Author’s Preface: I have revised my inductor coil table numbers and the narrative text pertaining to it. Clint KA7OEI posted a comment on the syndicated version at AmateurRadio.com. He questioned my Q estimates for the JPC-7 coil, based on his own tests as well as the very low numbers I reported. I owe Clint a debt of gratitude! After teaching measurement theory for years, I greatly respect questioning potentially specious numbers. Mine were and they arose due to the problems measuring coils that he himself acknowledges on his blog post about his measurements: getting a good reference plane for the device under test (DUT). I’ve inserted a brief methodology section near the table in this updated post, explaining what I did to revise those measurements to be reliable ones. I greatly appreciate Clint KA7OEI taking the time to point this out as I would rather be more accurate and embarrassed than not and propagate erroneous numbers to my readers. Thanks OM!
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 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.
New narrative is shown in gray background below:
After my first version of this article was posted and syndicated through AmateurRadio.com, Clint KA7OEI posted a comment there in which he questioned some of the Q values I reported, pointing out his own blog posts about the JPC-7 coil. He measured it with both the HP 4275a and an HP 4291a. Clint also questioned my use of 200KHz as the frequency at which I measured these impedance values since he prefers the frequency at which they will be used. I encourage readers to look at his blog post on the JPC-7 coil here. His goals and mine are different as he performed an in-depth study of this one coil (even rewinding his with different wire). Mine were to show readers how to go about modifying the EiffelTenna into a center-loaded vertical for 40 meter operation and what to look for in such a coil.
I’ll describe my procedures. I now know why I initially reported some “goofy” Q values. It was the same struggle that Clint describes in his blog post: getting a stable reference plane for the device under test (DUT) with minimal interference from the immediate environment. He very accurately says there, “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.” Clint used an HP 16092a Spring Clip fixture with a short wire attached to the JPC-7 (he cites the JPC-12 but they’re largely the same). The metal-cased measurement device is close to the coil which, as he notes, can affect the readings, especially at higher frequencies. He performed measurements at 1, 2, 4 and 10 MHz and up to 30MHz using the HP 4191a. (The 4275a maxes at 10MHz.) He does a great job describing his measurements and results. The reader would benefit from a study of that blog post.
Why did I report inductance values measured at 200KHz? Here’s why. My goal was to assist readers in modifying a bespoke vertical antenna design (details forthcoming) into a center-loaded one. The vast majority of readers won’t own an LCR meter. Clint, I and others are blessed to have built labs or have access to some lab grade test gear. The inexpensive LCR meters on the market typically measure a maximum frequency of 100KHz with a few, like the Peak LCR meter, at 200KHZ. Even well thought of meters like 800 series by B&K and the DE 500 LCR devices top out at 100KHz. My choice was to provide mainly inductance (L) values to the reader but added the Q for a fuller picture. Clint’s approach is more exhaustive since his focus was on the wire composition and improving this one coil by Chelegance. Mine was merely an assistance to potential buyers of a coil where manufacturers do not disclose the actual values in their advertising or user material.
Regardless of differences in goals, reliable values reported for Q should be made. I failed to do that as I held the connectors to the ends of the coils while measuring. Like Clint’s machine being close by, my hand cupping the coil and somewhat unreliable mechanical connection likely contaminated the Q values. I should have recognized the very low Q values. So, I re-measured all of the coils with my HP4275a and Matrix MCR-5200 LCR meters again. Both were at 200KHz for reasons stated above.
I used the Kelvin Clip fixture sold with the HP 4275a, the HP 16048C, and the similar one sold by Matrix for the MCR-5200. The four-wire measurement fixture calibrates at the clip ends (open-short, with a gold-plated shorting plate) to remove most of the extraneous factors from a single wire contaminating the inductance measurements. It was sold explicitly for the purpose of measuring devices like this. Instead of holding the clips to each end and cupping the coil, I used a “practical reference plane” to temporarily attach the clips through a bolt or nut attached to the respective ends of each coil. This is exactly how each coil will be used in the field. On the Wolf River Coil Silver Bullet 1000, I connected the clip directly onto the end of the exposed wire while using the nut on the other end to shorten the lengthy attachment built into that end of this coil. This provided a sturdy, continuous connection after calibrating the 1-meter Kelvin Clips to the LCR for measurement. The same “test jig” was used with the Matrix MCR-500 Kelvin Clips while the coil setting was the same.
Table 1 has been revised to include measurements, all at 200KHz, from each LCR meter at the respective low, middle, high or fixed settings. My Lab stays at a humidity level of 45-55% and 72-75 degrees F almost year-round due to an AC/Heater with Dehumidifier unit I installed in that small room off my garage. This helps maintain stability in measurements as well as my GPSDO for timing reference. The LCR meters were warmed up for two hours prior to taking the revised measurements vs an hour for the original tests. The 4275a LCR meter has been calibrated as per the service manual for that device. While both sets of measurements are shown in the table, I discuss only those from the HP LCR device. I hope these details on the measurement approach and choices help the reader better understand why I revised this table. Many, many thanks to the diligence and collegial communication from Clint KA7OEI! I encourage you to read his blog as well.
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 0.5 to 16.7 uH with corresponding Q values of 26.2 to 92. (Since I have three of these coils, I measured all three. There was little production batch variation so the first measurements are reported.) Air coils tend to have higher Q values which is why Martin Jue decided on that form. While the Mad Dog adjustable coil (sturdily built, I might add) has a wider inductance range (0.48 to 27.9), it has somewhat low Q values (5.7 to 9.7). The Chelegance JPC-7 also has a wide range of inductance settings, from 0.3 to 21.2. Like the Mad Dog coil, the JPC-7 Q values are not as good as an air coil at 2.3 to 5.2. Here’s where one coil, larger than the rest, shines in this table. The Wolf River Coils Silver Bullet 1000 has inductance values from 0.6 to 81.0, allowing a larger frequency range for loaded vertical antennas. The Q values for this coil range from 2.3 to 14.3. 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. Some would have higher loss than others as the Q values suggest.
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 measured inductance value is 8.4 uH. There is a clone from China that measures 7.9 uH. Perhaps because of different manufacturing processes, the WRC coil has a lower Q value at 10.5 than the clone from China has at 16.8. 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. But manufacturers just say they’re “for 40M.”
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!