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July 15th 03, 04:32 PM

(Richard Harrison) wrote:
Floyd Davidson wrote:
"---the look angle for a geosynchronous satellite here is about 12
degrees if the satellite is directly south."

Floyd is in Alaska. Floyd`s example of anomalous propagation from
reflection to a terrestial transmission was in Arizona. On the equator,
the "look angle" may be nearly straight up.

The reason a satellite dish is less susceptible to earth reflections is
that the satellite dish is not aimed to pick the reflections up. The
satellite dish is aimed at the sky.

The reason your example is poor is that it assumes something
which *clearly* is not always true. Satellite dish antennas are
not always aimed significantly far away from the earth's
surface.

A terrestrial microwave dish aimed directly at the satellite dish is
likely not transmitting an interfering frequency, but if so, it is
unlikely to be aligned well enough or above the horizon of the satellite
dish.

That is not necessarily true. And in fact I've seen 4 gig
terrestrial microwave systems cause grievous interference to
satellite systems. (In one case, by reflections off a metal
building across the street from the satellite dish, which caused
the weird effect of the interference coming from a microwave
that was 20 miles distant directly *behind* the direction the
satellite dish was pointed!).

The approximate specifications of a 6-foot dish for beamwidth and gain
versus frequency a

This doesn't account for side lobes, and hence gives a *very* false
indication of the actual susceptibility to interference arriving at
angles off the main lobe.

1.3 GHz 9 deg. 25 dbi

2.3 GHz 5 deg. 30 dbi

3.5 GHz 4 deg. 33 dbi

6 GHz 2 deg. 36 dbi

10 GHz 1.5 deg 43 dbi

25 GHz 0.5 deg 50 dbi

Sources of the above are the "RSGB VHF-UHF Manual" and the ARRL Antenna
book (they agree).

In moderate latitudes, the satellite earth station antenna is really
looking up. It is quite likely terrestrial signals are not within range
of its bandwidth, beamwidth, or distance.

But not all microwave systems requiring path engineering are
located so convenient for your specifications. It is simply false
to claim that, even at moderate latitudes, the satellite antenna
is *necessarily* looking at a high angle above the terrain. Locations
with small latitudes *can* see some satellites at high angles, while
locations at high latitudes *never* see a geosynchronous satellite
at a high angle. But in either case there are *many* satellites
at lower angles.

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