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Registered Member #71
Joined: Thu Feb 09 2006, 08:23AM
Location:
Posts: 63
The internationally recognised frequencies for emergency radio beacons are 121.5 and 406 Mhz (there are some more but forget about that). Although satellite tracking for 121.5 Mhz transmitters will be terminated in 2009, they still work and to my knowledge nothing but emergency signals should be present +/- 6khz around that frequency.
I have a scanner which regularly picks up transmissions on frequency 121.5 Mhz, clearly from Airservices Australia (things like QNH, cloud cover and radar are mentioned in transmissions). And its not a scanner inaccuracy, because I've tested it with amateur transceivers to check that the right frequency is displayed. Also, the close call RF capture doesn't receive anything from the planes, so that may rule out the possibility that the receiver is being swamped.
Why is a government department using an internationally recognised distress frequency as air traffic control? Is it because transmission modes are different? I'm pretty sure EPIRBs are meant to let off a sweeping audio signal..? I'm missing something here, what is it?
Registered Member #71
Joined: Thu Feb 09 2006, 08:23AM
Location:
Posts: 63
The signal is rather loud and clear though, and unless my scanner has one hell of a crappy image response rejection ratio, it wouldnt be. I'm not saying your wrong though, personally I have no idea what to think.
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
I'm almost certain that it is an image response from the scanner. I've heard police communications clear as a bell smack in the middle of the 144Mhz amateur radio band. It took me several days to figure out that it was a scanner issue, and that the signal I was hearing was actually on 154Mhz.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Small handheld scanners probably have really pants image rejection. Doing image rejection right takes big hefty filters, and doing it over a wide range of frequencies takes a huge collection of these big filters. So to make the radio nice and small, they just leave them out.
I used to have an Icom IC-P4E which is a tiny 70cms handie. It was fine with the rubber duck antenna, but would pick up transmissions from ambulance radio as if they were on the local 70cms repeater, when I used it with an outdoor antenna. I tried using a big cavity filter and the problem went away, so I knew it was images.
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Cospas-Sarsat emergency frequency
