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Discussion of general interest including club business and events. Please post ARES discussion in the SC4ARES group.

Common mode chokes for noise reduction

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    There is a lot of information available on radio frequency interference (RFI), including identifying it, tracking it down, and eliminating or blocking it. The ARRL has a page at There is also a lot of information available from Jim Brown, KJYC, at

    For a complete understanding of the whats and why of common mode chokes, I recommend RFI, Ferrites, and Common Mode Chokes For Hams at This document is fairly technical, but it well written and reasonably understandable.

    However, most of just want to add a common mode choke to our antenna feed lines to reduce noise, prevent transmitted signals from getting back on the feedline, and save the technical reading for when the bands are dead. In that case, start with the Cookbook for Transmitting Chokes For 160-10M at This document has a series of ‘recipes’ for chokes, based on band and power.

    For most of us, the starting point will be a ferrite toroid made of Type 31 material, say 2.4″ outside diameter.
    This is Fair-rite part number 2631803802 These ferrites are available from many sources and often with high markups. I typically order from Mouser, and the current single piece price is about $5. In general, we want to buy a batch of these for the best price and minimum shipping. (At the moment, Mouser is out of stock. 1 month to field day – no surprise.)

    With toroids in hand, the chokes can be wound. 12 AWG THHN solid copper wire works fine – i typically use the black and white pair from a piece of Romex. It takes two lengths of wire, about 4 feet long. Check the cookbook for the number of turns, but in general 13 – 14 turns is about right for the HF bands. For 160M, you will need closer to 16 or 17.

    After winding, we need to add connectors and some protection. There are many ways to do this – I typically solder an SO-239 to one end of the choke, and use a short length of coax with a PL-259 at the other.

    Here are a couple of pictures to get the idea across:

    One caveat – these designs are intended for use in antenna feedlines to reduce common mode current at HF frequencies. They will help prevent your transmitted RF from getting back to the transmitter on the outside of the coax. If you have RFI coming from other sources, or RF interfering with computers or other equipment, different chokes or techniques might be required.

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