Common Mode Current Choke

Posted by Fred C (W6BSD) on Dec 17 2022

A couple of weekends ago, I took my trapped vertical antenna apart to clean it up, ensure every component was in good working condition, and fine-tune it. While working on the antenna, I noticed that the Common Mode Current choke box had a crack due to the hot California summer sun making the ABS1 dry and brittle. That crack might develop into a water ingress rendering the choke unusable after a while. It is why I have decided to build a new one.

What is a choke?

The choke I am building is a device inserted between the antenna and the radio that reduces the RF current flowing along the outside of the coaxial cable. That current is called common-mode current. The choke must have a high impedance for RF currents, usually more than 1000Ω. The common-mode current flowing on the feed line can cause RFI2 problems in the shack or elsewhere in your house. A choke will ensure that the coaxial cable's shield does not become part of the antenna and unintentionally or receive unwanted signals, such as RF noise generated by the electronic devices in your house. You can find more on the benefits and how they work on the common-mode chokes page on Wikipedia.


I found a few ferrite toroids in my drawers, and I had two types of ferrite mix to choose from Mix-31 and Mix-43. I also needed to determine how many turns I needed to have a high impedance on the HF frequencies I use.

To characterize each ferrite core's attenuation, I wrapped between 8 to 14 turns of RG58 coax and measured the attenuation with a NanoVNA.

The following two graphs show the attenuation in dB for each Mix with 8 to 14 turns of coax. One of the lines on the graph shows the attenuation of an Air Core Balun with twelve turns of coax around a 15cm form (6 inches). I have included that graph for comparison. My goal is to get a minimum 30dB attenuation across the entire HF spectrum.

As you can see, the toroid with Mix-31 has a slightly better attenuation than Mix-43 in the lower frequencies (below 10MHz), but they are almost equivalent in performance for a common mode choke. Both ferrite mix options are good candidates for my choke.


Tests with Mix-31
Tests with Mix-31


Tests with Mix-43
Tests with Mix-43

Building the choke

For the final construction, I used an RG400 coaxial cable. This cable is made of Teflon; it has a small bending ratio and can resist high temperatures. I built two chokes, one with Mix-31 and one with Mix-43. Each choke has a total of 11 turns. As shown in the graphs below, Mix-31 has more attenuation on the 80-meter and 40-meter bands. The choke using the Mix-43 has a better attenuation on the higher bands. With an attenuation of almost 40dB on the 15-meter and 10-meter bands.

Five turns on each side of the cross-over. The cross-over also counts as a turn.

Test setup
Test setup

On the right, you will find a picture of the final choke before mounting it into a weatherproof project box. The crossing is just there for convenience. It allows having both wires on the opposite side, making it more convenient to install the choke in a box. In the following graphs, you can see the attenuation I get with both chokes.

At the base of my antenna, I am using the common-mode choke with the Mix-43. It will offer me an attenuation of 30dB to 40dB and a choking impedance above 2kΩ and up to 9kΩ on the higher bands. It is perfect for my Hustler 4BTV antenna.

I can always replace it with the choke using the Mix-31 if I need more common-mode current attenuation on the 40-meter band.

Common Mode Current choke

Choking Impedance
Choking Impedance


The common-mode chokes made with Mix-31 or Mix-43 ferrite material have a wide bandwidth and offer an excellent choking impedance for the HF bands3. For comparison, I have included an Air Core Balun in the previous graphs, also called an Ugly Balun. As you can see on the chart below, the Ugly-Balun has a very narrow bandwidth and several points of resonance.

6 inch - 12 turn - Ugly Balun
6 inch - 12 turn - Ugly Balun

The Air Core Choke in this example will have a reasonable attenuation on the 40-meter, 15-meter, and 10-meter bands. But it has barely any attenuation on the 80-meter and 20-meter bands. If you use a mono-band antenna, an Air Core Choke with a sharp resonance at the frequency of your antenna could be effective as long as you build it right.


  1. Acrylonitrile Butadiene Styrene (ABS) is an opaque thermoplastic 

  2. Radio Frequency Interference 

  3. The HF bands range from 3 MHz to 30 MHz 

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