Wire diameter vs Impedance
 

Hi to all.. It is well know that a reduction in the diameter of the
wire must be compensated by a higher antenna length to maintain
resonance.

I am looking for an explanation of the reason for this. Why the total
reactance becomes more capacitive? I know math formula showing the
variation of the inductance of the wire vs its diameter, but I a
looking for the real reason, not the mathematical consequence.

I suspect that a higher diam cause higher transormation of AC to
electromagnetic energy on a segment delta(l) so that a shorter
physical length would be needed to include the full electrical 180
degrees of a dipole.. but not really sure of this.

Any comment would be welcome

Thanks, and 73 de Pierre ve2pid

Wire diameter vs Impedance
 

Try increased capacitance from larger wire diameter, larger surface (plate)
area.
Capacitance goes up, inductance (length) has to come down in order to
maintain resonance - LC circuit in standing wave environment.

73 Yuri, K3BU, VE3BMV, VE1BY

"Pierre Desjardins" wrote in message
...
Hi to all.. It is well know that a reduction in the diameter of the
wire must be compensated by a higher antenna length to maintain
resonance.

I am looking for an explanation of the reason for this. Why the total
reactance becomes more capacitive? I know math formula showing the
variation of the inductance of the wire vs its diameter, but I a
looking for the real reason, not the mathematical consequence.

I suspect that a higher diam cause higher transormation of AC to
electromagnetic energy on a segment delta(l) so that a shorter
physical length would be needed to include the full electrical 180
degrees of a dipole.. but not really sure of this.

Any comment would be welcome

Thanks, and 73 de Pierre ve2pid

Wire diameter vs Impedance
 

On Fri, 28 Apr 2006 12:31:42 -0400, Pierre Desjardins
wrote:

Why the total reactance becomes more capacitive?

Hi Pierre,

The total reactance before you shrank the wire diameter was balanced
at zero (presuming a resonant structure). After the wire diameter was
made smaller, for the same length, the inductance was lowered. Less
inductance to balance the existing capacitance leaves an excess
capacitance you observe. Of course, by making the wire thinner also
changes capacitance, the change in inductance moved further.

73's
Richard Clark, KB7QHC

Wire diameter vs Impedance
 

Oh, Richard, Richard...

A smaller wire diameter has MORE inductance, not less, in the same
environment. Think for a moment about coax: reduce the inner
conductor diameter, and the impedance goes up while the propagation
velocity stays the same. That means that C goes down and L goes up.

For Pier something else to ponder is that the change for resonance
(zero reactance) in a half-wave dipole is considerably less than the
change in a full-wave ("anti-resonant") dipole, for the same wire
diameter change.

I don't think that simple concepts of the antenna behaving like a TEM
transmission line are going to cut it here, and I'll wait for a better
explanation than that.

Cheers,
Tom

Wire diameter vs Impedance
 

On 28 Apr 2006 13:28:56 -0700, "K7ITM" wrote:

Oh, Richard, Richard...

Hi Tom,

A smaller wire diameter has MORE inductance, not less, in the same
environment.

Yes, I did invert the relation of thickness to inductance - for a
short wire. However, the feedpoint observation speaks of common
results offering a different perspective. This is the question.

It does not intuitively follow to describe less capacitance for the
same size, but now thinner antenna makes an antenna more capacitive,
does it? [A transform is at work.]

Think for a moment about coax: reduce the inner
conductor diameter, and the impedance goes up while the propagation
velocity stays the same.

This analogy begins to break down for antennas in that as the antenna
grows thinner/thicker, the propagation velocity does change. On the
other hand, and agreeing with your example, Z tracks (lower w/thicker)
with an antenna. This is in conflict.

That means that C goes down and L goes up.
with a proviso:
I don't think that simple concepts of the antenna behaving like a TEM
transmission line are going to cut it here, and I'll wait for a better
explanation than that.

No, it didn't.

For an antenna with an with an element circumference of 0.001
wavelength, the Vf is 0.97 to 0.98. Compared to an antenna with an
element circumference of 0.1 wavelength, the Vf is 0.78 to 0.79.

Velocity factor is a property of the capacitor's insulative medium
(relative permittivity), which has never changed. [I would argue that
the medium has in fact changed by the presence of the radiator, but
that is another thread.]

Large structures near resonance confound small component analytical
results. So, we will both wait for Reggie to explain it in what he
calls english; or for Cecil to explode with a new SWR analysis.

73's
Richard Clark, KB7QHC

Wire diameter vs Impedance
 

On 30 Apr 2006 09:57:57 -0700, "AC7PN" wrote:

Tom is correct here, a smaller wire has more inductance than a larger
wire.

Hi Robert,

You are late into this cycle of discussion.

The resason is that the current paths on a large conductor become
far enough away from other current paths on the surface of the same
conductor, that their magnetic flux lines begin to not totaly include
each other.

And yet this does nothing to answer the question, does it?

Speaking of antennas by the way, have a look at my 5 element log cell
20 meter beam. I have a photo of it on QRZ. http://www.qrz.com/ac7pn

As pictures go, it is a good one.

73's
Richard Clark, KB7QHC

Wire diameter vs Impedance
 

AC7PN wrote:
Bottom line, smaller wire means more inductance and a shorter lenght
for the same resonant frequency. Bigger wire means less inductance and
a longer length for the same resonant frequency.

Why does EZNEC report that increasing the wire diameter results
in lowering the resonant frequency?
--
73, Cecil http://www.qsl.net/w5dxp

Wire diameter vs Impedance
 

A wonderfully logical explanation. But there's something wrong with it
because the conclusion it reaches is demonstrably wrong.

An antenna made with a larger diameter wire must be made shorter, not
longer, than one with a smaller diameter wire to maintain the same
resonant frequency.

Roy Lewallen, W7EL

AC7PN wrote:
. . .
Bottom line, smaller wire means more inductance and a shorter lenght
for the same resonant frequency. Bigger wire means less inductance and
a longer length for the same resonant frequency.
. . .

Wire diameter vs Impedance
 

Your are right Roy & Cecil. I just confirmed using NEC and looked it up
in the ARRL hand book as well. I' happy to not be carrying around that
misconception any more. I'm just a little embarased having passed
myself off as some kind of expert.

I've spent a lot of time using NEC and have actually built a lot of the
antennas I've designed using it, with excellent results. It just goes
to show that successfully using CAD doesn't neccessarily impart wisdom.

I'm sure the larger conductor has less inductance but as Yuri
Blanarovich pointed out earlier the bigger conductor has more
capacitance to free space and that effect must dominate the effect
inductance reduction.

Wire diameter vs Impedance
 

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.

Roger

Wire diameter vs Impedance
 

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.

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.

Wire diameter vs Impedance
 

On 30 Apr 2006 20:12:15 -0700, "AC7PN" wrote:

I'm sure the larger conductor has less inductance

Hi Robert,

For a wire, that is not in dispute.

but as Yuri
Blanarovich pointed out earlier the bigger conductor has more
capacitance to free space and that effect must dominate the effect
inductance reduction.

And yet it does not. Components that are significant in size with
relation to wavelength do not exhibit the same qualities. That much
is glaringly obvious.

The problem here is one of a Transform acting upon the expected
outcome. It is generally cautioned here not to treat an antenna as a
transmission line, but this is cautious to the point of ignoring the
solution.

Schelkunoff developed a general formula for the dipole by employing a
biconical structure. This structure operates in the TEM mode and fits
radial expressions for fields naturally described in Maxwell's curl
equations which would be tedious to describe here - so we simple cut
to the chase.

Schelkunoff reveals, mathematically, that this transmission line
analysis presents a finite terminating condition for the current
traveling radially (that is, along the wire out towards the end).
Hence, the biconical form as transmission line never terminates in an
open.

In developing this model towards the thin radiator, the angle of the
cones of the biconical structure fall to a very small value. With
this, the biconical math also simplifies. This simplification does
approach the transmission line condition of an open termination.

The thick radiator falls in between as it is obviously neither thin,
nor conical in shape. As a consequence, neither is it a transmission
line that has an uniform Zc along its length. The formulas usually
used to describe its Zc are an average.

The easy answer comes from this. The two conditions of going from
thick to thin involve two different mathematical basis (providing you
aren't simply going from kind-of-thin to kind-of-thick). This
mathematical basis is transmission line math built on wave mechanics,
not inductors and capacitors. Those are components whose geometries
and size wavelength has condemned to less than useful analogies.

73's
Richard Clark, KB7QHC

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

Thanks for fulfilling my expectation.

Cheers,
Tom

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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.

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

What do you mean by "true" length?


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

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance - correction
 

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

Wire diameter vs Impedance - correction
 

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?

Wire diameter vs Impedance
 

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.

Wire diameter vs Impedance - correction
 

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.

Wire diameter vs Impedance - correction
 

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.

Wire diameter vs Impedance - correction
 

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.

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

"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.

Wire diameter vs Impedance
 

Cecil Moore wrote:

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?

Complex propagation constant is ? = ? +j? :

Whe

? is the attenuation in Nepers/wavelength

? is the phase shift in Radians/wavelength

Did I pass ??????????

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

Dave wrote:

Complex propagation constant is ? = ? +j? :
? is the attenuation in Nepers/wavelength
? is the phase shift in Radians/wavelength
Did I pass ??????????

If I remember correctly, SQRT(Z*Y) results in a
dimensionless quantity.
--
73, Cecil http://www.qsl.net/w5dxp

Wire diameter vs Impedance
 

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

Wire diameter vs Impedance
 

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.

Wire diameter vs Impedance
 

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.

Wire diameter vs Impedance
 

"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.

Wire diameter vs Impedance
 

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