Date: Tue, 5 Jul 2016 02:40:24 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fair rite materials for choke baluns
It's my understanding that a choke of (theoretical) infinite Z does not
disconnect the antenna. It would reduce the current flowing on the
outside of the coax (common mode) to zero at that point. It has no
effect on the interior currents
The choke does nothing to the balance of the antenna. It just stops
(more correctly reduces) the CM current caused by a reasonable imbalance
such as a sloping dipole, or that caused by nearby objects. The greater
the imbalance the greater the power dissipated in the core material.
The difference between sloping, center fed dipoles on 80 Vs 40 is very
pronounced
Different core materials will likely have different dissipation values.
OTOH it does not prevent that same exterior shield from picking up
signals if long enough, or of the right (or wrong) length. Then a second
and possibly third choke of smaller values might be advisable making
the shield appear as if it were broken up with insulators.
Unless referring to a specific antenna, I'd not expect Z to be
relatively important. For a specific type of antenna Z & R vary with
height, particularly on 160 and 75. Sometimes by large amounts.
73
Roger (K8RI)
## IF your sloping half wave dipole is UN balanced...then why insert a CMC
at the feed point at all ??
I would dump the CMC at the feedpoint, then perhaps use a CMC way downstream
, like just
before the coax meets the tower..or at the base of the tower..or both. Id
assume a half wave sloping dipole
would be almost 100% unbalanced...the same as a vertically polarized dipole.
Having said that, I have seen
CMCs used at the feedpoint of 80M ground planes.... where the feedpoint is
8-12 ft above ground, with the
usual 4-6 radials..and coax running up the side of a wooden 6 x 6. Those
simple CMCs didnt heat at all, zero
heat with qro power.
Jim VE7RF
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