Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!well!nagle
From: nagle@well.UUCP (John Nagle)
Newsgroups: sci.electronics
Subject: Re: TV RF transmitter wanted
Message-ID: <11983@well.UUCP>
Date: 3 Jun 89 18:39:39 GMT
References: <190700040@trsvax> <107726@sun.Eng.Sun.COM>
Reply-To: nagle@well.UUCP (John Nagle)
Lines: 45

>In article <190700040@trsvax> bb@trsvax.UUCP writes:
>>
>>I am interested in transmitting a TV RF signal over a distance of at least
>>2000 feet.  I realize I will probably need some kind of FCC license.  Does
>>the FCC grant licenses to send a TV signal of that strength?  What is the
>>size, cost, weight, and wattage that I am looking at to send 2000 feet?
>>Has anybody out there (such as robotics types) tackled the same problem?

      I have a setup on my bench doing exactly this.  But it is nontrivial.
I'm using an FM TV transmitter running on 1.245GHz, with a power of 50mW.
The equipment is a Wood and Douglass 1240TVT modulator, and the receiver
is a Wyman Research WR-FM4912R.  The receiver is front-ended with a
GaAs RF preamp.  Antennas are drooped verticals.  An external preemphasis
unit is used at the transmitter.  I also have a 5W linear amp from Wyman,
but it has never worked.  I'm licenced to operate this as an experimental
station under Part 5, as KA2XXF.  Such licences are straightforward to get
for research purposes, and cost nothing, although the paperwork is something
of a nusance and it takes several months for frequency coordination.  At
low power levels, though, the FCC will generally grant an experimental
licence.  See 47 CFR 5 for the regulations.  Alternatively, a ham licence
will allow this operation, and under fewer restrictions.

      Overall results are mixed.  It's definitely good enough for manually
driving a robot by remote control.  But reflection nulls are frequent, so
as the transmitter moves, the image wavers.  What's needed is a pair of 
receivers and a diversity switch.  It would also help to use a circularly
polarized antenna.

      The Wyman gear suffers from poor linearity in the video section, and
this translates into distortion of the grey scale in the video transmission.
There is also cross-coupling from the audio section into the video in the
receiver, but it's in the audio amp for the speaker in the receiver, so
if you don't use the internal speaker, this problem can be avoided.  The
audio channel is good from about 100hZ to 20KHz (with significant attenuation
above 10KHz), so it could be used for data.  The audio subcarrier is 
FM, mixed with the video.  The RF system is all FM, so noise isn't a a problem
as long as you have enough signal strength for capture.

      The gear at the transmitting end is small.  The modulator is about
1"x4"x6", the preemphasis unit is about 2"x3"x1", and the linear amp is about
3"x3"x3", including heat sink.  Everything runs on 12v DC.

					John Nagle

					John Nagle