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Re: [TowerTalk] POWER LINE RFI

To: "Towertalk" <>
Subject: Re: [TowerTalk] POWER LINE RFI
From: "Jim Brown" <>
Date: Tue, 25 Mar 2008 09:39:35 -0700
List-post: <">>
On Mon, 24 Mar 2008 21:05:39 -1000, Lee Wical wrote:

>>Jim K9YC:
>         In your booklet, "A Ham's guide to RFI, Ferrites and Audio 
>Interference", on page 23, figure 31, you show a photo
>of Chokes on a power line.
>         I need to find out from you what values for ferrites are 
>best used (#43 or #31 material) and are these Fairite company

These are the #31 "big clamp-on" described in the text, with Fair-Rite 
part numbers listed in Appendix One.

>The picture shows six >(6) chokes. Is that what I need for 20M thru 10M?

These chokes WILL suppress COMMON MODE radiation by the power line. They 
will NOT suppress differential mode trash (but that is usually not the 
major problem anyway). General guidelines for suppression are in the 
tutorial -- at least 500-1,000 ohms at the frequency of interest, and 
breaking the "antenna" (the power line is an antenna) up into lengths that 
are too small to be efficient radiators. A half wave is a great radiator, 
even with an open circuit at the end, while a quarter wave is not, so try 
to place clusters of chokes such that the longest "antenna" is something 
like a quarter wave (or even 3/8) at the highest frequency that the power 
line is radiating. A quarter wave at ten meters is 2.5 meters. 

Note that you can only do this when the toroid surrounds all of the power 
conductors so that the total differential mode field (the power to your 
house) cancels in the toroid, and the toroid sees only the differential 
current (hopefully, only noise). In the photo, it's a 120-0-120 feeder, 
with the bare wire being the neutral. 

>Any particular part number? And finally, what vendor do you recommend?

Recommended vendors are Newark, Allied, Lodestone, and Kreiger. 


Jim Brown K9YC


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