RadioBanter - View Single Post

November 17th 05, 05:21 AM posted to rec.radio.amateur.antenna

wrote:
What kind of "ground" should I use to EZNEC-model a flat-top dipole antenna
of which one side will be about 7 feet above and parallel to the peak of
my house roof and the other half will be about 30 feet above the yard.

My house is a story-and-a-half (the usual attic was a dormitory for my
long-gone kids) over a full basement (which just happens to have both
a Ufer ground in the basement-wall footings and a "ring ground" around
the outside and at the level of the footings). Thus most of one-half
of the antenna will be over the usual Kansas soil (about a foot of fertile
stuff above many feet of more clay-like stuff) while the other half will
be above the roof above the attic/dormitory above the ground floor above
the basement above the footings.

So will this be a "good", "bad", "average", or ??? ground for EZNEC-
modeling purposes?

The short answer is that it doesn't matter.

I do recommend using "Real, High Accuracy" ground for all models unless
it's necessary to make a zero-resistance connection to the ground plane.

When using "high accuracy" ground, the ground quality is used by the
program for two purposes: 1. To determine the antenna impedance and
currents, and 2. To calculate the pattern, which depends on ground
reflections. The former doesn't make much difference if the antenna is
more than about 0.1 or 0.2 wavelength high. The latter takes place
(except for very high angle signals) well beyond your house or other
nearby objects, and for horizontally polarized antennas doesn't make
much difference anyway (again except for high angle signals).

Whatever effect your house will have on the antenna, you won't be able
to effectively model it. It will be chiefly due to the conductors in the
house -- wiring, pipes, rain gutters, etc. -- and it's just about
impossible to know enough about them to model them decently.
Fortunately, they're unlikely to have much of an effect unless something
happens to be nearly self-resonant. If that happens, you're likely to
know about it via other means (e.g., TVI or other RFI, "hot" conductors
or appliances when transmitting, or even lights that go on when you
transmit), and will be wanting to spoil the resonance rather than simply
modeling the offending conductor.

Roy Lewallen, W7EL