Radio Broadcast (May-Oct 1922)

154 RADIO BROADCAST that the number present in any given area of the wave grows smaller as the distance from the sending station increases. Thus the farther a receiving station is from the sending station, the less energy it receives and the more difficult it becomes to pick up signals. The exact manner in which distance affects the energy in a wave may be expressed by a complicated equation. For our purpose it is sufficient to remember that the energy decreases much more rapidly than the distance increases. That is, a receiving station twice as far away from a transmitting station as another receives much less than one half as much energy as the nearer station; one three times as far away receives much less than one third as much energy. This is the usual thing but sometimes "freak" results that do not follow this rule occur. To sum up, the energy received by a station depends upon the energy sent out by the transmitting station and upon its distance from the transmitting station. WAVE LENGTH SELECTION A SENDING station transmits radio waves of a definite wave length. A receiving station is so arranged that it can pick up signals (energy) from a definite wave length. Of course the receiving station may be adjusted to many different wave lengths but when adjusted it picks up energy on one wave length onl\'. But it picks up all energy on that wave length. It is the fact that receiving stations can so select energy from one wave length that allows more than one radio message to be sent through the ether at the same time. For example, suppose there were different stations transmitting, one on 300 meters, one on 400 m.eters, one on 500 meters, etc. Then a receiving station could be adjusted to select the 300-meter energy only and would not get any energy from the 400 or the 500-meter stations. But if there were two stations in the same vicinity transmitting on the same wave length, say 300 meters, then the receiving station would pick up energy from both transmitting stations. This would cause interference. This result would be very similar to that obtained when two people are talking at once on the same telephone line, or if you had two people talking to you through the same speaking tube at once. There is, or was, in a certain city an amateur who had a radio telephone transmitter. Each night he turned on a phonograph and transmitted the whole evening. No one else in that vicinity could hold communication on that wave length because of the interference produced. A definitewave length, then, must be thought of as a definite channel of communication through which one signal may pass but not two. If two signals are sent, the result is only confusing interference. Of course, if the transm'ning stations are far apart, one signal may become so weak that there is no interference. There are certain limitations in the radio transmitters and radio receivers which will not allow these wave-length channels to be too close together. That is, with ordinary apparatus, there cannot be one channel on 300 meters and another on 301 meters. This is because the apparatus is not perfect. This result may be approached however. The writer knows of one set which has been designed that allows nine channels of communication in a wave-length range of only two meters; from 74 to 76. Here, then, lies the reason that wave lengths are the subject of discussion at the radio conventions. There are so many radio stations of different kinds that some control over their wave length must be exercised, otherwise a great many stations will be using the same wave length with consequent interference. Wave lengths must be controlled by someone