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Re: [TowerTalk] Best coaxial to make Chokes

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Best coaxial to make Chokes
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 12 Nov 2013 18:14:50 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 11/12/2013 5:28 PM, Peter Voelpel wrote:
Any choke is inductive.
If not it is no choke at all.

WRONG. A proper common mode choke inductively couples loss from the ferrite core. A parallel resonance is formed by the inductance of the winding (the wire passing through the core) and the stray capacitance between the winding. The coupled resistance is what forms the common mode choke, and because the core material is very lossy, the circuit Q is quite low (typically on the order of 0.5), so the resonance is quite broad.

A single turn through a ferrite core resonates in the 150 MHz range. Like any coil, winding multiple turns multiplies L by the square of the turns, capacitance increases approximately linearly with the number of turns, the resistance, because it is coupled inductively is multiplied by the square of the turns. This moves the resonance to the part of the HF spectrum where choking action is needed, and it is the RESISTANCE that provides the most reliable choking action. A proper choke wound on a suitable core material can provide resistive choking Z in the range of 3K - 8K Ohms over a 2 octave frequency range (because the circuit Q is so low).

Again, any choke that is not strongly resistive at frequencies of interest will resonate with the line (which behaves as an antenna) at frequencies which depend on the length of the feedline and the reactance of the choke.

For a thorough development of these concepts, see my RFI tutorial, cited in my prior email.

73, Jim K9YC
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