"Roy Lewallen" wrote in message
...
That's interesting, but in real life you're going to have a jolly time
trying to make the antenna behave like the model, due to feedline
radiation -- unless, of course, you've included an accurate
representation of the feedline in your model. It's unfortunate, but
calling one piece of wire an "antenna" and another a "feedline" doesn't
make one radiate and the other not, especially when they're connected to
each other and positioned so they're in the same field.

You can reduce the current conducted to the outside of the coax with a
"current" or "choke" balun at the feedpoint. Induced current is another
matter, and is likely to take one, two, or more additional baluns to
bring to a low enough level to make the antenna behave like the nice,
no-feedline model.

The radiating feedline will also have an effect on the feedpoint
impedance.

Roy Lewallen, W7EL



Oh. Well, crap.


John wrote:


My EZNEC showed about 62-j114 when the radials are horizontal. BTW, Mr.
Street says in his article that he got the values from a table compiled
by
Hallen which gives an impedance of 75-j425. Not that it matters much;
it's
about the same work to match either impedance.

Here is a really interesting thing I discovered while trying to verify
the
numbers. Make a 5/8 wave vertical with 4 horizontal 1/4 wave radials.
Look
at the impedance and pattern. Nice pattern, especially with an elevation
at
about 2 wavelengths. Now take the radials away one at a time. The
resistance
gets a little greater and the reactance gets a little less. With only
one
radial the impedance is not that bad and the azimuth pattern has become
sort
of elliptical. The elevation angle of maximum radiation doesn't change
much,
either. If the last radial is pointing to the North, the azimuth pattern
is
squashed slightly so that East/West is best. So the E-W gain is a bit
better.

Moving down to an elevation of about 1/2 wave elevates the maximum
radiation
and the elevation pattern is broadened. I think, though, that it almost
doesn't matter much using any of 1 to 4 radials. But the 2,3, or 4
radial
patterns and impedances look very similar.

Ain't that a lick?

John