On Oct 24, 1:29*pm, wrote:
On Oct 24, 6:48 am, Michael Coslo wrote:

Trying to make a "readers Digest" version here....

If I'm following so far:

The lowered frequency of resonance is due to changes in the velocity factor.


so as the wire gets thicker the C per unit length goes up at some rate
and the L per unit length goes down at some other rate, fine so that
reduces the characteistic Z by some rate....but none of that changes
the wave velocity as was pointed out above in the coax example.

I think the shortening effect may all be due to the extra C of the end
surface, i.e it iss end effect. For a thick wire, the end is a circle
that has C and this is all extra C that is not present for the thin
wire. Is this extra C alone enough to create the shortening effect?

Mark

On Oct 24, 11:01 am, Mark wrote:
On Oct 24, 1:29 pm, wrote: On Oct 24, 6:48 am, Michael Coslo wrote:

Trying to make a "readers Digest" version here....

If I'm following so far:

The lowered frequency of resonance is due to changes in the velocity factor.

so as the wire gets thicker the C per unit length goes up at some rate
and the L per unit length goes down at some other rate, fine so that
reduces the characteistic Z by some rate....but none of that changes
the wave velocity as was pointed out above in the coax example.

I think the shortening effect may all be due to the extra C of the end
surface, i.e it iss end effect. For a thick wire, the end is a circle
that has C and this is all extra C that is not present for the thin
wire. Is this extra C alone enough to create the shortening effect?

Mark


No.
And, "end capacitance effect" is a poor model for what's really going
on. It's been used as an "explanation" for the observation that an
antenna that is slightly shorter than half a wavelength is resonant(as
in has no reactive component at the feedpoint). The problem is that an
infinitely thin dipole is resonant at less than 1/2 wavelength, and in
that case, there's no real "end" to have an effect.

On Oct 24, 12:11*pm, wrote:
On Oct 24, 11:01 am, Mark wrote:



On Oct 24, 1:29 pm, wrote: On Oct 24, 6:48 am, Michael Coslo wrote:

Trying to make a "readers Digest" version here....

If I'm following so far:

The lowered frequency of resonance is due to changes in the velocity factor.

so as the wire gets thicker the C per unit length goes up at some rate
and the L per unit length goes down at some other rate, fine so that
reduces the characteistic Z *by some rate....but none of that changes
the wave velocity as was pointed out above in the coax example.

I think the shortening effect may all be due to the extra C of the end
surface, i.e it iss end effect. *For a thick wire, the end is a circle
that has C and this is all extra C that is not present for the thin
wire. *Is this extra C alone enough to create the shortening effect?

Mark

No.
And, "end capacitance effect" is a poor model for what's really going
on. It's been used as an "explanation" for the observation that an
antenna that is slightly shorter than half a wavelength is resonant(as
in has no reactive component at the feedpoint). The problem is that an
infinitely thin dipole is resonant at less than 1/2 wavelength, and in
that case, there's no real "end" to have an effect.

?? I have been under the impression that in the limit as the
conductor radius goes to zero, the resonance does go to a freespace
half wavelength. You have to make the antenna _really_ thin to get
anywhere near that, though. Even a million to one length to diameter
ratio won't do it.

There's another empirical point, though, that may be worthwhile
considering to convince folk that Jim's exactly right that you can NOT
just figure things from "capacitance" and "inductance" and the
resultant propagation velocity. For the resonance of a nominally half-
wave dipole in freespace, the resonance changes by only a small amount
as the wire becomes thicker. With the wire length/diameter ratio at
10,000:1, resonance is about 2.5% below freespace half wave. For l/d
= 1,000:1, it's about 3.7%. At l/d = 100:1, it's about 6.1%. But for
the same l/d ratios operated at full-wave (anti)resonance, the factors
are respectively 7%, 9.3% and 17.5%. It would be tough to reconcile
that difference using the simple L and C per unit length model.

Ronold King made a career out of developing the theory of linear
antennas. I find the "Antennas" chapter he wrote for "Transmission
Lines, Antennas and Waveguides" to be a valuable source of insight
about antennas. It's presentation is more empirical than theoretical,
but I've found that his explanations there pretty much always give me
better insights into what's going on. It can be tough to find the
book, but I do have a PDF photocopy... If you want to get seriously
into the theory and math, one of his other books might be just the
ticket. Though I like the way he presents the material, I know of
others who are turned off by it, so "ymmv" as they say.

Cheers,
Tom

K7ITM wrote:

Ronold King made a career out of developing the theory of linear
antennas. I find the "Antennas" chapter he wrote for "Transmission
Lines, Antennas and Waveguides" to be a valuable source of insight
about antennas. It's presentation is more empirical than theoretical,
but I've found that his explanations there pretty much always give me
better insights into what's going on. It can be tough to find the
book, but I do have a PDF photocopy...

Not too tough. Amazon has 7 copies from $9.96 -
http://www.amazon.com/gp/offer-listi...4901677&sr=1-8
They appear to be the original 1945 hardcover book. Mine is a soft cover
Dover reprint published in 1965. Alibris has 17 copies, for $7.00 up -
http://www.alibris.com/search/books/...ave%20gu ides.

The quick web search that found these also found others. I picked mine
up at Powell's some time ago for $15.00.

I also highly recommend this book.

Roy Lewallen, W7EL

On Oct 24, 4:31 pm, K7ITM wrote:
On Oct 24, 12:11 pm, wrote:

And, "end capacitance effect" is a poor model for what's really going
on. It's been used as an "explanation" for the observation that an
antenna that is slightly shorter than half a wavelength is resonant(as
in has no reactive component at the feedpoint). The problem is that an
infinitely thin dipole is resonant at less than 1/2 wavelength, and in
that case, there's no real "end" to have an effect.

?? I have been under the impression that in the limit as the
conductor radius goes to zero, the resonance does go to a freespace
half wavelength. You have to make the antenna _really_ thin to get
anywhere near that, though. Even a million to one length to diameter
ratio won't do it.


Half wavelength of zero radius has a feedpoint impedance of about
73.1+j42.5 ohms
(see, e.g. page 639 of Orfanidis's online electromagnetics book
(chapter 16), which gives a one page derivation)
(http://www.ece.rutgers.edu/~orfanidi/ewa/)

Roy's right about the spiral not converging to 377+j0.. I misread the
graph.

Jim