[TowerTalk] Fair rite materials for choke baluns

Jim Thomson jim.thom at telus.net
Tue Jul 5 10:34:11 EDT 2016


Date: Tue, 5 Jul 2016 02:40:24 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>
To: towertalk at 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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