Antenna Matching

Here is a fact some uninformed operators might dispute; if your installation doesn't require any matching, and the input SWR is under 1.6:1 or so, you either need a better antenna, or a better mounting scheme. If this is the case, you should read these articles; Antennas, Commercial, Antenna Mounts, Antenna Efficiency, and Grounds, RF & DC.

Matching a mobile antenna to the requisite 50 ohms is a requirement for several reasons. For example, modern solid state radios are designed to reduce their output power when the input SWR reaches 2:1. Some will handle a little more, some a little less. The SWR doesn't have to be flat, so anything below 1.6:1 is close enough.

One very important point needs to be mentioned at this point. If you're using a remotely controlled HF mobile antenna (HiQ, High Sierra, etc.), the motor leads (and reed switch leads if used), must be properly bypassed (choked). If they're not, I'll guarantee you'll have problems. Do yourself a favor, and read this article: Antenna Controllers.

Important Caveat

Improper choking of the motor leads will effect the impedance of your antenna. Therefore, the motor leads should be disconnected at the antenna before attempting to adjust any matching method. Once matched, if reconnecting the motor leads changes the matching point, it is a good indication that the motor lead choke impedance is too low. In other words, you need more turns, a different ferrite, or both.

Reactance vs. SWR

Here is one very important item to remember. At resonance, the input impedance of a decent-quality, correctly-mounted, HF mobile antenna will be about 25 ohms. By definition, the resonance point is where the reactive component equals zero(X=Ø) . Since the requisite impedance of our feed line is 50 ohms, the resulting SWR would be 2:1. However, if you adjust the antenna to a frequency higher or lower than the transmit frequency, the SWR will decrease, perhaps to 1.5:1. This occurs because the resistive component rises faster than the reactive component. For this reason, you should use an antenna analyzer with a reactance readout, when adjusting the matching coil in the following section.

Again, you should look for the lowest reactance, not the lowest SWR when adjusting the matching device. Once the matching device is properly adjusted, the minimum SWR point on your transceiver (or external SWR meter) will be very close to the actual resonance point of the antenna.

Digressing for just a moment. The reactance readout on an antenna analyzer may not be exactly zero at it's lowest obtainable setting. This is due to several factors, not the least of which is the basic accuracy of the instrument in question.

Matching Input Impedance

Matching the antenna impedance to the line impedance is important, especially if you're using an automatic antenna controller. If your HF antenna doesn't require matching to provide a low SWR (with the possible exception of 10 and 12 meters), then the antenna you're using isn't much more than a dummy load on a stick! Further, proper matching reduces the chance of RF flowing on the outside of the coax feed line.

Literally thousands of articles have been written about antenna impedance matching. What ever method you choose to achieve a match is fine, as long as the antenna is DC grounded. There are two reasons for this.

First, DC grounding helps control static discharges from the antenna, thus reducing some of the received hash we all put up with. Secondly it is a safety issue. If the antenna were to come into contact with a live overhead power line, DC grounding will help prevent damage to your equipment and perhaps to you as well. Newcomers might think that a DC grounded antenna won't work, but this is not the case. Just because we DC ground the antenna, doesn't mean it is RF grounded too. There are several ways to accomplish a DC grounded, RF match.

If you're planning on using a remotely tuned antenna like a HiQ, High Sierra, Scorpion or one of the many other remotely controlled designs, and an automatic antenna controller like the Better RF unit, then inductive matching is your only choice if you're seeking fully automatic operation.

Inductive matching does DC ground the antenna. At left is a photo of the MFJ-908 L match unit. Building one rather than buying one won't save you any money, but you might learn how they work if you do. The ARRL Antenna Handbook is a good place to start.

However, you really don't need a switched inductor like the aforementioned MFJ unit. Instead a simple inductor, like the one shown in the right photo (or upper left), will suffice. One end is attached to the antenna feed, and the other end is connected to ground. The ground end of the coil should be collocated with the coax shield ground.

The coil in the right photo has 9 turns, 1 inch inside diameter, and wound with # 12 or #14 Thermalese^® (enameled) wire. You can also use building wire, but it's a little harder to work with. In actual use, the turns are spaced a little further apart to adjust the inductance. Depending on the input impedance of the antenna, the coil needs to be about 1 uH. In the real world, the value may be between .5 uH and 1.5 uH depending on the actual input impedance at resonance.

How does it do the matching? The coil, and some capacitive reactance from the antenna (when tuned slightly above the operating frequency), form a highpass, LC network. This network transforms the antenna's low impedance (typically 25 ohms or so) to that of the 50 ohm feed line.

The coil's form factor should be kept close to 1:1 (length to diameter). Long skinny coils do not work nearly as well. You should also avoid those commercial units which surround the mast.

If you own an antenna analyzer, you can adjust the turns spacing to provide a good compromised match (<1.5:1) across the operating range of the antenna. Once you've installed the coil, here's the tuning method to use. By the way, this process is very difficult to accomplish with just an SWR bridge, but it can be done. It just takes about 10 times longer, and you have to actually transmit on the air to do so.

Move the antenna to the 80 meter band. The actual frequency isn't important. Adjust the frequency on the analyzer until the X equals zero, or as close to it as you can (the SWR reading is not important at this point). Read the R value. If it is less than 40 ohms, stretch out the coil slightly, and readjust the frequency so X again equals zero. If the reading is lower than the first try, squeeze the coil together, and try again. Repeat the process until the R reading is about 40 ohms when X equals zero. This relates to an SWR of 1.25:1. Remember, the only time the SWR readout will be accurate (and significant), is when X equals zero, and R is close to 50 ohms.

Move the antenna to the 40 meter band, and adjust the analyzer's frequency until X equals zero, and read the R value. In most cases, it will be at about the same spot as it is on 80 meters. If it is not, it may be necessary to compromise between the two bands.

Once you've complete the 80 and 40 meter settings, move the antenna to the 20 meter band. Adjust the frequency on the analyzer until X equals zero, and read the R value. It will almost always be closer to 50 ohms than either 80 or 40 meters. You can check the rest of the bands if you desire. Most of the time, they will read very close to the 20 meter R value.

If a compromise can't be reached between 80 and 40, or the 20 meter R reading is far removed for 50 ohms when X equals zero, then chances are there is an inadequate ground plane under the antenna, or there is too much stray capacitance between the antenna and the body of the vehicle, or both!

UNUN Matching

You can also use an UNUN (UNbalanced to UNbalanced) RF transformer. Like the LC network above, it provides a DC ground, and the requisite impedance match. The overall system losses are low, so the UNUN can be mounted near the radio rather than at the antenna. Here is an article on how to build an UNUN. Or, you can buy one ready made like the MFJ-907 unit shown at left.

Keep in mind, if you use a remotely controlled antenna, you will have to change taps between bands. In most cases, you can use one tap for 80 and 40, another for 20 and 17, and straight through for 12 and 10. If you don't like the idea of changing taps, then make one of the aforementioned inductors.

Capacitive Matching

I don't recommend capacitive matching for two reasons. First, the method doesn't DC ground the antenna. Secondly, as the frequency increases, the reactance (in ohms) decreases, which means you have to use a different value capacitor for each band, and sometimes within a band.

If you're already using capacitive matching and don't want to bother changing it, at least add a 10K resistor across the antenna terminals (high power will require several resistors in series). While this will not protect you from live overhead wires, it will help tame the static discharges. A correctly sized RF choke will to the same.

By the way, MFJ offers two sizes of capacitive match boxes; one rated at 300 watts, the other one 600 watts, but both models are stressed at half their power rating.

Measuring SWR

Several different companies manufacture remote SWR bridges so you don't have to run coax hither and yon if your radio is remotely mounted. MFJ sells several different models, like the Ameritron above right. Most of these units utilize cross-needle meters to display SWR conditions. All well and good, but I had problems finding a place to mount the head of my Icom IC-7000, much less finding space for a 4 inch square meter. I opted for a different solution.

Left is a picture of LDG Electronics TW1 talking wattmeter. There is a review of it in the November 2005 issue of QST starting on page 77. It runs on a nominal 13.8 VDC, and draw less than 90 uA on standby so a power switch isn't needed, although you should properly fuse it. Incidentally, LDG has discontinued the TW-1, but they're still available from their retailers.

Here's something to remember about accuracy. The very best of wattmeters are rated at 5% of their full scale reading, and you pay for the privilege! Most, like the AWM-35 above, aren't much better than 10%, if that good. For example, its low scale is 250 watts, so your 100 watt transceiver may indicate anywhere between 75 and 125 watts! Further, what ever accuracy they exhibit diminishes when the load becomes reactive.

If you are willing to pay for accuracy, then Alpha Products' 4520 is what you're looking for. Its space-age design sports an accuracy rating of 3% of full scale. With an auto-ranging scale of 10 to 5000 watts, it is currently one of the best available, and at $1,200 plus shipping, it should be!

Odds & Ends

I cover this in detail in my coupler article, but it bares repeating here. Either an internal or external auto coupler may be used to match a mobile antenna's input impedance to 50 ohms. And, they can be used to extend the bandwidth of a monoband antenna. However, using one with a remotely tuned antenna presents some operational problems. If you use one, keep the following in mind.

The antenna in question should be adjusted to resonance (lowest SWR is close enough) before the auto coupler is turned on. This is especially important with external couplers with their greater matching range. Under the right circumstances, failing to resonant the antenna close to the operating frequency can cause the RF voltage to be high enough to arc over most base insulators, and might even exceed the ratings of the coax between the coupler and the antenna.

Another good reason to DC ground a mobile antenna is for lightning protection. It won't necessarily protect your radio from damage, but does offer a level of personal protection. Don't laugh, I've been hit several times! The last time was May 25, 2007. The corona ball now has a new hole, the the whip nearly burned in two, and the matching coil was damaged. The radio was on at the time, and suffered no damage.

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