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

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

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

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

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

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

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

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.
. . .

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.

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