Re: Common mode chokes on receiving loops

Chris Moulding

Very interesting Ray.

Up until the Covid-19 lockdown we used to manufacture a Mains Filter that combined an IEC mains filter with a ferrite toroid to provide very high suppression of conducted HF RF interference on the electricity supply.

The web page showing the details and spec of our filter is:

The GM3SEK pages are interesting but he follows the conventional radio amateur practice of only using Fair-Rite or Amidon toroids. This is fine in the US but the prices when imported into Europe are high after tax and shipping is included. We now use Ferroxcube ferrite toroids from Poland which to us in the UK are around 1/4 the price of Fair-Rite or Amidon. It's also a higher spec product with a greater range of ferrite materials and often with epoxy coating of the toroid for extra electrical safety.

On the PA0NHC web page the description of how a common mode choke works with coax is incorrect. If it worked how he describes it then RF wouldn't flow through the coax in differential mode when a toroid is used on the outside of the coax.

Now that I feel like I've marked your homework if you are a RSGB member you might be interested in the article I've written in RSGB RadCom this month. Using loop counterpoises and an isolation transformer I've described a vertical antenna that is resistant to local RF noise.

I've got the agreement of the Technical Editor of RadCom to share the article next month when the next issue comes out.

I know from the emails I'm receiving that a lot of RSGB members are trying out the design with some success. The Ferroxcube distributor Arrow Electronics has now run out of the toroid I specified in the article and I'm now suggesting the next size down. They have 800 in stock so they should last a while.



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