Australian Broadcast TV Channel 5A
For those living in Australia who wish to receive APT
images from NOAA
Satellites from within the 137-138MHz Space Downlink Band, there is
something you be aware of: TV Channel 5A. The
standard TV Channel numbers that Aussies are familiar with, and their associated radio
frequencies, were devised during the 1950's before the advent of
the satellite age. Consequently there wasn't then a concern with
interference to LEO
satellite transmissions, and TV Channel 5A in particular is very badly
placed in radio frequency. For those living nearby a powerful
Channel 5A TV transmitter (e.g. the entire city of Newcastle, NSW),
it's basically a Show Stopper for 137MHz satellite reception.
radio frequency spectrum is regulated by the Australian Communication Authority
(ACA) and they are well aware of the conflict between Channel 5A and
137MHz, as described in a 1997
ACA technical report entitled "Little LEO Satellite Systems:
Potential for Interference to Earth Station Receivers in the Band 137 -
138 MHz". This report concludes that 137MHz LEO reception is
likely to be wiped-out anywhere near a Channel 5A TV transmitter.
B of this report further describes the whereabouts of 5A
transmitters within Australia (circa 1997), with useful maps showing
transmitter power distribution surrounding various 5A transmitter
towers. If one is considering setting up a 137MHz station, it is
worth checking the current list
of Australian TV Channel 5A transmitters (page 5) beforehand.
Likewise if one is already operating a NOAA receiving station and
persistent 'buzzing' radio noise (i.e. the TV 'vision carrier'), and 5A
interference is suspected, tune up to 143.75MHz and listen out for the
FM sound component of 5A. If you hear TV sound at 143.75MHz, you
have a problem.
The good news is that Channel 5A, along with all VHF TV channels, are
on the way out. Digital broadcast TV, and the ACA's long
to protect the spectrum around 137-138MHz, mean that the
present 5A transmitters will gradually be phased out over the next few
years. Even though the ACA technical report above is a little
gloomy regarding 5A interference, there is a very useful antenna trick
which might permit reasonable satellite reception in some
circumstances. Some antennas have pronouned 'nulls'; specific
directions in which they are essentially deaf to radio signals.
If an antenna's deaf spot can be directed accurately at an offending
interfering transmitter, a relatively faint satellite signal might stll
be heard. This is a
that my friend Colin used with his Lindenblad
antenna to beat a powerful 5A signal.
But if you thought that 5A interference was bad, there is another
of radio interferece which is worse still; pagers! For
broad-spectrum radio interference and general offensiveness, the
communciate with portable pagers are difficult to top. There is
no shortage of Web comment
about pager transmitters, so I won't elaborate. But if you
are an intending 137MHz listener, hope very much that you are not
located nearby one of these infernal devices.