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Cables and prop shafts are a windup
© Chris Youlden, 2013

On Friday 1st May 1970 we left our base at BBC Bristol to travel to Exmouth for the recording of Heat 2 of the series It's a Knockout to be transmitted on BBC1 the following Wednesday, 6th May. This was one of the most entertaining programmes that I worked on. The series was notable for the contestants being dressed in absurd large rubber suits. The winners of each heat went on to take part in Eurovision's Jeux Sans Frontieres.

This series was made in colour given that BBC1 had commenced transmissions on UHF the previous autumn. This put an extra demand on our ancient S T & C microwave equipment. Designed for monochrome use, the valve-based equipment had to be tweaked carefully to avoid non-linear operation giving rise to poor differential phase and gain figures. We had been used to setting a relatively high deviation of 8Mc/s (I'm using English parameters as these were current at the time!) for the FM equipment, since this helped noise figures over some of the paths we used. However, this was reduced to around 4Mc/s because of non-linearity.

The receivers consisted of a head unit which was mounted behind the dish and which required a feed to be screwed into it through the dish. If there was no head unit behind the dish with a feed attached there was a possibility of a 4 foot diameter dish being caught in the wind. It was a sight to see if the wind ever caught such a dish at a certain angle. It would take off like a flying saucer and if unlucky smash itself into fibreglass bits.

Down in the van was a control unit and a discriminator unit. It was possible, by tweaking drift tubes volts carefully, to slew the frequency slightly so as to improve the non-linearity whilst watching a differential phase or gain waveform. Another successful way of bringing differential measurements into limits on a multi-hop link was to look at the individual measurements on each hop and cross-feed them so that problems cancelled out. Inverting amps were also available for a similar purpose.

This could take some while and often end up with one vision circuit in tolerance and the reserve link way out of tolerance. But it was an exercise we were required to make even though it would cost an extra day's setup time in many cases.

We travelled and parked up on Friday. First thing on Saturday morning setting up the link began immediately, because Exmouth was a 3 hop link.

The OB site at Exmouth fired the signal up to the first midpoint at Mamhead Obelisk, Haldon and then to Cothelstone Hill on the Quantocks near Bridgwater. From there the signal was routed into the masthead receivers dishes at Wenvoe which can be seen in this mb21 photograph.

The first midpoint at Haldon was a picturesque place. Set in woodland by an obelisk it was situated on the edge of a ridge with magnificent views out over the Exe estuary past the Jurassic Coast and Lyme Bay, sweeping around to the Quantocks and Exmoor. It was a site I used to take my girlfriend to at that time - she liked to see the obelisk apparently. However, the radio links van could not get to the edge where the two tripods were ground rigged so parked almost 200 feet away. As there were two receivers and two transmitters, four lengths of 200 foot BICC polypole cables were required. The excess lengths of each cable were laid in the time honoured tradition of figure of eight coils in front of the van.

Whilst setting up the first hop, interference was seen on the receivers. Whatever we did would not get rid of it. There was some scratching of heads as to the source. We were asked to switch off at the OB and the rest of the link looked at the output of Haldon's receivers. Suddenly the RT crackled - they were seeing an upside down tv picture. The penny dropped. Link frequency allocations in the 7GHz band had resulted in the receivers being close to those of the Eggardon Link On this occasion the solution was to unofficially shift the first hop frequencies away from the licensed spot frequencies we were supposed to use. Such was the beauty of having tuneable SHF gear.

Typical BBC Radio Link in 1989

A typical BBC Radio Links midpoint in 1989

It was the practice to employ a night watchman at remote sites like this. They would usually arrive in a camper van and watch the set up throughout the night. On this occasion a young man arrived in a small van with few facilities - he was shown the rig and the engineers left for the night.

We arrived early on Sunday morning at the OB site to start the line-up for transmission. Then all hell let loose. We heard there was a crisis at Haldon which required every spare length of polypole available. We were the only site with spare cable since it would take too long to obtain more cable from Cothelstone or Bristol. It was fortunate that we had a van roof rig at the OB site using only 30 foot polypoles, so our compliment of two 200 foot cables were free. I was despatched with these two reels of cable in short order as transmission loomed.

On arriving at Haldon everything became clear. The night watchman was ashen. His van was parked on top of the polypoles. He had left the site late the previous evening to purchase a pint of milk. On returning he unwittingly parked on the cables in the darkness.

As he went to leave the site in the morning the vans wheels spun on the damp cables and grass. Foolishly he revved up and ended up with the cables wrapped around the prop shaft. Result: four stripped cables and a badly damaged van.

The cables I delivered enabled a single vision channel to be set up and so we made it to air, but with no reserve until replacement cables arrived.


mb21 by Mike Brown
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