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