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Re: [TowerTalk] common mode chokes, baluns and multiband, doublets

To: towertalk@contesting.com
Subject: Re: [TowerTalk] common mode chokes, baluns and multiband, doublets
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 12 Dec 2017 13:34:00 -0800
List-post: <mailto:towertalk@contesting.com>
On 12/12/2017 12:32 PM, David Robbins wrote:
"The inductance of a coil of coax is in series with the rest of the transmission 
line, and if that line looks capacitive (by virtue of its length), the coil of coax DOES 
NOT WORK to provide isolation. Indeed, it increases common mode current on the line 
rather than reducing it."

This statement I would debate.... mainly the 'by virtue of its length' 
statement... that starts mixing the lumped component (coil) with a transmission 
line model.
David,

Looks like you still haven't bothered to study the link I gave you several times. I took the time to read your book, and learned some things from it. Perhaps you might benefit from studying my work.

The coil is NOT part of a transmission line model, it is part an ANTENNA model, where the coil is in series with a WIRE (the feedline as a common mode element) connected between the feedpoint and the shack (usually ground).  If it's coax, that wire is the outside of the coax shield, and has the VF of a wire having the diameter of that shield and the insulation of the coax outer jacket. And that coil is not only an inductor, it is a parallel LC circuit, where C is the stray (parasitic) capacitance between turns. When it's a simple coil of coax, the Q is pretty high. When the coax is wound through a lossy ferrite core, it's a very LOW Q resonance.  Either coil is modeled in NEC by first adding the wire, then adding the coil/choke as a Load. To do this, of course, we must know the R, L, and C values. The references show how I've done that for some practical (measured) chokes.

In order to properly analyze it you must include the rest of the environment, 
the distributed capacitance to the surroundings from the shield, the 
distributed inductance of the shield, losses or pickup from it coupling to 
stuff around it, and locations of connections to the shield from connections to 
antenna booms, towers, lightning grounding, etc.

One may, of course, include as much of that detail as one has time for. The common mode circuit for the feedline of a high dipole is quite different from the feedline of a beam mounted on a tower, with coax bonded to the tower top and bottom.  A feedline bonded to the tower top and bottom and run either within the tower or taped to a leg will be in parallel with the tower. The tower/coax run will be in series with the coax between the top bonding point and the antenna feedpoint. Considerable RF current can be induced in that vertical run, but a choke in the coax run between the antenna and the feedpoint prevents that current from coupling to the antenna, where it would fill nulls and couple noise.

73, Jim K9YC
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