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ASSEMBLING A MOTOR COACH WIRELESS SET.

4th December 1923
Page 26
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Page 26, 4th December 1923 — ASSEMBLING A MOTOR COACH WIRELESS SET.
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A New Series of Wireless Articles in which Detailed Instructions for Coachmaking an Efficient and Compact Motor Coach Receiver will be Given.

THOSE readers who have carefully followed the. articles that have been running more or less regularly in The Commercial Motor for the past seven months or so, will, by now, have a fair grasp of the theory and fundamental laws that govern the design and construction of radio receivers in general, and also the particular problems that confront those who -wish to adapt this latest of scientific blessings for use on mobile vehicles. The Editor of The Commercial Motor feels, therefore, that the time has now come to publish more specific information on this most fascinating of subjects, and has arranged, therefore, that a radio receiver, specially designed for use on a vehicle, shall be described in this and subsequent articles. The intention is to give such detailed and clear instruction that anybody with the necessary small amount of mechanical skill will be able to build one of these receivers, our aim being the equipment of a large number of motor coaches by the garage staffs engaged on the overhaul of the vehicles during the coming winter months.

Before going on to an actual description of the receiver proper, it will be well if we briefly survey the special requirements for the particular use to which we intend to place our set. In all radio work—as in most other things --it is very rarely that one can embody in any one design all the desirable features that one might wish. For instance, if one wants the last ounce of efficiency one must be content to have a very complicated and delicate receiver, and one, moreover, that is tricky to handle and hard to tune. On the other hand, if simplicity and ease of operation are the first considerations, then one must be prepared to sacrifice at least a little in the way of efficiency and sensitivity. The whole question is one of eompromise, and upon the skill with which this compromise is carried out depends the success, or otherwise, of the receiver for the specific purpose for which it is intended.

Now, in our particular case, a high degree of efficiency is very necessary. First, we are dealing, at the best of times, with an extremely poor aerial. The utmost that we shall be able to do in this direction will fall far short of even a mediocre aerial on a house. Secondly, the vehicle on which the set will be installed will be changing its location frequently, which means that the conditions under which the aerial will be working will also be changing, practically from Moment to moment. Especially is this the case with regard to screening, either from hills, embankments, trees,or buildings. Lastly, the distance over which we may wish to receive is also constantly changing, as is also the direction from which reception will have to be made.

All these things call for a high order,of efficiency in our receiver—much higher, indeed, than the re

quieements of the average " broad catcher.And yet, at the same time, almost as irnportantAit is that our receiver be extremely easy to operate. Here are two almost diametrically opposed desiderata, and only very careful and well-thought-out design can meet these conflicting requirements with any measure of success. In The Commercial Motor receiver, about to be described, a very high degree of efficiency has been attained, and yet the operation of tuning and the adjustments have been reduced to the simplest movements.

This circuit, and the general arrangements of the parts a n d form of construction have been designed as the result of many years of experience in wireless transmiSsion and reception. Commercial Motor has no

commercial interest whatever in the manufacture of sets, but places the information a n d advice freely at the service of its readers.

The next iinportamt requirement for a coach receiver is compactness. One hears from time to time of extraordinarily small receivers being built, but it is a golden rule in radio that if you want the best results you must give all the components of the set plenty of air. Especially is this the case if one uses -much note magnification. Here, again, it is a matter for judicious compromise, guided by an understand

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A cross-section of the case and panel of the wireless set advocated for use on motor coaches. ing of which parts may be safely placed close together and which must needs be given ample room. The Commercial Motor receiver measures only 184-ins. by 6t ins. by 6 ins., yet all the valves are inside the receiver and all the components are arranged that they cannot possibly interact detrimentally.

Then, of course, our coach receiver should be so designed that all delicate parts are protected from accidental damage. The most vulnerable portions of a wireless set are the valves, and these, in The Commercial Motor redeiver, are placed right inside, and completely shut up so that they cannot be broken, unless the containing box is also smashed. Coupled with this provision is that of accessibility to the valves in case hurried replacement becomes necessary.

Lastly comes the question of protecting the set against the weather, and this largely depends upon wnat type of body the coach is fitted with, since, as it will readily be understood, a set used in an open or semi-open coach. will need much greater protection against the weather than will one that is used in a closed vehicle. The Commercial Motor receiver is designed so that the receiver proper is quite independent of the containing case, and it can, therefore, be boxed to suit any of the varying conditions under which it may be called upon to function.

One of the illustrations shows the front view of the receiver. It will be seen that it is shaped so that it may conveniently be installed on the dashboard of the vehicle. On the loft of the panel will be noticed the two dials which control the aerial and anode tuning respectively. Next to them comes the knob which adjusts the reaction coupling between the reaction coil and the anode coil. (The reaction method employed is that permitted by the Postmaster-General for broadcast reception.) .Proceeding towards the right, we have the main switch, which controls both high and low-tension circuits, so that, when this switch is off, the set is absolutely dead. Finally, we have a master filament resistance.

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