(Richard Harrison) wrote:
Floyd Davidson wrote:
"This thing (TV microwave relay station) was located 50 miles from town
at 8500 foot up on a mountain top. They had a 50 foot tower (at the TV
studios I suppose)----they turned it on and it worked great."

So, if it wasn`t broke, why did they fix it?

Floyd also wrote:
"But somebody had the smart idea to see what happens if they slide the
dish down the tower to see if the signal would improve. It did!."

I wasn`t there, so I can only speculate, but I might have not been
surprised by those results.

On a 50-mile path with plenty of mid-path clearance, propagation is
similar to communications with a satellite. One difference at the
Arizona latitude is the vertical angle the dish path makes with the
Earth. The low angle the dish on the terrestrial path makes with the
Earth, makes it vulnerable to reflections from the Earth. The higher the
dish is placed, the more vulnerable it becomes. That`s a reason to go
high / low on a reflective path, and not high/high. (Low/low won`t make
the trip on most long paths due to Earth curvature).

A 50 foot tower on top of an 8400' mountain. What are you
talking about high/low etc etc. I wasn't there when the
decision was made to put it on a 50 foot tower, and neither you
nor I have any idea why that was done. Perhaps the topo maps
were wrong, and some obstacle they assumed was there didn't
actually exist. Perhaps the engineer made a mistake. I don't
know and you don't know. But, you don't suppose the engineer
knew exactly what he was doing, eh???

A 40 foot change in elevation suggests there simply were no
obstacles, so one has to wonder what the 50 foot tower was
supposed to accomplish... other than allow a range of adjustment
to find the best point for signal strength. Because this was
not a 50 mile shot, it was much closer to 100 miles and no doubt
they were very interested in optimizing the signal strength.
With no obstacles, not even earth bulge, calculating reflections
isn't so easy... unless you move the antenna vertically to find
the right position.

But, it doesn't emulate "a satellite" shot even in the slightest.
Or, not for real satellite shots at least.

E.g., the look angle for a geosynchronous satellite here is only
10-12 degrees or less depending on where the satellite is. I
don't agree at all that a 50 mile microwave path with no
obstacles is approaching similarity to a satellite shot! Some
satellite links have *less* clearance.

Of course, your exposure to satellites might not be that great.

--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

Floyd Davidson wrote:
"But, it doesn`t emulate "a satellite" shot in the slightest."

A satellite is a microwave repeater in the sky which is usually not
susceptible to earth reflections.

Terrestrial microwave paths are often designed to minimize earth
reflection.

Reason is, on a long path, the lengths of the direct ray and the Earth
reflected ray are almost the same. Reflection reverses the phase of the
reflected ray so that the reflected ray detracts from the direct ray
when they combine at the receiver. They arrive out-of-phase, at least at
some antenna altitudes.

The phase of the signal undergoes 180-degrees of phase change as we
examine the signal either forward or backwards from the plane of the
signal. That is to say, the phase of the signal is a function of
distance along the path. When a second signal source is created by Earth
reflection on the path, the combination of the direct and reflected rays
produces strata of stronger and weaker signal intensities as altitude
changes. No reflected signal, no significant variation of signal with
altitude, given sufficient path clearance. This is a worthwhile design
goal.

No matter which end of the TV relay path was varied to produce a
significant signal strength change, the attenuation at the higher
elevation was probably caused by an Earth reflection. Another reflecting
surface near the microwave path could have been the reflector. I`ve seen
that in the city canyons I`ve had to squeeze signals through. But on
most paths, the Earth is the most likely reflector.

Best regards, Richard Harrison, KB5WZI