In message , Cecil Moore
writes
Ian Jackson wrote:
Are you sure it's as high as that, Reg? I once did a Smith Chart plot
of the impedance at the centre of a dipole, the valued being taken
from a table 'compiled by Wu' (LK Wu?). These only catered for a
lengths up to a few wavelengths. As the plot progressed round and
round the Smith Chart, it seemed to be heading for something around
350 to 400 ohms.

Maybe 377 ohms? Remember that any finite length dipole is a standing
wave antenna and the feedpoint impedance is (Vfor+Vref)/(Ifor+Iref)
where Vfor is the forward voltage phasor, Vref is the reflected
voltage phasor, Ifor is the forward current phasor, and Iref is
the reflected current phasor.

For a 1/2WL resonant dipole the feedpoint impedance is low:
R = (|Vfor|-|Vref|)/(|Ifor|+|Iref|) ~ 73 ohms

For a 1WL (anti)resonant dipole the feedpoint impedance is high:
R = (|Vfor|+|Vref|)/(|Ifor|-|Iref|) ~ 5200 ohms (EZNEC)

An infinite dipole would not be a standing wave antenna. It would
be a traveling wave antenna (as in a terminated rhombic). So the
feedpoint impedance of an infinite dipole would be Vfor/Ifor=Z0.
Since the reflections modify the feedpoint impedance, we might
suspect that Vfor/Ifor falls between the feedpoint impedance for
a 1/2WL dipole and a one WL dipole. Seems to me, the Z0 of the
dipole, i.e. Vfor/Ifor, must be in the ballpark of the square
root of the product of those two feedpoint impedances.

Yes, I did think of 377 ohms (which I understand is 'the impedance of
free space'), but I'm no expert in these matters.

As you indicate, the impedance must lie somewhere between 73 and 5200
ohms. You suggest that this might be something like the square root of
the product of those two feedpoint impedances (the geometric mean),
which gives 616 ohms. However, you would see 600 ohms simply by looking
into an infinite length of 600 ohm feeder, which has parallel,
non-radiating conductors. If the length of the feeder was relatively
short (compared with infinity!!), pulling the conductors apart would
increase the impedance (probably to a lot more than 616 ohms). The
question is, 'when does radiation start to influence the impedance?'

If you look at K6OIK's paper at
http://www.fars.k6ya.org/docs/antenn...nce-models.pdf
and look at, for example, page 22, you can see how the feed impedance at
odd halfwaves increases, and at even halfwaves, decreases. I only found
this paper this morning, and haven't had time to look to see which (if
any) of the many formulas was used to obtain the plot. It must be
possible to get close to the infinity condition by entering values for a
very, very long dipole.

Cheers,
Ian.

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