Page:Popular Science Monthly Volume 90.djvu/169

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How to Become a Wireless Operator

V. — Increasing the Range of the Receiver by Tuning

By T. M. Lewis

��THE December article of this series gave directions for completing and operating the receiver which was designed for working over distances of about one mile. The apparatus was in- tended for use with the small transmitter which was described in the October number of the Popular Science Monthly; when a larger transmitter is used, the possible working range is, naturally enough, con- siderably greater. It is necessar>' for each receiver to be adapted for the wireless waves sent out by the transmitter to which it is listening, however, if the best results are to be secured.

Production of High- Frequency Oscillations

Every transmitter for wireless telegraphy which is permitted to operate under the present radio laws must send out what is called a pure and sharp wave. That is to say, the sending apparatus must be so adjusted that its radiation has a single and definite wavelength or frequency. The main purpose of the regulations insisting upon this condition of sharpness and purity of wave is to enable a receiving station to "tune-in" a sending station without inter- ference. Senders which emit waves neither sharp nor pure are the cause of interference, and sometimes prevent all other stations in their neighborhood from working effectively.

To understand this matter of tuning, one must realize first of all that the currents in a wireless telegraph antenna are of the high-frequency alternating sort, which change in direction very rapidly. In a simple transmitter, with the spark-gap directly in series between the antenna and the ground, as described in the October and December articles, the effect of the induction-coil is to charge the aerial by storing in it a quantity of electricity just before each spark takes place. The induc- tion-coil secondary tries to charge the antenna up to its maximum pressure, or to drive into it all the electricity possible. Before this top-point is reached, however, the spark-gap breaks down and the charge of electricity rushes across it to the ground. I Because of the electrical property of the

��aerial wire (and of the coils in series with it) called inductance, the charge overshoots itself somewhat, and the antenna is left charged in the opposite direction for an instant. Therefore, in the natural attempt to restore equilibrium or electrical balance, the charge rushes back out of the ground into the aerial ; this time it overshoots also, but not by so much. The electrical energy thus oscillates back and forth, like a swing left to itself, until it is all used up in radiation, or in losses in and near the circuits.

Period and Frequency

A certain amount of time is required for the electrical charge to travel from the top of the antenna to the ground and back again, just as a certain time is required for a pendulum to swing from one end of its beat to the other and back again. This amount of time, measured in seconds, is called the period of the oscillation. The longer the wire, the longer the time for each trip of the current, and the longer the period. The number of times the electrical charge makes the round trip in one second is called its frequency, and this of course may be calculated by dividing the period, in fractions of one second, into one second. For example, if the period of oscillation of an antenna is one millionth of a second — which merely means that the charge takes that long to travel up and down the antenna once — the frequency is one-millionth second divided into one second, or one million. This is the number of trips the charge will make in one second.

Wavelength

Knowing the frequency of any electrical oscillation or high-frequency alternating current, one can immediately compute the wavelength which it will produce if it flows in a suitable wireless- telegraph antenna. The rule is simply to divide the frequency per second into three hundred million. The answer to this little problem in Arithmetic gives at once the wavelength in meters. For example, taking the frequency of one million per second quoted at the end of the paragraph immediately above, it is

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