Multi station offset frequency VHF cover.
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?