Richard Fry wrote:
"Cecil Moore" wrote
Adding or subtracting loading-coil degrees is what
happens while one is tuning a screwdriver antenna.
At resonance, the screwdriver is electrically very
close to 90 degrees in length.

Note that the electrical length and the physical length
are nowhere near the same value. The electrical length
can be 90 degrees at resonance while the physical length
is only 13 degrees for a 75m mobile antenna.

It may have the reactance of an unloaded ~90-degree, self-resonant
radiator. But in normal applications that doesn't make a screwdriver the
radiational equivalent of that full-sized radiator, because the
radiation resistance of the physically/electrically short screwdriver
whip is less than a full-sized antenna -- and much less on the lower bands.

I agree 100% and have never disagreed. I have already stated
that the radiation characteristics of an antenna depend upon
its *physical* length while the feedpoint impedance depends
upon its *electrical* length. A screwdriver antenna may be
only 13 degrees long *physically* on 75m. Of course, it is
NOT going to radiate like a physical 90 degree antenna. It
is going to radiate more like a 13 degree (short) antenna.

You have apparently misunderstood what I am trying to say.
I have made *zero assertions about radiation patterns* except
to answer your earlier posting on that subject.

A dummy load can have the reactance of a resonant screwdriver, too, but
a dummy load is not a very good antenna. I doubt you would claim that
it is electrically 90 degrees in length, just because it has the same
reactance as an unloaded ~90 degree, self-resonant monopole.

A dummy load's feedpoint impedance is not (Vfor+Vref)/(Ifor+Iref),
i.e. not a virtual impedance, so your comment is irrelevant in
this context. The IEEE Dictionary distinguishes between those
two definitions of impedance, (B) for an antenna, (C) for a
dummy load.

That conclusion applies to a screwdriver antenna system, as well.

Since it is possible to tune a screwdriver antenna to the
270 degree mode, the following will assume the screwdriver
antenna system is used only in the 90 degree mode:

A screwdriver antenna system has radiation characteristics
appropriate for its *physical* length of, e.g. 13 degrees.
A screwdriver antenna system with a low resistive feedpoint
impedance is electrically 90 degrees long because
(Vfor+Vref)/(Ifor+Iref) is resistive. The only way for Vfor
and Vref to be 180 degrees out of phase is for the antenna
to be electrically 90 degrees long. The only way for Ifor
and Iref to be in phase is for the antenna to be electrically
90 degrees long. That's simple wave reflection model physics.

In abandoning the wave reflection model, many people have
abandoned any possibility of understanding what happens
in a standing-wave antenna. Sooner or later, their short
cut methods bite them in the posterior. The W8JI and W7EL
current measurements are an example.

Anyone who never looks for the "missing" phase shifts in
a mobile antenna will never find them. Side 1 of the
argument assumes they are not there. Side 2 of the argument
assumes they are there in the loading coil. Both sides
are wrong. I have gone looking for the "missing" phase
shifts and have found them. Here is a lossless transmission
line example which is *physically 45 degrees long*:

---Z0=600, 22.5 degrees---+---Z0=100, 22.5 degrees---open

What is the impedance looking into the stub? Where are the
"missing" 45 degrees?
--
73, Cecil http://www.w5dxp.com