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Richard Buckby G3VGW

The BBC/Merlin HF station at Woofferton celebrated its 60th birthday in October last year, having been set up in 1943 using six imported RCA 50 kW transmitters, although the ship carrying some of the bits from the USA was torpedoed and they had to improvise to get two of them working. The Engineer in Charge for many years was Laurie Ivin, (G5IC) who used his amateur radio expertise to construct two working transmitters out of the parts that had not gone to the bottom of the Atlantic.

The transmitters as supplied by RCA used two triodes in push-pull in the final. Senders 83 to 86 were complete, but Senders 81 and 82 were a problem as vital parts were missing. One transmitter was built with a "dummy" valve operating in push-pull with a real one! When I was transferred there in 1963, Sender 81 still had a system of ropes and pulleys to alter the PA coil coupling!

For much of its life, Woofferton was a Voice of America relay station, being operated by the BBC on their behalf and earning valuable dollars for Britain. The VoA programmes were received off-air at the now defunct BBC receiving station at Tatsfield, Surrey and sent by landline to Bush House, London and then up to Woofferton. The quality of the received
signal was often awful, so by the time it was re-transmitted by Woofferton and landed in Eastern Europe it must have been dire! Latterly, high quality satellite feeds were used.

Woofferton is situated right on the Shropshire and Herefordshire border, the actual county boundary runs across the aerial field, which must cause a few problems with charging rates on the place! Driving south from Ludlow on the A49 you will soon spot the 250 foot aerial masts on the right as you head for Leominster. The first BBC short wave station at Daventry was built on a hill top site, as it was thought that hills would give a better 'take-off' for signals but it was soon found that a flat site was better, so that you could predict the angle at which signals were sent, relative to the ground. Woofferton is actually in a valley, surrounded by hills, but these are below the 7 degree minimum beam elevation.

The P.A. stage of the 250 kW Marconi BD272 'senders' that were installed at Woofferton in 1963 but are still going strong. The copper tube in the centre supplies 11,000 volts to the centre point of the final anode coil. In this case the 'coil' (1 turn!) is for the 25 metre band. The coil connects to the anodes of the two BY1144 valves ('tubes' if you are reading this in the USA!). The anodes sit in a tank of distilled water, the whole thing sitting at 11kV potential. The water in the tank is heated to boiling point by the waste heat from the valves and the steam produced is taken off to a heat exchanger which is cooled by ordinary water. Under the anode coil you can see the coupling coil connected to the antenna feeder. The coil is mounted on a truck which can be moved by a motor back and forth, so the coupling can be adjusted from the front panel. The anode current taken by the valves is a modest 26 amps!

In 1962 the station was modernised and Senders 81 to 84 were replaced by six Marconi BD272 250 kW transmitters. These are impressive beasts, the pair of triodes in the final running 11 kV at 26 amps anode current.

Wavechanging was fun, you had less than 15 minutes to do it and it involved man-handling very large and very hot coils and a lot of knob-twiddling. Aerial switching was also manual.

Each array had 6 feeders going to it. If you can imagine that most arrays have two separate curtains (called 'bays') connected together by a 'bay' feeder.

Half a wavelength behind the front curtain is an identical one that acts as a reflector.

If you feed the RF from the sender to the centre of the bay feeder, then the beam will be at right angles to the curtain. (This is much easier to explain with a diagram!)

If instead you feed the RF at a different point on the bay feeder, the beam will be offset to the left or right. You can therefore 'slew' the beam by about 14 degrees either side of the natural bearing. So an array intended for a bearing of 114 degrees (for the Middle East) can be slewed to 100 or 128 degrees. Then of course if you feed energy to the back curtain instead, the beam goes in the opposite direction, in this case 294 degrees. (for the U.S. eastern seaboard), plus you can slew it to 280 or 308 degrees.

This one shows the coupling 'coil' (1 turn!) that connects to the antenna feeder via a very large TVI filter. The coil is mounted on a truck powered by a motor so that it can be moved back and forth to alter the coupling to the anode coil. Mounted on the truck are also some Jennings vacuum capacitors. You can see some have a 'knife switch' and are only in use on certain bands. Another thing you have to remember to switch when wave-changing the beast!

The man who was on aerial duties had to go out into the field, (on a bicycle!) and at night all the light you had was a lamp on your hat. Because of the way the schedule worked, quite a few arrays had to be switched at around 03.00 GMT when we stopped transmitting to North America and started to broadcast to the Middle East. Thus we needed to reverse the beam from 294 to 114 degrees.

You waited by a telephone out in the field until you were informed from the control room that the array was off power. Then you unhooked the flexible piece of feeder at the gantry, using a pole with hooks at the end and hooked it on to the feeder for the new bearing. You cannot leave the other feeders simply disconnected as this would upset the array, so the unused feeders have a short circuit placed across them. If you place a short circuit on a feeder, then a quarter of a wavelength away it appears as an open circuit, so is effectively invisible to the RF energy. This was achieved by moving a 'shorting pole' around.

This picture shows the other coils used in the BD272 senders, sitting waiting for the next wavechange. The two on the left are the two halves of the anode coil for 41& 49 metres. They fit in place of the 31/25 metre band coil that was show in the previous pictures. The fat coil to the right is the single coil for 41/49 metres that fits on to the truck that carries the coupling coil. The small coils on the right are used in the preceding 'driver' stage of the sender.

Doing this at night was quite tricky. To add to the problem, I got quite nervous out there as with the wind making weird noises blowing through the wires it was quite 'spooky' and to make it worse, sheep grazed in the field and your lamp lit up their eyes!

Now, all this is done by remote controlled air operated switches. This is a very good thing for the people at the Kranji station in Singapore. When I visited there, they told me the hazard in the aerial field is not sheep but snakes and crocodiles!

Being built in wartime, the station incorporated precautions against enemy bombing. Each pair of senders was in a separate cubicle surrounded by very thick walls. The roof was hinged so that if a bomb fell in one cubicle, the roof would open and the blast would be dissipated upwards, leaving neighbouring senders unharmed. This was the theory but was never put to the test. However when the walls were due to be demolished to make way for the new senders, the contractors had a terrible job knocking them down!

The RCA Senders 85 and 86 survived for several more years. Anyone familiar with the old HRO receiver would have felt at home, as they were festooned with those wonderful HRO dials. The station has since been modernised once more and now has the latest 300 kW transmitters which are capable of remote control with no manual tuning required.

This photograph, circa 1965, showing when we used to have to switch antennas manually. Each array had 6 feeders going to it. If you can imagine that most arrayshave two separate curtains (called 'bays') connected together by a 'bay' feeder. Half a wavelength behind the front curtain is an identical one that acts as a reflector. If you feed the RF from the sender to thecentre of the bay feeder, then the beam will be at right angles to thecurtain. (This is much easier to explain with a diagram!) If you instead feed the RF at a different point on the bayfeeder, the beam will be offset to the left or right. You can therefore 'slew' the beam by about 14 degrees either side of the natural bearing. So an array intended for a bearing of 114 degrees (for the Middle East) can be slewed to 100 or 128 degrees. Then of course if you feed energy to the back curtain instead, the beam goes in the opposite direction, in this case 294 degrees. (For the U.S. eastern seaboard) Plus you can slew it to 280 or 308 degrees. The man who was on aerial duties had to go out into the field, (on a bicycle!) and at night all the light you had was a lamp on your hat. Because of the way the schedule worked, quite a few arrays had to be switched at around 03.00 GMT when we stopped transmitting to North America and started to broadcast to the Middle East. Thus we needed toreverse the beam from 294 to 114 degrees. You waited by a telephone out in the field until you were informed from the control room that the array was off power. Then you unhooked the flexible piece of feeder at the gantry, using a pole with hooks at the end and hooked it on to the feeder for the new bearing. You cannot leave the other feeders simply disconnected as this would upset the array, so the unused feeders have a short circuit placed across them. If you place a short circuit on a feeder, then a quarter of a wavelength away it appears as an open circuit, so is effectively invisible to the RF energy. This was achieved by moving a 'shorting pole' around. They are the white poles in the picture, they also act to ground or earth that feeder at that point. Doing this at night was quite tricky. To add to the problem, I got quite nervous out there as with the wind making weird noises blowing through the wires it was quite 'spooky' and to make it worse, sheep grazed in the field and your lamp lit up their eyes! The switch in the picture is that for array 807 which used to fascinate me as it turned out that one of the shorting poles had to be exactly where a stream crosses the site, so they had to build a bridge for it! You can see the bicycle too. The stream actually forms the boundary between two counties, Shropshire is on the left and Herefordshire on the right.

With the end of the Cold War, the VoA reduced its overseas broadcasts and as the main function of Woofferton was to relay their signals through heavy jamming into Eastern Europe, its VoA role has considerably reduced and there was surplus capacity. This in part
prompted the decision to close the Daventry HF station and transfer its BBC World Service transmissions to Woofferton.

I was lucky enough to get an invitation to the 60th. birthday celebrations at Woofferton short wave site on the 17th. of October. You will recall that it used to be a BBC facility before the HF sites were sold off to Merlin Communications - a management buy-out. Since then Vosper Thorneycroft have acquired the company. I had been to the 50th. birthday celebrations and really could not believe that 10 years had passed - I must be getting old!

When I worked there first, in 1963, the then new Marconi BD272 250 kW were just being installed. I was very surprised when I got there in 2003 to see that they are still there and going strong and still looking very modern!

What has changed is that when I was first there, we had 9 people on each shift. Now there are just two people on the night shift.

The Marconi units need a minimum of 15 minutes to change them on to another band, as it involves changing several sets of heavy and hot coils. Thus the Woofferton schedule is now designed so that where possible they are operated on one band for long periods, with wave changes reduced to a minimum.

As it turned out there WERE a lot of old faces there, including one chap whom everyone thought had died! It was quite a gathering.

73, Richard G3VGW @ GB7NOT (Ambergate, Derbyshire, UK)

Click here for more photographs of the BBC Woofferton transmitters

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