From the ARRL Contest Update from 12 Oct 2011:
Long may it wave...but not much longer. The transmissions of BBC's station Radio 4 on 198 kHz emanate from a pair of meter-high "valves" that are irreplaceable. Once the remaining store of the big bottles is gone, the broadcasts may well go dark. On the air since 1926 and serving areas out of range of FM stations, transmission moved to 200 kHz in 1934 and then to 198 kHz where it can be found today. (Thanks, Tomas NW7US)