Amos Keag wrote:
But, what does feedpoint impedance have to do with
resonance?

On a standing wave antenna, like a center-fed dipole, the
feedpoint impedance is (Vfor+Vref)/(Ifor+Iref) where Vfor
is forward voltage, Vref is reflected voltage, etc. and
the plus sign denotes superposition, i.e. phasor addition.

On a wire dipole, the resonant feedpoint impedance will
occur only when all the phases line up, i.e. If Vfor is
at zero degrees, Vref must be at 180 degrees, Ifor must
be at zero degrees, and Iref must be at zero degrees. That
way, we get minimum voltage divided by maximum current with
a resultant phase angle of zero degrees. Eureka! The dipole
is resonant because the feedpoint impedance is purely
resistive.
--
73, Cecil http://www.qsl.net/w5dxp

On Fri, 24 Feb 2006 19:28:37 GMT, Cecil Moore wrote:

Amos Keag wrote:
But, what does feedpoint impedance have to do with
resonance?

On a standing wave antenna, like a center-fed dipole, the
feedpoint impedance is (Vfor+Vref)/(Ifor+Iref) where Vfor
is forward voltage, Vref is reflected voltage, etc. and
the plus sign denotes superposition, i.e. phasor addition.

On a wire dipole, the resonant feedpoint impedance will
occur only when all the phases line up, i.e. If Vfor is
at zero degrees, Vref must be at 180 degrees, Ifor must
be at zero degrees, and Iref must be at zero degrees. That
way, we get minimum voltage divided by maximum current with
a resultant phase angle of zero degrees. Eureka! The dipole
is resonant because the feedpoint impedance is purely
resistive.

Cecil, are you saying that a resonant dipole must have a low
impedance, and that modes where the feedpoint impedance is purely
resistive but high are not "resonant"? That seems to be what your
formulae above and explanation suggests.

Owen
--

Amos Keag wrote:
Resonance has NOTHING to do with impedance.

jX is only zero at resonance.
--
73, Cecil http://www.qsl.net/w5dxp

Owen Duffy wrote:
Cecil, are you saying that a resonant dipole must have a low
impedance, and that modes where the feedpoint impedance is purely
resistive but high are not "resonant"? That seems to be what your
formulae above and explanation suggests.

Yes Owen, that's what I am saying. When I was at Texas A&M in
the dark ages, we called the feedpoint impedance of a one-
wavelength dipole an "anti-resonant" impedance. It is explained
at: http://dx.doi.org/10.1036/1097-8542.041800
"Antiresonance - The condition for which the impedance of a
given electric ... system is very high, approaching infinity."

Semantics strikes again. To distinguish the left-most low resistance
point on an SWR circle from the right-most high resistance point, we
mid-20th-century Aggie EEs called the leftmost point, "resonant", and
called the rightmost point, "anti-resonant".

If you and I were ever to agree on definitions, I have no doubt
that we would also agree on concepts.
--
73, Cecil http://www.qsl.net/w5dxp

On Fri, 24 Feb 2006 20:52:11 GMT, Cecil Moore wrote:


Semantics strikes again. To distinguish the left-most low resistance
point on an SWR circle from the right-most high resistance point, we
mid-20th-century Aggie EEs called the leftmost point, "resonant", and
called the rightmost point, "anti-resonant".

If you and I were ever to agree on definitions, I have no doubt
that we would also agree on concepts.

I am sure we are talking the meaning of the terms (semantics) rather
than the underlying concept.

Narrowing the term resonance to only apply to the resonance that
exhibits a series resonance equivalent behaviour seems to me to
unnecessarily limit the meaning of resonance (though I note it is used
in optics to some extent).

I think of the high impedance of a dipole with zero reactance at some
frequencies also as a resonance, and I think you did too when you said
recently to Amos "jX is only zero at resonance." If that is to mean
that jX is "only ever" zero at resonance, then if jX is zero, you have
resonance, whether R is high or low.

On that basis, one would have to say that a full wave centre fed
dipole exhibits (at the feed point) resonance similar to a lossy
parallel tuned circuit and should be considered a resonant radiator.

Owen
--

Amos Keag wrote:

Resonance has NOTHING to do with impedance. Resonance is resonance; it
has a harmonic response.
. . .

Resonance has everything to do with impedance. Resonance is defined as
any frequency at which the impedance is purely resistive; that is, where
the reactive part of the impedance is zero. And that is all resonance is.

You can change the resonant frequency of an antenna by simply adding a
series or parallel inductor or capacitor at the feedpoint. This doesn't
change the antenna characteristics in any other way than to alter the
feedpoint impedance.

Roy Lewallen, W7EL

Owen Duffy wrote:
On that basis, one would have to say that a full wave centre fed
dipole exhibits (at the feed point) resonance similar to a lossy
parallel tuned circuit and should be considered a resonant radiator.

I know that is what your gut feeling wishes were true. But a
large portion of the RF engineering community considers "anti-
resonance" to be the exact opposite of "resonance" and indeed
it is the exact opposite on a Smith Chart, being the opposite
side of the SWR circle. Semantics strikes again. I'm sure that
our Russian counterparts have a completely different word for
exactly the same effects.
--
73, Cecil http://www.qsl.net/w5dxp

Roy Lewallen wrote:
Resonance has everything to do with impedance. Resonance is defined as
any frequency at which the impedance is purely resistive; ...

In the distant past, when I had a dinosaur for a pet, resonance
was defined as the frequency at which the impedance is a purely
low impedance. The frequency at which the impedance was a
purely high resistance was known at the anti-resonant point,
the exact opposite of resonance, and indeed, it was the exact
other side of the SWR circle on a Smith Chart.
--
73, Cecil http://www.qsl.net/w5dxp

Egad. Calling it antiresonance is asking for (communications) trouble,
since not eveyone uses the same terms. Just call it "half-wave
resonance" and "full-wave resonance". I don't think I've EVER heard
anyone call a parallel-tuned circuit "anti-resonant." I do regularly
hear people distinguish between series and parallel resonance, however.
I'm not likely to soon adopt "antiresonance" for either condition, as
it sounds way too much like something opposing resonance.

Cheers,
Tom

K7ITM wrote:
I'm not likely to soon adopt "antiresonance" for either condition, as
it sounds way too much like something opposing resonance.

In a transmission line with reflections, antiresonance is
indeed plus or minus 90 degrees from resonance and "never
the twain shall meet". Resonance and antiresonance cannot,
by definition, occur at the same point, i.e. if a point
is antiresonant, it cannot, by definition, be resonant.

Quoting "Transmission Lines and Networks", by Walter C.
Johnson, PhD. (one of the heavyweight gurus of the mid-20th-
century) page 156: "When the lossless line is an odd number
of quarter wavelengths long, the sending-end impedance is
theoretically infinite (inversion of the receiving-end
impedance). The actual impedance, considering losses, is a
very large resistance, and the line is said to be ANTIRESONANT."
(Capitals substituted for italics for obvious reasons)

So your argument is with Walter C. Johnson, PhD, ex-chairman of
the Department of Electrical Engineering at Princeton University,
not with me.
--
73, Cecil http://www.qsl.net/w5dxp