[TowerTalk] Fair rite materials for choke baluns

[jimlux]

On 7/5/16 7:34 AM, Jim Thomson wrote:
> Date: Tue, 5 Jul 2016 02:40:24 -0400
> From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>
>
>
> 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.
>

> 73
>
> Roger (K8RI)
>
> ##  IF your sloping  half wave dipole is  UN balanced...then why insert a CMC  at the feed point at all ??


Think of the choke as being equivalent to putting an insulator to break 
up a guy wire.  the idea is to prevent segments of the feedline from 
coupling to the antenna and reradiating because of the current induced 
in the shield.

In some cases, there's no real point in trying to break up the feedline 
(a 100 foot piece of coax running alongside a 100ft tower... the tower 
is the big conductor in the near field of the antenna)

If you've got a sloping dipole, and the feedline comes off at right 
angles, in free space, the coupling is small, and, you're right, 
probably don't need a choke at the feedpoint.

on a real antenna, where the feedline probably isn't exactly 
perpendicular, and there's a big conductive supporting structure, and 
there's a somewhat conductive ground plane underneath.. maybe there is a 
significant current induced, maybe there isn't.

One could easily model this and *see* if there's significant current on 
the feedline in a non-ideal scenario.  If I ran the model, and I saw 
that the current everywhere in the wire that represents the feedline was 
<1/10th the current at the dipole feed, I'd call it done and go home. 
Nobody is looking for 20dB nulls from a dipole, and that's what that 
stray current would do.

It's sort of the same as the "is that rain gutter/downspout" going to 
cause a problem modeling.