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.

Australian 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.  Attachment 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 noticing a 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 term desire 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 trick 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 source of radio interferece which is worse still; pagers!   For broad-spectrum radio interference and general offensiveness, the transmitters that 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.