The roof is concrete with rebars in it and I think therefore can be modeled
as near perfect ground. Anyway that does not explain why the input impedance
of a single element delta loop fed for vertical polarisation and with apex
angle of around 100 degrees increased from 60 ohm (when over the ground) to
nearly 160 over the roof.
My another question:
Is there a point to install antenna with vertical polarization in such a
circumstances?
I mean vertical 30 meters above the ground radiates too much straight into
the clouds.
On the other hand, is it true (and it is according to models) that there is
no point in putting any stacked antennas on the roof just because the lower
antenna is too high and therefore one does not get "fill in" for certain
angles.
___________________________________________________________
Hi,
You certainly will have to deal with the realities of the building in
modeling the antennas up there.
You haven't mentioned the composition of the roof. If it is metal, or has
relatively dense metal bracing under it, or is concrete with rebar in it,
you might want to model the roof as a near perfect ground underneath.
Certain programs will allow you to place more than one ground structure
beneath it, e.g., near perfect ground for the first 20 meters, and poor
ground 30 meters lower after that. This can be done in EZNEC-2, as one
example.
If the roof is not conductive, or except for fairly wide spaced steel
crossbraces, then any metal content of the building within 30-50 meters of
the antenna can affect the pattern and/or impedance of the antenna in highly
unpredictable ways. That, of course, is an antenna modeler's nightmare.
Try modeling your antenna as if the roof were a perfect ground and see if
the predicted impedance approaches what you measure, if so then try the
first scenario I described.
An anecdotal aspect, when I was K3FKJ in Washington DC in the early 60's, I
lived in a row house. The roof of my house and the neighbor's roofs up and
down the block were tar covered tin and copper. I had a multiband vertical
with radials about a foot above the roof. It was the only vertical I ever
used that really worked well. Done right in good circumstances, the roof of
a building can do very well as an antenna farm.
Good luck, 73, and a prosperous New Year.
Guy.
- ----------
Guy L. Olinger
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