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BELL SYSTEM TECHNICAL JOURNAL 



in the circuit shown in Fig. 4, the receiver is still bridged by the transmitter. 

 It is a fortunate fact that the internal impedance of many magnetrons 

 (the most common type of transmitting tube) becomes very low when they 

 are in the inoperative condition so that the tube is nearly the equivalent 

 of a short circuit. By adjusting the length L», until this equivalent short 

 circuit position is an odd number of quarter wavelengths from the junction 

 point "B", the shunting impedance at ''B" can be made very high so that 

 only a small part of the received energy is lost. In the event that this change 

 of impedance of the transmitter is not sufficient, a second TR switch com- 

 monly known as an ATR, may be introduced to perform this function as 

 will be described later. During the receiving period, some loss will occur 

 in the TR box resonant cavity as a result of the inherent resistive and 

 dielectric losses. An additional loss will occur immediately after the 



Fig. 4 — The elements cf a shunt branching circuit 



transmitting period because of the loss producing particles (free electrons) 

 which remain for a time in the discharge gap. The combined losses must be 

 kept small so as not to impair the performance of the system. 



Most modern radars employ series branching circuits instead of the 

 shunt branching circuit just described. A coaxial line example of such a 

 system (from the SCR-545) employing the 721A tube is shown in Fig. 5. 

 As shown in Fig. 6 the cavity is coupled to the coaxial line by means of a 

 window which can be slid along on a slot in the outer conductor of the coaxial 

 line leading from the transmitter to the antenna. Fig. 7 is an exploded view 

 of the cavity. Such a cavity is in effect in series with the line as the currents 

 existing in the outer conductor of the coaxial line are interrupted by the 

 window-. During the transmitting period the low impedance at this window 

 limits the voltage across it to a small value and prevents serious loss of 

 transmitter power. 



Reception in the series branching circuit of Fig. 5 is achieved by adjusting 



