You often hear about it and it is a hot topic on forums, repeaters and club gatherings. I have interference (RFI) on my equipment or that of my neighbors caused by my own radio signal. What can I do about it? First of all, the best strategy is to tackle the problem at the source, which is the most effective and usually the cheapest. In almost all cases it starts with the antenna and feed line.
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The RFI (Radio Frequency Interference) problem can often be traced back to particular antennas. Notoir are the EndFed, vertical, dipole and its variants like the OCF dipole (Windom). The most simple and effective solution to tackle RFI is placing a common mode choke. But it’s all about where you place it. Here are some tips for optimal placement. But first… wy are these antennas a source of RFI?
Why does and EndFed antenna cause so much interference?
The best known EndFed is a single wire which is half a wavelength long on the longest operationial wavelength. It’s also known as EFHW and is sometimes electrically lengthened with a coil. This is an asymmetrical antenna that uses a counterpoise to radiate. On the housing of the 49:1 impedance transformer (other lengths also use 64:1 or 9:1) is often a ground terminal screw to which you can connect a counterpoise (such as a metal mast or non resonant length of wire).
If you do not use the grounding terminal, the EndFed will use the coax as a counterpoise. As a result, currents flow over the coax’ outer shield, better known as common mode currents. These currents flow along the entire length of the coax, all the way to the shack and all equipment connected to your transceiver. As a result, the entire coax becomes an active part of the antenna and radiates. This causes RFI in apparel.
The solution; isolating antenna from the coax
The solution with the EndFed, as with many other antennas, is to “isolate” or decouple” the coax from the antenna by suppressing common mode currents. You do this by installing a common mode choke (1:1 current balun or line isolator). With an EndFed, there are basically two options:
Option 1: If you have a separate counterpoise connected to the ground screw, place the common mode choke as close as possible to the coax connection of the impedance transformer. A short cable in-between of say 30-50 cm (12-20 inches) is not a problem.
Option 2: If you do not have and cannot make a separate counterpoise, then the outer shield of your coax cable will serve as a counterpoise.
Place a non-resonant length (not ¼ of the shortest wavelength) of coax about 2 m (6-7 ft) or 4 m (13-14 ft) long between the common mode choke and the coax connector on the impedance transformer. Only that part of the coax will then become an active part of the antenna. Disclaimer: Hanging that part of the coax close to your shack can still cause interference.
The vertical; 1/4, 1/2 and 5/8 groundplane antenna
The vertical is basically the same as the EndFed. However, there is a difference between the 1/4 wave antenna and the 1/2 and 5/8 wave antenna. The latter two, also known in the world of 27 MHz as the GPA, Antron 99 and I-Max 2000, must be treated the same as the EndFed. So you can also follow option 1 or option 2 with the EndFed.
With the 1/4 wave vertical, things are slightly different. It uses 1/4 wavelength elevated radials, as a counterpoise, or a network of radials on the ground often combined with a ground rond. Therefore, this vertical is also called a groundplane antenna.
This antenna also uses the coax’ outer shield as a counterpoise. The antenna “sees” the coax shield as a “radial”. But we don’t want that, so we need to isolate that coaxial cable. Therefore, in the 1/4 wave vertical, just as with the EndFed, you place the common mode choke (= 1:1 unun) as close as possible to the feed point of the antenna.
A ‘1/2 wave groundplane’ should be treated the samen as the half wave EndFed.
The dipole fed via coaxial cable
The dipole with its “two legs” is a so-called symmetrical or balanced antenna. If you feed it with an asymmetrical or unbalanced feed line, such as a coaxial cable, you will get common mode currents even with this antenna. To prevent this, place the common mode choke (which is a 1:1 balun) directly at the feed point.
Antennas such as the Yagi, Cobweb, Fan Dipole and Hexbeam also use one or more dipoles as radiators and thus also need to be equipped with a common mode choke.
In variants where the antenna is partially fed with open line (ladder line), such as the G5RV, ZS6BKW and Doublet, you place the common mode choke where the coax transitions to the open transmission line. So where asymmetrical transitions to symmetrical.
The OCF dipole; as asymmetric as it gets
The OCF dipole (off center fed or Windom) is a special case. In this antenna, one leg of the dipole is shorter than the other. As asymmetric as it gets! The OCF dipole produces significant common mode currents. A common mode choke (aircore balun or ugly balun) by coiling coax usually gives too little suppression of common mode currents for this antenna. For such an antenna, use a common mode choke based on a ferrite core with bifilar windings or coax windings. Install, as with the “regular” dipole, directly at the feed point.
Want to know more about common mode chokes?
Common mode chokes come in different shapes and sizes. The design depends on the application, frequency range and transmission power. Want to know more about the different types of chokes? Then read this article.