On (09 Feb 97) Stephen Kellat wrote to All...
 SK> I vaguely remember a Hints & Kinks item from QST within the past six
 SK> years concerning how to receive ATV signals on a cable ready TV using
 SK> something like a paper clip for an antenna.  Does anyone know of any
 SK> such way do something like this?  TNX in advance for any assistance.
 SK> It will be appreciated.
Yeah. The usual 70 cm band frequency for ATV repeater output is 421.25
Mhz. That is the same frequency as *CABLE* channel 57. *Not* UHF or VHF
but *CABLE* frequency. Each TV channel is 6 Mhz from the next one so
427.25 = Cable channel 58. 433.25 = ch 59 and 439.25 = ch 60. It is also
fairly common to make the "in band" ATV repeater input on 439.25. And
sometimes direct ATV communication (not thru a repeater) are done on
channels 58 and 59. You can also do this with a VCR that goes at least
up to *cable* channel 60.
There are a lot of "cross band" ATV repeaters because even with 18 Mhz
of separation (repeater output 421.25, and input at 439.25) it is
difficult to not "desense" your receiver when your transmitter is on the
air. So, in order to be able to "see" yourself on the output while
transmitting on the input many favor putting the repeater output on 70
cm and input at 900 Mhz or 1.2 Gig. These are called "cross band" ATV
repeaters.
Typically, ATV signals are polorized horizontally, just like regular TV.
And to get color you need a "broadband" antenna because the ATV (or any
TV) signal is about 6 Mhz wide. Remember the little UHF "coat hanger" or
"bow tie" antenna that usually came with some TVs, particularly the
portable ones? The one that was shaped like a bow tie? This kind of wire
antenna *is* broadband and will work for receiving ATV signals if
oriented horizontally (just like you would wear a bow tie and "face" the
repeater output antenna).
Your antenna *must* be line-of-sight from the repeater output antenna
(or station on direct) to get a reasonable picture. Remember also that
a TV or VCR tuner has a very poor receiver. ATV signals that can be
barely seen, with no color at all, can be easily seen with a gas fet
pre amp tuner like the popular P C Electronics ATV downconverter (about
$89.95 or so). But I would not bother buying one unless you can see
*something* with a bow tie type of antenna first. If the signal is *not
there*, the best amp in the world won't help it.
The typical outside UHF TV antenna like everybody sells, including Radio
Shack, will work well for receiving ATV signals. Those antennas have
"fat" elements. Fat elements *are* broad band. I built a "quagi" antenna
using 5/8 inch copper tubing for the two "quad" elements and 4 gauge
copper ground wire for the "yagi" elements to the dimensions for an 8
element 70 cm wire element quagi shown in the ARRL "antenna hand book"
that I put in my attic to receive our club ATV repeater signals on
421.25. It works great. About 13 db of gain. I got "P5" signals with it
from the ATV repeater until the ATV mavins in the club decided to make
the ATV repeater output vertically polorized. Since then, I haven't
bothered to climb up there to roll it 90 degrees for vertical
polorization. Now the signal is about P1 (and no color) proving the 21
db loss you get with cross polorization of receive and transmit
antennas.
Remember also, that at these frequencies, line loss in the feed line is
a *big* factor. So first connect a bow tie antenna to your set using the
300 ohm "twin lead" that came with it and just hold it up, as high as
you can, with the side of the bow tie facing the ATV signal and see if
you can see anything at all. If you can, *then* start worrying about a
better antenna or buying a down converter. If your TV input has a 75 ohm
coaxial connecter (only) use one of those little 300 ohm to 75 ohm
"baluns" to connect it. BTW, 300 ohm twin lead is *very* low loss at
these frqs as compared to *any* coax, including so called "hardline".
Thats why all the outside TV antennas come with twin lead instead of
coax.
Did you really want to know all this?
... Took an hour to bury the cat. Damn thing kept moving.
--- PPoint 2.00
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