GAS-DISCHARGE T RAN SM IT-RECEIVE SWITCH 83 



of the external cavity type of tube, is that of certain radar systems which 

 were required to be capable of receiving signals on occasion at a frequency 

 differing from their normal tuning. This was done by a solenoid-operated 

 plunger which could be preset to alter the tuning of the cavity by the de- 

 sired amount whenever the solenoid was energized. 



The Tube Design 



The 702A and 709A vacuum tubes, as previously mentioned, were put 

 into service with little or no consideration of their real suitability. With 

 these stop-gap designs in production the basic design problem was given 

 serious consideration, with separate studies being made of the mechanical 

 design considerations as they relate to the size and shape of the discs and 

 glass of the tube, and of the gas filling. 



The exact shape of the disc is determined first by the total tuning range 

 which is to be required of the tube, and second by the necessity for main- 

 taining the Q of the structure as high as possible. It has been shown that 

 in a spherical resonator with coaxial cones the maximum Q occurs when 

 the cone half-angle is nine degrees. The copper-disc tube can only roughly 

 approximate the ideal spherical resonator; nevertheless it appears desirable 

 to use cones of this angle. The disc spacings and diameters are so chosen 

 that the tube resonates at the shortest wavelength at which it is to be used 

 in a "square" cavity; i.e., one in which the inside diameter approximately 

 equals the height. Such a cavity is about the closest practical approach to 

 a sphere. The glass diameter is made as large as mechanical considerations 

 permit so it is as far as possible removed from the region of high electric 

 field intensity. 



The experimental results of Fig. 24, previously noted, indicate that the 

 leakage power of a TR box decreases as the gap spacing decreases; thus one 

 is tempted to make the gap extremely small. Too small a gap is very 

 troublesome, however, since such a gap has an unreasonably rapid variation 

 of resonant frequency with gap separation, making the tuning extremely 

 subject to change as a result of dimensional variations due to processing or 

 to temperature changes. Accordingly one chooses a compromise gap separa- 

 tion. The electrode radius at the gap must be large enough to permit the 

 radio frequency glow discharge to dissipate the required power without ex- 

 cessive spreading, and must be determined by experiment. 



Rather than attempting to hold all of the mechanical variations in the tube 

 (including glass thickness) to the necessary tolerances to insure the desired 

 uniformity in tuning, the tubes are pretuned before exhaust by deforming 

 the copper discs. The tubes are placed in a special cavity and tlie disc 

 inside the envelope distorted by a tool until resonance is obtained at a speci- 



