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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