Jeff Liebermann wrote:

Maybe some examples might help.

1. I want to model a UHF (440 MHz) vertical "ground plane" antenna
mounted on a pole on my roof. The roof is 20ft high and the antenna
is mounted on top of a 10ft pole. How high above "ground" do I make
my antenna model?

About 30 feet, i.e. the height of the radials.

2. Same antenna, but with a #12 solid ground wire running to a ground
rod pounded into the ground. How high above "ground" do I make my
antenna model?

For the same antenna, the same height.

3. Assuming the pole is made from fiberglass, and RF power amp is
mounted at the antenna (common for cellular TMA installs), will a free
space model work?

Depends on the actual height above the ground in wave lengths and what
else is around.

4. If the 10ft pole it transplanted to the ISS, is there an "earth
ground" and how high?

The ground would be free space, but for accuracy you would need to
model the ISS.

Scroll down for my answers.









Looks like I'll be busy for a few days, so I might as well answer my
own questions now:

1. 10 ft. At UHF, the house is considered part of the earth ground
especially if the roof is corrugated steel or full of chicken wire.
Same with any LARGE mounting structure (in terms of wavelengths).

My roof is all non-conductive so I can ignore it. Your milage may
vary.

2. 10 ft. The ground wire is presumably not part of the radiating
parts of the antenna and can be ignored if shielded by the ground
plane wires. If long enough, it looks more like an RF choke
(inductor) than an antenna element.

Having actually modeled such, I can say that such a wire will have
little to no effect on the antenna unless it happens to be about
1/4 wavelength long.

3. Yes, free space will work, although nearby metal structures and
wires need to be considered if they can be "seen" by the radiating
elements.

We agree totally on this one.

4. Toss a coin and 10 ft. The ISS is large enough (in terms of
wavelengths at UHF) to be considered a perfect ground, as in my
monopole examples. However, if you need an accurate model, the ISS
has enough complex shapes that can re-radiate RF, and should probably
be modeled as part of the antenna system, in free space and without
any influence from the planetary rock.

I would think that the shape of the ISS is such that you would need
to model it and take into concideration the location relative to the
solar panels and their orientation.



--
Jim Pennino