Re: Radio 4 198 sibilance
Most of the present arrangements are likely to date from the 1980s, when the original 1934 transmitter was replaced and the distribution circuits re-engineered.
198kHz really was an important part of UK civil defence planning then - it was not expected to survive an all out nuclear war, but was to be the main channel of central government communication in any national emergency less serious than that, such as a limited nuclear strike, a civil insurrection or a complete failure of the national electricity supply. It was also used as an indicator of national survival to UK ballistic subs on patrol (though it's a myth that sub commanders would launch a nuclear strike solely on the basis of the transmitter going off air).
Because of this, the programme distribution arrangements are different to those of the standard BBC transmitter networks. They were (and possibly still are) classified, and are likely to involve lots of 1980s defence comms technology. I believe this is the reason why faults are difficult to fix and keep recurring.
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