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

Jim Brown jim at audiosystemsgroup.com
Tue Dec 12 16:34:00 EST 2017

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.

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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