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Birth of the BT Tower

The Telecom Tower in London, previously the Post Office Tower, is regarded as the hub of the BT microwave radio network, but the decision to build the tower was made some time after the Post Office started providing microwave radio links.

There was no initial plan for a nationwide microwave network. The network was built up piece-meal, the first link being from London to Birmingham in 1948 to enable the BBC to expand its TV service to the Midlands. This link would normally have been provided by coaxial cable because in those days, all inland communications were provided by cable, radio being used only for overseas communications. The London - Birmingham TV link was provided by radio because at the time there were no spare coaxial tubes between the two cities and more importantly, for much of the route no spare duct space for new cables. The BBC required the link to be from London’s Broadcasting House to their Birmingham Studios. Museum Telephone Exchange and Birmingham’s Telephone House were the most suitable buildings within easy cabling distance of the BBC buildings, and which could accommodate towers and equipment cabins on the roof.

The link was not seen as the fore-runner of a new radio network, and when the BBC wanted to extend to Manchester, coaxial cable was used. I am not sure of the chronological order of the next BBC expansions, but links to Wenvoe for S.Wales and the West Country, and to Tacolneston for East Anglia were provided by cable. The next radio link was from Manchester to Kirk o’Shotts for Scotland, this link also feeding Pontop Pike for NE England and Sandale Fell for the north west. Southern England was served by a "direct pick-up" link which consisted of a station near Alton in Hampshire which, with a 300 ft mast picked up the BBC Alexandra Palace signal directly "off air" and relayed it by microwave to the BBC transmitter on the Isle of Wight At some time thereabouts the Eurovision radio link was provided from Tolsford Hill near Folkestone across to Lille. This was linked back to London by cable.

Then came ITA (now ITV). Most of the early provision was by cable, radio being used only for London to East Anglia, Carlisle to Scotland and Northern Ireland, Bristol to Plymouth and Southampton to the Isle of Wight..

Thus we had a number of disjointed radio links connected together by cable with no apparent plan for a national radio network. Then came Backbone, a system planned to provide secure communications between strategic Government locations avoiding cables which, by the very nature of the network, passed through important towns and cities. The Backbone network deliberately avoided built-up areas and thus did not provide any basis for a national city to city radio network. Top management in the Radio Branch were somewhat irritated by this.

Then came, BBC2, 625-line colour television. The then current 0.375 inch coaxial cable systems had a bandwidth of 4Mhz whereas 625-line colour needed 6Mhz. These were pre-digital days. Upgrading the coax systems would have entailed reducing repeater spacing from approximately 6 miles to 4. There was under development a 12Mhz system using 1inch tubes but there was no way that this system could be in place in time to meet the BBC’s time scale. SOS to Radio Branch. Could microwave radio handle 6Mhz. Answer "Yes", so plans were at last laid for a national city to city microwave radio network. The Government also authorised the transmission of BBC1 and ITA in 625-line colour, thus requiring at least three channels per link.

Next question, if a national radio network were established for TV, would adding telephony channels to radio links be cheaper than laying more cables? Cost studies showed this to be the case. Hence long term forecasts were prepared for circuit capacity requirements for 20 years. These forecasts revealed that forty aerials would be required at London. This number obviously could not be accommodated on the roof of Museum Telephone Exchange. The alternative was a purpose built tower. By now tall buildings were beginning to arise in London, so links from a central tower could be in danger of obstruction. The then Ministry of Public Buildings and Works, which in those days handled all GPO building works, forecast with great confidence that it would be uneconomic for anyone to build commercial buildings in excess of 300 ft. To be on the safe side it was decided that the lowest aerials should be at 355 feet. Such a tower was obviously going to be very costly and so the alternative scheme of four towers of lower height and lower capacity on the outskirts of London, with cables into the centre, was considered. Cost comparisons showed the central tower to be the most economic. The chosen location for the tower was near Museum Telephone exchange because a television switching centre had been established there and all local and regional links, both cable and radio terminated there.

The application for planning permission to the then GLC was referred to the Government which sanctioned the go ahead provided that some public facilities were provided at the top of the tower to make it a public landmark. The Government did not however, offer any financial contribution. This threw the cost comparison out of the window but the scheme went ahead despite that. Unfortunately of course, access for the general public was withdrawn some years ago due to a bomb attack.

The conclusion from all this is that we have mainly to thank the coming of 625-line colour television for the existence of BT’s microwave radio network and the BT Tower. It would be interesting to know what proportion of London’s telecommunications traffic is now carried by the Tower in the present digital and glass fibre era.

Optional additions:
A similar cost comparison resulted in the central tower in Birmingham, albeit without the public facilities. For each of the other cities a single tower on the outskirts was provided except at York and Darlington, where towers were provided on the exchange buildings. I do not know whether they are still there.

There are now many buildings in London that are well over 300 feet in height. I can only assume that digital techniques are able to overcome any problems these may cause.

Formerly of the pre BT Inland Radio Planning & Provision Branch
Engineering Dept
PO Telecommunications Headquarters

Construction of the BT Tower, London, c1963

Picture location: London, c1963
Date: 1965
Sent by: LRN Mills
Category: The network and street furniture

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