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ATV and the Weak Signal/Oscar band plan....



Hello fellas,
     Just read some of the comments on the subject of ATV and the weak
signal stuff and thought you might like some personal experiences...

     Here in the Phoenix area we have had a local 434.00 input to our ATV
repeater for the last 10 years or more.  Problems of interference to the
Satellite users and weak signal guys has been NON EXISTANT.
Interference to the ATV input because of signals from satellite users
were a real problem at one time but have diminished to infrequent at
best.  The local military base radar system does more damage to the
input and to weak signal users  than anything else.
     Here are a few tips that we used in our design that helped to
minimize problems.
1.    Use the opposite polarization.  Most all weak signal stuff is
Horizontal so we used a vertical antenna.  Isolation is better than 20Db
or more.
2.    Use vestigial sideband filters on both the input and output of the
repeater along with a trap for the weak signal prime freq of 432.1Mhz.
3.    We are power limited in AZ and even though we have a special high
power permit allowing us to run as much as 250 Watts output, we have
kept thing reasonable and used a VERY clean and linear amp at about 80
Watts on sync tips.  After feed line losses, the isolator and the
filters get thru with it you have about MAYBE 26 watts reaching the
antenna.  Now to maintain the correct ratio of signals, that puts the
sound carrier at -15 to -18Db down from the picture carrier so you have
about 500 mw. of audio sub carrier with approx. 25 kHz deviation.
Output freq is 421.25Mhz.
4.    Input is filtered in the same way as the output and of course must
keep the usual NTSC video specs.
5.    A typical station from home doesn't use VSB but look at the real
levels of signals when you figure the average user is using about ONE
WATT PEP of picture carrier.  Very few use more than say 10 Watts.  If
we are talking about the one watt level, then the audio sub carrier is
down to between 25 and 30 mw with most of the information in the color
burst area perhaps not more than say 125 mw.  Pretty small potatoes.
     Now let me say that I too am a weak signal user and in fact most of
the TV folks I know are also into satellites and weak signal stuff.
(actually, ATV is also a weak signal mode of sorts, just wider
bandwidth.)  My intent here is to help ease your fears that suddenly all
Hell is going to break out and you won't be able to work the "Birds",
EME, or do any form of weak signal work just because there is a new
"user" within the band. As I have already pointed out, it is more likely
that the TV folks will have a harder time dealing with the level of
interference to their input during a satellite pass than you will have
from them.  As one other fellow mentioned, if you figure that the number
of ATV users is very small even by comparison to the number of weak
signal/ satellite users in any given community of Hams and that ALL
parties involved will be using as directive an antenna as applicable
(typical station here uses a FO-22ATV in VERTICAL polarization) unless
you are pointed at each other or VERY close together physically, you
will probably not know each other is on the air!
     If you take the Phoenix Metro area as an example, there are perhaps
as many as 75 people CAPABLE of operating ATV, and of those, maybe half
are fairly regular users who get on during the weekends a few times a
year and occasionally during the regular "net" times, and compare it to
the number of weak signal folks and fairly regular satellite users, and
figure that we have had NO complaints of interference to the weak signal
/ satellite users over a period of more than 10 YEARS, I believe that
your fears of interference can be put to rest.
     As far as the legality of it, if it wasn't legal to operate an ATV
repeater on 434.00 Mhz, as an input, it would have NOT been approved by
the FCC all these years.  If you look thru the repeater directory under
ATV repeaters and count the number of them who have a 434 input you will
find there are quite a fair number of them.  Are they all illegal?  I
don't think so.  Do they have an interference issue to deal with?  Why
not drop their Freq Coordinator an E-Mail and ASK?  Why not drop the ATV
clubs using 434.00 as an input an E-Mail and ask them directly if they
have had any interference complaints?
     Quite frankly guys, there is always someone who will cry "the sky is
falling" before they have all the facts.  Good and accurate
communication is what we are all about within this HOBBY so the very
first place someone should start is getting the FACTS correct and
maintain an OPEN dialog with each other.  Who knows, both sides in the
issue might just find they have more in common than they originally
thought.  COMMUNICATE!  It is what we are supposed to do best isn't it?
     Just my 2 cents worth of input to help quell a problem before it
starts.
     If you have any questions about what I have just written please feel
free to drop me an e-mail and I'll make an effort to help both sides in
this issue...
     Best 73 to all..
     Hope to "See you" on the air!
     Mike Baker   President.  Arizona Amateurs on TV
     K7DD  DM33VN


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