THE CONTROL OF TRANSPORT BY WIRELESS.
Page 19
Page 20
If you've noticed an error in this article please click here to report it so we can fix it.
The Possibilities of Directional Transmission, and Its Use in Giving Partial Secrecy to Communications.
IN THIS series of articles we have so far shown that it is desirable that there should be a method of employing the broadcasting stations for the transmission of control messages from the owner of a motor coach, lorry, or van to the driver thereof, and also that the system is practicable and virtually reliable. We have shown the course that has to be taken by such messages from despatcher to receiver, and the manner of acknowledgment. We now come to a consideration of what must, to many people, have occurred as one of the chief drawbacks to wireless in eonnection with transport control. How are we going to ensure a reasonable amount of secrecy in our wireless communications to our drivers.?
As we all well know,, any message sent out into the ether is the property of everyone possessed of the electro-magnetic ear. It is this feature that has made broadcasting of music and other entertainments so fascinating—but it is exactly this feature that is likely to cause most misgiving with potential users, and thus( to delay the adoption of what really is one of the greatest potential aids which the transport business has yet been offered. Before going into the methods that may be employed to secure a fair amount of secrecy, let us ask ourselves just how much is needed, and in what circumstances it should be demanded
The people who listen-in, and from whom it is the object of secrecy to guard our wireless transport messages, are roughly of three categories : operators of commercial wireless stations and ships : amateurs who either hold an experimental licence or are merely 'broadcasting ,patrons; and those engaged in transport like ourselves. The amateur holding an experi. mental licence will listen attentively to softie of our messages, especially when she is young to radio., but he is not likely to remember what is said, and, as he grows older in wireless wisdom, stray messages will loose their attraction for him. Moreover, he is legally debarred, under the terms if his licence, from divulging anything that he may chance to hear. The broadcasting amateur has a set that operates only on a fairly narnow band of wave lengths, and it is prob able that transport work would be conducted on a band sufficiently remote from the broadcasting allocation to make it very unlikely that it would be heard on these sets.
This leaves only the interested party from whom it may be desirable that our messages are shielded. It is not for a moment imputed that many transport firms will make a deliberate practice of trying to filch one another's instructions, but where competition is keen, and a niessage relative to possible business happens to reach a person other than the one for whom it -was intended, it is only human nature to take advantage of such a fortuitous happening. We have, therefore, narrowed the necessity for secrecy down to one class of individual, and we can now further narrow it down to practically one class of message ; that which relates to tentative business. Mere service instructions to drivers will rarely be of any interest to other than the rightful recipient. But it is quite possible that the chance of a load to be picked up will lead to pirating unless means are taken to prevent it.
Now, in radio, as we know it to-day, there are only two methods of ensuring anything like secrecy—the first (which, even if certain technical difficulties are overcome, can only give partial immunity from leakage), is known as directional wireless, and the second (which, if properly used, ensures absolute privacy) is the use of a code.
Directional wireless is still, more or less, in an experimental stage, although Marconi made some experiments along these lines almost at the beginning of wireless history. Briefly, it consists in projecting the ether waves in the, form of a beam instead of letting them radiate from the trans/litter in all directions. Wireless ether waves do not differ in any respect from light waves except in their length. It therefore follows that they are capable of being both reflected and refracted, or bent, as are these latter. The difference in the length of the two kinds of wave, however, is very great—the average length of light waves being only a small fraction of a millimetre, while wireless waves, in everyday use, range from 300 metres, which is nearly the fifth of a mile, to 14,000
metres or more, which is about nine miles'. To bring about the phenomena of reflection and refraction, it is necessary that the surface reflecting and the substance refracting be very large relative to the wavelength being operated on. This condition is easily' met in the case of light, but, when it comes to even the shortest wireless waves, the problem becomes formidable.
Recently, however, experiments have been carried out, making use of very short wireless waves—in the neighbourhood of 14 metres—and employing a parabolic reflector consisting of a number of parallel wires stretched on a parabolic structural-steel frame. By means of such an apparatus erected at Hendon, touch was maintained with a motorcar despatehed to Birmingham throughout the whole of its journey, and
satisfactory communication was maintained. Without the reflectors, such short waves would have been stopped before they had travelled more than a dozen miles. The experiments are still being continued, especially in connection with ships, but sufficient has not yet been achieved to warrant our regarding directional wireless as being within the realms of practical politics. Moreover, the degree of secrecy that it -confers is only partial ; any receiver situated in the path of the beam would be able to intercept messages, and the beam itself is pretty broad and somewhat divergent, so that it probably measured nearly a mile across by the time it reached Birmingham. Codes are the only absolutely sure way of keeping the subject matter of messages secret. In the next article various methods of making and using codes and cyphers will be discussed. '