Tom Bruhns wrote:
I certainly agree that the point where a reactance arc is tangent with
an SWR circle is the maximum reactance on that SWR circle, but that's
not necessarily (and generally is NOT) also a point on the curve
representing antenna feedpoint impedance versus frequency. -- Did I
miss something fundamental here?

I suspect you missed that my input to the discussion assumes a thin wire
at HF frequencies. Try an HF dipole with a thin wire and see what you get.
It will be approximately the same curve as Fig. 10, page 2-10, ARRL Antenna
Book, 15th edition. Since this is the graph of an end-fed antenna, the
dipole response can be obtained by multiplying the frequencies by 2.
Everything I have said has been extrapolated from that graph in the
ARRL Antenna Book which is, as I said before, an "imperfect circle or
imperfect spiral".

Are we not discussing how the
antenna feedpoint impedance changes with frequency, and specifically
the frequency between half and full wave resonances at which the
antenna feedpoint reactance is maximum?

Yes, that's true. Now try it with a thin wire on HF. I believe what
you will find is that at the maximum reactance point, the resistance
is approximately half of the one-wavelength (anti)resonant value.
At the maximum reactance point, the resistance and reactance are
approximately equal. Since the maximum reactance value lies between
two points of pure resistance, doesn't it make sense that it might
be approximately where the resistance is half of the maximum value
of resistance?

Maybe you should just tell us why you disagree with Fig. 10 in the
ARRL Antenna Book, 15th edition, page 2-10.

If you want to see a dipole feedpoint impedance Vs frequency, it
is illustrated in Figs. 2-5, Page 2-3,4, ARRL Antenna Book CD,
version 2.0. Unfortunately, they plotted reactance on a linear
scale and resistance on a log scale and thus messed up the
shape of the "imperfect circle or imperfect spiral". Even with
that, one can see that the point of maximum reactance approximately
equals the resistance at that point and is approximately half the
value of the maximum resistance.
--
73, Cecil http://www.qsl.net/w5dxp



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