Even more about Rhombics – Eric Vast

I read John’s article in the October VS with interest and thank him for the compliment. Quite likely at 88 my memory may be at fault, but I doubt whether the army would have induced me to take all the trouble I did for 12db! I think the promise must have been higher.

In those days it was usual to compare a signal strength with an Isotropic Radiator, i.e., a hypothetical sphere with equal power distribution over the whole surface. Such a one’s power is reduced by a square law of the radius, as it is increased, i.e. 6db for twice the radius.

What all the text books, including the Bell Labs monograph, lack when considering Rhombics, is where you place the phantom isotropic radiator and where exactly you make your measurement of the Rhombic lobe in order to compare the two.  The aerial is a great sprawling device and the lobe may be equally ill-defined until it has travelled some way.  The further apart the measurement is made from the defined position of the Isotropic Radiator, the greater the gain will be.

Consider the brightness of an electric light bulb made equal to that of a laser. You then have the light bulb as the “Isotropic Radiator” ready to be compared with that of the laser. With this set-up you can make the gain of the laser what you like according to the distance you separate them.

I once did a seminar with RCA on aerials and we were shown numerous slides on how they checked their designs with full-scale equipment, I expect the Bell Labs did the same. They used wooden towers in all sorts of positions and made field strength measurements.

Anyway, perhaps a copy of the monograph will turn up one day and all will be revealed. I have a Terman 1st edition which is equally vague, and was issued about the same date. As to single long wire aerials, I had an encounter with these in 1944. Apparantly the UK could not receive the Russian controlled stations at Bucharest, Belgrade and Moscow which worked on medium wave frequencies during daylight hours. It was heard that they were putting out anti- British propaganda. They in fact were not. The authorities thought that we might do better in Egypt and, please they would like a transcript. Somehow it was decided that I should put up three long aerials say 4 – 5 wavelengths long and try.
The longest would be about 2 miles. I was a bit puzzled as to where to aim them as most authorities gave a long wire aerial two lobes, one on each side of where it is pointing. A preliminary run indicated that the maximum signal was exactly in the point where the aerial was aiming.

Accordingly, up went three aerials, each pointing to one of the stations, on poles about 20 feet high, on a very flat peice of desert. This was a sort of pocket of sand with no rocks protruding and might have been very deep. The sand was a mixture of different sized particles that packs well and a sample indicated a dielectric constant of about 3. Conductivity was, as far as I could make out, nil.

We then installed about 20 foreign gents in a hut with typewriters, etc., and the paperwork began to roll. As far as I know, they were still able to receive a good signal up to the time the stations closed down at night.

Since then I have had little faith in double lobe characteristics for long wires.

I discussed this with a professional type who had got caught up into uniform and he suggested that perhaps the received signal was a ground wave with a funny angle of wavefront due to the slower speed of the signal in the non-conducting sand, the wavefront tipping forward as it were!

Who knows?

Eric Vast.

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