Mobile Matching Networks
Most transceivers run about the same power, 100-130 watts, so that part of the equation will be ignored. The antenna and matching of whatever antenna you choose to your transmitter will determine how well your signal is heard. Since the antenna impedance can be anywhere from 3 ohms or lower on 160 or 75 meters to somewhere near 30-40Ω on 10 meters, some method of getting the antenna impedance matched to the transmitter output impedance which is 50Ωs.
To start, keep in mind the transmitter output impedance is constant and cannot be changed, at least not without modifying the transmitter. Also, nearly all solid state transmitters have a vswr protection circuit that keeps the final transistors from being destroyed if there is a mishap with the antenna and the reflected power increases out of sight. The old tube rigs just spat fire and hissed at you if anything happened to the antenna, but the transistor rigs just die expensively if there is no swr protection.
So with this in mind an examination of some of the circuits that are used to match the antenna. The object of the game is to get the transmitter to see a 50-ohm load no matter what frequency you are using. If you don’t, the transmitter will shut down. For the sake of simplicity, we will use a center-loaded whip. All the matching networks will work with all of the other antennas as well. Only body mounts will be considered, no bumper mounts or other clamp on type of mount as these are not as effective as far as having a low resistance rf ground connection.
The first type of network we will look at it is the shunt inductor. This has been around for a long time, and there are several variations, some more effective than others. So, take a look at the diagram and see the different variations and we will look at the relative merits of each. These are strictly inductive circuits, and any stray capacitance is not going to be considered to keep things simple.
The network in Diagram A has been around a long time and was sold commercially for a while by Master Mobile and works very well, and is easy to adjust. It should be mounted at the base of the antenna, outside of the car if possible, as it will contribute to the radiated energy since it radiates also. The network in Diagram B also has been around for a long time also, and works well also and it is easy to adjust for multi-band use.
Diagram C is a more efficient method than the other two, since it allows the transmitter to work into a constant 50 ohm load and the antenna is to be tapped up from ground until the correct impedance point is found for the antenna impedance. This particular matching network is found in a lot of the commercial marine antenna couplers and a lot of auto-tune military couplers, notably the Collins C/U 358, and others.
The value of the inductor will depend on the particular installation, but can be determined by using one of the antenna impedance noise bridges that are on the market. Usually, 12-15 tpi, 2 to 2 1/2 diameter will be fine. If you wind your own coil for this use the largest wire size you can, as the currents at the base of a low impedance antenna can get quite high even with 100 watts. The use of 1/8 soft drawn copper pipe would be quite appropriate here.
So you tap up and down until your transmitter sees an impedance that it likes and the swr protection allows the power to go to its normal output. Which does not mean that the antenna is matched correctly, but it does work, and a lot of hams have used this system for years with success.
In Diagram C, your transmitter will see a constant impedance and the antenna is tapped up from the ground end of the coil until the correct impedance point is found that matches the antenna impedance. As has been mentioned before this system has been used for years in a lot of commercial marine antenna couplers and in a lot of auto-tune military antenna couplers. Keep in mind the military and commercial marine operators cannot afford anything that is not the best, as there is no room for anything else when the boat is in danger of sinking, or the bad guys are getting the upper hand on the battle field.
The next matching method we will examine is the shunt capacitor method where a shunt capacitor is connected from the base to ground, which is the body of the car. This is what Diagram D is depicting. For each band that you operate, a different value of capacitor will be needed. This matching system usually works better on the lower frequencies, 160-40 meters, than on 20,15,or 10 meters. It is broad band though, and one value of capacitance will usually cover the entire 75-meter phone band or the entire 40 meter band. By using a good grade of high current rotary switch the matching network can be switched when you change coils for another band. This method is simple and moderately efficient, but there are some losses due to circulating currents in the car body.
Now to the more complicated networks:
Diagram E is of the ” Backwards L ” so-called because in the usual configuration the capacitor is on the end of the coil that goes to the transmitter and in this configuration, is used to match a high impedance antenna. In the backward configuration, the L network is used to match low impedance antennas. As a matter of interest in both the Icom AH-2 and the Icom marine, the L network is used with relays that can change the configuration to meet the requirements of the antenna system as the operating frequency is changed.
Since the two couplers are designed to operate with single conductor antennas such as long wires and vertical whips, the impedance of these types of antennas can change drastically with large frequency excursions. So the configuration must be changed to fit the antenna impedance requirements.
The parallel tuned circuit in Diagram F is a matching system that has not seen too much use in the mobile antenna matching game. It is seen in a lot of older transistor radios to match a tuned circuit to a transistor base. Since a transistor base is low impedance with respect to the collector, some sort of matching network was needed to get things to playing properly. It seems that this particular network would do a good job matching a low impedance mobile whip to a 50Ω transmitter, as well as be of a fairly high efficiency, due to the resonating of the coil by the tuning capacitor. The high Q and the resultant high circulating currents of the tuned circuit should result in a very high percentage of the transmitter power being transferred to the equally high Q antenna. It is something to speculate on and experiment with.
A ten-meter mobile operator will wonder why bother matching a ten meter whip antenna since it is naturally resonant. Well, a 1/4 wave whip or any other 1/4 wave antenna still has a feed point impedance of about 36 ohms, so despite what your swr bridge says, a matching network will help your overall efficiency. If you are using a converted cb mag mount, like one of the ASP series, don’t worry about matching as they are already matched internally. Just tune it for minimum swr.
So now you have seen several different types of matching networks that can be used to better match your mobile 160-10 meter antenna system, now it is up to you to do the experimenting to see which does best for your antenna and particular installation.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 13th March 2024