[RFI] 20M Heterodyne

Jim Brown jim at audiosystemsgroup.com
Sat Sep 20 14:27:09 EDT 2008

On Sat, 20 Sep 2008 11:06:11 -0400, David C. Hallam wrote:

>Is it my receiver or is there a heterodyne of some sort on 20M? 

I'm a CW guy, so I know the products that show up in the CW bands. 
Do you also hear it around 14,029-14,030?  Around 21,052? Those are 
common mode radiation of products of a wired Ethernet system. There 
are also some on 30M, and the bottom of 10M and 6M. They can be 
attenuated by winding multiple turns of the Ethernet cable around a 
toroid core to form a common mode choke. You need to use a number of 
turns that places the broad resonance of the choke in the frequency 
spectrum where you're bothered by the interference. For cables 
longer than a quarter wave, you may need a choke on both ends. 

See http://audiosystemsgroup.com/RFI-Ham.pdf for measured data on 
toroidal chokes that will fit this need. They will, of course, work 
for other common mode sources as well.    


Jim Brown K9YC

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