Thanks for fulfilling my expectation.

Cheers,
Tom

K7ITM wrote:
Thanks for fulfilling my expectation.

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.

Tom, when you can determine the position and velocity of
every electron in the system, please get back to us. :-)
--
73, Cecil http://www.qsl.net/w5dxp

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

I know perfectly well how to use EZNEC to determine the relationship
between the conductor diameter/length ratio and resonant frequency.
EZNEC does not tell me anything about "velocity factor" as far as I
know. I don't need EZNEC to tell me the resonant-frequency and
conductor diameter/length ratio relationship; I have that in detail
from other sources. Those sources also don't tell me anything about
"velocity factor" as far as I can tell.

I don't expect those who are totally invested in and entangled by
"velocity factor" to understand this. But they continue to fulfill my
expectations. (Richard C. will probably even predict with some
accuracy their next card to be played...)

Cheers,
Tom

Those sources also don't tell me anything about
"velocity factor" as far as I can tell.

I don't expect those who are totally invested in and entangled by
"velocity factor" to understand this. But they continue to fulfill
my
expectations. (Richard C. will probably even predict with some
accuracy their next card to be played...)

Cheers,
Tom

=======================================
Yes, the velocity factor doesn't change with Length/Diameter. But it
is sometimes convenient to discuss the effect as such.

Actually everything happens at and near the ends of the wire. The
short length of wire to be pruned to bring about a state of resonance
is the same regardless of the number of half-waves in the anenna.

It is sometimes referred to as the "End Effect".

Think in terms of the directions of the electric lines of force at the
wire ends. They are not all radial lines of force. Some of them
extend outwards in the direction of the wire. In the same way as
magnetic lines of force appear when a bar magnet is sprinkled with
iron filings.

This, at the ends, and only at the ends, has the effect of increasing
capacitance to the rest of the Universe. The wire behaves as if its
longer than it actually is. Hence pruning is necessary.

When several half-waves are connected in series it is not necessary to
prune each of the half-waves. The electric lines of force are all in
radial directions at their junctions.

The "end-effect" occurs with any length of antenna. There are only two
ends. Obviously, as the diameter/length ratio increases so does the
effect. The flat ends of the antenna support a greater number of
lines of force in line with the antenna.

The effect slightly reduces efficiency. When the antenna is pruned to
bring it into resonance it is accompanied by a reduction in radiation
resistance. This is most noticeable at UHF and above where very fat
cylindrical antennas are used. Sometimes elipsoids are used for high
power transmitting antennas.

I trust my description/explanation has not further confused the issue.
----
Reg.

Reg Edwards wrote:
The "end-effect" occurs with any length of antenna. There are only two
ends.

Is the lack of an "end-effect" why a full-wave loop has
to be made longer than 2*468/f?
--
73, Cecil http://www.qsl.net/w5dxp