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[RFI] Common Mode RFI

To: <rfi@contesting.com>
Subject: [RFI] Common Mode RFI
From: douglass@dcwis.com (Edward F. Douglass)
Date: Wed, 04 Dec 2002 11:44:39 -0600

I was reading reviews of various kinds of filters on the eHam web site.  A 
fellow ham reported that his TVI problem disappeared when he installed RF 
Inquiry common mode chokes on his rig.  He used the 5 kW unit ($125) after 
his linear and the 250 W ($90) unit between his transceiver and his linear.

In the RFI tech material published by the League, I don't recall seeing any 
mention of the use of common mode chokes on transmission lines.  There are 
a couple of exceptions.  One is to coil up some transmission line just 
below the feed point of a beam.  The other is to wind power cords and small 
diameter coax through toroid cores to block the flow of RF into equipment.

1.  Is it likely that common mode RF flowing on the outside of coaxial 
cable would radiate into domestic telephone systems and computer modems, 
causing RFI?  How would one determine if this is the case (without buying a 
common mode choke, first, and experimenting)?

2.  I am going to e-mail Steve Ford, WB8IMY, because in looking over the 
list of product reviews in QST, I did not find a review of common mode 
chokes like the ones made by RF Inquiriry.  I would be interested in 
reading a technical evaluation.

3.  Radio Works sells something they call a "Line Isolator."  I will e-mail 
them to see if these are common mode chokes.

4.  If common mode chokes on transmission lines could be part of the 
solution to RFI, I would imagine a lot of hams would be interested in 
knowing that and knowing how and where to install them for maximum reduction.

Looking forward to your reply.

73 de Ed, AA9OZ

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