Scottish Information, 119.875 (approx!)
Any air traffic (tels) engineers on here? Flying from Carlisle I use Scottish a lot on 119.875. My aircraft radios have no problems receiving the transmissions for my area, which I believe come from Green Lowther. I'm aware that in order to get large area coverage, multiple transmitting sites are used and these are separated in frequency by a few kHz to avoid heterodynes within the audio spectrum.
At a recent presentation by the Scottish Information FISOs one of the audience mentioned that he could only receive Scottish very poorly, with bad distortion. This started me thinking about the transmitter offsets and I did some tests myself, which reveal that the local transmission is actually on 119.8675MHz, some 7.5kHz LF of the nominal channel centre frequency.
It seems to me that that is rather a lot! With 25kHz channel spacing I assume that the aircraft receiver IF bandwidth would be of the order of 15kHz, in order to provide 5kHz of guard channel either side of each channel. But a 15kHz filter would mean that the local Scottish carrier would be right on the edge (realistically 6dB or so down the edge) of the filter. That's with a 25kHz channel receiver. What about these newfangled 8.33kHz units? No chance receiving on the specified channel I would think.
So I tentatively conclude that the trouble some people are having is because their filters are zapping at least one sideband and perhaps most of the carrier as well. In effect, trying to listen to an SSB transmission without a BFO!
Can anyone comment on this hypothesis? And why is such a large frequency offset required? It would be very interesting to know where the various transmitters are and what frequencies they are on. Finally, has anything changed recently? This seems to be a recent phenomenon.
Thanks for any insight!
John.