Page:Popular Science Monthly Volume 88.djvu/647

 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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