Richard,
I would recommend a vertical on the roof, as tall as possible, but hopefully
at least 40'. If you can use toploading (several wires from top of vertical
to corners of building) that would be helpful. Use a coil (1/4 inch tubular
copper which you can buy at the hardware store) at the base to resonnate
around 1830 and tie your ground system (several radials to corners of
building if possible) into the metal frame of the building (important). I
would tie the ground system into all exposed metal on top of the building:
electrical grounds, lightning rods, water pipes, etc.
If you are able to accomplish the above, you should have a VERY
respectable transmitting antenna system. Your bandwidth will depend mostly
on the heigth of the vertical. If you can not tie your ground sysytem into
the building frame, etc. then use two, above building ground, resonant
radials resonated to about 1830 with inductors. Even if you can tie your
ground system to the building's metal frame, etc., you might want to add the
resonant radials.
The above antenna might also be a good receiving antenna depending on
local noise sources, but you might add a low dipole at ground level or
sloping dipole from roof to ground for receive. A slopping dipole might also
be a good transmit antenna, especially in the direction of the slope.
My two cents worth. I've always wanted to construct a low band antenna
on top of a high rise, but never had the opportunity. Will be interested in
hearing what you do and how it works out.
Doug W0AH
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