GAS-DISCHARGE TRANSMIT-RECEIVE SWITCH 53 



separate cavity type in which contact through the vacuum envelope is 

 made by means of thin copper discs. More recently other designs of tubes 

 have appeared in which the entire cavity is evacuated. Over 400,000 tubes 

 of the three types discussed in this paper were manufactured in 1944 alone 

 and substantially all of the American-made radars which saw active service 

 employed one or more of these tubes. 



The TR tubes used in American radars are, of course, no more essential 

 than are the magnetrons, the beating oscillators, and the many other special 

 parts which go to make up the modern radar. Nevertheless it is interesting 

 to note that the 721A tube was an essential part of the radar equipment 

 on ahnost every major ship in the United States Fleet, that the 724B tube 

 was an essential part of the bombing equipment on nearly every bomber 

 used against Japan, including the planes which carried the atomic bombs, 

 and that the capture of Okinawa, to name a single case, would have been 

 much more expensive in men's lives without equipment depending upon the 

 1B23 tube. 



Method of Operation 



The 709A tube as shown in Fig. 1 was operated in what has come to be 

 known as a shunt branching circuit. Its operation can be explained in 

 terms of Fig. 4. During transmission, energy flows from the transmitter 

 along the coaxial line toward the antenna. Some of this energy enters the 

 branch leading to the receiver where it encounters the TR box. This con- 

 sists of a resonant cavity with a pair of spark gap electrodes arranged so that 

 the maximum resonant voltage is built up across the gap. Since the voltage 

 across the gap is then limited by the discharge voltage and the voltage ap- 

 plied to the receiver is still further reduced by an equivalent step-down ratio 

 of the output coupling in the resonant cavity, the receiver input power is 

 held to a small value. The power dissipated in the gas discharge, and there- 

 fore abstracted from the transmitted signal is kept small by the impedance 

 mismatch. The discharge itself takes the form of a small pale blue glow 

 between the electrodes. The effect of the discharge is to place a low im- 

 pedance (predominantly resistive) across the maximum impedance point 

 of the cavity. This results in the appearance of a still lower apparent 

 impedance across the input to the cavity. If the length Z-i is an odd num- 

 ber of quarter wavelengths, the apparent impedance of the receiver branch 

 at the branching point becomes very high in comparison with the impedance 

 of the antenna and is therefore unable to abstract much power from the line. 



At the end of the transmitting period, the conductance of the gas discharge 

 falls rapidly to a very low value since the small received voltages will be 

 insufficient to maintain the discharge. Signals arriving at the antenna 

 can then be transmitted through the TR box to the receiver. However, 



