BBC Radio 4 long wave, which transmits on the 198 kilohertz frequency, relies on ageing transmitter equipment that uses a pair of the valves – no longer manufactured, to function.
The 198Kilo Hertz transmitter (1500metres in old money) uses 2, 1metre high Thermionic Valves. These Valves, which can last from 1 year to 10 years (pot luck on the quality as they are donkeys years old)
The BBC has bought up the last ten in existence.
Nobody make them any more and haven't for years, thus the skills and technology has been lost. The BBC tried to get some made in Germany, but they couldn't Guarantee to match the quality of the old ones.