The broadening of the radio frequency frontier towards the visible spectrum has made possible the development of mobile train radio service. Following extensive field studies, the Federal Communications Com-mission in the United States has, within the last few months, allocated the band 158-162 mc. for railroad use, in end-to-end, train-to-train, and wayside-to-train service; and on a non-interference basis, for yard and terminal communication. The bands 44-72 mc. and 186-216 mc. are also available on a sharing basis with television, for yard and terminal radio systems. This action by the F.C.C. is significant and of vital importance, and cannot be overlooked in considering the potentialities of VHF train radio. It is to be expected that Canada will consider similar action in assigning frequencies for this type of service. The opening up of the VHF band for this service has resulted in many advantages; they can be briefly summarized as follows:
(1) The first and most important consideration is the desirable propagation characteristic of 160 mega-cycle radiation. Due to the absence of ionispheric reflections, it is possible to control the range a the requirements of railroad application might dictate. Then, too, the earth and objects upon it act as more and more efficient reflectors as we proceed upward in the spectrum to 160 megacycles. This results in perfectly usable signal levels entirely beyond the line of sight and , on the basis of recent tests, it appears that solid communication on 160 megacycles can be maintained in long tunnels.
The short wavelengths employed, slightly less than two meters, permit the erection of small but highly efficient antennas having directional or non-direc-tional characteristics as desired. This makes it possible to erect antennas on locomotives and cabooseswell withinthe railroad’s clearance pattern. This is one of the major reasons why tests at lower frequencies in the past have been somewhat un-satisfactory.
A third and most important feature concerns electrical interference. It was originally feared that the electrical noise level in huge Diesel-electric loco-motives and in industrial areas would be severe. Recent operating tests have indicated that at frequencies of 160 megacycles, noise from sparking commutators, high tension lines and other such sources was almost non-existent. It is safe to say that atmospheric noise is almost entirely absent and that any man-made noise encountered is of the impulse type and rendered innocuous by the use of radio noise limiters.
Due essentially to the three preceding factors, it is not necessary to employ high power to provide a consistent noise-free communication circuit. Low power can do an excellent job. This means smaller equipment operating at lower temperatures, lower power input, lower tube cost and lower overall cost.
It also makes practicable the employment of pack sets and handy talkies.
160 megacycles also represent a “happy combina-tion” of things. The frequency is high enough to secure the advantages of controlled range, small antennas, low noise level and lower power, yet the frequency is low enough to permit routine manu-facturing and maintenance techniques and use of standard components. This means that thoroughly reliable equipment is available today.
The MRT-lA communication unit is one of the latest post-war designs of a combined mobile transmitter-receiver for railway use. The case is of moulded, reinforced aluminum coastruction with a rubber gasket between upper and lower halves to give a water-and-air-tight housing. The overall dimensions are 27 in. long. 14 in. wide and 10 in. high, and the com-plete unit weighs 55 lb. This equipment operates in the 108-162 mc. range, the lower end of the band being available for aviation radio. It is frequency modulated and has a transmitter output of approx, 12 watts.
The associated antenna is of novel design; relatively short in any event due to the high frequency, but made still shorter to meet the space limitations of the railroads by the top loading disc, and carries its own ground plane in the form of a cartwheel.
/end Mobile train radio use, design and electronic, nee electric details in Canada 1916