Every band has it's own propagation characteristics. At the lower
frequencies they are similar to HF, like F layer propagation on 6 (if the
cycle is a good one) and Es on 6-222. Ducting can often happen up to 1296.
The upper bands mainly use the various scatter modes to push things past LOS
(Both stations looking at the same piece of air). Over 300 miles is doable
on 5GHZ if there's enough power on both ends and sites are carefully chosen
for the least interfering geography and a clear "near field." (a bush at a
yard is worse than a forest at a mile). Not a lot of science in these
comments but a fair amount of experience.

Phil, KB2HQ

"Dave Shrader" wrote in message
. net...
Alan wrote:
Can anyone explain to me why UHF links typically are not LOS while
microwave a millimeter wave links typically
are. Does UHF propagate differently? Is it that UHF will scatter off
buildings and stuff while microwaves
get absorbed or something? Are there any good books that explain this?
Thanks.

Alan

This has been just sitting for a day with no responses so I'll offer an
opinion.

Within the radio Line of Sight Horizon both types of signals are truly
LOS. However, microwave signals generally have a much narrower beamwidth
from the antenna and requires more effort in aligning the antennas at
both ends.

UHF signals have several over the horizon propagation modes which
include ducting, refraction, and weather front modes.

Microwave signals MAY have some atmospheric absorption. This is caused
by self resonance of the nitrogen and oxygen molecules [I believe] in
the atmosphere, and by water vapor absorption. There are major
absorption peaks around 10 GHz and in the high 20s GHz.

Anyone else want to jump in? I'll stand to be corrected on the Physics
issues because I'm an old [really old EE].

Deacon Dave, W1MCE