Hmmm! Let's remind ourselves of Nuradi's situation. The roof is 110m
high, 45m x 33 m. Corner to corner is less than a wavelength on 80M, more
than a wavelength on 40M, but the distance to a corner from a wire strong
between the two corners is less than a quarter wave on 80M, less than a
half wave on 40. Assuming an ideal conductor on the roof, it's going to
act as a reflector going upward, but the low angle pattern will be
determined in the far field.
A dipole on a real tall building roof is worth a try (because it is so
simple and easy), but it is not a simple predictable system.
First, any conductor approaching a resonant length in the near field is a
real problem for antenna pattern modification. Second, another issue is
absorption from everything in the building. Third, there is likely noise.
A building is generally a real complicated mess of long conductors that run
both horizontally and vertically, and thick lossy dielectrics of all types.
A building typically cannot be modeled as a flat sheet the dimensions and
height of the building roof. On receiving, a building is typically full of
multiple noise sources.
There can be buildings that work OK through luck, and they are generally OK
on frequencies where the antenna can be a long distance in fractional
wavelengths away from the building wiring, but they are almost never
anything like a pole or mast support for pattern, and they are rarely good
for noise.
Usually it is a better idea to get the wire out away from things that might
be problems, or at least put a nearfield null in the direction of the
problem.
I had a friend who managed a high rise apartment building on a hill. We
never could get a good 80 horizontal antenna on the roof, but it was killer
on 20 meters and up (he had a 20-30 ft tower on the roof). The only thing
that seemed to work well on 80 was a wire hung off the side out away from
the building. We used the metal flashing as a ground, and end-fed a real
Zepp antenna. It was like a J-pole laid on its side.
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