At the end of World War II overcrowding in the medium waveband was becoming a problem and it was apparent that only a move to a higher band would solve it. This would also give the opportunity for higher audio quality and a wider frequency range to be broadcast as any new plan wouldn't be restricted to a 9kHz channel spacing. 

Experiments in the USA had already indicated a possible improvement if FM (frequency modulation) were to be used instead of AM (amplitude modulation). A brief series of tests on ch1 from Alexandra Palace (45MHz) appeared to confirm this, however, in view of the enormous implications if a wrong decision was made, it was decided to set up a full comparison between AM and FM at the new frequency (Band II: 87.5 - 108.5MHz).

After some theoretical and practical site testing, it was decided to establish an experimental high power station at Wrotham in Kent and construction began in 1949.

Tests to compare the relative merits of the AM and FM systems were run for several years with the same programme on each, generally the Home Service and Light Programme on alternate days with the Third Programme each evening. AM/FM receivers were provided to various staff members, the trade, and others and eventually FM was chosen as the superior system. It was then decided to start covering the whole country with a three programme VHF/FM service starting with Wrotham in 1955.

With the availability of stereophonic programme material in 1958, experimental broadcasts took place after regular closedown, using two of Wrotham's transmitters. The Third Programme transmitter broadcast the left hand channel on 91.3 MHz and the Home Service transmitters broadcast the right hand channel on 93.5 MHz. This developed into regular transmissions (still officially experimental every other Saturday morning with Third Programme transmitters (VHF and MF) broadcasting the left-hand channel and television sound transmitters broadcasting the right-hand channel. These broadcasts went on for several years while much thought was given to a stereo coding system. Once the Zenith-GE system was chosen, experimental broadcasts were made from Wrotham's Third Programme transmitters from August 1962 until regular public service started in 1966.

Above: The original Wrotham mast showing the cylindrical slot aerials producing the horizontally polarised signals