History
Along with Fairseat, Harrow Weald and Kelvedon Hatch, Bagshot served as the "first hop" from London along a route which was planned in the late 1950s but only completed once the Post Office Tower was operational in the late 1960s. Bagshot differs from the other three as the site is the point at which two of the new routes diverge, towards Southampton or the main route to the west.
A 1959 network map shows a direct link between London (Museum) and Stokenchurch was "planned", along with those towards the other three "first hop" sites, some of which were already in use for early televison links. A fifth route, via Golden Pot to the Isle of Wight was used for television distribution but it seems from correspondence between the IBA and Post Office that this was not entirely satisfactory and the 1959 map shows a "programmed" route running south west from London, turning south, and then west again towards Southampton. This appears to correspond with the Bagshot/Butser Hill/Thornhill route.
Perhaps to reduce the number of antennas needed on the central London tower the route to Stokenchurch was diverted towards Bagshot when the network was eventually built. Possibly it was realised the Southampton route was unlikely to carry significant traffic in comparison to that serving Bristol, Wales, the South West and the international links via Goonhilly.
A 1967 article in the Post Office Electrical Engineer's Journal, discussing the use of horn antennas, comments: "In 1960 it was decided to introduce...horn aerials capable of carying more than one band of microwave frequencies. [...] the first fully-loaded route, operating in the 4 and 6 GHZ bands, is being taken into service between the Post Office Tower, London, and Bagshot radio stations, carrying the London-Bristol and London-Southampton microwave systems".
The article hints that horn antennas would normally be used in pairs, therefore it would seem Bagshot originally carried six horns to cater for the split routes.