Making a common-mode choke

Over the winter I bought a couple of different EFHW antenna kits, not all of which are assembled yet. (More about these in the weeks & months to come!) I have been wishing for a common-mode choke to work with them because of all the stories that one reads about RF-on-the-coax-shield interfering with the operation of the radio, but nothing in that line has come to my attention crying out “Buy me! buy me!” and my mind finally worked itself around to the idea of just making one from off-the-shelf parts. Here you see the result:

The core is this FT-240-31 item from Amazon; there are not many -31 cores on there sized between the 240 (2.4″) and the 82 (0.82″), and I needed one that could pass a BNC connector so that I could use a pre-made RG-316 cable. I thought about using a 3 foot cable, but then upsized to 4 feet “just in case”. The wire-ties are from my bench, I always have an open bag of them around.

Allowing a tasteful 6″ of lead-in, I attached the cable to the toroid and started winding, taking care to maintain light tension as I proceeded. 15 turns later there was about 6″ of cable remaining and I applied another wire tie to hold it all in place, then added another to each end for insurance. It will add another 5.4 ounces (including the barrel adapter on one end) to my field pack according to the kitchen scale. I will report on performance when it gets used in an activation.

Thanks for visiting!





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