The ARRL Antenna Book puts it much more succinctly than I just did -
Page 3-20 of 21st Edition:
"In short, far-field losses for vertically polarized antennas are highly
dependent on the conductivity and dielectric constant of the earth
around the antenna, extending far beyond the ends of any radials used to
complete the ground return for the near field. Putting more radials out
around the antenna may well reduce ground-return losses in the reactive
near field for a vertical monopole, but will not increase radiation at
low elevation launch angles in the far field unless the radials can
extend perhaps 100 wavelengths in all directions! Aside from moving to
the fabled "salt water swamp on a high hill" there is very little that
someone can do to change the character of the ground that affects the
far-field pattern of a real vertical".
73,
Steve G3TXQ
On 05/01/2011 22:38, Steve Hunt wrote:
There are two distinct and different issues here. Take a look at how the
ARRL Antenna Book tackles them quite separately.
1. Antenna efficiency: directly affected by the ground conductivity in
the Reactive Near Field. This is the parameter we are usually improving
by laying down a good ground radial system
2. Elevation angle pattern: determined by soil characteristics in the
Far Field where ground reflections are taking place. Do some simple
geometry and you'll see that at really low elevation angles - 5 degrees
say - the ground reflections from an 80m quarter-wave vertical will
extend out to at least 750ft from the base of the vertical. A typical
ground radial system will do nothing about the conductivity there.
You can check this using EZNECs ability to have two different ground
media. Set up Media 1 as salt water extending for about 50ft around the
vertical, and Media 2 as rocky soil beyond. Look at the elevation
patterns - they will be those which pertain to poor ground. Now
gradually extend Media 1 outwards until you get the elevation pattern to
match that over salt water - that will tell you how far out the
reflections are taking place and how extensive your radial system would
need to be.
Please note - I'm not saying a good ground system doesn't help. I'm
saying that the usual ground radial system predominantly affects antenna
efficiency and not elevation pattern.
Please also note that nothing I have said conflicts with the Exam
Question answers.
73,
Steve G3TXQ
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