The Empire Short-Wave Service 

23 September 2022 tbs.pm/76392

A Talk by the Chief Engineer of the B.B.C., Broadcast from the Short-wave Station at Daventry on February 28, 1934

From World-Radio for 9 March 1934

PREVIOUSLY I have made some remarks about the distribution of British short-wave receivers overseas, and some remarks concerning requirements of short-wave receivers in general, mentioning some undesirable features which existed in certain types. One listener, who, incidentally, lives in England, heard parts of this, but not all of it, no doubt due mainly to the fact that at short distances fading is nearly always severe and erratic. He heard the remarks about the distribution of British receivers, and about the undesirable features possessed by certain receivers, and had linked the two together and come to the conclusion that I was pointing out short-comings in British receivers. It seems hardly necessary to point out that I was not doing anything of the kind, but in case the same thing should have happened to any other listener, I thought it advisable to mention the matter.

Put very briefly, the real object of my previous talk was to suggest that listeners should obtain the best possible receiver they could afford, because a really good receiver was more important for short-wave than for ordinary broadcasting.

Automatic Volume Control

You may remember that I mentioned one or two matters which I could not talk about owing to lack of time. One of these was automatic volume control, and I propose to say a few words about this now. Perhaps some listeners may not be very familiar with automatic volume control, or A.V.C., as it is usually called, so that I will begin by explaining its main object.

Everyone knows that short-wave reception is usually troubled by fading. This fading is due to changes in the field strength arriving at any particular point from a distant station, which are normally equal for the whole band of frequencies sent out by the transmitter. On the other hand, it may be of the differential type, by which is meant fading which is not the same, both for the carrier frequency and all sideband frequencies. In other words, all the frequencies sent out do not fade together, maintaining their normal relative strengths. Except during certain periods of very peculiar fading, which can be neglected, automatic volume control gives a very definite improvement, in fact, when the mean signal strength available is high enough, it is almost an entire cure for the first type of fading. The second type — i.e. differential fading, however, is not completely curable by this means.

Mr. Noel Ashbridge, Chief Engineer of the B.B.C.

It is not practicable to describe in detail over the microphone methods of applying A.V.C., but the general idea is entirely simple. There is usually an extra valve, across the grid and filament of which is applied a voltage proportional to the strength of the field arriving from the distant station. When the voltage on this valve tends to decrease, the anode circuits are so arranged as to cause an increase in the gain or magnification of the main high-frequency amplifying valves, the latter being of the variable-magnification type. Thus, there is a tendency to counterbalance changes in strength of reception – i.e., a decrease in field strength by greater magnification, and vice versa; the final output from the detector valve being theoretically constant.

Thus, if one is listening on a good receiver with this system of working, instead of hearing the strength rise and fall, one hears a constant signal with an increase or decrease of the background of atmospheric noises. The reason for the latter is fairly obvious, because as the magnification of the high-frequency stages goes up so does the background go up, and vice versa. Put in another way, the signal-to-noise ratio is constantly changing in sympathy with the changing field strength.

Unfortunately, however, things don’t work quite so simply as this, because at times the signal may fade out practically entirely, in which case all one hears are loud atmospheric noises produced by the receiver making frantic efforts to produce enough magnification to receive a station which, for the moment, has become un-receivable. Again, the receiver may not, in any case, have enough gain to admit of an increase and decrease above normal strength of reception, which makes it necessary to work at full sensitivity all the time. In this case, all that automatic volume control can do is to prevent the receiver being badly overloaded, when, due to the effects of fading, there is a temporary increase of strength from the wanted station.

As a matter of fact, several receivers embody an automatic volume control which always works on this “downwards only” principle. It is somewhat analogous to the old limiter, which was used many years ago to prevent any signal rising above a certain value — the object being to reduce the effect of jamming when an unwanted station was stronger than the wanted one — but this was used for telegraph stations mainly. Such a type of volume control is rather more useful than it sounds at first, and it is, as a matter of fact, in fairly common use.

The supervisory position in the control room, Broadcasting House

On the whole, there can be no doubt whatever that A.V.C. is highly desirable, and greatly increases the pleasure of listening, and the intelligibility of what is said. Of course, the alternative is to control the volume by hand. This is quite practicable when conditions are good, and fading is slow and regular. At other times, however, it becomes irritating. When A.V.C. is used in conjunction with spaced or diversity aerials, the results can be nearly perfect, but such aerials are hardly suitable for the average listener.

Aerials

The other subject I mentioned last month, but did not say very much about owing to lack of time, was aerials. It is very difficult, if not impossible, to say what kind of aerial should be used, unless one knows all about the receiver with which it is going to work.

In the course of the past year I have tried a number of short-wave receivers. Most of these receivers have so-called aperiodic aerial circuits, in other words, the aerial is coupled to the first tuned circuit very loosely, so that it does not affect the tuning of this circuit to a great extent. This type of coupling is very convenient where ganging is used. It is true, of course, that variations of the aerial capacity are frequently compensated for by a trimmer condenser, but this, as a rule, does not compensate for large changes.

At any rate, when working with sets embodying this arrangement I have found that the longer the aerial the better the result. In fact, aerials with a horizontal portion up to about 300 feet seem to be quite satisfactory in some cases, and it even seems to be advantageous to bring the far end of the aerial down towards the earth in order to increase the capacity, keeping the middle of the horizontal part as high as possible. Such an arrangement has of course a tendency to be directional, nevertheless, apart from this, it seems to give a great increase of signal strength as compared with that obtained on the ordinary broadcasting aerial about 100 feet long, moreover it does not seem to interfere with the selectivity of the set. All this does not sound very scientific, but it seems to work in practice, which is probably due to the fact that a high-capacity aerial is more suitable for the very loose coupling to the first tuned circuit.

No doubt many listeners have considered using a special type of aerial such as a vertical half-wave with or without a reflector, tuned to one particular wavelength. Such an aerial may be inclined or vertical, or it may be in the form of a horizontal dipole. Of course, there can be no doubt that such aerials give a very large gain over what may be called the ordinary unscientific kind of aerial which we have just been discussing, but they give no advantage unless the circuits in the receiver are suitable. Some manufacturers provide for this and I have seen advertised coupling units to make ordinary aerial circuits suitable for half-wave aerials of various types.

Before leaving this subject perhaps I should mention that it is not much good putting up a half-wave aerial near to some object which might prevent its working as such. For example, an aerial which has been erected near a house probably will not tune to the wavelength which it should according to its dimensions. This is an additional reason why a special aerial sometimes seems to give no better results than the more rough and ready kind. Where, however, there is plenty of space, and the aerials are properly erected, there is bound to be a very considerable gain from the scientific variety.

It is very difficult to include all one would like to say about receivers for short-wave broadcasting, but if any outstanding point arises from correspondence with overseas listeners I will include it in a future talk.