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

"Cecil Moore" wrote in message
om...
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
=======================================

Cec, I do wish you would stick to metric dimensions instead of feet
and inches. It would make life much easier.

You've read about it in a book. Have you ever measured it? Has
anybody else ever measured it?

Although a continuous loop has no ends it does have a small opposing
mutual impedace (both L and C) between one side of the loop and the
other. This affects the velocity factor. The mutual impedance does
not exist when the wire is all in one straight line.

What explanation do YOU have to offer?
----
Reg.

Reg Edwards wrote:
Cec, I do wish you would stick to metric dimensions instead of feet
and inches.

Sorry, Reg, I'm with the English on that one. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Thanks, Reg. It certainly didn't 'further confuse' the issue for me,
though I can't speak for others.

Never one to let things rest without doing a bit of thinking about
them, I had a look at some info I have from R.W.P. King, and also did a
bit of NEC2 simulating. What I find is that for half-wave and 3/2-wave
dipoles, the shortening effect is nearly the same length (constant
frequency and wire diameter), but for full wave and 2-wave long
antennas, the shortening is greater. King and NEC2 agree pretty
closely for the half and 3/2 case but differ noticably for the full and
2-wave case, though again, the shortening is (somewhat more roughly)
the same for a given model when comparing the full and 2 wave cases.

I suppose the difference between the models, and the difference between
the resonant (odd-half-waves) versus anti-resonant (even-half-waves)
cases, can be accounted for by the terminal conditions. After all, the
electric field is quite high at the center feedpoint of the
even-half-waves antennas, so details of the terminal conditions (wire
diameter and spacing) are important there, much more so than with the
relatively low electric fields in that region for the odd-half-waves
antennas. The terminal conditions act roughly like a capacitor across
the feedpoint, and that has little effect with the low feedpoint
impedance of the odd-half-waves antennas, but a much larger effect with
the higher feedpoint impedance of the even-half-waves antennas.

I also used NEC2 to simulate the effects of a small top-hat: it was 4
radial wires at the top of a vertical, 0.001 wavelengths long. The
vertical diameter was .000001 wavelengths (as were the radials forming
the top hat). I found that adding that top had reduced the length for
resonance by exactly the same length in each case, for 1/4, 2/4, 3/4
and 4/4 wave tall antennas, probably within the accuracy of the
computing engine. (The differences among the shortenings was less than
0.01% of a wavelength; the shortening effect of the top hat was 0.4%.)

I suppose there are some higher-order effects going on too, but this is
close enough to satisfy my curiosity--for now. Thanks to Pierre for
posting an interesting question that has nothing to do with "velocity
factor."

Cheers,
Tom

There's another interesting thing about very fat antennas.

Again considering them as transmission lines, their Zo is quite low
and so the input, or end impedance of a half-wave dipole is relatively
low.

Thus it is possible to place two half-wave dipoles in series and feed
them in the middle. (In the same way as feeding one half-wave dipole
in the middle).

What does the radiation pattern of a full-wave, exceedingly fat dipole
look like? Can the modelling programs cope? What's the feedpoint
impedance at resonance? What's the bandwidth?

What about a 4 or 5-to-1 ratio for Length / Diameter?
----
Reg.

Tom wrote -
I also used NEC2 to simulate the effects of a small top-hat: it was
4
radial wires at the top of a vertical, 0.001 wavelengths long. The
vertical diameter was .000001 wavelengths (as were the radials
forming
the top hat). I found that adding that top had reduced the length
for
resonance by exactly the same length in each case, for 1/4, 2/4, 3/4
and 4/4 wave tall antennas, probably within the accuracy of the
computing engine.

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

The "End Effect" is thereby proved.

Marvellous things are computing engines!
----
Reg.

"End effect" looks to me like a description of a result, not an
explanation of a cause. Its "proof" consists of the observation that
fatter dipoles have a shorter resonant length than thin ones. I'm afraid
a real explanation of why the "end effect" occurs requires much deeper
physics and math.

I don't believe that Tom's result with the top hat is a demonstration of
the same phenomenon that makes resonant fat dipoles shorter than thin
ones. Here's what the top hat experiment means:

Suppose you have a thin antenna of any length. Look at the current
distribution on the last few degrees of the antenna. I believe you'll
find that it's the same regardless of the antenna length. Then replace
the wire with a top hat. Again you'll find that the current distribution
on the top hat is the same regardless of the length of the antenna below
it. So it shouldn't be surprising that you can substitute one for the
other and get the same result regardless of the antenna length.

This proves that you can replace a part of an antenna with a capacitive
hat, and that the relationship between the length of wire and size of
top hat is, at least to first order, independent of the antenna length.
It's not clear to me what else it proves.

Incidentally, does this have anything to do with the "true length" of an
antenna? No one has stepped forward yet with an explanation.

Roy Lewallen, W7EL

Reg Edwards wrote:
Tom wrote -
I also used NEC2 to simulate the effects of a small top-hat: it was
4
radial wires at the top of a vertical, 0.001 wavelengths long. The
vertical diameter was .000001 wavelengths (as were the radials
forming
the top hat). I found that adding that top had reduced the length
for
resonance by exactly the same length in each case, for 1/4, 2/4, 3/4
and 4/4 wave tall antennas, probably within the accuracy of the
computing engine.

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

The "End Effect" is thereby proved.

Marvellous things are computing engines!
----
Reg.

Roy Lewallen wrote:
Suppose you have a thin antenna of any length. Look at the current
distribution on the last few degrees of the antenna. I believe you'll
find that it's the same regardless of the antenna length.

For a 1/2WL dipole, it is also the same at the center of the
antenna and at all other points anywhere on the antenna. The
standing wave current phase cannot be used to measure phase
shift in a wire or a coil or a top hat or a stub. The phase
of standing wave current is meaningless.

If one makes the top hat large enough, one should see an
abrupt ~180 phase reversal in the standing wave current.
This happens on each side of a current minimum point.
--
73, Cecil http://www.qsl.net/w5dxp