View Single Post
Old 04-09-2016, 10:05 AM   #8
NN5I
Carl, nn5i
 
NN5I's Avatar
 
Join Date: Sep 2011
Location: Tallahassee, FL
Posts: 1,441
Default

Quote:
Originally Posted by N3LYT View Post
When you get into the higher freqs radial design becomes an issue because a longer radial can become dominate in relation to the desired radiator. The other factor is take off angle so it's not just the creation of a "ground plane".
Actually it is. The takeoff angle is determined by the location of the ground plane (how far it is below the antenna) and by its reflection characteristics (how good a reflector it is). This is true for both vertical and horizontal antennas.

At UHF and above, and sometimes for VHF, we can build such things as rotatable parabolic reflectors that can be aimed both in elevation and in azimuth. For these there is no ground plane; but at HF such antennas are impractical. For HF antennas that aren't on spacecraft there is always a ground plane, and it always determines the takeoff angle.

This oversimplifies a bit -- there isn't just one "takeoff angle". There is a radiation pattern that gives varying signal strength at different takeoff angles. That pattern, in its variation by elevation angle, is determined almost exclusively by the location (and reflection characteristics) of the ground plane.
__________________
-- Carl
NN5I is offline   Reply With Quote