GAS-DISCUARGE TR.WSM IT KF.C ElV E S\V ITCH 81 



similar to Fig. 31 but transformed to the magnetron side of the ATR junc- 

 tion. This may be done by subtracting the ATR impedance from the values 

 read off of Fig. 31 corresponding to desired values of F and replotting these 

 on the reflection coefficient plane. As an example, the in-tunc value ofZ 

 for one typical 724B ATR cavity is 8 + jO. Points lying on the i?= 8 

 circle on Fig. 31 will then lie on the magnetron J? = circle, the region 

 inside being distorted and expanded to fill the entire positive R region on 

 the reflection plane. The results are shown in Fig. 32. From this plot it 

 is evident that the maximum possible low-level loss chargeable to the trans- 

 mitting branch would be slightly more than 0.52 db and that this would 

 occur only for a restricted range in the value of magnetron impedance. As a 

 matter of practical interest the "cold impedance" of the usual magnetron 

 is such as to give at most a 20-db standing wave. This restricts the possible 

 range in impedance values to the area on Fig. 32 within the dotted circle, 

 thus limiting the maximum loss to slightly less than 0.52 db, and imposing 

 a minimum loss limit of 0.22 db. 



This type of analysis may be extended to consider the ATR loss during 

 the recovery period if desired although the problem becomes rather com- 

 plicated as a result of the simultaneous variation in input impedance of both 

 the TR and the ATR. 



TR Box Design Considerations 



The desired electrical properties for a TR box can of course be achieved in 

 a variety of different physical structures. A construction technique which 

 separates the gas-discharge tube from the rest of the TR box cavity offers 

 many advantages. In the first place the cost of the entire device is kept low 

 by reason of the fact that it is not necessary to transmit the tuning motion 

 through the vacuum-tight tube enclosure. The replacement cost is also 

 greatly reduced since the more complicated part of the TR box is a perma- 

 nent part of the equipment. Then, the same tube structure can be used 

 for a variety of different types of equipment operating in different wave- 

 length bands and requiring different amounts of receiver protection by the 

 use of different size cavities and different size coupling windows. This 

 greatly simplifies the problem of maintaining replacement stocks. An addi- 

 tional factor, which was of importance during the early days of the war, 

 is that the design of such a tube can be frozen at an early stage, before all 

 the possible circuit aspects of the TR problem have been solved since changes 

 in the external parts of the TR box can be made independent of the design 

 of the replaceable tube element. The widespread use of the 721A and 724B 

 vacuum tube is, in a sense, proof of the essential soundness of the arguments 

 for the external cavity type of construction. 



