I think it's a BIG mistake to be writing about "velocity factor" in
this thread (and perhaps also in some current, related threads). The
reason is that it presupposes behaviour that is just like a TEM
transmission line, and clearly it is not when you get to the fine
details. Until we better understand just what is going on, I propose
that we simply say that resonance occurs for a wire shorter than 1/4
freespace wavelength, when that wire is fed against a ground plane to
which it is perpendicular, and that the thicker the wire, the shorter
it is at resonance when compared with the freespace wavelength. The
effect can be described with an emperical equation, of course. But to
invoke "velocity factor" assumes something about the solution which may
well lead you away from the correct explanation.

I don't really expect many will take this seriously--there seems to be
too much invested in explaining everything in terms of behaviour that
seems familiar. It's a bit like saying a photon is a particle (or a
wave). It is not--it is simply a quantum; and it behaves differently
from particles we know, and behaves differently from waves we know from
our macro-world experience.

The transmission-line analog is a very useful one for practical antenna
engineering, just as considering loading elements as lumped reactances
(perhaps with parasitic lumped reactance and resistance as appropriate)
is useful for practical engineering. But that doesn't mean it fully
explains the behaviour in detail.

Cheers,
Tom

K7ITM wrote:
But to
invoke "velocity factor" assumes something about the solution which may
well lead you away from the correct explanation.

For the feedpoint impedance to be purely resistive, i.e.
resonant, for a standing wave antenna, the reflected wave
must get back into phase with the forward wave. Velocity
factor is a way of explaining how/why that happens. The
diameter of the conductor no doubt appears in the VF
equation.
--
73, Cecil http://www.qsl.net/w5dxp

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?

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