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BELL SYSTEM TECHNICAL JOURNAL 



In practice the generator may conveniently be built up from an 

 oscillator unit, a piston assembly and an adjustable iris, all of the same 

 diameter of pipe fastened together by exterior metal clamps as shown 

 in Fig. 12C. The open end of this generator may be connected to a 

 guide over which transmission is desired or it may be coupled loosely 

 to some nearby laboratory apparatus on which measurements are to 

 be made. 



The total length of the chamber and hence the piston setting will of 

 course depend on the frequency to be generated. In general this will 

 be roughly an integral number of half wave-lengths. The relative 

 position of the oscillator along the length of the chamber will depend 

 on its impedance characteristics and to some extent on the diameter of 

 the iris opening. For a piece of laboratory apparatus where frequency 

 variability is desired these various dimensions should preferably be 

 adjustable as shown. If a source of single frequency is desired, the 

 resulting apparatus may be greatly simplified as all of these dimensions 

 may be fixed at the time of construction. 



The Tuned Receiver 

 By reversing the principle used in the generator above, replacing 

 the oscillatory source by a suitable indicator the resonant chamber 

 becomes effectively a simple tuned receiver. If the indicator is ap- 

 propriately located along the length of the chamber, substantially all 

 of the incident power will be absorbed and the device as a whole will 

 be a veritable sink of wave power. It may be clamped to the end of 

 a long wave guide, thereby constituting a termination, or it may be used 

 to pick up short radio waves of not too small amplitude. See Fig. 13. 



TO METER OR 

 AUDIO OUTPUT 



Fig. 13 — A tuned receiver based on (he resonant cavity principle. 



