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An inexpensive and effective Common Mode Choke for Long Wave listening.

One of our Short Wave Listener (SWL) clients recently request a Common Mode Choke suitable for long wave reception from low KHZ to around the Top Band radio spectrum.

Never wanting to miss an opportunity to help a customer we decided to research a toroid mix suitable for his request.

Having spent an hour or so looking through the Fair-Rite website we decided to pick the Fair-Rite mix-75 toroid (5975011121) as a candidate for the request.

Details; Type 75 ferrite mix Plastic 75 Ferrite Core Toroid

Length Width 2.978" (75.65mm) Diameter 0.535" (13.60mm) Height Weight: 188 (g)

An order was placed for some Mix 75 toroids and when they arrived we made a simple choke for our customer.

The choke itself is one core with approx. 40 turns of RG174 passing through the core with a crossover half way and two BNC Male connectors on the ends.

Long Wave Common Mode Choke

The core was then hooked up to a Nanvo VNA to check the return loss on S21.

To be a good CMC a minimum of -20db loss should be present. Here are the results.

Narrow scan 10khz to 2mhz, you can see exceptional choking below 1.25mhz to 70khz.

Wide scan 10khz to 20mhz, the choke does have some effective choking upto 7mhz; however a Mix31 series choke with ideally a Mix43 in series would give the best possible choking setup for HF on receive.

As you can see this will make an excellent CMC for LW communications reception.

The client will now place the Common Mode Choke (CMC) in series with his existing CMC from a MIX 31 core. This should give very good choking characteristics for LW and MW listening.

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