MICROWAVE RADAR TESTING 475 



between testing components. Microwave attenuators and pads are of 

 two general types (a) those which employ dissipative elements to absorb 

 power and (b) nnn-dissipativc devices which introduce propagation or 

 C()U])ling loss. 



For the shorter microwaves the most convenient form of attenuator is of 

 the dissipative type, employing a strip or vane of dielectric coated with a 

 resistance material, as for example, carbon-coated bakelite. This is placed 

 in rectangular wave guide with its plane paralleling the side of the guide. 

 The attenuation is varied by varying the depth to which the vane is inserted 

 in the guide (Fig. 23a) or by changing its position in the guide. A valuable 

 feature of such attenuators is that the minimum loss can be made substanti- 

 ally zero. For good impedance match the strip must be tapered. By 

 using two strips the over-all length of the attenuator can be reduced. 

 Extremely satisfactory attenuators of this type covering a frequency range 

 of 8 to 12%, with loss variable from to 35 or 40 db, have been obtained 

 in the frequency range 4,000 to 24,000 mc. 



For the longer microwaves, where wave guides are inconveniently large, 

 attenuators of the wave guide-below-cutoff type are very useful. These 

 consist of a section of round wave guide whose diameter is small compared 

 with wavelength and whose length is adjusted by telescoping (see Fig. 23b). 

 The TMo,i mode has been found very satisfactory, and TEi,i has also been 

 used. Connection is made to the attenuator by a coaxial circuit at each 

 end, with disk excitation for the TMn,i mode and loop coupling for TEi,i. 

 The attenuation formulas are:^^ 



r£i, . A=^-^ ^\ - (^^^J ./Vmeter (13) 



where D = diameter of wave guide in meters. Because of the effect of 

 other modes when the coupling is close, a minimum loss of 20 to 30 db is 

 required before the attenuation becomes linear with displacement. The 

 attenuation differentials are substantially independent of frequency. At- 

 tenuators of this type present a large impedance mismatch at either end, 

 the effect of which may be alleviated by padding or by a termination. 

 Types of pads employed in microwave work include the following: 



(1) Flexible coaxial cable, usually with high resistance inner conductor. 



(2) Coaxial -k with carbon coated rod and discs. 



(3) Coaxial with carbon coated rod as inner conductor. 



(4) Resistance strip in wave guide. 



(5) Directional coupler. 



