Jeff Liebermann wrote:

snip

Somewhere on my computah is a series of models that I built for a
simple 20 meter dipole, that started with an idealized free space
model, and progressed towards a real installation which by coincidence
resembles my house. I threw in everything that might have an effect
on the pattern to see what might happen. I got stuck at including the
sloping hillside because NEC2 seems to only include a flat earth. I've
also done similar studies for commercial antennas mounted on very real
and quite cluttered towers. Doing these incrementally is an excellent
introduction into the difference between ideal antenna patterns, as
found in the literature, and the nightmarish reality of real antenna
installations.

Yes, this is another thing typically ignored by amateurs.

There are lots of plans out there for J-poles made of pipe where one
of the legs is extended at the bottom to provide the mounting, typically
by bolting it to a mast. The assumption is there is no current in that
bottom leg.

If you model that you find that there really is current in that bottom
leg and it can REALLY screw up the antenna characteristics.



--
Jim Pennino