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Re: [TowerTalk] decoupling RF from control lines

To: "Jim Brown" <>,<>, "Pete Smith" <>
Subject: Re: [TowerTalk] decoupling RF from control lines
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 16 Aug 2005 11:13:51 -0400
List-post: <>
used for 13 volts out to the relay box.  The .1 caps were
suggested by TopTen for best effectiveness at 160, but what
I was really unsure of was the inductance of the RF chokes.
I'll certainly have a look at your app notes.>>

As for component values Pete, they all depend on the
application. The voltage or current sensitivity of the
device to RF, the waveshape change on control lines the
device can tolerate before switching or component life
becomes a problem, and so on. There is no real universal
value of filter that works.

You are significantly better off with an absorbing material
like a ferrite with a very low Q at HF rather than
inductors. The bypass caps mostly establish a low shunt
impedance for the series impedance of the chokes to work
against. In virtually all cases .01uF or .05uF caps would be
more than adequate for 160 meters. A .01 is under 9 ohms on
160 meters. Anything less than several ohms reactance is
really just poor engineering. If it takes less reactance
than five or ten ohms to prevent RFI in a LV circuit the
system clearly needs some series impedance!

It is very unlikely any value disk cap when the wire leads,
ESR, and connection path in the equipment is included would
have less than five ohms impedance from 1.8 to 30 MHz. Why
pick a cap that might have .9 ohms reactance on 1.8 MHz only
when perfect (no ESR) with zero lead length?

73 Tom


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