10-14] PRESSURIZATION 545 



The consequences of operating a system whose reflection coefficient is a 

 sensitive function of frequency include frequency selectivity of the trans- 

 mission line causing spectrum distortion of pulse transmission, variation in 

 pulling effect as frequency is changed, variation in transmission loss, and 

 the possibility of resonances which might cause magnetron moding. High 

 VSWR resonances, which may be overlooked in spot frequency measure- 

 ments of VSWR, are sometimes responsible for unaccountable breakdown , 

 in high-power transmission line systems. The testing method specified for 

 broad-banded systems must be designed to avoid this possibility. A 

 swept-frequency reflectometer^^ is invaluable for revealing resonances of 

 this nature. 



Many broadband radar systems can operate over a 10 per cent band or 

 more, but considerable effort is being directed toward development of very 

 wideband components which will permit design of systems to operate over 

 the full 40 per cent band of a dominant mode rectangular waveguide. 

 Operating bandwidths even greater than this are possible with strip line 

 and ridge waveguide. 



10-14 PRESSURIZATION 



The microwave plumbing of airborne radar systems is generally pres- 

 surized for two important reasons: first, to prevent RF breakdown^^ at 

 altitude pressures, and second, to eliminate problems of moisture con- 

 densation and corrosion in the transmission line due to breathing. To 

 completely seal the microwave assembly involves capping the antenna feed 

 in some manner with a dielectric cover or window, which then becomes an 

 integral part of the feed itself and is involved in any measurement of the 

 electrical characteristics of the antenna feed. Pressure-sealing gaskets are 

 required at each transmission line coupling, and all branch lines must be 

 sealed with some form of pressurizing window. A TR tube, for example, 

 can serve this function if properly gasket-sealed. Most magnetrons and 

 some of the newer local oscillator klystrons also can be pressure-sealed to 

 the waveguide system. A wide selection of broadband windows are 

 commercially available for pressurizing waveguide systems. Most of these 

 are rated for a differential pressure of 30 psi or more, when mounted with 

 the glass seal on the high-pressure side. Where breakdown power level is 

 not raised sufficiently by pressurization alone, sulfur hexafluoride gas^* 

 (SFg) has been used to raise significantly the power-handling capability of 



26N. L. Pappas, "Measuring Microwave Impedance with the Reflectometer," Tele-Tech 15. 

 No. 72 (May 1956). 



2^Lawrence Gould, Handbook on Breakdown of Air in Waveguide Systems, Microwave 

 Associates, Inc., Boston, 1956. 



"^Sulfur Hexafluoride, Technical Bulletin TB-85602, General Chemical Div., Allied Chemical 

 and Dye Corp., Philadelphia, 1955. 



