Transmitter Operations - Reminiscences
|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). 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!
|The inside of the P.A. compartment of the
250 kW Marconi units. Shown is the coupling 'coil' (1 turn!) that
connects to the aerial 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 other coils used in the BD272
senders, sitting waiting for the next wave change.
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.
|One of the BY1144 valves sitting in its water jacket.|
|The 'new' Sender 83 - a much less interesting blue box!|
That was the modern(ish!) stuff, now for the ancient.
Going back to 1963 when I first transferred to Woofferton from Daventry, the first of the BD272s was just being installed and the sender hall resembled a building site.
When the station was set up in 1943, they used 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! Sender 81 still had a system of ropes and pulleys to alter the PA coil coupling!
Six new Marconi senders, numbered 91 to 96, were installed but 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. Eventually they too were replaced by modern 300 kW transmitters which are capable of remote control with no manual tuning required. However there is one link with the past as the new senders took on the old numbers, so Sender 83 for instance still lives - at least in name!
|The first picture shows a very young me tuning up Sender 85. The triode valves were water cooled and you can see some of the 'plumbing' near my feet.|
|The second picture is Rod Viveash, sitting at the control desk for 85 and 86.|
|The amount of radio frequency energy leaving on the aerial feeders is quite impressive but not all of it used to find its way there, as some leaked out of the cabinets, as illustrated by Rod Viveash holding a brightly lit fluorescent tube flashing in time with the modulation!|
|The programme control desk was entirely manual, there was no automatic switching unit, so it relied on the operator to switch at the right time. This picture seems to have been taken at 18.29 GMT on the 4th. of July 1964. The paper cover over Senders 95 & 96 buttons suggest they were not yet in use.|
Another task undertaken by staff when I worked there was manual slewing of the aerial arrays. The routing of individual senders to the correct array was done by air operated switches.
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. You can probably see from the following pictures that the station is actually in a valley, surrounded by hills, but these are below the 7 degree minimum beam elevation.
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.
If you instead 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.
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. 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!
Now, all this is done by remote controlled air operated switches. This is a very good thing for the people at Kranji. When I visited
there, they told me the hazard in the aerial field is not sheep but snakes and crocodiles!
||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. You have to get permission from the County
Councils to erect tall masts, so with arrays crossing the border, that
must cause wonderful bureaucratic problems!
|A colleague strains with the switching pole.|
Happy Days. Over 40 years on from first working at 'Wof' I reckon I could still remember how to wave change Sender 96 !