Tom Bruhns wrote:
Cecil Moore wrote:

alhearn wrote:
My question is what determines where that peak occurs?

Mathematically, it will be where the SWR circle is tangent
to the reactance arc.

That is not in general true; do you have reason to believe it's true
for the impedance of a dipole?

When the dipole is at maximum feedpoint reactance, can the SWR be
calculated? Of course. Will that point, when plotted on a Smith Chart
lie on the SWR circle? Of course. Will it also lie on a reactance arc?
of course. Will the SWR circle be tangent to that reactance arc?
Of course. Will the un-normalized value of the maximum feedpoint
reactance be constant? Of course, assuming nothing changes except Z0.

Consider what happens when you change
the reference impedance for the SWR measurement.

It doesn't matter. The Smith Chart reference changes and therefore
the SWR changes but *the value of the antenna feedpoint impedance
stays the same*. The new SWR circle is still tangent to the reactance
arc at the same value of un-normalized reactance even though the
normalized reactance value has changed. Xmax/Z01 is different from
Xmax/Z02 but Xmax has not changed (assuming Z0 is the only change).

The higher the SWR, the higher the
maximum possible reactance and the closer it is to the
anti-resonance point.

What's the reactance at the anti-resonance point?

Always zero, by definition. Examples: A full-wave center-fed dipole
or a half-wave end-fed monopole. ("Anti-resonant" is what we called
such antennas at Texas A&M 50 years ago.)

Is highest SWR at
anti-resonance, or at maximum reactance, or at some point between?

If the Z0 of your transmission line is equal to the feedpoint impedance
at anti-resonance, the lowest SWR will occur at anti-resonance. If the
Z0 of your transmission line is equal to the feedpoint impedance at
maximum reactance, the lowest SWR will occur at maximum reactance. :-)

Depends upon the Z0 of the transmission line. If you choose a Z0 that
is the square root of the resonant impedance times the anti-resonant
impedance, the SWR at resonance and anti-resonance will be the same.
For a dipole that Z0 value is usually between 450 ohms and 600 ohms.
That's why anti-resonance is no problem for ladder-line/open-wire.
--
73, Cecil http://www.qsl.net/w5dxp



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Cec, & Co.

Zo of an antenna wire seems to be an important parameter in your
discussions. Knowledge of its value would appear to be essential before
continuing with calculations. Otherwise nobody will get nowhere.

So how is the value of Zo obtained (without bringing Terman et al into it)?
----
Reg, G4FGQ

Reg Edwards wrote:

Cec, & Co.
Zo of an antenna wire seems to be an important parameter in your
discussions.

Why do you say that? The only thing I have dealt with is
a constant feedpoint impedance at any one frequency.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
Cec, & Co.
Zo of an antenna wire seems to be an important parameter in your
discussions.

Here's an example of what I have been talking about. The feedpoint
impedance of my 130 ft. dipole (according to EZNEC) is:

60+j0 ohms at 3.63 MHz - resonance

2513+j2314 at 6.575 MHz - maximum reactance

5000+j0 at 7.118 MHz - anti-resonance

Assuming the antenna is fed with Z0 = 550 ohm open-wire line, the SWR
ranges from 8.5:1 to 9.2:1 and the impedance at a current maximum
point on the ladder-line ranges from 60 ohms to 65 ohms. The feedpoint
impedances from 3.63 MHz (resonance) to 7.118 MHz (anti-resonance)
describe an imperfect semi-circle on a Smith Chart normalized for 550 ohms.

If one plots the feedpoint impedances for all the different bands on a
Smith Chart normalized for SQRT(Rresonant * Ranti-resonant), it will
resemble an imperfect SWR circle (or imperfect spiral). By feeding my
dipole only at the current-maximum points, I achieve a near-perfect match
on all amateur HF bands without an antenna tuner.

At least for my multi-band dipole, it appears that the anti-resonant
feedpoint impedance is about 100 times the resonant feedpoint impedance.
The feedpoint impedance at the maximum reactance point is about 5000/2+j5000/2,
i.e. the R is about half the anti-resonant resistance and the Xmax is about
half the anti-resonant resistance times 'j'. These are my rules-of-thumb for
my dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote in message
...
5000+j0 at 7.118 MHz - anti-resonance

making up terms again cecil?? I have never heard of 'anti-resonance' and
don't see it in my references here... i would say it is just resonant at the
second harmonic, just because the impedance is high doesn't mean it is
'anti' anything, or anything else special. it is just plain resonant at
7118.

making up terms again cecil?? I have never heard of 'anti-resonance' and
don't see it in my references here

I have heard the term used before. I typed anti resonance into Google and got
296,000 hits. I don't think the term was made up.
73 Gary N4AST

"JGBOYLES" wrote in message
...
making up terms again cecil?? I have never heard of 'anti-resonance' and
don't see it in my references here

I have heard the term used before. I typed anti resonance into Google and
got
296,000 hits. I don't think the term was made up.
73 Gary N4AST

it would appear from a quick check that many of those 'anti-resonance'
references are mechanical (preventing resonance), quantum mechanical(some
kind of spin resonance), optical(something about lithography masks), or at
best electro-mechanical (lots of piezo-electric references in there), i did
see one that was explaining electrical resonance as mechanical analogies
that used that term. and, oh yes, there was another discussion group that
had latched onto it as an alternate term for a 'parallel resonant' circuit.

Dave wrote:
... and, oh yes, there was another discussion group that
had latched onto it as an alternate term for a 'parallel resonant' circuit.

According to the IEEE Dictionary, that's exactly what it is. And the
feedpoint of a one-wavelength center-fed dipole acts a lot like a
parallel resonant circuit. Somehow, saying that the resonant feedpoint
impedance of your dipole is 6000 ohms leaves something to be desired.
--
73, Cecil http://www.qsl.net/w5dxp



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Dave wrote:

"Cecil Moore" wrote:
5000+j0 at 7.118 MHz - anti-resonance

making up terms again cecil?? I have never heard of 'anti-resonance' and
don't see it in my references here... i would say it is just resonant at the
second harmonic, just because the impedance is high doesn't mean it is
'anti' anything, or anything else special. it is just plain resonant at
7118.

There's lots of thing of which I am ignorant but I don't make postings
about those things. The first four books I picked up had information on
anti-resonance. Are you familiar with books? :-) A web search for
"anti-resonance" turned up 700 hits. Perhaps you don't need a book.

_Radio_Handbook_, 20th edition, by Bill Orr: "In radio circuits, parallel
resonance (MORE CORRECTLY TERMED ANTIRESONANCE) is more frequently
encountered than series resonance; ..." emphasis mine

_The_ARRL_Handbook_, 17th edition, page 14-5: "A capacitor connected across
a cavity provides an ANTI-RESONANT notch below the resonant frequency (f0),..."
emphasis mine

_Electronic_Fundamentals_and_Applications_, John D. Ryder, page 287 under
section 9-13. Parallel-Resonant Circuits: "The total admittance of the
parallel-resonant (SOMETIMES CALLED ANTIRESONANT) circuit is ..."
emphasis mine

_IEEE_Dictionary_: "antiresonant frequency - Usually in reference to a
crystal unit or the parallel combination of a capacitor and inductor."

Since the resonant feedpoint impedance of a 1/2WL dipole acts somewhat
like a series resonant circuit, at Texas A&M we called that the resonant
frequency. Since the resonant feedpoint impedance of a one-wavelength
dipole acts somewhat like a parallel resonant circuit, at Texas A&M we
called that the anti-resonant frequency. Differentiating between the resonant
frequency and anti-resonant frequency gives one additional information
about the impedance to be matched. "Resonant" = low, "Anti-resonant" = high.
It's a useful concept. You really should try it.
--
73, Cecil http://www.qsl.net/w5dxp



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Nope! Cecil's not making up terms.

You just missed a point in your education.

Dave wrote:

"Cecil Moore" wrote in message
...

5000+j0 at 7.118 MHz - anti-resonance


making up terms again cecil?? I have never heard of 'anti-resonance' and
don't see it in my references here... i would say it is just resonant at the
second harmonic, just because the impedance is high doesn't mean it is
'anti' anything, or anything else special. it is just plain resonant at
7118.