[sci.electronics] Diversity reception for UHF

jbn@glacier.STANFORD.EDU (John B. Nagle) (09/20/88)

      I'm struggling with the problems of getting high-quality video
from a mobile robot.  I'm using FM TV on 1258.25MHz at 20 milliwatts.
I have plans to increase this to one watt, but as yet have not done so.
The receiver, at most a hundred feet away, but often with some
intervening walls, has a low-noise GaAs FET preamp at the antenna, which
is just a quarter-wave whip with a ground plane.  All antennas are omni
directional, of course.  (Actually, I've been using a 50-ohm dummy load
on the transmitter for this short-range testing.  When I go to higher power,
I hope to increase the range to a thousand feet or so.)

      Generally, results are good, but there are occasional losses of signal
as the transmitter moves.  These are presumably due to reflections.  When
at a null, moving the receiving antenna a few inches usually clears up the
problem.  Since the wavelength is 23cm, this is not suprising.

      So I'm thinking about diversity reception.  My first thought was to
get a second receiver, bring out the AGC lines from both, feed them into
a comparator, inserting a little hysteresis to avoid excessive switching,
and use the output to control analog switches on the video output.
Are techniques such as this ever used by hams in ATV?  Is it necessary to
extract the sync and constrain the switches to occur during the vertical
interval?  Would it be better to switch at the IF level, rather than
at the video output?  And what analog switches are suitable for video
switching?  Advice would be appreciated.

      Incidentally, this is not a ham activity, but an experimental one,
under Part 5 (experimental), licenced as KA2XXF.

      I'm still looking for a 1200MHz 1 watt power amp, by the way.  5 watts
I can get, but that's overkill, and the unit is too big.  Suggestions?
And, please, if you suggest some vendor so tiny they're not in Thomas's
Register, give a phone number or address.  Everybody talks about those
Mitsubishi RF bricks, but no one seems to actually sell them.

					John Nagle