Tom Bruhns wrote:
o In other words, the SWR circles are useless
for finding the maximum-reactance point on
the impedance curve.

Useless for finding the *exact* point, yes. Quite useful enough for a
lot of ballpark work. For instance, Zs and Zm are usually as close
as EZNEC's prediction of antenna feedpoint impedance is to the
actual real-world antenna feedpoint impedance.

Example: Zm = 1000+j800 ohms

Where did this impedance come from? A full wave dipole's feedpoint
impedance is usually around 5000+ ohms. The value above does not
look like the Xmax impedance between 1/2WL resonance and one-wavelength
(anti)resonance for HF thin-wire dipoles. Such a dipole's Xmax impedance
is around 3000+j2000 ohms.

Does Zm represent the impedance at the Xmax point? It's not the Zmax point.

Ref. Impedance: 50 ohms: SWR = 32.82; Zs for that circle = 821.25+j819.73
Ref. Impedance: 300 ohms: SWR = 5.59; Zs for that circle = 865+j811.31
Ref. Impedance: 600 ohms: SWR = 3.; Zs for that circle = 1000+j800

(The 600 ohm case illustrates that there is no requirement that Zs and Zm differ.)

That's better than I expected out of my ballpark rule-of-thumb, Tom.
Note that Xs is within 2.5% of Xm in all three cases and right on for the
type of transmission line that I use. Zs is within 9.5% of Zm. Looks like
my estimates were twice as accurate as I had imagined. Thanks for proving
my original point.

The purpose for my rule-of-thumb is to get me close enough to achieve
cut-and-try tuning from that point on. That's also the purpose to which
I put EZNEC. In my latest experiment, EZNEC missed the actual resistive
component of the impedance by 125% but that's OK because I am close
enough to know which way to go from there.
--
73, Cecil http://www.qsl.net/w5dxp



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