[RFI] 20M Heterodyne

Jim Brown jim at audiosystemsgroup.com
Sat Sep 20 16:39:59 EDT 2008

On Sat, 20 Sep 2008 15:00:20 -0400, David C. Hallam wrote:


>Yes I hear it around 14.030.  The next spot I really hear it is at 14.152.

>My Ethernet system has been in place for several years and it's only in the
>last couple of weeks that I have been hearing the heterodyne.  I wonder if
>changing from a HF6V vertical to a end fed Zepp antenna has anything to do
>with it.  

Probably so. It's a simple matter of coupling from one antenna to another 
(that is, the Ethernet cable is an antenna), and the proximity and directional 
patterns of each antenna have a strong effect on what you'll hear and at what 

>I made the change at about the same time I started hearing the
>noise.  I am in the process of moving my vertical and put up the Zepp as a
>temporary antenna.

I'd go ahead and suppress the Ethernet wiring. You should should be able to 
knock it down by at least 20 dB. Now, you may ALSO hear your neighbors' 
systems too, depending on proximity, their wiring, and your antenna. I did 
when I lived in Chicago, where I had a tenant downstairs and houses 25 feet 
either side of mine and four more within 100 ft. 


Jim K9YC


>> -----Original Message-----
>> From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com]On
>> Behalf Of Jim Brown
>> Sent: Saturday, September 20, 2008 2:27 PM
>> To: RFI List
>> Subject: Re: [RFI] 20M Heterodyne
>> 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.
>> 73,
>> Jim Brown K9YC
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