Multi Circuit Coupler

I use the rather crude-looking, but practical, breadboard contraption described here to determine what type of antenna coupler I should use for any particular antenna that I may dream up. It is designed to allow the interconnection of all components for many different configurations, such as PI, L, T, and any other design you may come up with. By doing so, you will be able to find out what the most efficient design will be to allow maximum power transfer into the antenna of choice. This will allow you to make up the coupler and install it at the base of the antenna where it belongs. Contrary to popular belief, an antenna tuner located in the shack will not match the antenna to the transmitter, unless you are feeding your antenna with twin lead or ladder line. If you use a tuner at the rig and use coax to feed the antenna, all you do is tune out the VSWR that the transmitter sees on the coax. You do not eliminate the VSWR—it is still there, only your transmitter does not see it.

In Figure 1, the components making up this particular experimentation unit can be seen. Each of the components have flexible leads with an alligator clip on both leads. Each lead is long enough to reach all over the extent of the board, thus allowing many different configurations to be tried without modification. There is an RF ammeter shown, but a field strength meter can be used instead. RF ammeters can be found on the surplus market and have made my tuning much easier. You should also use an VSWR bridge for tuning as this will also tell you when your VSWR is at a minimum. Moreover, the forward power position can help you in getting the maximum forward power into the antenna. A crossed needle VSWR meter is ideal for this application, because it lets you to see when the VSWR is at the minimum and power at the maximum.. Each adjustment will make a difference in VSWR as well as forward power.

Figure 2 demonstrates a configuration used for short antennas. It is what I use with my marine whip, except I do not use the series capacitor. L-1 is tapped for minimum VSWR and L-2 is tapped for maximum power into the antenna. Since I operate only one frequency on 160, this arrangement is fine. If I did a lot of moving about, I would have to reconfigure the coupler to allow me to do this. I would probably add the capacitor in series and set it for maximum capacity and tune up the whip for the lowest frequency I wanted to operate. As I go higher in frequency, the capacitor can be tuned for maximum field strength, or antenna current. It is possible that tapping the coils again would be required, but since I don’t shift frequency much, I am not concerned.

Figures 3, Figure 4 and Figure 5 below show other configurations.

In the photo at the top, the coupler breadboard can be seen. This coupler was made out of surplus components and should only be used as a guide for a general layout The two coils at the bottom are in series, as I could not get enough inductance for my experiment. The two series coils are mounted at right angles to the other coil which is a length of miniducto I had laying around. This is to prevent coupling and interaction between the two coils when adjustments are made to one of the coils. I also used a vacuum variable. As soon as I can find the mounting brackets, I will install another vacuum capacitor, of about 1000pf. This will allow me to make a PI-type network or any other type of circuit that requires two variables. I also have some large high current mica and fixed vacuum capacitors I may stick on the board, just in case I should need a large value of capacity. The usage of the vacuum variables is not required, but I had on hand having picked them up surplus some years ago

This is a project that will enable you to find out what will work best before-hand with any given antenna you may want to put up. For matching the lower bands, 160, 80 and 40 meters, where you may be restricted as to area for an antenna, the tuning and loading up of a shortened antenna for these bands can be a real problem. But, by running your coax out to the antenna, connecting this tuner to the rig and the antenna, you can determine experimentally what antenna matching circuit will be the best for your particular antenna. Then you can build one, put it in a weatherproof box and mount it at your antenna where it should be.

OF course, I trust that you will not try to make connections with clip leads while the transmitter is key—you will get an rf burn instantly! SO, DO NOT MAKE ANY CONNECTIONS WHILE THE TRANSMITTER IS KEYED TO KEEP FROM GETTING A PAINFUL BURN. This holds true even with low power. Speaking from experience…

Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF

Last Updated : 24th April 2024