Wire Antenna Tuners
Sometimes circumstances limit the choice of an antenna to a wire, long or short, that must be strung between two poles, trees or whatever is convenient. It is unknown as to what the impedance that the antenna will be, or where it will be resonant, unless you tried to cut it for a particular band. Thus, you will need an antenna tuner of some sorts to be able to radiate a signal. The automatic tuners like the SGC and similar ones will work nicely, but they will also work wonders on your pocket book. So something less expensive will be required.
Over the years, a variety of antenna tuning circuits have been used—some were good and some were not. This article will discuss some of those various circuits used down through the years and are presented here for you to consider in loading up that wire antenna. These tuners are not for loading up the coax to the antenna, but are meant for a wire antenna that comes into the shack. A counterpoise and/or good ground are also required to make this type of antenna work as well as it can. If you intend to use one of these tuners in the shack to tune your backyard antenna connected to a length of coax coming into the shack, forget it. You will be loading up the coax as well as the antenna. Not a good way to go. Despite this warning, there are hams that have done it and thought they were really resonating the antenna. Wrong! A lot of power is lost in the coax to the antenna since the VSWR is still there between the output of the tuner and the coax. Again, the best use of these antenna tuning systems is with wire antenna that must come into the shack. That is not the best situation, but there is often just no other way to configure an antenna to allow you to get on the air. With that in mind, on to the diagrams!
Figure 1 is of the Pi-L tuner, which is a very good tuner and could match almost any impedance antenna that you could connect to it. It is not found in any equipment made today. The two inductors are ganged together and tuned by a single knob. The tuning capacitors are separately tuned, and are in the range of 10 to 1000 pf. They can be made up of either a large capacitor or one of medium size with fixed capacitors being switched in as required. This circuit was found in several WWII transmitters and was a product of Collins Radio. The inductors can be whatever size you can use as long as they are identical in value and ratings. A variation on this is the Pi Network tuner, which is made by removing L2 from the tuner. It is also a very good matching system, but does not quite have the impedance range of the Pi-L Network.

Figure 2 is of the very familiar capacitive T-network. Nearly all of the manual tuners sold today are of this type. They are cheap and easy to build. The efficiency of this coupler varies and my own personal experience has shown me under certain conditions, this type is rather inefficient. An article in one of the ham radio magazines on this type of tuner discussed its shortcomings, but, it is a cheap way to make a tuner.

Figure 3 is of a variation of the T-Network and uses a variable inductor in the line instead of the two capacitors and a capacitor to ground. I do not know any equipment using this circuit today, but several hams I knew used this tuner and seemed to get out well on Field Day expeditions. I do not remember what kind of antennas that they were using, but they were happy with the results.

Last but not least, in Figure 4 we have the L network. By making it so you can switch the variable capacitor from input to output, you can match a large range of impedance variations over a large frequency range. It is a very versatile antenna tuning system.

I have not put any values on these diagrams since I do not know what you may have to work with, but the capacitors in the Pi-L and the L network were in the range of from 10 to 1000 pf. The inductors were rather large, but I do not remember what range the values were. Perhaps some other old time hams could refresh my memory on this. Basically, whatever you can come up with will require some experimenting with the different component values to see what will work or improve your particular configuration.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 29th April 2024