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

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

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

Kenneth Norton worked for the old Bureau of Standards back in the 30s
through the 70s. He published many monographs on propagation, and
eventually covered the entire RF spectrum. Many of his papers were also
published in IRE Proceedings, or IRE Antenna and Propagation Society
Transactions. Those papers are classics, and have all the information you
seek. Unfortunately, short of a good technical library, I do not know where
to send you to read them.

A simple start is to read the Propagation chapter in any recent edition of
the ITT Handbook, and then look at as many of the end of chapter references
you can access.

--
Crazy George
Remove N O and S P A M imbedded in return address
"Alan" wrote in message
.. .
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

Alan wrote:
"Can anyone explain to me why UHF links typically are not LOS---?"

UHF links are LOS, but there is LOS and there is LOS on steroids with
which the atmospheric refraction blesses UHF and VHF.

Space waves which travel through the atmosphere just above the earth are
best for terrestrial microwaves. The propagation mode is called line of
sight or LOS.

The surface of a propagating wavefront effectively consists of an
infinite number of tny radiators sending signals in various directions
away from the wavefront. So, at any instant there are an infinite number
of paths from a given wavefront to its receiving antenna. These can be
represented by a "target" with its bull`s eye centered and impaled on
the direct ray between the transmitting and receiving antennas.

The bull`s eye itself represents the first Fresnel zone. The ring
adjacent to and surrounding the bull`s eye represents the 2nd Fesnel
Zone.The second concentric ring reperesents the 3rd Fresnel zone and the
third concentric ring represents the 4th Fresnel zone, etc.

At a grazing point for our LOS signal along the surface of the earth, a
reflection from the surface of the earth may occur. The reflection
itself shifts the signals phase by 180-degrees.

Back to our target figure. Within the bull`s eye, all of the waves are
approximately in-phase. All of the waves in the first ring (2nd Fresnel
zone) are approximately out-of-phase with those in the 1st Fresnel zone.
The reason? Those in the 2nd zone have traveled approximately
180-degrees farther than those in the 1st zone.

The microwave path designer adds a Fresnel zone allowance to the
geometrical grazing clearance of his signal where it is tangent to the
earth. The area of the Fresnel zones here is determined by their
distance from the transmitter and the receiver, and the operating
frequency. The higher the frequency, the smaller the 1st Fresnel zone
and the others around it.

There are an unlimited number of Fresnel zones, with each succeeding one
contributing less energy than the one before.

Only the first few zones are significant. No matter what the Fresnel
zone sizes are as a result of the scaling to a particullar frequency,
the percentage of the total energy each may contribute to the energy
transferred between antennas is the same.

Smaller Fresnel zones (at higher frequencies) cause obstacles in the
radio path to obstruct a greater percentage of radiated energy. They
also cause more severe and frequent cancellations of energy between the
direct and reflected rays when these are bent by the earth`s
atmosphere..

In the paths I designed, I chose 0.6 of the first Fresnel zone as an
arbitrary addition to grazing clearance, as I now recall.

Fresnel zones have other effects. In periscope antenna systems which use
a reflector up on a tower and a dish near ground level, the reflector
may be sized to only accommodate the 1st Fresnel zone to avoid
cancellation caused by the out-of-phase 2nd Fresnel zone. This
gratuitous overflow of microwave energy into free space may be a cause
of decreased popularity of periscope antenna systems.

The question of "Does UHF propagation propagate differently?" is yes.
But, there is more to the difference than path loss.

Path loss is significantly different. Path loss is computed from:

Loss = 10 log f squared (d squared)

It`s seen that signal power not only follows the inverse square of the
distance, but the inverse square of the frequency too.

Thanks to the Lenkurt Demodulator for the above information.

Best regards, Richard Harrison, KB5WZI

I second what George has said about Norton's papers (and those of his
associates). 1950s IRE is prob. where one should look.
At lower UHF, scattering can be an important mode. I did estimates
of these modes to predict interference to a radio-quiet site long ago.
As a general rule, lots of power needs to be involved.
At all of the frequencies, diffraction over the surface is
significant. A standard scheme was to assume that the earth had a
larger radius than is the case and them to plot LOS paths on that basis.
Used to do that with topo maps by hand correcting the height of each
contour (cutting the path) for how far away it was. The last time I did
it - to help a college radio station - I used electronic topo maps -
what progress!
73 Mac N8TT

--
J. Mc Laughlin - Michigan USA

Ken Bullington's seminal 1947 paper in BSTJ and IRE also covers UHF
propagation pretty well, also, but doesn't extend to microwaves. There is a
later paper which does, but I do not have it handy.

--
Crazy George
Remove N O and S P A M imbedded in return address
"J. McLaughlin" wrote in message
...
I second what George has said about Norton's papers (and those of his
associates). 1950s IRE is prob. where one should look.
At lower UHF, scattering can be an important mode. I did estimates
of these modes to predict interference to a radio-quiet site long ago.
As a general rule, lots of power needs to be involved.
At all of the frequencies, diffraction over the surface is
significant. A standard scheme was to assume that the earth had a
larger radius than is the case and them to plot LOS paths on that basis.
Used to do that with topo maps by hand correcting the height of each
contour (cutting the path) for how far away it was. The last time I did
it - to help a college radio station - I used electronic topo maps -
what progress!
73 Mac N8TT

--
J. Mc Laughlin - Michigan USA

In message , J. McLaughlin
writes
I second what George has said about Norton's papers (and those of his
associates). 1950s IRE is prob. where one should look.
At lower UHF, scattering can be an important mode. I did estimates
of these modes to predict interference to a radio-quiet site long ago.
As a general rule, lots of power needs to be involved.
At all of the frequencies, diffraction over the surface is
significant. A standard scheme was to assume that the earth had a
larger radius than is the case and them to plot LOS paths on that basis.
Used to do that with topo maps by hand correcting the height of each
contour (cutting the path) for how far away it was. The last time I did
it - to help a college radio station - I used electronic topo maps -
what progress!

I used to do it for microwave links by plotting the path on '4/3' paper.
That is, a path curved by 4/3 the earth's radius.

Mike
--
M.J.Powell

Mike Powell wrote:
"I used to do it for microwave links by plotting the paths on "4/3"
paper."

Shows that different folks use different strokes. I alwaws used K=2/3
paper. This is quite pessimistic, but my paths usually worked under
propagation conditions (early morning hours when the fog sets in) that
bent the lines of wave travel away from the earth.

I also tried hard to keep distance between repeaters closer to 20 miles
than to 30 miles. Also, when I had a choice, between 2 GHz and 6 GHz, I
chose 2 GHz.

Best regards, Richard Harrison, KB5WZI