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Review of common mode choke Fair-Rite Mixes 31 and 75

I contacted Chris at Radio-Stuff for advice on choosing a common mode choke to reduce electrical noise in my shack. My hobby is transatlantic DXing so the lower the noise level the better with these distant signals. I use an RSPdx Software Defined Radio along with a Wellbrook loop antenna.

I didn’t realise, but Chris actually lives quite close to me and he very kindly offered to come round to my house and advise on the best way to solve my issues. He brought with him a selection of parts and took time to understand my needs. We started off with two mix 31 Ferrite Toroids, then later on we added a mix 75 (which Chris ordered in specially for me) for the lower frequencies. We found that the mix 75 needed more turns than the mix 31 so Chris researched the best thinner coax cable we could use and soldered on the connectors for me as well.

I can’t praise Chris enough for his level of service and support. I have recently passed my Foundation License exam and will certainly go to him for antennas and spares in the future.

The following are tests that I carried out with Redhill Hospital Radio which has a power of 1W and broadcasts on 1431Khz. The first image is with the Mix 75 and Mix 31 ferrites in series. An average SNR of 26.4 was achieved.

Removing the ferrites and replacing with a standard coaxial cable results in a much lower SNR of 21.9 on average (as shown in the second image),

We used the following table for Mix 31 and chose 17 turns on 2 FT-240-31 ferrites:

For mix 75 we chose 35 turns as described in a paper called ‘Noise Suppression for the Antenna Feed Line’ by Nick Hall-Patch:

Peter Jones M7EFU

February 2023

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