On 7/2/20 7:41 AM, Richard (Rick) Karlquist wrote:
On 7/1/2020 6:00 PM, dj7ww@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.
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