DIY Common Mode Choke

19 Mar 2022

Balun Designs Choke Impedance

Common-mode choke impedance graph

For a long time, I have been advocating the relatively simple practice of using common mode chokes in antenna feedlines to reduce or eliminate the unwanted coupling of noise sources in the shack to receive antennas outside, or transmit energy from outdoor antennas to devices in the shack.

I have been using some very good quality chokes from Balun Designs recently, but they are designed for transmit powers up to several kilowatts, and for some receive-only cases (like my Beverage antennas), this is clearly gross overkill.

Balun Designs Common-Mode Choke

The well-regarded guru Tom Rauch, W8JI has some comments regarding the construction of common-mode chokes and matching transformers for Beverage antennas, and I’ve started to construct a receive-only common-mode choke based on his advice.

I am using a binocular core with type 73 ferrite material, as W8JI recommends and since it seems to be getting more and more difficult and expensive to source specialist types of wire (i.e. Kynar insulated), I decided to just use some wire from discarded Ethernet cables, which is roughly the correct size, and has the added advantage of being pre-twisted which is needed for a bifilar transformer like this.

DIY Choke

I am not certain the inverse phasing he references is needed in my case, but the much lower cost of these transformers will allow me to put one on every feedline, and perhaps even at the feedpoint of some of the antennas which should greatly increase the common-mode rejection.