Topband: Low Angles

[Richard Fry]

Steve K0SR wrote:
>...As I stand at the base of my mighty Butternut, I look in any direction 
>and see obstructions. Cars, trucks, garages, hills, telephone poles, sheds, 
>trash cans. The very base of my vertical can "see" about 40 feet away. What 
>does that do to the radiation pattern of this antenna? ... My gut tells me 
>I need higher angles of arrival or I'm out of business.

Probably the only objects in your list that would have a significant affect 
on radiation patterns would be telephone/power poles, if they have a wire 
running down the side of the pole connecting to a ground rod.  Such 
scenarios are rather easy to model in NEC software to get some idea of their 
net effect.

On the lower bands, especially 160m, wavelengths are usually quite a bit 
larger than the vertical dimensions of even house-sized obstructions, so 
they don't have much affect on low-angle radiation from a monopole.  The 
link below shows a fairly extreme example of this.

The groundwave fields of AM broadcast stations in the "expanded band" 
(1610-1700 kHz) can cross entire cities and still have very nearly the same 
value on the far side of them as if the cities weren't there.  So 
radiation/reception by a monopole at elevation angles below 10 degrees also 
won't be much affected by those obstructions.

http://i62.photobucket.com/albums/h85/rfry-100/Part-15_AM_WhipElevated_Frame.gif