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Re: [RFI] Receiver settings for hearing RFI?

To: rfi@contesting.com
Subject: Re: [RFI] Receiver settings for hearing RFI?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 11 Mar 2012 09:38:32 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On 3/10/2012 10:17 PM, Gary K9GS wrote:
> Can I assume that the best receiver settings are AM mode and as the
> widest bandwidth filter settings?

David Robbins and Paul Christianson are right -- AM detection is only 
the method of choice when the noise source is impulse noise. When the 
noise sources are clocks and their harmonics in various kinds of 
electronic gear, including power supplies and battery chargers, you want 
to sample as much spectrum as you can. This makes a panadapter or other 
spectrum analyzer the weapon of choice, and failing that, the broadest 
detector on the most sensitive radio you have.

However -- it's also possible to do some chasing with any 
battery-operated portable that can tune the bands affected by the noise 
sources. A ham talkie or battery QRP rig that can tune the HF bands and 
is small enough to drag around can be used with a small whip or rubber 
duck to probe around suspected noise sources.

Most HF noise is conducted as a common mode signal on cables connected 
to he noise source and radiated just like any other RF current. This 
means that noise sources don't radiate much until those cables are 
connected to something that gives that current somewhere to go (like 
ground, or the chassis of equipment), or makes the cable long enough to 
itself be a decent HF antenna.  It means that when you probe for noise, 
you want to probe those cables, and you may observe wavelength-related 
effects simply because like any other antenna, there is a voltage and 
current distribution on that cable that varies along its length. The AC 
power line can be one of those cables; the DC cable that carry a power 
supply's output is another.

The computers and computer devices in our homes and the electronics in 
our living rooms are all potential noise sources, and all of the wiring 
connected to them are potential noise radiating antennas, INCLUDING coax 
cables that connect to a cable TV system, satellite downlink, or other 
outdoor antenna.  The act of grounding this stuff does NOT usually 
reduce noise -- tha ground wiring also radiates whatever RF current it 

What DOES usually help are serious common mode ferrite chokes on the 
"antennas" formed by winding enough turns around the ferrite core to 
move the resonance of the choke to the frequency range where the noise 
is being heard.  These chokes work by placing a high impedance, low Q 
parallel resonant circuit in series with the common mode circuit, which 
greatly reduces the RF current.  It has been my experience that a choke 
like this is usually more effective than a good commercial power line 
filter (like the Corcom units someone posted).  These filters are 
designed to be bonded to the shielding enclosure of the noise source 
they are filtering by very short leads, and when they are not, they are 
generally not effective.

For more on this, including advice on winding suitable ferrite chokes,  
see http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim Brown K9YC
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