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

[jimlux]

On 7/2/20 7:41 AM, Richard (Rick) Karlquist wrote:
> 
> 
> 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.

"way more complicated" is an understatement. Although, the basic math 
behind it was known in the late 19th century.


> 
> 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.

Well, you do need to take into account the air having a permittivity of 
1.00059.

And if you should be running enough power (or be operating in a high UV 
or radiation environment) to partially ionize the air in between the 
conductors, then it gets real interesting, because the permittivity can 
be negative, and that leads to all sorts of interesting propagation speeds.

Lest anyone think this is of no importance - not for amateur radio 
transmission lines perhaps, but it's what non-straightline propagation 
is all about. And there are people who worry about it for transmission 
lines - but they're probably over on the time-nuts mailing list or some 
other exotic application.