a @ 200 KHz) to measure the value or range of values for several commercially available inductor coils. The results are in the table below. I have included three adjustment settings for the variable coils and the Q value. One definition of Q is “The quality factor (Q factor) is defined as the ratio of reactance to resistance, indicating efficiency at a given frequency.” For us, the importance of Q is “A higher Q value signifies lower losses and better suitability for high-frequency applications, as it implies a smaller ratio of resistance to inductive reactance.” So Q is an additional measurement about that inductor’s value that shapes how effectively it works.
While the MFJ open-air coil is no longer being manufactured, it is in wide circulation in the amateur radio community. It has a wide range, from 0.4 to 17.1 uH with corresponding Q values of 0.5 to 5.8. While the Mad Dog adjustable coil (sturdily built, I might add) has a wider range (0.73 to 28.3), it has somewhat low Q values (0.3 to 0.6). The Chelegance JPC-7 also has a wide range of inductance settings, from 0.5 to 22.8. Like the Mad Dog coil, the JPC-7 Q values are not great at 0.33 to 0.18 (double checked this figure). Here’s where one coil, larger than the rest, shines in this table. The Wolf River Coils Silver Bullet 1000 has values from 2.73 to 80.3, allowing a larger frequency range for loaded vertical antennas. Equally impressive is that the Q values range from 4.3 to 13.5 at the same time. All of these adjustable coils would fit the requirement of adding a 12.1 uH value at the center point of the vertical antenna shown above.
I included another coil from Wolf River, their fixed value Sporty Forty. They don’t tell the buyer what value it is, just that it’s an accessory for their ground-mounted whip antennas to get them to also work on 40 meters. I have two and they’re well built. Their value is 8.3 uH. There is a clone from China that is also 8.3 uH. Perhaps because of different manufacturing processes, the WRC coil has a much higher Q value at 8.6 than the clone from China has at 2.5. For these fixed value coils, it is key to realize what inductance value they have because neither would work in the center-loaded vertical example used here.
There is a very neat “bypass” trick created by Michael KB9VBR, published on his Youtube Channel. My version is shown at left. It’s simply a set of pigtails attached at the top and bottom of the coil with Power Pole connectors on each end. Plug them together, the coil is bypassed. Unplug them, and it’s in the driven element. Takes about 15 minutes or so with materials that you likely already have it you’re an antenna builder. If not, these parts are very inexpensive via online vendors.
This bypass trick can be used with any inductor coil so keep it in mind if you build a center-loaded vertical like I’ve done here. I don’t have to bring down the full vertical whip by unscrewing it, physically removing the coil, and replacing the whip. I can just reach up, plug or unplug the pigtails, and the vertical is either on 20 or 40 meters. This assumes that I’ve already done two things in the case of the Eiffeltenna center-loaded vertical.
Getting it tuned spot-on for 20 meters is fairly easy using the Faraday Cloth for the counterpoise field. It is a precursor for switching in the adjustable coil, such as the JPC-7, as shown above in my driveway. This is so that the coil can than then be adjusted to the correct uH value to load the antenna for 40 meters using an antenna analyzer. Once this is accomplished, marking the coil makes the process almost automatic during setup in the field. Checking it with an antenna analyzer, though, is always a good thing (ask me how I know, lol).
These vertical antennas can be configured in many ways but I hope that this article is useful to the portable operator who wants to operate with multiple band options using a quick setup vertical antenna. The Eiffeltenna, inspired by a tripod experiment published by Jim W6LG on his popular Youtube Channel, and further work by Jason VE5REV, fits that bill. Extend the tripod, add the coil and whip, placed it on the Faraday Cloth rectangle, connect the ground wire to the Cloth and the coax, and you are largely ready to go.
I’ll be publishing more about this very portable antenna once I’ve completed testing it. However, getting a load of the principles in this article applies to many, many vertical antennas. Get a the load of the coil you’re buying before the purchase!
5 Responses to “Get a load of this…”
 Ham Radio Deluxe |
 W5SWL Electronics |
 Ham Radio Prep |
 KB3IFH QSL Cards  Hip Ham Shirts  HamRadioAuctions Reliance Antennas Enigma Shop |  morseDX  Ni4L Antennas  R&L Electronics antennas.us Ears to Our World |
- Matt W1MST, Managing Editor
I peruse the Newsletter for items of interest, and this is the first that has really resonated with me. Thanks Frank for sharing your research and findings. 😁
Hi Larry,
Thank you so much for your kind words. This blog post is part of a larger antenna design project that I mentioned within it. I’ll have info on that EiffelTenna once it is complete.
73,
Frank
K4FMH
Hi Frank,
I’ve written extensively about the JPC-7 and JPC-12 in my blog (Google will find it), describing its good and bad points – the latter being mostly related to the loading coils that are used, the same being used for the JPC-7 and JPC-12.
As supplied, they are originally wound with non-austenitic stainless steel wire which, as you can imagine, is very lossy compared to copper – potentially resulting in 20-50% power loss as heat depending on the tap selection, antenna configuration and frequency. I obtained an “extra” coil (keeping it as stainless for comparison) and rewound the other coils with silver-plated 18 AWG (1mm) “jewelry” wire (from Amazon) reducing the losses to just a few percent. I measured a 110F/61C temperature rise on the stainless coil with 70 watts at 40 meters as opposed to about 3F/2C for the silver-plated.
While not as much of an issue with the JPC-12 vertical where the impedance can be juggled somewhat by its configuration, using silver-plated wire for the coils on the the JPC-7 loaded dipole does – at least on the lower bands (40, 30) present a matching problem in that with the natural feedpoint resistance of a physically-small, loaded dipole would be very much lower than 50 Ohms. Whereas the loss of the stainless steel coils are significant – enough to keep the feedpoint resistance high enough for <2:1 VSWR, using low-loss wire in the coils lowers this to =10 MHz. It was gratifying to get very similar results between the two different instruments on frequencies where the measurements overlapped – being a bit of a “sanity check” that reassured me that both the instruments and my measurement methods were likely consistent.
What I noticed in particular where the “Q” measurements where my readings and yours differ by a factor of roughly 100 for the JPC-12 coil. If you take the definition of “Q” – which is the ratio of reactance to the actual ohmic resistance measurement – a “Q” measurement of below 1 would catastrophically bad, indeed – and this number “seemed” to me to be unrealistic: Indeed, such a low-Q inductor would have presented the operator with a pleasantly wide bandwidth, but with unpleasantly high power losses!
I did notice that your readings were taken at 200 kHz – a frequency on which I made no measurements: Instead, I concentrated on the frequency ranges closer to those on which the antenna would be operated – going down only to 1 MHz as part of a “sanity check” – but still observed radically different readings of “Q” – namely 41 @ 1 Mhz and about 28 @ 2 and 4 MHz at the 40 meter paint mark (about 19uH in my measurements) for the stainless coil. At lower inductances (e.g. the 20 meter mark) the Q at 10 and 15 MHz was closer to 35 for the same stainless coil. Again, these readings were “sanity checked” with the two instruments at the frequencies at which both could be operated.
As a comparison, while the “Q” of the stainless steel coil was typically in the 30-50 range at the general frequency ranges at which one would use the selected taps, the “Q” of the silver-plated copper wire was, in very rough numbers, five to eight times that value – typically in the area of 200 or so at the intended operating frequency – explaining the far lower losses that I observed in the re-wound coils.
It’s worth noting that when one starts measuring values of “Q” over 100, it gets exceedingly difficult to make accurate, repeatable measurements as the slightest amount of extraneous “R” induced by connecting leads will dramatically reduce the reading: Even a few 10th of an ohm can do it meaning that care had be taken to use very short, low inductance connections between the coil under test and the instrument to avoid short-changing the readings: I suspect that the the actual “Q” of the silver-plated coil at the higher frequencies is larger-still than I measured owing to the physical awkwardness of interfacing a large coil to the HP-4191A. At least the errors of “Q” measurement are likely to result in conservative readings rather than wildly optimistic ones.
Of course, in situ, the actual “Q” of the antenna *system* will be lower than that of the (more or less) unloaded “Q” of the coil itself so the Q of the coil alone is more of a predictor of its loss rather than that of the bandwidth of the antenna (e.g. the silver-plated coils are easily narrower than those of the stainless-steel coil).
As usual, your articles are both thoughtful and thorough, but the large disparity between my “Q” readings – made with an HP-4191A and HP-4275A (and also using W7ZOI’s methods in his article “The Two Faces of Q”) has me wondering.
73,
Clint, KA7OEI
Hi Clint,
I had ICQ Podcast duties yesterday so it’s taken me a couple of days to respond. THANK YOU! I read your own blog post on this coil. I wish I also had an HP 4291a to go above 10MHz but I do not (yet).
You are correct in that the Q values for several coils I reported were bogus. It is due to the reference plane not being adequate, something you point out on your blog: “Measuring a physically-large coil at these frequencies is awkward: The test equipment itself is metal, meaning that its proximity affects the measurements but we cannot use long wire leads to space it far enough away to avoid this effect as this would affect inductance and also the Q, so we can only do the best that we can.” While you used the HP 16092A Spring Clip Fixture putting the coil next to the device, I used the Kelvin Clips sold for this purpose: HP 16048C. The calibration does try to neutralize what single wire clips would do to reduce the accuracy of the measurements. But what I erroneously did in my initial measurements was to simply hold the clips on each end of the coil, resulting in my hand contaminating them as well as an unreliable connection. At least, it seems that is what happened.
I have revised my blog article at k4fmh.com (won’t re-sync to amateurradio.com) to include two sets of new measurements. I explain why I use 200KHz as the measurement frequency there. Measurements were take with the 4275a and a Matrix MCR-500 LCR meter, also with Kelvin Clips. These clips produce measurements very close to standard value components for R and C. It’s very difficult to get a good (not highly expensive) standard for L. There I explain in more detail the methods use and thank you kindly for your communication.
As I say in my revised post, I would rather be critiqued and have correct numbers than not but propagate incorrect ones. I hope the current estimates—the L values were not much different—are indeed reliably near values inherent in the coils.
73,
Frank
K4FMH
Sorry for the delay in replying.
Thanks for the kind words Frank!
You and I seem to be of similar mind: We try not to make mistakes – but when we do (and I *certainly* do!) we look forward to learning the what, why and how – and will likely knowing more when all is said and done than we otherwise would have.
Keep up the good work and 73,
Clint, KA7OEI