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

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

Reg Edwards wrote:
wrote in message
ups.com...

From page 22.2 of the 2005 ARRL Handbook

"CONDUCTOR SIZE"

"The impedance of the antenna also depends on the diameter of the
conductor in relation to the wavelength. If the diameter of the
conductor is increased, the capacitance per unit length increases

and

the inductance per unit length decreases. Since the radiation
resistance is affected relatively little, the decreased L/C ratio
causes the Q of the antenna to decrease so that the resonance curve
becomes less sharp with change in frequency. This effect is greater

as

the diameter is increased, and is a property of some importance at

the

very high frequencies where the wavelength is small."

Lots of interesting graphs and charts in the ARRL Antenna Handbook

as

well.

======================================
A nice summary.

But to be more precise, it is the ratio of conductor diameter over
length which matters.

Inductance and capacitance change very slowly with diameter/length.
The changes are hardly noticeable.

L = 0.2 * Length * ( Ln( 4 * Length / Dia ) -1 ) microhenrys.

C = 55.55 * Length / ( Ln( 4 * Length / Dia ) -1 ) picofarads.


So, if Length / Dia equals e / 4 (about 2.7183), then C = infinite?


Zo = Sqrt( L / C ) = 60 * Ln( 4 * Length / Dia ) -1 ) ohms.

Antenna Q = 2 * Pi * Freq * L / (Distributed Radiation Resistance).

For a half-wave dipole the distributed radiation resistance is 146
ohms, or twice the feedpoint resistance.
----
Reg.



John

John - KD5YI wrote:
Reg Edwards wrote:

wrote in message
ups.com...

From page 22.2 of the 2005 ARRL Handbook


"CONDUCTOR SIZE"

"The impedance of the antenna also depends on the diameter of the
conductor in relation to the wavelength. If the diameter of the
conductor is increased, the capacitance per unit length increases


and

the inductance per unit length decreases. Since the radiation
resistance is affected relatively little, the decreased L/C ratio
causes the Q of the antenna to decrease so that the resonance curve
becomes less sharp with change in frequency. This effect is greater


as

the diameter is increased, and is a property of some importance at


the

very high frequencies where the wavelength is small."

Lots of interesting graphs and charts in the ARRL Antenna Handbook


as

well.


======================================
A nice summary.

But to be more precise, it is the ratio of conductor diameter over
length which matters.

Inductance and capacitance change very slowly with diameter/length.
The changes are hardly noticeable.

L = 0.2 * Length * ( Ln( 4 * Length / Dia ) -1 ) microhenrys.

C = 55.55 * Length / ( Ln( 4 * Length / Dia ) -1 ) picofarads.



So, if Length / Dia equals e / 4 (about .67957), then C = infinite?




Zo = Sqrt( L / C ) = 60 * Ln( 4 * Length / Dia ) -1 ) ohms.

Antenna Q = 2 * Pi * Freq * L / (Distributed Radiation Resistance).

For a half-wave dipole the distributed radiation resistance is 146
ohms, or twice the feedpoint resistance.
----
Reg.




John

So, if Length / Dia equals e / 4 (about .67957), then C = infinite?

====================================
C even goes negative for smaller values of Length/Dia.

I'll let you into a secret - the formulae are approximate and don't
apply when antenna length is less than about 5 times its diameter.

When was the last time you saw an antenna wire only 5 times longer
than its diameter?

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:
So, if Length / Dia equals e / 4 (about .67957), then C = infinite?


====================================
C even goes negative for smaller values of Length/Dia.

I'll let you into a secret - the formulae are approximate and don't
apply when antenna length is less than about 5 times its diameter.

When was the last time you saw an antenna wire only 5 times longer
than its diameter?



You should supply your "secrets" along with your formulae.

You should supply your "secrets" along with your formulae.
=====================================

At my time of life I don't have time to write a book!

You'll just have to read between the lines. ;o)
----
Reg.

Reg Edwards wrote:
You should supply your "secrets" along with your formulae.

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

At my time of life I don't have time to write a book!

You'll just have to read between the lines. ;o)
----
Reg.




Fine. From now on, I will assume you have no time to explain your "secrets"
when you post so I will ignore your formulae. This approach is much better
than being misled if I do not read between your lines properly.

K7ITM wrote:
Those sources also don't tell me anything about
"velocity factor" as far as I can tell.

What RF engineers call "velocity factor" is related to the phase
constant in the complex propagation constant embedded in any
transmission line equation in any decent textbook. Do your
sources tell you anything about the complex propagation constant?
--
73, Cecil http://www.qsl.net/w5dxp