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Re: [TowerTalk] Current Choke for Ladder Line

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Current Choke for Ladder Line
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 12 May 2015 11:28:45 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Tue,5/12/2015 11:01 AM, Hans Hammarquist via TowerTalk wrote:
Gentlemen,


I have seen many articles how to build and/or buy common mode chokes for coax 
cables but I don't remember any articles how to build (or buy) and choke for a 
400 ohm ladder line.

That's because no one has thought of one that works. An important component of the problem is that the Zo of practical two-wire chokes wound on ferrite toroids is much lower than 400 Ohms. For example, close-spaced enameled wire yields about 50 ohms, close-spaced THHN (house wire) yields about 90 ohms.

Another important component of the problem is that, because the feedpoint is off-center, the common mode voltage gets VERY high if you're running much power, so the dissipation in the choke is likely to fry it.

A third reason is the concentrated differential mode dissipation in the turns of line when the antenna is poorly matched to the line.

The common mode current on a ladder line is causing the same problem as a 
common mode current on a coax.

Exactly right.

i have a choke between my tuner and radio which reduced noise quite good but 
wonder now how much RF power I radiate and signal I receive from the CM current 
in the feeder to my OCF dipole.

The only other practical way to kill common mode on high Z line would be with a conventional 2-winding transformer capable of handling the power. Fair-Rite #61 might be good enough for moderate power levels below about 12 MHz, but its losses increase above that frequency. Fair-Rite #67 would be a better candidate -- its losses remain quite low up to about 40 MHz, and are not bad at 50 MHz. On the graphs below, u's is the mu that defines inductance, and u''s is the loss component in a series equivalent circuit.

http://www.fair-rite.com/newfair/materials61.htm

http://www.fair-rite.com/newfair/materials67.htm

http://www.fair-rite.com/cgibin/catalog.pgm?THEAPPL=Inductive+Components&THEWHERE=Closed+Magnetic+Circuit&THEPART=Toroids#select:freq1

73, Jim K9YC

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