By Dan Cranefield, Senior Engineering Manager, BBC Tel OBs
Dan Cranefield recalls the days before satellite uplinks.
During the 1960s, before the days of satellite uplinks, the vision signal from an OB was carried back to the receiving points, usually transmitter masts, by microwave links, often involving several hops as every link had to be line-of sight, with no obstructions in between. The receiving points in London which had permanent receiving equipment were Swains Lane in Highgate, which had a 90ft-high mast, and Crystal Palace where the equipment was about 300 ft. up. These receivers could be frequency-tuned remotely and the dishes rotated remotely of course.
London Tel OBs’ base at this period was the Palace of Arts, Wembley, and there was a separate Radio Links department. There were three sorts of link in use in the mid 1960s which were a BBC designed UHF, an EMI type where the transmitter or receiver was mounted on the front of the 4ft diameter slotted dish and a TRT type where the equipment was mounted at the rear of a similar diameter solid dish.
If the distance from the OB to the studio was short then BT cables could occasionally be used instead but with equalisers installed at intervals. Sometimes the distance from the OB to the BT exchange involved BT vans every few hundred yards! I have known this to happen but this was a fairly rare occurrence. Some regular venues had permanent cabling to the BT exchange.
Depending on the location of the OB the first transmitter could be mounted on a suitable roof, or on an “Eagle Tower” which was a lorry-mounted extendable mast reaching up to 60 feet. There were also originally two “PTAs” each of which had a Merryweather fire-appliance-type ladder which reached to 100ft but this was limited in capacity and rather wind-dependent. An Eagle Tower had to be rigged while the mast was in the vertical position at 30ft and the PTA was rigged in the lowered position using a ladder. So staff, like me, working on Radio Links, had to be fit and not afraid of climbing vertical ladders or of heights! I don’t ever remember being asked about this. I also had to take a BBC driving test because Land Rovers, often with towed 1-ton generators on the back, were frequently needed at mid-points.
There usually needed to be a mid point, or sometimes more than one, if distances or land contours demanded it and this would be on high ground and sometimes a water tower was often used, which are always built on high points. At a mid point the roof of the radio link van could be used in exposed places, or an Eagle Tower if trees or buildings were a problem. The longest individual link I remember was Walbury Hill, outside Newbury, to Crystal Palace which was about 50 miles if I recall. At each mid point a receiver converted the signal back to video for monitoring and then converted it to a link on a different frequency for onward transmission. A generator would be needed, towed to site by the radio link van and a spare generator would be needed if there was no mains supply which would be towed to site by a Land Rover. This vehicle became the crew transport in some cases.
To set up each Radio link each crew had an “RT” set on a frequency of 74.7 MHZ for their communication. Each would be given co-ordinates to the previous or next location and the appropriate frequency to use for each link. When ready to set up the transmitting crew would point in the correct compass direction and the receiving crew would pan around until the signal was found and final adjustment would be done at each end until the signal-noise ratio was at its best and within the standards required. All calculations were done using contour maps and calculations by the office staff beforehand and, in the case of a link or series of links from a site not used before, (and there were many!), a test would be carried out well in advance to check that it worked.
© Dan Cranefield 2019