"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.

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

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"