WIRELESS AS AN AID TO VEHICLE CONTROL.
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In this Second Article upon the• Recent Developments in Wireless Telephony the Possibilities of its Usefulness in Vehicle Control are Outlined and Various Forms of Aerial are Described.
IN THE FIRST article under this heading the fundamental construction of matter and its relation to the electrical phenomena which make wireless communication possible were discussed. Before proceeding to a detailed description of wireless apparatus a word or two will be said about the scope of radio, more particularly from a transport point of view.
What Radio Can Do and What it Cannot.
, Theoretically speaking, wireless communication should be passible wherever there is ether. In practice this is not strictly the case. Although we set the ether in vibratory motion, and these vibrations spread out exactly like the ripples on the sur face of -a pond when we throw a stone into it, yet local conditions profoundly moderate the perfection with which the ether ripples, or waves (as they are generally called), are propagated. The laws governing these aberrations are, even at the present day, only imperfectly understood, and the causes for the various curious effects that have been noticed are not well understood at all. In this article it will suffice if we catalogue the more 'commonly met cases so far as these may have a bearing on transport work in connection with -wireless.
The Greater .Range at Night-time of a Distributing Station.
First of all, then, it is generally found that communication is „better, and can be carried out over a greater range • during the night. In the old days, before the present extremely sensitive receivers were invented, this difference was very Marked. So much so, indeed, that most wireless stations were said to have two ranges—a day range and a night range— and often the night range was ten times as long as the day range.
A striking instance of this came before the writer's notice during the war, where, in a particular island in the Indian Ocean, it was possible to pick up the Berlin Press news during the night—over a distance of nearly six thousand miles—whilst in the day-time it was a hard job to get Durban—scarcely fifteen hun-' dred miles away. Against this must be set the Net that at night-time, and especially up to midnight, and more in summer than in winter, radio is troubled with what are known as atmospherics. • Atmospherics are noises heard in the telephones, caused by natural electrical disturbances in the atmosphere. A lightning,. discharge is exactly analogous to the interrupted flow of electrons given in last week's article. It follows, therefore, that it will affect the ether in the same way. Moreover, since the power represented in a single "flash of lightning may be anything up to half a million horse-power, it follows that a lightning discharge may be capable of setting the ether in very violent motion.
'Luckily, in Great Britain the atmospherics are seldom—even in midsummer—so heavy as to prevent wireless communication altogether, but if they are at all persistent they considerably curtail the range at which understandable signals can be read. They usually come up as the sun goes down and continue until well after midnight. In their case the Sensitivity of the modern receiver is a disadvantage, as it is liable to bring in loud atmospherics that the earlier type would hot pick up at all.
The Interferences are Not Important.
For some reason that cannot at present be explained, atmospherics are more troublesome on long wave-lengths than on short, and since it is on the latter that telephony for transport control Would be practised, the nuisance will probably not be too great from this cause.
The remaining interference Thich is likely to be met with is from fading and blind spots. Fading is a phenomenon noticed when short waves are employed. It is not universal, and the same station may fade badly at one place and remain perfectly steady at another. Its cause is quite unknown. It will be an important factor to be taken into account in applying wireless to transport control, however, and it may be necessary to prepare fading charts for these islands, so that when a, vehicle is in a district wher,e fading is prevalent due allowance May be made for the fact.
Blind spots are, as their name indicates, places Where wireless communication cannot be carried on at all. They are, luckily, very rare, but a considerable area in the United States is known to be absolutely blind, whilst nearer home it is said that there is a blind spot at Didcot, which is but a few miles from London. They come under the same category as fading, and it would appear that the only remedy —from a transport point of view—is to locate them all and make proper allowances for them. • With the exception of the above-nientiened hindrances, communication by wireless-phone' can be carried out anywhere and with certainty, provided that the transmitting apparatus has a sufficient power to cover the distance specified, and that the receiving gear is sufficiently sensitive to make full use of it. This brings us to a consideration of the aerials that can be employed on vehicles.
What an Aerial is and How to Fit it to Your Vehicle.
We must bark back, just for a moment, to the explanation given last week. We showed there that the ether was jogged each time that an electron was
made to stop moving, and we gave as suitable apparatus a pocket flashlamp battery and a piece of wire. Now it must be understood that the energy transferred to the ether from stopping an electron depends entirely upon how quickly we can tarry out the operation, and in thisrespect the electron resembles very much the case of a man falling out of a window. If he land on a stone courtyard it is generally the worse for him. Ji he fall on a feather bed he may get off with a bad shaking. The difference in the two cases is due to the suddenness with which he is brought to rest. If, instead of falling out of the window on to a stationary object, the man fall down a lift shaft on to a lift that is coming up, the result is still more tragic. So with our electrons. The pocket flashlamp battery has not the necessary pressure (voltage, to make the electrons move very fast, nor is our method of making and breaking the current conducive to a very
quick stop. The result is that not very much energy will get into the surrounding ether. In order to get a high voltage, Marconi, in his original apparatus, employed an induction coil (similar to, but considerably larger than, the apparatus used on a Ford car for the ignition), and in order to get the maximum energy into the ether he arranged that the electrons should not only be stopped, but should be reversed Many thousands of Limes a second. He did this by having the spark from the coil take place between, not two points (as in our sparking plugs), but two metal balls of considerable diameter, one connected to the earth and the other to an elevated metal plate. This plate, now generally replaced by a system of wires, is known as the aerial, and, although Marconi's apparatus has been replaced by entirely different methods, the necessity fqr aerials, both at the transmitting and receiving ends, still remains.
Types of Suitable Aerial
Aerials are of two types, open and loop or frame. These function in an entirely different manner from one another, and the exact why and wherefore will be considered later. The open aerial is the most efficient, provided that it can be erected a fair distance above the ground; the ground in the case of a vehicle being thehighest point of the bodywork. The frame aerial is not affected by its position, but it is very sensitive to direction and needs a much more elaborate receiver to give the same effects as the open aerial.
The open aerial should be as long as possible, and should consist of from four to six wires parallel to one another connected together at the end from which the down lead to the receiver is taken. In the case of a flat platform vehicle, it may be supported on two short masts, so as to be at least a foot above the top of the Toad. The aerial should be arranged to roll up out of the way when_ not in use, se as to obviate any danger of damage during loading or unloading.
The same arrangement can be applied to tipping wagons, but it would, of course be necessary to remove the aerial before tipping. In the case of tilt vans, the aerial may be supported either on top of the tilt or at the sides. In either case it will be necessary to keep it as far from the tilt as space permits. The illustrations show several methods of adapting an open aerial to various types of body. The frame aerial, if used, must be arranged to pivot in a vertical plane, since it will only receive at all effectively when it is lying so that a plane passing through it also passes through the transmitting station.
In the case of a tilt van it may be fastened to the back door of the van, but this arrangement is not entirely satisfactory, as the door would need to be open the whole tiree that communication was going on unless it so happened that the transmitting station was in a direction exactly at right angles to the vehicle. A frame aerial of conventional type is shown in one of the illustrations,
Th and it will be seen that it is more difficult to house without taking up load space than is the open aerial.