Radio Broadcast (May-Oct 1922)

RADIO BROADCAST 7 400 375 360 METERS 345 WAVELENGTH o 0' 740 750 760 770 780 790 800 810 820 830 840 850 860 870 880 890 900 FREQUENCY IN THOUSANDS OF CYCLES PER SECOND A chart illustrating the changed conditions which would exist if one of the four sending stations radiated a "broad" or poor wave centraling on 373 meters wavelengths (800,000 cycles wave frequency) but spreading its energy over a range of 1 50,000 cycles. No matter how sharply the receiver may be tuned (the four stations still bemg assumed to be of equal power and at equal distances), some interference will be found on all wave frequencies from 725,000 cycles to 875,000 cycles. Because of the difference in intensity the signals of the fourth (345 meter wavelength) station could be understood in spite of this interference, but it would be much harder to understand the 400 meter station's messages. So much interference (indicated by the shaded area) would be had when tuned for 833,000 cycles (360 meters) that it would be very difficult to interpret the words sent at that frequency graphical zone without interfering with each other. This is because receiving apparatus has been developed which is highly selective as to wave frequency. With such instruments it is feasible to arrange matters so that signals will be received from all reasonably distant radio transmitters which operate for example at a frequency of 500,000 per second, but that no signals will be heard from equally distant and equally powerful sending stations using frequencies a little above or a little below this value. Design of receiving apparatus has not yet progressed to the point at which it would be possible to select between transmitters using frequencies so close together as 500,000, 500,001, 500,002, 500,003, and 500,004 vibrations (or cycles) per second. Such exactness cannot be even approximated to-day, but, where radio senders emit waves of reasonably pure character and when the arriving signals are of about the same intensity, it is not difficult to discriminate between wave frequencies which are 10,000 cycles apart, such as 500,000, 510,000, 520,000, 530,000, etc., per second. It is easy to see that if wireless transmitting apparatus is so designed and adjusted that its radiated waves are quite closely of a single frequency, and if radio receivers are "tuned" so as to exclude waves more than 5,000 cycles above or below the main received frequency a large number of channels for independent communication can be had. One pair of stations can intercommunicate at 3,000,000 cycles per second, another at 2,990,000 cycles, another at 2,980,000 cycles, another at 2,970,000 cycles, another at 2,960,000 cycles, and so on down to the last few pairs at 40,000, 30,000, and 20,000 cycles per second. Each of these frequencies, chosen at 10,000 cycles separation, would constitute what amounts to a radio "private wire "or independent communication channel for the pair of stations using it. Where a division of working time could be made between a number of pairs of stations, all could be operated upon the same wave frequency (which would then become the equivalent of a "party wire" through space). Thus there could be provided room in the ether for a vast number of radio stations to work simultaneously and without interference. Unfortunately this splendid condition cannot yet be realized in practice. There are three important technical difficulties still to be overcome, as follows: 1. With present-day apparatus the higher frequencies are not suitable for economical and dependable transmission over long distances. 2. Many transmitters now in use, particularly those of the spark type, radiate a large