'Exstat' aerial transformer

[Panrock]

I attach two pictures of this aerial transformer, found inside a 1934 RGD radiogram.

The terminals 'A' and 'E' go to the set as you'd expect. There is 4 ohms between them. 'E2' at the cable clamp outgoing to the aerial appears to be directly connected to 'E'. Screened feeders were originally used both ways I see...

There is an open circuit between 'W' and 'B' and these terminals are also isolated from 'A' and 'E'. However there clearly are capacitors in there, which would account for this o/c.

What does 'W' and 'B' stand for? To what sort of aerial should they be connected? Maybe a doublet?

Thanks,

Steve

[AC/HL]

I'm not sure, but I believe there may have been another part fitted to the aerial, forming a kind of balun. This matched a long wire to screened cable, and then back again at the wireless end.

[AC/HL]

This is what I mean, although this one is much older (first published 1938)

Antiference are still in existence (I hope!), so they might be able to provide some archive material.

[Panrock]

Thanks. I presume this was a way to 'step down' the signal to a lower impedance, in order to make it reasonably free of capacitative losses and interference during its trip down the garden, then step it up again at the set.

This is the opposite of the practice used for power grid transmission, presumably because then resistive rather than capacitative losses become dominant.

[Synchrodyne]

Hi Panrock, I think what you have is the lower transformer part of an “antistatic” aerial system, which was evidently once quite common in the UK, although unknown in this part of the world. Certainly, “Exstat” was the name that Antiference used for its antistatic systems, as evidenced by “Wireless World” advertisements of the 1950s. I’ll plan on scanning some of these and including in a future posting. Belling & Lee (“Eliminoise”) and Aerialite (“Antistatic” and “Mastatic”) were also active in this field. Aerialite seems to have kept antistatic aerials in its catalogue until the late 1970s at least. The Aerialite entry in the FM/VHF Aerials section of the 1979 Hi Fi Year Book includes the note “Aerialite also manufacture a range of vertical rod type aerials and anti-static systems for AM reception.” This note does not appear in the 1980 HFYB.

As you have surmised, the transformers are essentially impedance converting devices. At the top end, the aerial (rod, long wire, even a TV aerial structure RF-insulated from earth) is connected to an impedance step-down transformer that sends the signal into a screened, balanced feeder; which conveys the signal through the (presumed) interference field to the receiver; the reverse impedance conversion is done at the receiver end. The earthy end of the aerial transformer primary is taken to an earth point by a pathway that ideally avoids the interference field. Where this is not possible, then it can be connected to a centre-tap on the aerial transformer secondary. A corresponding centre-tap on the receiver transformer primary is then connected to the earth point. The balanced cable screen connected to the lower earth point only. Whilst it would be possible to use the balanced cable screen as the earth path from the aerial transformer primary, this would tend to defeat the purpose, as any noise picked up would then be in the aerial primary circuit.

A good explanation of the workings of antistatic aerial systems is provided in the 3rd (1960) and 4th (1963) editions of “Radio and Televsion Engineer’s Reference Book”, and probably in earlier editions, too. And I recall that the Gordon King book, “Practical Aerial Handbook” well-covered the subject (my copy never came back after being loaned out many years ago, and I have yet to find a replacement on ABE, etc., or pre-ABE, on visits to Hay-on-Wye).

Evidently the design challenge for the transformers was to obtain good matching across the band 0.1 to 30 MHz, without undue losses and without resonances.

More recently – say 25 years or so ago – more-or-less the same idea was resurrected with the so-called longwire balun (actually a unun, I think), particularly by RF Systems. However, the execution of these seems to have been less elegant, using coaxial rather than screened twin cables, and in some cases the coaxial screen as the primary earth path.

Also, there was an evident European practice to combine an AM vertical rod aerial with a TV aerial, using the TV coaxial cable as a joint feeder, and with appropriate matching aerial and receiver transformers/frequency dividing networks to get the AM signal into and out of the feeder.

Back to your transformer, the “W” and “B” terminals probably connect to the two wires in the screened feeder. Perhaps it is a simple as the two wires were “white” and “black”, although why correct polarity would be important here I don’t know. There also appears to be a fifth terminal/cable clamp that may also have been connected to a centre-tap on the primary winding.

Cheers,

[Synchrodyne]

Here are some advertisments and other items about antistatic aerials from late 1940s and early 1950s issues of "Wireless World". After that period, aerial ads seem to be oriented mostly to TV and FM equipment.

Cheers,

[Synchrodyne]

And some more.

The September 1952 Belling & Lee "Eliminoise" ad is interesting. From the picture, it seems that coaxial cable is used to connect the lower unit to the receiver, with a change to 4 mm plugs just before the receiver. Possibly balanced-to-unbalanced conversion occurs in the lower unit, with impedance matching taking place at the end of the coaxial cable. Or maybe all conversion takes place in the lower unit, and the relatively short length of coax was not considered prejudicial to performance.

Cheers,