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Electric wave propagation along copper tubes.
A system of 'guided-wave' transmission which is neither radio as we know it nor yet ordinary wire transmission, is at present the subject of intense research-work by scientists associated with, among others, the Bell Telephone Company, The American Telegraph and Telephone Company and the Massachusetts Institute of Technology: This new system of guided waves commences, so far as wavelength and frequency are concerned, at just about the point where ordinary radio waves leave off, that is to say at about 15 centimetres (2,000 MHz), and, as is well known, ordinary wireless transmission has not, at present, been conducted on shorter wavelengths than that, so far as commercial work is concerned. Some guided-wave workers are, however, experimenting with wavelengths somewhat greater than 15 centimetres.
Dr G C Southworth, one of the pioneers of 'Guided-Wave' transmission, at work with some of the apparatus in the Bell Telephone Laboratories.
Tubular Metal Guides
These guided waves are propagated along hollow metal tubes and, although a great variety of types exist, four are of particular interest. It has been found that in the case of any given type, the diameter of the tube bears a definite relationship to the length of wave which can be propagated. For instance, in the case of a 10 centimetre wavelength, the tube can, so far as one particular type of wave is concerned, have a diameter of 2.30 ins. If, however, the tube is filled with a dielectric having a constant of 5, then a tube of approximately one inch in diameter would suffice, and so on.
Now for three of these types of transmission the attenuation per mile drops to a minimum as the frequency is increased and then slowly rises, and for one of these three the attenuation remains practically constant at this minimum value over a band width of four thousand MegaHertz. Actually, the variation is less than half a decibel per mile over this band width. The fourth type of transmission is, however, very remarkable insomuch that attenuation appears to become progressively less as the frequency is increased. It happens, however, that this type requires, for a given guide, a higher range of frequency than any other. This puts it, therefore, in a frequency range where it is more difficult to deal with and it is, as a matter of fact, considerably less developed than the others.
In much the same way that an air column may resonate to certain sound-waves, so may a short section of wave-guide tube be made to resonate electrically to the frequencies which are to be propagated. In its role as a resonator it behaves as does a tuning circuit consisting of coil and capacitor in the case of ordinary radio waves. The necessary energising impulses may be generated by any source of sufficiently high frequency such as a Barkhausen or magnetron oscillator suitably coupled to the resonating chamber according to the particular type of waves being used.
In this photograph, two of the copper-tube transmission lines erected by the Bell Telephone Co can be seen. Dr G C Southworth is seen holding one of the 'resonating chambers'.
The open end of a conductor may be 'flared' and caused to radiate electric wave energy just as the flare of a loud-speaker horn radiates acoustic wave energy, Also it may be arranged in this way to form a highly efficient load for the acoustic generator with which it is associated.
The great question now arises, to what use can this guided-wave system be put? For long-distance work the situation is that it is not yet sufficiently far advanced for any use to be made of it. For transmission over short distances, for use as projectors of electric waves or as selective elements under certain conditions, the wave guides have definite possibilities. The system has also great potentialities as a connecting link between the transmitting apparatus and the aerial, or even as the aerial itself in the form of an electro magnetic 'horn', such as has been mentioned. The unusual characteristics of the system are well adapted for transmitting the wide-frequency range demanded by television in cases where transmission of television signals has to be carried out from point to point in a city or even, with further development, from city to city. There is, of course, little or no external field and consequently a pleasing absence of interference from radio stations or from external noises.
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