Multi station offset frequency VHF cover.

[coalface]

I operate offshore helicopters in the north east of Scotland and regularly use Aberdeen Information on 135.175 and Scottish Info on 126.5. Both, I believe, use multiple transmitters on an "offset frequency" system. Regularly we have problems of lack of reception of ATC transmissions unless we switch our on board radios to the "test" position thus disabling the squelch filter. The problem doesn't seem to be one of excessive range from the transmitters as we often get good reception when seemingly at extemities of published coverage but when we get into the middle of the area of coverage we get into what we call "black holes" for 30 or 40 miles. Is this a recognised problem and what can be done about it? . MOR's have been filed in the past and there is talk of extra transmitters being provided but as the problem often occurs when we are less than 50 miles from the nearest transmitter at all altitudes I don't see how extra transmitters will do much good. Can any radio experts tell me what is causing this problem? Is it an interference problem?

[HEATHROW DIRECTOR]

I'm sure that you must have run the test on different a/c but I'm curious that everything is fine when you disable the squelch. That suggests that the squelch control on the rx is set too high. Come back the days when there was a little knob to twiddle, eh? If you can receive the ground station quite clearly when the squelch disabled it doesn't sound like interference on the actual frequency but maybe a strong transmission on an adjacent channel? (Hopefully one of my Tels colleagues will provide a more knowledgeable answer). The multi-carrier technique of transmitting the same channel from several stations on slightly fiffrerent frequencies (7khz if memory serves) has been employed for many, many years and should not cause this problem.

However... that said, you may be interested to know that some quite serious investigations, including flight checks, are in progress to determine the cause of loss of communications in the London area. A number of pilots have reported losing contact with ATC - and we've noticed it too (got the grey hairs to prove it). It has certainly affected one or two Heathrow frequencies but I don't know which others.

Lastly, one very serious case of QRM in the London TMA, which caused problems on certain frequencies for some time, was traced to....... yes, a baby alarm!!

[Data Dad]

Coalface,

This issue is under investigation. However, it has emerged that the problem only appears to affect one operator - I take it you fly the "blue" ones? If so, I believe that your technical people have/are approaching the competition to see what is different with their comms configuration/operation.

I hope thats Leuchars Radar you are talking to on 126.5 not Scottish Info! :D :D .....and don't forget - if you are receiving on test only - its a Flight Information Service only!

DD

[400 Hertz]

Ah, the magic of the wireless.

In the North Sea area at the moment we are using 8kHz offset freqs for the REBRO system. A clever idea, that does work.

The VHF 20A's that you are using (I guess) may need tweeking. In the pre-rebro days, the squelch thresholds were set differently (higher), so more signal was needed to let you hear it.

If you manage to get into the workshop, try adjusting your carrier squelch threshold to 8uV instead of the old 20 uV. The Carrier/ noise squelch threshold can stay where it is at 3 uV. That should do the trick.

Happy adjusting!

[Spitoon]

Just a bit of lateral thought that 400 Hz may have the answer for.

What happens when 8.33 MHz channel spacing is extended? Will there be interference or are the rebrpo frequencies protected in some way?


:confused: :confused: :confused:

[Flying Adj]

There is a problem with cover in the North Sea Offshore sector RTF due to a combination of extreme range and coverage gaps in the REBROS system. The worst offender seems to be on 131.3 (Sumburgh Radar) as aircraft in the Beryl area are at the extremes of RT cover. The REBROS system (Aberdeen Information) does have "black holes" on the 048 HMR between 80 and 90 miles and further south between the western edges of coverage of the Fulmar and Forties transmitters.

As Data Dad has said these problems are well known and are being addressed, however those who control the purse also control the solution to our problems.

The seemingly good cover at extreme ranges can sometimes be due to Tropo Ducting...a phenomenon of High Pressure systems.

Do however note that the Scottish transmitters are located at Sumburgh and Windy Head (where one of the 134.1 transmitters is located)...so you are often right on the limits of their RT cover too!

We are aware that using "Test" is a pain and is noisy, but a bit of noise and relevant traffic info is surely better that climbing into the unknown. Controllers try to address the RT problems of a/c descending in field by asking for a check passing through a specific level. This is to ensure we don't lose two way before you have transferred your flight watch.

Why not pop across to the tower one day and have a look around. We do have quite a fetching chart of the offshore area showing theoretical RTF cover for Aberdeen Info at 1500 feet, and you can listen in to the freqs and get an idea of where the trouble spots are!

Please bear with us...we get as frustrated as you do!! :)