Rhombic aerials – Eric Vast

In early 1941 in Cairo, I was given the job of looking after 3GHQ Experimental Signal Section. The previous owner had picked himself a very nice number in the local S.O.E., putting some sort of order into their Signal System. Accordingly I went with him to to the workshop (which had once been a dairy), to see what it was all about. There were some ten lads, mainly ex-Post Office, quite clued up, whose main bread and butter was finding spares which would work in the numerous locally bought transmitters of many different makes.

In any spare time, they were also making two copies of the Post Office Secraphone, a primitive scrambler. Of the fortunate ten there were two regular soldiers, one of whome had been a licensed Ham.

I asked the chap who was leaving, what next?  He said that he had it on  his mind that Signals in the Middle East had not got round to auto, and that in fact, all cypher traffic was handled by the Marconi people’s big station down river.  The snag was he said, that the most desirable arrangement was diversity reception using rhombic aerials which were God’s answer for long distance fixed point to point working.  The Post Office and Western Electric in the States, had at that time gone over to them. I think they had become the work-horse of the long distance stations. However, he said, to set up rhombics you needed the Bell Lab’s Monograph on the subject, and he had been unable to find one in the Middle East.

The rhombic aerial is a balanced job of two wires arranged as the name implies, directional in the way that the long diagonal points.  The four sides can be several wavelengths long and the best results can be obtained by duplicating the wires from each end and spacing them some 15 feet or so in the middle. The height above ground is important, as the angle of the main lobe depends mainly on this.  We used frequencies in the range 6 – 7.5 Mhz for Egypt to UK traffic.

The nominal impedance is 600 – 700 ohms and the distant end has to be terminated in a 600- 700 ohm resistance.  The effective gain in the lobe pointing skywards is of the order usually of some 45db.  It should of course be calculated to suit the angle of the received sky wave.  Nice and simple provided you had the Bell Lab’s Monograph.  The aerial can also be used for transmitting.  Alas, half the power is dissipated in the terminating resistor. However, it hardly matters with 45db gain available.  If you find this loss distasteful, you can stick a similar rhombic on the end, and so on, and the loss is reduced.  The main users apparantly did this, but it does of course use up a lot of land area.

I wrote one of the recently invented aerographs to my Mrs, I asked her to get in touch with my late boss at Western Electric UK, and see if he could find the desired publication.  He was one of those American citizens who disobeyed his government’s instructions to leave the UK when the German bombing started, and he had good contacts. Incidentally the British government were so pleased with those Americans who stopped, that they gave them British passports and open-ended work permits in perpetuity. He came up trumps and a book was received by Mrs V. She then had a go at the Ministry of War and despite being given the run-around, found a little man whose job it was to act on such occasions.  I got the Monograph within a month.

One worry remained, the only masts available were 60 footers. However, on a mound of sand behind Maadi camp, we knew there was a wet highly conducting layer of salt earth some 30 feet down. We assumed that this would make the masts effectively 90 feet high. We really needed a bit more height, but it worked out well.  Meanwhile I had managed a visit to Marconi where they were using curtain arrays and were switching to the highest signal from three such aerials (triple diversity) with a very fast switcher. The whole process could be followed on meters. I was rather shaken by the speed at which the signal did switch, every few seconds, and the amplitude of the fades.  However, we decided to make a start using double diversity and for this start two aerials were erected pointing at the UK. The sides of each aerial were about 250 yards and the separation of the two aerials about the same distance. They were not aligned with each other.

We then acquired two HRO’s and commoned the AGC voltage. When connected to it’s own aerial, the one with the larger signal produced the greatest AGC voltage and pushed the other one’s gain down, reducing noise. Listening tests were very hopeful.  The output of the two HRO’s was fed into a flip flop, after which came a re-perforator which we had acquired. This particular version perforated a new tape and also printed the signal on a secondary tape.

Tests were arranged with the UK using 75 wpm Post Office Standard. On the night we monitored RJS1 and upon switching on at 6pm, out rolled “RJS1 vvv GHP3” repeated many times and followed by a load of cypher. I had a piece of this first tape for many years, but lost it in the move from Coghurst to Hoadswood Road.

The arrangement described went into service straight away. The angle of the lobe had been calculated for two hops from the UK, which we were assured was the expected path of the signal. Later I doubted this, I set up some gear for measuring this angle. My results rather indicated that we received a mixture of two, three and four hops, of which three was usually the largest. A most acrimonious argument broke out later with the UK experts which was never settled.

Some year or so after, the UK people asked for a better signal from our end.

The main station at Polygon, Abassia, had got round to using rather small rhombics as they were cramped for space. By this time we were a separate unit on our own. It was decided to put up a big rhombic on the other side of our camp, away from Polygon, roughly the same size as the Maadi receiving ones. By this time taller masts were available. We used heavy gauge stranded copper wire on the feeders and the aerial. The drive was a 15 KW RCA transmitter which at 600 ohm load means 5 amps at 3000 Volts.

These particular transmitters were fairly common in the Middle East. They were not a compact unit like the Marconi SWB12, but were set out in a fenced area with the various components. They were extremely severe on people who did not set out the component parts exactly to the book of the words, becoming quite unstable. They had I believe been designed for broadcasting, and with their 809/810 output valves at 10,000 Volts and

2 Amps DC, were formidable machines. If an audio frequency run was made, the output stayed extremely steady, but the nominal 2 Amp anode current fluctuated rather alarmingly. One interesting feature was a feedback arrangement of a chain of high resistance 2 Watt carbon resistors in series with a 200 ohm one. The whole chain connected between the common valve anodes and earth. Feedback from the 200 ohm function was taken directly back to the very input of the audio, which was at very low level.

With the aerial operating we at first put the dummy load terminator under the aerial. Here it would have been vulnerable to thieving Arabs and the weather. So a lightish 600 ohm feeder was run back all the way to the transmitter and the terminating load installed next to it. As we were getting a lot of interference locally from the feeder I decided to test it for balance. Four similar RF meters of 10 Amps were acquired and placed in a row in the transmitter area. Two were arranged, one in each conductor of the outgoing feeder to the aerial, and the other two in the two leads of the feeder coming back to the terminating 600 ohm load. To everybody’s delight, the first pair “key down” read exactly 5 amps each and the second pair 3 amps each which was I suppose about right as the second feeder was a bit lossy.

Later we were to run out of 809/810 valves and I had to start re-filamenting them which was difficult.

Eric Vast.

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