[TowerTalk] common mode chokes, baluns and multiband, doublets
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
73, Jim K9YC
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