Trap those Radials
After reading more than one article where the author had problems with a vertical antenna on different frequencies, due to unexplained VSWR increases, a light came on. The problem was the vertical height and radials length had ended up where the feed point was essentially at the center of a virtual full wavelength antenna. By this I mean that the combined length of the vertical and the radials was equal to a full wave antenna. When this happens, the feed point impedance jumps up greatly making for a very high SWR.
A good example is the multi-band vertical with a radial system. If you put out a ¼ wave radial system on 80 meters, then when you go to 40 meters, the radials are going to become ½ wavelength long resulting in an impedance mismatch in most cases. Same thing on 20 meters if you have a radial system for 40 meters. So after thinking about this for a while, the realization came to me that if you put traps in the radial system, all of this unpleasantness could be avoided.
To fill in the blanks on how a trap works, look at Figure 1. This is a conventional parallel-tuned circuit, consisting of a coil and capacitor. Now the characteristic of the circuit we are interested in is that it turns into a high impedance at resonance. Remember all of the tube plate circuits were parallel-tuned circuits of one form or another. This is because tubes are high impedance devices. So if we were to take the circuit in Figure 1 and feed it with the frequency where it resonates, between points A and B there would be very little or no current flowing through the circuit. Some would flow, but not enough to make a difference. In other words, it would act like an insulator. Now for the purposes of this discussion, only one radial will be used in the examples, but the principal applies to all radials in a radial ground system.
Therefore, if you had a radial of ¼ wave for 75/80 meters, you could go out from the antenna end to where the radial would be ¼ wave on 40 meters, cut the radial and put in a parallel-tuned circuit for 40 meters. Then from the antenna end to the 40meter tuned circuit, that portion of the radial would only be seen by the antenna system. The other section of the radial effectively would be cut off as far as any 40 meter current flow is concerned. Then, going back to 75 meters, the circuit would not be resonant and would allow the 75 meter energy to flow the entire length of the radial.
So if you were to install traps in each of your radials, then you would have less chance of a problem with impedance changes affecting your antenna system. Figure 2 shows how you could do this, with the total length of the radial (L) being for 80 meters and trap A being inserted at the correct length for 40 meters and trap B for 20 meters, trap C for 15 meters and trap D for 10 meters. The traps would have to be installed in each radial for this to work correctly. The WARC bands were not shown, but would have to be installed the same way.
In Figure 3, one trapped radial is illustrated and the effective length is shown by the colored line below it. In the case of the 75 meter band, the entire length is shown by the blue line below it. The 40 meter band is shown by the green line and the active length of the radial on 20, 15, and 10 meter bands are shown by the violet, red and green lines. This is not drawn to scale, but is only an illustration of how current flow in the radial is controlled by the traps in the radial.
If you want to construct the traps for your radial, it is rather easy to do. First, you will need a coil form made of some insulating material to wind your coil on. After you decide what you will use for a form and have wound the coil, and if you have a variable capacitor and know its value, connect it across the coil. Then connect an antenna to one end of the circuit and your rig to the other. Figure 4 shows how you should have this connected, using the diagram in Figure 1 as an example, with A or B being connected to the antenna, (it doesn’t matter which) and the rig to the other terminal. Then, set your rig to the band that you want to operate on. After you have done this, tune the capacitor and listen for a null in the received signals as you tune. When you have found the null, that is where the circuit is resonant. Next you will have to substitute a fixed capacitor for the variable, getting a value that will be as close to the approximate value of the air variable. A certain amount of guesstimation is required here as the variable is not going to be calibrated in capacity, but if you know the value of the variable, then when it is half open, the fixed capacitor needed is half of the full value of the variable capacitor.
If you don’t have any variables, then you will have to use a fixed capacitor and short out the coil until you find resonance. Once you get the values for the capacitor and coil, then make four of them for each radial you have. Put them in a weatherproof container and install them at the appropriate place in each radial and you are in business.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 16th May 2024