EZNEC can be used to verify the relationship of conductor
diameter to velocity factor. Once the conductor diameter
exceeds a certain limit, the standing wave current at the
ends of that conductor undergo a 180 degree phase change,
indicating a longer length than resonance.

========================================

A cylinder has a flat circular end. Antenna wires and rods are
cylinders. You should be reminded that the true length of the antenna
is its straight length PLUS the radius of the flat circular end.
----
Reg.

Reg Edwards wrote:

A cylinder has a flat circular end. Antenna wires and rods are
cylinders. You should be reminded that the true length of the antenna
is its straight length PLUS the radius of the flat circular end.
----
Reg.

What do you mean by "true" length?

Roy Lewallen, W7EL

What do you mean by "true" length?


You know very well what I mean. Have you nothing else better to do
with your time?

Reg Edwards wrote:
What do you mean by "true" length?


You know very well what I mean. Have you nothing else better to do
with your time?

No, I don't know what you mean. And your response doesn't give me a
great deal of confidence that you do, either.

The reactance of an infinitely thin half wavelength dipole is 42.5 ohms,
meaning that it isn't resonant. An infinitely thin dipole of length
0.496 wavelength, or about 1% shorter, is resonant. So my first question
is whether the "true length" of an infinitesimally thin resonant dipole
is 0.496 or 0.5 wavelength. (If 1% is too little to quibble about, why
are we concerned about a length difference of a wire diameter?)

If we increase the diameter of the antenna to 1/50 its length, the "true
length" would then be 1.02 times the "true length" of the
infinitesimally thin dipole. Yet we have to reduce the antenna length by
nearly 7% to maintain resonance. So the "true length" doesn't have
anything obvious to do with resonant length, nor does it provide a way
to predict the resonant length based on wire diameter.

If the meaning of "true length" is obvious, most other readers must know
what it means. Would someone please be so kind as to explain to me what
it means and how it's used?

Roy Lewallen, W7EL

Indeed...

And--how is the resonance affected by using a tubular conductor that's
open on the ends? What if the bottom end of a monopole fed against a
ground plane (or the meeting ends of a doublet) is conical with perhaps
a 30 degree included angle, out to the uniform diameter of the tube?
Does it matter whether the upper end (outer ends) of the tube is open
or has a disk shorting across it? (A wire-frame simulation suggests
that a disk shorting the top has a small effect, but less than half its
radius.)

But certainly as Roy says, the effect on resonance is much greater than
considering the length to be one diameter longer than the end-to-end
length of the conductor.

These aren't details that are likely to matter in a ham antenna
installation, but they are interesting to me from a theoretical point
of view.

Cheers,
Tom