Scottish Information, 119.875 (approx!)

[The Westmorland Flye]

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.

[mnttech]

As a long time avionic's tech, I think some body's radio or test set is off freq. Here in the states the 25 KHz radios are to meet 30 ppm or better. If my math is correct (big if) 119.875 * 30 = 3596 Hz, or 3.596 KHz which make more sense.

I agree that if the aircraft receiver is off freq, the farther away you are from the station, the worst the received signal will be.

Here in the states, with remote sites, all of the transmitters are on the correct center frequency. Center can key the one closest to the airborne aircraft. Not sure about the other side of the big pond.

Oh, and I think BFO is typically only used for CW communications, not SSB. Or at least that was the way it was when I was doing position reports on HF.

Hope this helps.

[mono]

Ah. This is what's known as Climax operation or offset carrier broadcast. Originally this was not possible under the new 8.33kHz spacing but there are plans to re-introduce it. For more details look here

http://www.icao.int/anb/panels/acp/meetings/acp1/ACP.1.WP.024.2.en.doc

[The Westmorland Flye]

Thanks for the replies so far.

mnttech: I'm using amateur radio equipment that I've calibrated and know to be accurate to within 100Hz or so. It is very evident that the local Scottish information ground transmitter is way off the channel nominal centre frequency, as it would have to be to avoid audible heterodynes with other transmitters in the same service. The approach this side of the pond is that all transmitters are keyed simultaneously.

You're right that the term BFO has rather gone out of common use but the effect is the same - on CW it is used to produce an audible heterodyne, on SSB it is placed at zero beat to replace (insert) the missing carrier. I think the in vogue term for this is now Carrier Injection Oscillator. I've been playing with radio for 40+ years and I'm showing my age I suspect!

I don't think it's to do with the aircraft Rx being off frequency - it's a known fact that the ground Tx is deliberately off frequency. My question was how the magnitude of the delta from channel centre frequency relates to 25kHz channel spacing, Rx filter widths and the eventual onset of 8.33kHz channel spacing.

Do you happen to know what the bandwidth (say at the 6dB points) is for a standard 25kHz chanel aircraft Rx?

mono: Thanks for the link - yes that's the beast and it goes some way to explaining what's going on. I guess that the local Scottish Tx is two "offsets" away from the channel centre.

Perhaps what has happened is that it was until recently only one offset away. Now it's two offsets away, local people are reporting troubles. It still seems mighty odd to offset a Tx by 7.5kHz when the channel is only 25kHz wide. That assumes that the aircraft Rx filter is at least 20kHz wide to accommodate both sidebands of the offset transmission (up to 2.5kHz sideband), which leaves precious little guard space between adjacent channels.

Interesting stuff.

John.

[mnttech]

Just when I think I have this avionics stuff figured out, they come up with something new. :ugh: I have never heard of that type of Climax. Wow, a ground station being set off freq. I understand the why, but it would seem to cause a lot of problems, just as you have stated.

I did some digging here, and found a current radio spec sheet showing 6 dB at +/-8 KHz for the 25 KHz radio, and 2.778 KHz for the 8.333. Hope this helps.

I would have thought that under Climax they would have only used 1 offset, never two.

[The Westmorland Flye]

Thanks mnttech. So 16kHz at the 6dB points - that is about what I thought it should be and it means that with the Tx offset by 7.5kHz, one is only, in effect, listening to one sideband. It only needs the filter to be a tad narrower than the standard 16kHz and the carrier starts slipping down the side of the passband. I think I shall do some bandwidth tests with the offending receivers. I feel a grumble to our CAA coming on! := :ugh:

John.

[13370khz]

Hi !

Interesting post. Perhaps UK AIC 91/2002 (Pink 40) needs updating -
..--..

[Aerials]

Hello all,

TWF provoked me to giving some thought to this and on the few occaisions I have strayed from the 'Ham Bands' to listen on the Av band, I have heard transmitters definitely 'off freq' and wondered why. Now I know! Ashamed to admit as an Avionic Tech that I hadn't heard of this system before.

I wonder why this carrier offset technology is being persued however. Surely one would not hear hetrodyning if all of the transmitters' carriers were synchronised using a standard such as MSF or GPS? In much the same way as 7.5 Khz is outside the receivers' audio passband, so would close on zero beat be.

Aerials

[mono]

It wouldn't matter if you synchronised the carriers so that the phase relationship was identical as it left the antenna. There would be beat notes because the phase relationship at the aircraft would change according to its location and speed. Offset carrier was the only way to go. Older 25khz rx's have a special circuit to deal with the beat notes created (climax squelch circuits) I guess the newer 8.33 units don't. Although if the system is re-introduced they will need it.

[The Westmorland Flye]

Well I got a reply from a Tels guy at NATS and he has confirmed the 7.5kHz spacing. I've sent him my own observations and await his reply with interest!

It seems that there are a few radio amateurs around here... I am G3WGV or N3GV when I'm on the other side of the pond.