EQUIPPING A COACH FOR WIRELESS RECEPTION.
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The Earth Connection, and the Importance of Screening the Engine to Prevent Waves from the Magneto Reaching the Set.
BEFORE GOING into the question of the choice of a set and its actual operation, we must con
sider the remaining work that must be done offtside of the s'st proper, in order that wireless may be received. The first thing, after the aerial and lead in are completed, is the earth connection.
The " earth " in electrical parlance, as most of our readers know, does not necessarily mean the globe that we inhabit, but refers to the general mass of any object. Thus, the earth on board a ship is the hull and all metallic masses in contact therewith, and the earth on a• vehicle means he chassis in generat and all things connected , thereto Such as engine, gearbox, etc. In dealing with the high-frequency, high-voltage, oseillatory. currents that carry radio telephony, the earth also includes many nonmetallic masses such as the bodywork, because, in contradistinction to steady or direct currents, oscillating currents have the faculty of being shunted away by means of a condenser almost as readily as they pass over metallic objects. Since an electrical condenser consists of a " sandwich" of a piece of insulating material such as mica. or ebonite, between two conductors such as sheet copper, it follows that the insulating materials on a vehicle—the woodwork, glass and rubber—can •also cause •a condenser action to be set up, if they be sandwiched between a wire 'carrying high-frequency current and the metallic chassis to which they are attached. This is the reason that we have laid stress on (in previous articles) the necessity for keeping all high-frequency wires, such as the lead in, so far as possible away from bodywork, etc.
Screwed tight Z:sweated
;a+ YO. 11 ro earTh terminal oti iristivnetit,
1r..„ frame.hyeakr Fig. 11.—A method of making an " earth " connection between instrument and chassis frame.
How to Secure an ".Earth" Connection from a Vehicle.
If you want to get the very best results from any wireless receiving set, you must make sure that the earth connection is as good as possible. Here, again, we are up against it on a vehicle. All the earth that we can get is that represented by the metallic masses of the chassis, and these, in comparison with the real earth, are very small. It is, therefore, doubly important that the actual earth lead, i.e.., the wire that will run from the terminal marked E on the set to some part of the chassis, is connected to the frame in as perfect a manner as possible.
Select a part of the actual chassis, or of the engine, or the gearbox, for making. this connection. Do not hitch the earth wire to the first thing that comes handy. The place selected should be easily accessible and in such a position as to make the earth lead as short as possible. It should be thoroughly cleaned, drilled and tapped si3i in. Whitworth and a brass screw fitted with a washer inserted; the wire should be clamped under this washer and, after it is Secured in place it is best to warm up the part with a blowlamp and to sweat the whole connection up solid. This stops slow electrolytic action bhat is otherwise bound to take place with a gradual diminution of signal strength. (See Fig. 11.)
The Efficacy of a Length of Chain.
Since our chassis is on rubber tyres, it is more or less insulated from the earth proper and, in fact, with its rubber tyres and the earth, it forms a condenser the capacity of which is in series with the capacity between our aerial and the chassis. As one end of the radiating system at the broadcasting station is earthed, this extra capacity• is .bad, and, although we shall be able to receive messages using the chassis as an earth alone, we shall make our set have a better range and behave more uniformly if we take steps to minimize this second.capacity effect The easiest way in which to do this is to hang a short piece of chain from the lowest part of, say, the gearbox and make this chain sufficiently long to allow two or three links to trail on the ground. This is a similar arrangement to that used on rubber-tyred steam wagons in order to prevent the accumulation of static charges due to evaporation in the boiler. (See Fig. 12.)
Screening the Magneto. •
We now come to a very important part of our procedare. The magneto on the engine, containing as it does two spark gaps and a considerable amount of capacity and inductance, and operating at a pretty high voltage, is no inconsiderable wireless transmitter. It is true that the waves it sends out are untuned and are of very short length; nevertheless, they axe of the whip-crack variety and are, therefore, capable of affecting a tuned circuit in their vicinity— especially, as is the case with us, if the receiver is very close. It is, therefore, very essential that we
adopt some means for screening these waves so tbat they shall not reach the receiving set. In attempting a solution of this problem, there are two ways in which we may set to work. Firstly, we may enclose the actual set in a wave-proof case—say, of aluminium or copper. This would prevent waves from the magneto reaching the set direct. But it would be found that by far the greatest amount of interference arrives from the magneto via the aerial, and, since it is impossible effectively to screen this without seriopsly impairing its signal efficiency, the only practical solution of the problem. is to screen the disturber.
To do this we shall have to cover the entire engine —not just the magneto alone, because the greatest amount of radiation is from the secondary, highvoltage circuit of the magneto, and this circuit includes the high-tension leads to and the sparking plugs themselves.In fact, the screening should cover every conductor on the car that carries alternating or intermittent current.. Even the magneto earthing switch is better for being thus screened, especially if the set is on the dashboard and near to it. (See Fig. 13.) Luckily for considerations of weight and cooling, it is not necessary to use a Cover of solid metal. It has been found that screening made of moderately fine copper gauze acts just as well as a preventer of radiation. No hard or fast instructions for screening can be given, as so much depends on the design of the engine and bonnet, but the screen should be made so as to fit right down on to the undershield or the chassis members so that the entire engine is metallically covered. Moreover, the screen should be earthed by having two flexible wires attached which are connected to the chassis in two aifferent places.