[TowerTalk] Common Mode RF choke on a 50 MHz yagi

[Richard (Rick) Karlquist]

On 7/1/2020 6:00 PM, dj7ww at t-online.de wrote:
> 95% velocity factor will probably be allright, a thin wire is typically 3%
> shorter for quarter wave resonance then free space calculates.
> 
> 73
> Peter
> 

Be careful not to conflate dipole impedance theory with
conventional transmission lines.  Different rules apply.
Dipole impedance theory is extremely complicated and
various incorrect treatments of it were published for
decades.  The true story has been uncovered by the
noted antenna expert K6OIK.  Specifically, Steve notes
that the 5% (or so) shortening for dipoles is not
caused by the velocity factor being 95%.  It's
way more complicated.  Using insulated wire for
dipoles does actually affect the VF.

In any event, none of that applies to a transmission
line.  In a transmission line, uninsulated wire has
a VF of essentially 100% AFAIK, and doesn't vary with
diameter.

In any event, I like Jim Thomson's (VE7RF) tuning method
to get the choke at exact resonance while being
perturbed by whatever parasitic effects there are,
including the vinyl jacket on the outside of the coax
(which does affect the common mode VF).  We will be
certain the choke is optimized and don't need to argue
about fine points of theory.

73
Rick N6RK