Dot wrote:
On Fri, 24 Feb 2006 22:41:54 GMT, Cecil Moore wrote:

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.

These days, resonance is described as either:

a) the point at which Inductive Reactance and Capacitive Reactance are equal
or
b) the point at which a load impedence is purely resistive.

The two points are exactly the same.

Looking at your definitions I would suggest that "resonance" is really the
point at which the antenna mimics a series resonant circuit, exhibiting a
low impedence and "anti-resonance" is the point at which it mimics a
parallel resonant circuit, exhibiting a high impedence.

The high-impedance full-wave resonant point (for a dipole; half-wave
resonant point for a monopole) is sometimes called "anti-resonance", but
not commonly, and mostly in older literature. It's a true point of
resonance, that is, where the reactance is zero. I don't believe I've
ever heard the term "anti-resonance" applied to other high-impedance
resonant circuits, such as a tank circuit.

It would then be reasonable for a given wire perpendicular to a good ground
plane to exhibit "resonance" at odd multiples of a quarter wavelength and
"anti-resonance" at even multiples of a quarter wavelength... Translating
gives low impedence at odds and high impedence at evens, which is where I
started out in this discussion....

If you choose to call the high-impedance resonant points
"anti-resonance", that's true. But again, they're points where the
reactance is zero, just like the points you're calling "resonant". The
only difference is that the impedance is high and the antenna acts more
like a parallel tuned circuit at nearby frequencies rather than a series
tuned circuit.

Your semantics is correct if you are looking to define an antenna as "a
current fed device", but that's not always the case. There are end fed half
waves out there... they are voltage fed, they are resonant and they do work.
(Ask anyone who owns a "Ringo Ranger".)

No, the definition of resonance has nothing to do with how an antenna is
fed. The impedance of the antenna doesn't change with the feed method
(assuming of course that it has a single feed point), and therefore its
resonant frequencies don't change with the feed method. (You can, of
course, alter the resonant frequencies of an antenna *system* by adding
reactance at the feedpoint or elsewhere.) And an antenna doesn't have to
be resonant (that is, have a non-reactive feedpoint impedance) to
"work". Resonance is only an indication of the reactance of the input
impedance, and has nothing to do with an antenna's gain, pattern,
bandwidth, or other performance characteristics.

Roy Lewallen, W7EL