GAS-DISCHARGE TRANSM IT-RECEIVE SWITCH 49 



"early warning" equipment. For example, the go and return time to a 

 target at 500 feet distance requires approximately one microsecond. Switch- 

 ing times must, therefore, be measured in microseconds. Since these short 

 time intervals would at first sight seem to be too small to permit the use of 

 gaseous discharge devices, some work was done on the use of specially 

 designed vacuum diodes. It is possible to employ balancing circuits (some- 

 times called hybrid circuits) to achieve single antenna operation, but such 

 circuits require critical balancing adjustments and they dissipate a large 

 part of the available power in non-useful balancing loads. The need for a 

 still different approach to the duplexing problem was clearly indicated. 



Spark discharges either in air or in enclosed gaps bridged across parallel 

 wire transmission lines were used in some of the early experimental long- 

 wave radar sets. Dr. Robert M. Page of the Naval Research Laboratory 

 was one of the pioneers in this work. These devices were only moderately 

 satisfactory because of their erratic behavior and because of electrode wear. 

 However, it was observed that the recovery time of such discharges was not 

 as long as might be expected on the basis of a simple ionization and deioniza- 

 tion explanation of their operation. This led to the investigation of the use 

 of low-pressure gas discharges. These very early gas-discharge "switches" 

 were actually much more in the nature of "lightning protectors", their 

 principal function being to limit the power delivered to the receiver during 

 the transmitting pulse in a gross sort of way, with considerable reliance on 

 impedance changes at the receiver and on the rugged overload capabilities 

 of the first tube in the receiver. 



The trend toward shorter wavelengths and the desire for better protection 

 led to the development of a partially evacuated gas-discharge tube located 

 in a relatively high Q resonant cavity. In England, cavity type duplex 

 tubes were made by inserting gas in a then current type (Sutton Tube) 

 of local oscillator tube. These devices were called TR boxes (abbreviation 

 for tran.smit-receive) by the English, a designation which has continued. 

 It is a curious coincidence that some of the earliest cavity type duplex tubes 

 made in this country at the Bell Telephone Laboratories were also con- 

 structed by inserting gas in an American type local oscillator tube (the 712A 

 vacuum tube). This tube (later coded the 709A vacuum tube) was tested 

 in an operative system which was subsequently demonstrated to the Army 

 with such satisfactory results that the tube was adopted without change for 

 several radar systems. The 709A vacuum tube and its associated cavity 

 are shown in Fig. \. 



A similar structure, known as the 702A and shown in Fig. 2 (together 

 with the 709A tube) was used for longer wavelengths. The need for these 

 tubes was so very great that no time was allowed for their improvement 

 before production was undertaken by the Western Electric Company. 



