Reg Edwards wrote:
Cec, & Co.
Zo of an antenna wire seems to be an important parameter in your
discussions.

Here's an example of what I have been talking about. The feedpoint
impedance of my 130 ft. dipole (according to EZNEC) is:

60+j0 ohms at 3.63 MHz - resonance

2513+j2314 at 6.575 MHz - maximum reactance

5000+j0 at 7.118 MHz - anti-resonance

Assuming the antenna is fed with Z0 = 550 ohm open-wire line, the SWR
ranges from 8.5:1 to 9.2:1 and the impedance at a current maximum
point on the ladder-line ranges from 60 ohms to 65 ohms. The feedpoint
impedances from 3.63 MHz (resonance) to 7.118 MHz (anti-resonance)
describe an imperfect semi-circle on a Smith Chart normalized for 550 ohms.

If one plots the feedpoint impedances for all the different bands on a
Smith Chart normalized for SQRT(Rresonant * Ranti-resonant), it will
resemble an imperfect SWR circle (or imperfect spiral). By feeding my
dipole only at the current-maximum points, I achieve a near-perfect match
on all amateur HF bands without an antenna tuner.

At least for my multi-band dipole, it appears that the anti-resonant
feedpoint impedance is about 100 times the resonant feedpoint impedance.
The feedpoint impedance at the maximum reactance point is about 5000/2+j5000/2,
i.e. the R is about half the anti-resonant resistance and the Xmax is about
half the anti-resonant resistance times 'j'. These are my rules-of-thumb for
my dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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