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 



1.5.S 



