On Sun, 14 Jun 2009 00:46:50 -0700, Sal M. Onella wrote:

"Rich Griffiths" wrote in message
communications...

Until this past year (when rotator cuff surgery took me out), I had
been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz).
I was often impressed by how far over the horizon it would work with
an antenna only about 5 ft off the ground and about 1 W of power.

Antenna gain on both ends explains most of that.

To me, it's a partial explanation, but not most of it. Microwavers often
resort to quasi-scientific explanations like "troposcattering", as do VHF-
ers. And sometimes the explanations get even fuzzier, like
"enhancement". They (and ducting) are somewhat predictable, but as a
rover the operating mantra was always just 'Let's try it".


Granted we were working with MUCH lower signal quality requirements
than the TV stations, but I still am surprised by how poor our
reception is of DTV channel 12 (and earlier, ch9), which is
transmitting MANY kW from a multihundred-ft tower only about 16 km
away.

Shannon's equations provide most of the answers:
http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html

Shannon's equations don't actually tell you much that's useful, as a
Ham. And even in a lot of commercial situations, it can be hard to be
sure what assumptions to make about noise and signal strengths, for
example.


Some of that is hard to follow, but the net effect is that you need a
certain (minimum) amount of power to send a complex signal in a confined
bandwidth. With ATSC, they put about 20 Mbps into a 6 MHz channel. To
get a decent SNR (16 dB or better), they need MANY KW.

I did a little mickey-wave engineering, myself. Point-to-point is
easier than broadcast!

"Sal"

As an amateur rover, I generally found point-to-point to be more
difficult, because of simple real-world Ham issues that don't have a lot
to do with heavy theory. At 10 GHz, a 60-cm dish has a beamwidth of
about 3.5 degrees. The rovers can cope with that with some care, but it
seems many fixed stations are using rotators that don't provide that kind
of resolution. So it could be hard lining up. In contrast, over-the-
horizon signals smear out a lot horizontally. I once estimated another
rover who was 180 km away as having about a 45-degree signal width, and
found 20+ degrees to be common.

Besides that, 10 GHz, and sometimes 5.7 GHz, will occasionally do
downright freaky/cool things. Rainscatter is one fun example. And I
once worked a station (S9, SSB) over a 50+ km path with a 90-degree bend
in it, because the "line-of-sight" path was heavily obstructed by trees
etc. We told each other we were bouncing off downtown Cincinnati, which
was not visible over the horizon.

I guess I'd like to end this sales pitch with the note that the Ham
microwave bands can be great fun. The advantages, and Ham capabilities,
of playing around with "real radio" and the small antennas that involves
can be substantial.

50 MHz and up ... Everything else is DC.

--
Rich