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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!