Richard Harrison wrote:
Jim Lux wrote:
"It`s often explained as "extra capacitance from the bigger size", but I
think that`s not what`s really going on."

Arnold B. Bailey in "TV and Other Receiving Antennas" agrees with Jim.
Bailey writes on page 317:
"We should expect such thin rods to be resonant when their physical
length is slightly less than a free-space half-wave length. When the rod
is thick, the effective velocity along the rod is considerably less than
the free-space velocity, thus reducing the wavelength proportionally."


Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. And that might not be far from the truth
for a restricted subset of antennas.

All of this kind of confusion is trying to make one sort of model (a
transmission line) fit something else (a radiator). Just like the
things that treat the antenna as a lumped RLC.

On Mon, 20 Oct 2008 17:57:03 -0700, Jim Lux
wrote:

All of this kind of confusion is trying to make one sort of model (a
transmission line) fit something else (a radiator).

Hi Jim,

I've seen this kind of assertion made before, generally as a blanket
prohibition/warning/incantation/supplication/condemnation - but never
with any demonstrable problem that wasn't an example of designed-in
failure suited to the argument.

Lest there be any confusion: an antenna IS a transmission line.

The clarity to this confusion starts with the Biconical Dipole. S. A.
Schelkunoff describes it as a "Linear" antenna and used transmission
line math to build the mathematical model for the thin wire dipole in
his classic publication "Theory of antennas of arbitrary size and
shape," Proc. I.R.E., 29, 493, 1941 and S. A. Schelkunoff, "Advanced
Antenna Theory, " John Wiley and Sons, Inc., New York, (1952) I'm
inclined to allow the weight of his work stand until someone tips it -
or can demonstrate I incorrectly read his thesis. Somehow given the
weight of authorities (Ronold King being one) that cite him for this
very reading (specific to the correlation) are abundant, I will await
heavier lifting to tip the math.

Accessible reference work can be found by searching the PTO with his
patent number: 2235506.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
Lest there be any confusion: an antenna IS a transmission line.

In fact, there is a formula for calculating the Z0 of
a single horizontal transmission line wire above ground.
#14 wire at 30 feet is very close to 600 ohms. #14 wire
at 30 feet describes a lot of dipoles.
--
73, Cecil http://www.w5dxp.com

In message , Cecil Moore
writes
Richard Clark wrote:
Lest there be any confusion: an antenna IS a transmission line.

In fact, there is a formula for calculating the Z0 of
a single horizontal transmission line wire above ground.
#14 wire at 30 feet is very close to 600 ohms. #14 wire
at 30 feet describes a lot of dipoles.

Are there any calculations or charts for centre impedance of a dipole in
free space, starting from zero length, and going out to infinity?
--
Ian

In article ,
Ian Jackson wrote:

Are there any calculations or charts for centre impedance of a dipole in
free space, starting from zero length, and going out to infinity?

I think that what you're looking for is in Kraus "Antennas for All
Applications", page 446 - "Self-impedance of a thin linear antenna".
The formula given is based on the induced-EMF method... it's an
approximation which apparently works well for cylindrical antennas
whose length is at least 100x the diameter.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In message , Dave Platt
writes
In article ,
Ian Jackson wrote:

Are there any calculations or charts for centre impedance of a dipole in
free space, starting from zero length, and going out to infinity?

I think that what you're looking for is in Kraus "Antennas for All
Applications", page 446 - "Self-impedance of a thin linear antenna".
The formula given is based on the induced-EMF method... it's an
approximation which apparently works well for cylindrical antennas
whose length is at least 100x the diameter.

Thanks for that. I've found a free download of a PDF copy (18MB) at:
http://www.badongo.com/file/9893801
I'll have a look to see if it is what I want.

I would have thought that the feed impedance of a dipole at a wide range
of frequencies/lengths (ie 'very short' to 'very long') would have been
fairly typical rule-of-thumb required information for those interested
in antennas. However, it does not seem to be!
--
Ian

In article ,
Ian Jackson wrote:

I would have thought that the feed impedance of a dipole at a wide range
of frequencies/lengths (ie 'very short' to 'very long') would have been
fairly typical rule-of-thumb required information for those interested
in antennas. However, it does not seem to be!

Oh... if rule-of-thumb is good enough for your needs, then it's not
too difficult to summarize. There's a nice chart on page 2-3 of the
ARRL Antenna Book.

You should consider the resistive, and reactive portions of the
feedpoint impedance separately.

The resistive part rises from zero, up through a nominal 50 ohms or so
at resonance (just under 1/2 wavelength), up to several thousand ohms
at second (or anti-) resonance. If you plot the impedance-vs.-
resistance relationship with the doublet length on a linear scale and
the resistance on a logarithmic scale, it's not too far from being a
straight line through much of this range.

Between second and third resonance, the resistance drops back down to
around 100 ohms... between third and fourth, up to several thousand
ohms again, and so forth. As the doublet continues to get longer, the
feedpoint resistance oscillates between low (odd-resonant) and high
(even- or anti-resonant) values, with the oscillation becoming less
and less as the doublet gets longer (think of a damped sine wave). In
theory it'll eventually settle down to 377 ohms.

The reactive portion of the impedance also oscillates as the doublet
gets longer and longer. Between an even-numbered and odd-numbered
resonance it's capacitive, dropping from thousands of ohms of
negative reactance, to zero at the odd resonance. It then becomes
inductive, rising to several thousand ohms just before the next even
(anti-) resonant length is reached. As the even-numbered resonance
length is passed it falls abruptly from very positive (inductive) to
very negative (capacitive), and then begins to return slowly to zero
at the next odd resonance.

These excursions from positive (inductive) to negative (capacitive)
continue, and also fall in their absolute value as the doublet gets
longer and longer. Once the doublet is "sufficiently long" its
reactance pretty much vanishes and it looks like a 377-ohm resistance.

Near the resonant lengths, the value of the reactance is changing
rather more rapidly than the value of the resistance.

The same basic principles apply fairly well to doublets that aren't in
free space, but ground reflections, mutual coupling with other antenna
elements, etc. have a big effect on the actual values. Few of us
have the luxury of stringing up an 80-meter longwire doublet in free
space, alas :-)

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:

I think that what you're looking for is in Kraus "Antennas for All
Applications", page 446 - "Self-impedance of a thin linear antenna".
The formula given is based on the induced-EMF method... it's an
approximation which apparently works well for cylindrical antennas
whose length is at least 100x the diameter.

And, interestingly, a LOT of amateur antennas don't meet this
slenderness constraint. Wire dipoles hanging in the air do. Fans,
cages, etc., often don't.

No problem with the model, just awareness of the footnotes and
limitations (which often get omitted in the less rigorously reviewed
internet literature..)

On Tue, 21 Oct 2008 13:30:39 +0100, Ian Jackson
wrote:

Are there any calculations or charts for centre impedance of a dipole in
free space, starting from zero length, and going out to infinity?

Institutional memory here is so slight:

"Theory of antennas of arbitrary size and
shape," Proc. I.R.E., 29, 493, 1941 and S. A. Schelkunoff, "Advanced
Antenna Theory, " John Wiley and Sons, Inc., New York, (1952)

Accessible reference work can be found by searching the PTO with his
patent number: 2235506.

73's
Richard Clark, KB7QHC

In message , Richard Clark
writes
On Tue, 21 Oct 2008 13:30:39 +0100, Ian Jackson
wrote:

Are there any calculations or charts for centre impedance of a dipole in
free space, starting from zero length, and going out to infinity?

Institutional memory here is so slight:

"Theory of antennas of arbitrary size and
shape," Proc. I.R.E., 29, 493, 1941 and S. A. Schelkunoff, "Advanced
Antenna Theory, " John Wiley and Sons, Inc., New York, (1952)

Accessible reference work can be found by searching the PTO with his
patent number: 2235506.

73's
Richard Clark, KB7QHC

Thanks. As I said in my reply to Dave Platt, I would have thought that
the feed impedance of a dipole over a wide range of frequencies/lengths
(ie 'very short' to 'very long') would have been fairly typical
rule-of-thumb required information for those interested in antennas.
However, this does not seem to be the case.
--
Ian