The Tuning of Radio Telegraph Receivers 



By John Vincent 



THE article of this series which ap- 

 peared last month discussed the 

 difference between free and forced 

 oscillations in radio telegraph circuits, 

 and applied the laws of resonance to 

 several of the more common types of 

 sending apparatus. It is interesting to 

 note that the same simple fundamental 

 laws of tuning govern the operation and 

 adjustment of receiving apparatus, in 

 very nearly the same way. The need of 

 securing as:ree- 



Fig. 1. Simple 

 receiving aerial 



inent in frequen- 

 cy between the 

 arriving waves 

 and the receiving 

 __ aerial circuit is as 

 '^ great as the need 

 of tuning together any two circuits in- 

 volved in radio telegraphy. 



As has been shown, the traveling 

 electromagnetic wave which is sent out 

 in all directions from a transmitting 

 station has a definite wave-frequency. 

 It is more usual to speak of each par- 

 ticular wave as having a particular 

 wavelength, but it is just as accurate to 

 consider the wave-frequency. The fre- 

 quency of any wave may be found by 

 dividing its length in meters into 

 300,000,000, according to the examples 

 given in January. As has also been 

 shown, every antenna circuit has a 

 definite frequency of resonant vibration; 

 this frequency depends upon the effec- 

 tive inductance and effective capacity of 

 the entire antenna and connected in- 

 struments, and this frequency is that 

 which would be assumed by an alternat- 

 ing current (or free oscillation) set up in 

 the antenna system by first charging its 

 capacity and then allowing it to dis- 

 charge freely through the circuit to 

 earth. The frequency of this free 

 oscillation may be figured out, according 

 to the rule given in the March article, 

 when the capacity and inductance are 

 known. 



The frequency of free oscillation is 

 practically the same as the frequency of 

 the forced oscillation which will cause 

 the largest current to flow in the antenna 



circuit. The equivalence of these two 

 quantities, as explained in connection 

 with transmitters last month, holds for 

 receiving - circuits as well. In other 

 words, the resonant free-oscillation fre- 

 quency of an antenna system not only 

 represents the wavelength which will be 

 best radiated from that antenna, but 

 also the wavelength which will be re- 

 ceived with the greatest intensity. 



This law may be worked out for a 

 simple circuit arrangement such as 

 shown in Fig. i, where the antenna A is 

 connected to earth E through a variable 

 tuning inductance Li, and a current in- 

 dicator /. Suppose the instrument / is 

 a sensitive hot-wire ammeter of the sort 

 used in wavemeters, and that the aerial 

 is rather large and is erected within a 

 mile or two of a powerful transmitting 

 station. Suppose that the antenna is of 

 the flat-top variety, ha\ing four wires 

 hung on 30' spreaders and with a total 

 length of 150'; this aerial will have a 

 capacity of about o.ooi microfarad. If 

 the high power sender is in operation, at a 

 wavelength of 5000 meters, strong ether- 

 waves of frequency 300,000,000-^-5000 

 = 60,000 cycles per second will pass by 

 the receiving station. If, now, we tune 

 the receiving aerial to this frequency by 

 adding to the coil Lj until the total 

 antenna inductance equals about 6.94 

 millihenrys, the ammeter I will show 

 the greatest deflection. If either more 

 or less than this amount of inductance 

 is used, the current in the antenna will 

 be smaller, for the reason that 6.94 



Fig. 2. An inductively coupled receiver, 

 in which a secondary coil and tuning- 

 condenser make up a closed, oscillating 

 circuit. In operation, the antenna cir- 

 ^cuit must be tuned as in Fig. 1 



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