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Re: [RFI] KY filters

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] KY filters
From: "Jim Brown" <jimbrown.enteract@rcn.com>
Reply-to: Jim Brown <jimbrown.enteract@rcn.com>
Date: Fri, 19 Dec 2003 22:22:01 -0600
List-post: <mailto:rfi@contesting.com>
Thanks for the link, Rob. It's useful to know what options are

One of the toroids I recommended costs $10-$12, depending on where you
buy it. I would try a toroid before I spent more than twice as much on
one of these filters.

And this response to another comment in this thread. While a 43
material toroid is, indeed, optimized for higher frequencies, if you
use enough turns, its effectiveness at lower frequencies will increase
significantly.  Remember that inductance increases as the square of the
number of turns. I would wind the toroid with as many turns of the
telephone cable as I could, and I would put as close as practical to
the problematic telephone. If there are multiple phones with problems,
I would put it on the piece of cable connecting that telephone that
looks most like a receiving antenna to your transmitter.

Remember that you are simply trying to use the toroid to add a big RF
choke in series with the telephone line (or whatever wiring is feeding
the device that is detecting the RF). In general, the more turns, the
higher the impedance of the choke. You can, of course, hit resonance
with enough turns and stray capacitance, but you are unlikely to do
that below 30 MHz with most wire and most toroid forms. And, as
frequency increases, ferrites get increasingly lossy, so even if their
reactance has dropped they still offer significant attenuation for
another octave or two.

Jim K9YC

On Sat, 20 Dec 2003 02:14:32 +0000, Rob Atkinson, K5UJ wrote:

>Big thanks to everyone who wrote me about the KY filters.  I wish I had time
>to reply to each individually, something I normally try to do, but this time
>of year it's not possible.  I appreciate all the leads, tips and advice.
>For those who did not know what I was talking about, because I made the
>mistake of assuming everyone did, (I hate it when people do that) go to
>It's our best dial-up Internet access offer: 6 months @$9.95/month. Get it
>now!  http://join.msn.com/?page=dept/dialup
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