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Re: [TowerTalk] 2.4 inch diameter Type 31 ferrite cores

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Subject: Re: [TowerTalk] 2.4 inch diameter Type 31 ferrite cores
From: "Jim Brown" <>
Date: Tue, 27 Jul 2010 10:04:16 -0700
List-post: <">>
On Mon, 26 Jul 2010 20:22:16 -0500, Tod -MN wrote:

>I felt that I would like to get an estimate of the
>attenuation of RF energy that might be passing along the coax. Instead of
>measuring the voltage and calculating the impedance as Jim did, I used a
>calibrated power meter to measure the change in power level across the input
>of the power meter. Using that setup I measured the attenuation provided by
>seven turns of coax through six cores as well as the attenuation provided by
>fewer turns through the same set of cores.  I was pleasantly surprised to
>find that there was 40 dB of attenuation at both 160m and at 80m for the
>seven turn unit.

As I told Todd in private email, what he is measuring is NOT attenuation. 
Rather, he is using a test circuit like mine with different equipment, and he 
gets the raw data from which he can compute the impedance of the choke. 

The attenuation (of common mode current and noise) that you can get from a 
common mode choke depends on the common mode circuit into which you are 
inserting it, as well as the proximity of the noise source(s) to the antenna 
and the transmission line. That circuit is part of an antenna, so it must be 
modeled as an antenna to figure out what is going on. I would be very 
surprised at noise attenuation of more than 10-15dB in the real world. You 
might kill transmission line current, but the line is connected to an antenna, 
and the antenna will pick up noise. The benefits of using the choke are that 
you keep the directivity of the antenna to reject the noise, and it can be 
further away from the noise source (which may be in your neighbor's living 

I strongly encourage participation in this group purchase. I've helped 
organize several, and particpated in another. In today's increasingly noisy 
world, you can never have enough ferrites. :)

K6LL said:

>Arrow Electronics is a good retail source for these, $5.24 for the 
>2631803802 in unit quantity, but their stock is low right now.

That's a very good retail price for small quantity. Many thanks, Dave. 

73, Jim Brown K9YC


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