Last Modified: April 10, 2013
Contents: Preamble; Basics; Caveats; Coil Adjustment; Why 40 Ohms; Odds & Ends;
Any SWR less than 1.6:1 is good.
There are a few technical aspects every amateur needs to know. The most important one is how reactance works. Without this basic knowledge, addressing antenna, RFI, and matching problems results in a frustrating situation. Everything you need to know is in chapter 2 of the ARRL Handbook, entitled Electrical Fundamentals. Once you've read chapter 2 a few times, move onto chapter 20 entitled Transmission Lines. Once you've read chapter 20 a few times, you'll discover one very salient point, and that is, not to get hung up on SWR! While SWR has some importance, it is not a panacea, especially when we're dealing with antennas which do not represent a 50 ohm, non reactive load. To put this in perspective, when dealing with mobile HF antennas, the lowest SWR is not necessarily the best match! A fact which will become apparent as you read further.
If you're an ARRL member, you can down load M. Walter Maxwell's, W2DU (sk), white paper, Another Look At Reflections from their web site. Although technical in nature, it is the primer for understanding SWR. This is a quote from that paper:
"With center-loaded mobile whips of equal size having no matching arrangement at the input terminals, best radiating efficiency is obtained on models having the lowest measured terminal resistance (highest resonant SWR, model for model). Models having lowest SWR are wasting power in the loading coil, because of either a low value of coil Q or excessive distributed coil capacitance, or both."
The following coil adjustment procedure requires the use an antenna analyzer to measure the reactive, and resistive components of the input impedance of a mobile antenna. Whatever the SWR is at any given point, means nothing! Nonetheless, far to many get caught up in the SWR is King scenario, and pay more attention to that readout, than they do the reactive readouts. So here's a worthwhile suggestion. Take a piece of masking tape, and cover the SWR readout portion of the display, and forget about SWR altogether!
There is a specific procedure which must be followed if proper adjustment of a shunt matching coil is to be achieved (read that as a relatively low SWR over a very wide range of frequencies), as required when using a remotely tuned antenna. In addition, the coil must be correctly configured. For example, long, skinny coils do not work as well as ones where the length and diameter are approximately the same.
Further, for best results the coil should be installed as clear of surrounding sheet metal as possible, not enclosed in any material, even plastic (especially gray PVC!), and not short-tapped! When matching coils are mounted close to mounting brackets, vehicle sheet metal, enclosed in any material other than air, or have some of their winding shorted out, their performance and linearity will suffer! Incidentally, his may require relocating factory-supplied coils, or winding your own as outlined in this article.
The number of turns may also vary. The coil at left has 9 turns, is 1 inch inside diameter, and wound with #14 Thermalese®(enameled) wire. The one on the right is 7 turns, and .75 inches in diameter. The requisite number of turns, depends on the quality of the antenna, the mounting location and style, and the amount of ground losses present.
You can also use building wire for the coil, but it is a little harder to work with. In actual use, the turns are spaced a little further apart than shown, to adjust the coil's inductance. The coil needs to be about 1 uH, but in the real world, the value may be between .5 uH and 1.5 uH depending on the actual input impedance of the antenna in question, at resonance.
Matching is achieved by borrowing a small amount of capacitive reactance from the antenna (the antenna is tuned slightly above the actual operating frequency), and the inductance of the coil. Together, they form a highpass, LC network which transforms the antenna's low impedance (typically 25 ohms or so) to that of the 50 ohm feed line.
By the way, the coils shown were wound using a sparkplug socket wrench as a form. Whatever you use as a form, for best results, the coil should be about 1 to 1.25 inches ID.
Here is a page from the September 1969 issue of QST, submitted by Edwin Knowles, K6OX. It lists the requisite capacitance, and/or inductance required to match an HF mobile antenna using either an CL lowpass match, or an LC highpass match at varying input impedances. Since the thrust here is remotely-tuned HF antenna, we're only interested in the LC highpass match (shunt inductance) type of match. Readers should note the finite adjustment needed for a several octave wide matching arrangement. It should be apparent why fixed, limited adjustment range, enclosed, and short-tapped coils should be avoided, especially when using automatic antenna controllers.
The following adjustment procedure assumes you have a properly wound RF choke(s) on the motor leads, and the reed switch leads if used. The choke(s) must be mounted as close to the base of the antenna as possible, and not inside the vehicle! Here's why.
An improperly wound and/or installed choke, will affect the input impedance of the antenna. To circumvent this possibility, the choke and control wires to the antenna should be removed before any measurements are made.
Once the coil adjustment is completed, if reattaching the control wires and choke, changes the input impedance, no matter how small the change is, the choke is inadequate! For more information on winding the requisite choke, read the Antenna Controller, and the How To Wind A Choke articles.
If you do the following coil adjustment procedure correctly, you will be rewarded with a relatively low SWR (< 1.6:1) at any resonant point the antenna (80 through 10) can be tuned to. The coil will not need further adjustment unless you change the antenna's length, add a cap hat, or perhaps relocate the antenna.
The photos show the readout of a screwdriver antenna, on 40 meters, without any matching, and after the shunt coil was installed and adjusted (left and right respectively). Results on 80 meters will be similar.
If the impedance of your unmatched antenna measures close to those shown in the right photo, chances are the overall losses are great enough that you don't need any input matching. If that is the case, you need a better antenna, better mounting, or both!
As noted, the number of turns may vary from 7 to 10 turns. The actual number needed depends on a lot of factors; the actual input impedance of the antenna, the size of wire, the overall length of the coil, its diameter, and whether it is insulated or not. The dimensions suggested (1 inch long, 1 inch in diameter, and 7 to 9 turns) will suffice 98% of the time.
One common adjustment failing is the I'll break it syndrome. Some folks are just too shy about elongating the coil enough to get a decent match. If you're one of these, remember this; all the coil is, is a small piece of enameled wire, anyone can (re)wind in a few moments. It isn't magical, and it isn't critical in any respect. It may indeed end up looking like a funky corkscrew by the time you're through with the adjustment, but that fact alone won't affect the end results. By the way, any motor or alternator rewinding shop will have enough scrap laying around to wind hundreds of coils; a 3 foot length is plenty. Size really isn't important either, but #14 and #12 are the easiest to work with.
There is one important consideration when using an antenna analyzer, and that is broadcast interference (BCI). If you live near an AM or FM broadcast station, or close to a commercial installation, you should check to make sure you do not have BCI. In the case of the MFJ-259B, that's easy to do. With the MFJ-259B connected to your antenna, push the mode switch until the frequency counter displays. If the SWR meter significantly deflects, you probably have BCI. MFJ does sell an optional BCI filter unit for the 259B which eliminates the problem.
In the following steps, it may not be possible to get the X value to equal zero (or very close to it). There are several reasons why this may occur, including the basic accuracy of the instrument in use. Or, you might be adjusting the analyzer to the lowest SWR rather than the lowest X value. Or, the coax between the analyzer and the antenna is too long. It should be less than 18 inches. Or, you might have BC interference as noted above.
Do yourself a favor, and explicitly follow the adjustment procedure herein. If you do not, here is what will happen: You will not achieve a low SWR over your antenna's band width; You will have antenna controller malfunctions and/or intermittent operation; And, you will become frustrated, no end!
Move the antenna to the 80 meter band. The actual frequency is unimportant. Remove the control leads, and choke as outlined above. Adjust the frequency on the analyzer until the X=Ø, or as close to it as you can. If you've adjusted the antenna analyzer to the lowest SWR, rather than X=Ø, you've already made a mistake!
Read the R value (not the SWR!). If it is less than 40 ohms, stretch out the coil slightly (1/4 inch at a time!). Readjust the analyzer's frequency so X=Ø once again. If the R value is lower than the first try, squeeze the coil together slightly (1/4 inch at a time!), and try again. Repeat the process until the R reading is about 40 ohms when X=Ø. This process could take several tries, and must be completed before moving to 40 meters.
After you've adjusted the coil on 80 meters, move the antenna to the 40 meter band. The actual frequency isn't important. Remove the control leads, and choke as outlined above. Adjust the analyzer's frequency until X=Ø (not the lowest SWR!). Read the R value. In most cases, it will be about the same spot as it was on 80 meters. If it is not, it may be necessary to compromise between the two bands. This may require multiple measurements on both bands. If you hit is right the first time, consider yourself lucky!
As the case may be; if you can't stretch the coil apart enough to find a match, take off a turn or two. If you can't collapse it enough, add a turn or two. The requisite number of turns for a one inch diameter coil, will vary between 7 and 9, and the overall length between 1 and 2 inches (.5 uH to 1.5 uH).
Once you've completed the 80 and 40 meter adjustments, move the antenna to the 20 meter band. The actual frequency isn't important. Remove the control leads, and choke as outlined above. Adjust the frequency on the analyzer until X=Ø, or as close to it as you can. Read the R value (not the SWR!). It will usually be closer to 50 ohms than either 80 or 40 meters, and may actually be slightly higher than 50 ohms. You can check the rest of the bands if you desire, but they'll usually read very close to the 20 meter R value.
If a compromise can't be reached between 80 and 40, or the R reading for 20 meters (and above) is far removed for 50 ohms when X=Ø, then chances are there is an inadequate ground plane under the antenna (#1 cause), or there is too much shunt capacitance between the antenna (particularly the coil), and the body of the vehicle, or both! The solutions are obvious.
Why 40 Ohms?
Someone is bound to ask; why set the shunt matching coil for 40 ohms rather than 50 ohms? Basically, it is a compromise. If you use 50 ohms, you'll find the match on 17 and 15 will be over 80 ohms (>1.7:1 SWR) in most cases. The reason is related to the reactance of the shunt coil (including its distributed capacitance), as the frequency changes. Distributed capacitance is also the reason small, closely spaced coils don't work too well in this application, especially when they're mounted inside the antenna's mounting bracket.
Odds & Ends
As mentioned in the Antenna Matching, Odds & Ends, if your antenna is an 160 through 10 model (as opposed to an 80 through 10 model), there is a very good chance you won't be able to use a fixed shunt coil. Thus, a switchable inductor like the one shown at left, or the MFJ-908 shown at right, are the only viable solutions. Nonetheless, the aforementioned procedure should be followed.
If your antenna is an 80 through 10 model, and you can't get an acceptable match using the aforementioned methodology, it is probably due to improper mounting and/or mounting location. It is also indicative of a low Q loading coil, and short overall length.
An SWR bridge can be used to adjust the coil, but requires about 10 times the effort, and you have to transmit while you're making the adjustments (not always advisable). Starting with the suggested dimensions (1 inch long, 1 inch in diameter, and 9 turns), find the lowest SWR on 80 meters (the actual frequency isn't important). This should be done by adjusting the antenna's resonant point. If it is under 1.5:1, move to 40 meters (the actual frequency isn't important). If the lowest SWR is higher than the 80 meter point, squeeze or elongate the coil, and readjust the antenna to the lowest SWR. The lowest SWR point should be set half way between the 80 meter point, and the original 40 meter point. After each adjustment, you must once again find the lowest SWR by re-resonating the antenna. You will have to go back and forth many times between 80 and 40 to get the coil properly adjusted. Properly set, the SWR should be the same on 80 and 40, or as close to it as you can. Using an SWR bridge to set the shunt coil typically takes about 2 hours to complete. It should be obvious then, that an antenna analyzer is a much better tool for adjusting the shunt matching coil.
Lastly, you can use a shunt base coil to match any mobile antenna, even a monoband one. However, if you're using a remotely tuned antenna, this matching method is the only one that doesn't require any adjustments when switching between bands, save for the aforementioned issues.