Motor-Car Radio 

30 January 2023 tbs.pm/76866

 

Cover of World-Radio

From World-Radio magazine for 1 September 1933

The development of radio receiving apparatus during recent years having reached a point where technical efficiency and compactness can be combined in a practical manner, considerable attention is being given at the present time to the production of receding equipment for use in motor-cars — “automotive radio” as it is termed in some quarters.

Type of Installation

On the technical and practical sides, the introduction of a radio receiver in a motor-car brings with it some new problems of a nature quite distinct from those usually encountered in the type of apparatus used for normal home use. Possibly the first problem is that of shape and size. Cars vary so very much themselves in actual size that it is very difficult to produce a standard type of receiver and guarantee that it can be successfully installed in any car on the market. Nevertheless, practically all the motorcar receivers which have so far been produced commercially have been designed for permanent installation in the car.

Motor-car receivers were first introduced about three years ago in America, and the tendency, now that they are gaining sonic popularity here, is to follow American design. At the same time, however, we must not lose sight of the fact that conditions over here are different in a number of ways, and there are indications that the permanent installation may not prove to be the most popular type of receiver for every type of car. The owner of a small car, for instance, will probably not favour the idea of extra expenditure on a receiver which will only be of use when the car is actually being used and will have little or no value at other times. Some type of transportable receiver, of the correct dimensions for use in a small car, seems to be indicated here, because it would have the advantage that it could not only be used for the car but could also be used elsewhere. On the other hand, the owner of a larger type of car will probably not object to the extra expense of the second receiver, and a permanent receiver solely for use in the car could be used in this case.

 

Courtesy of British Pathé

 

In passing, let it be said that quite excellent results can be obtained in a car, providing there is sufficient room, merely by using an ordinary portable receiver in conjunction with a small aerial wire in the car itself and “earthed” to the car chassis. The object of the aerial wire is not altogether to increase signal strength — which it will nevertheless do to an appreciable extent — but to help to nullify the directional effects of the frame aerial in the portable receiver. Without this wire, signals are liable to disappear on turning a corner!

Although not very much work has been done in this direction in the past, there is really no reason why the home constructor should not attempt the construction and installation of a receiver for use in the car. The following remarks are intended to serve as a guide to amateurs who would like to undertake this new type of radio work, and it is hoped these notes will perhaps form a basis from which the amateur will be able to undertake the design and construction of his own car receiver. There arc no fundamentally new principles to be encountered, and any amateur who has had some past experience with the more normal type of receiver should be able to undertake the construction of a comparatively efficient car receiver and obtain good results.

The essential parts of a motor-car installation usually come under the heading of four separate units — the receiver itself, the high-frequency collector system, the reproducer, and the power supply. Technically, of course, the receiver is usually very similar to any standard type of receiver, the noticeable difference usually being in the size and shape of the actual container. These receivers usually employ from four to six valves, a powerful receiver being necessary owing to the limited permissible size of the aerial system, and also to some extent, to the shielding effects produced by the car chassis which tend to cut down the effectiveness of any practicable aerial system.

Special valves have been produced in America which render the design and use of a car receiver considerably easier, these being of the indirectly-heated type and taking about 0.3 ampere at 6 volts, thus enabling the valve filaments to be wired up directly to the car accumulator. The use of an indirectly-heated filament makes the valves less liable to breakage due to vibrations, and lowers the possibility of microphonic noises. Such valves, however, are not generally available over here and so, as far as the home constructor is concerned, some other method must be found of supplying the necessary voltage to the correct type of valves.

Power Supply

A man sits in a strange motorcycle wearing headphones

Police motor-cycle combination equipped with wireless receiver. [By Courtesy of Marconis W.T. Co. Ltd.]

In considering individual installations, probably the first question to be considered is how the necessary power for the receiver is to be obtained. There are a number of alternative methods, and on the method chosen depends the size and bulk of the receiver as a whole. Assuming that a minimum of four valves will be used, some of the available methods of obtaining the necessary amount of high and low-tension current are as follows: (a) Using the car battery to operate a generator which will supply the necessary high-tension current, a portion of the car battery being tapped off to provide filament current; (b) Tapping the car battery for filament supply and using a separate high-tension battery of the usual type; (c) Using an entirely separate set of batteries for both high and low-tension.

The latter method may sound rather wasteful when we have a more or less permanent source of current which can very easily be utilised for filament heating, but it has the very big advantage that it enables the receiver to be taken out of the car more easily and made use of on other occasions. The receiver and the batteries, for instance, could be in separate small cases connected by a multi-cable, and the two units employed as a transportable receiver, for use either outdoors or in the home. On the grounds of economy, this method would prove useful in many cases.

It may appear to be somewhat paradoxical to consider the power supply first and the receiver afterwards, but the fact that the type of receiver which can be used depends to a large extent on the power available, justifies this. The receiver itself provides a fair amount of scope for design, as we may make use of almost any of the usual types of circuits in normal use, providing sufficient care is given in deciding which arrangement will be the most suitable in individual cases.

Use of Headphones

The car user who does a large amount of “solo” travelling would perhaps find his requirements fulfilled by the use of a small two or three-valve receiver and a pair of headphones, but this raises the debatable point as to whether or not this arrangement would prove dangerous to the good driving and management of the car. In most cases, the driver would probably find that the use of a pair of headphones would actually be a hindrance and possibly not a little dangerous, but such an arrangement would probably be found very attractive for a passenger, and on the grounds of simplicity and cheapness a small receiver of this type would be an attractive proposition for the home constructor. However, owing to the severe limitations of a receiver of this type, there is not very much likelihood of it attaining a very big popularity.

Actually, the receiver must have a high level of sensitivity if it is to operate a speaker which will be heard comfortably above the general traffic noise level. Actual tests in a car have shown that a five-valve portable receiver, using five three-electrode valves and a frame aerial, will produce a greater strength than a four-valve receiver using a screened-grid and pentode valves with an aerial wire slung inside the car. It will be realised, of course, that it is impossible to say just how many valves will produce a satisfactory degree of signal strength, owing to the fact that the receiver will be moved about from place to place. The receiver will be used under all types of conditions, requiring a good degree of selectivity when used in the neighbourhood of a powerful Regional station, together with a satisfactory degree of sensitivity when in use at some distance from a transmitter. Signal strength will vary according to the presence of, for instance, a railway bridge, a large building, or when travelling perhaps through a thickly-wooded district.

 

Courtesy of Motorola Solutions

 

It will be seen, therefore, that a car receiver, to be really satisfactory, must be capable of putting up a good performance under really extraordinary circumstances! It will probably be advisable to concentrate, therefore, on a minimum of five valves for the average car receiver. Of the many valve sequences possible at the present time, that of a screened-grid high-frequency stage, followed by a triode detector which, in turn, is followed by a resistance-capacity low-frequency stage, a transformer-coupled stage and, lastly, a Class B output stage, will probably be found to be the most suitable. On the grounds of selectivity, two tuned circuits will be sufficient in practically all cases, because only a comparatively small aerial system will be used, and two correctly ganged circuits, plus a small trimmer condenser, should be satisfactory.

Commercial types of car receivers usually employ a small tuning unit which can be clamped on to the steering column and so greatly facilitates tuning whilst the car is actually in motion. As a rule, the tuning unit consists merely of a vernier tuning dial, a volume-control knob, and an on-off switch, the dial and volume-control knob being connected to the actual components by a length of Bowden cable. The home constructor will hardly feel inclined to build up an arrangement of this type, and so must concentrate on designing a receiver which can be used in such a position (normally, just underneath the dashboard, to the left of the steering column), that the receiver can be tuned or switched on or off without having to stretch an arm too far, or to take one’s eyes off the road.

The actual size of the receiver case and position of the tuning dials must, therefore, be left to the judgment of the builder, but suffice it to say that it is generally more convenient, both for actual use and for the arrangement of the components inside the receiver itself, to place the tuning dial at the extreme right end of the front panel with the volume control and switch as close to this point as possible.

Provision for automatic volume control is by no means essential, and the home constructor will probably prefer to omit any provision for this, particularly as battery-type valves will usually be employed. In many cases, sufficient control of volume will be obtained by the use of a reaction control alone, but where this is not sufficient, an auxiliary control, taking the form of a bias control for a variable-mu valve, may be used.

 

Courtesy of vk3ase

 

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