Topband: Best wire antenna for roof top location

[Tom W8JI]

> 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.