MICROWAVE RADAR TESTING 473 



and much better values are obtained in the laboratory. Some of the nu- 

 merous designs of directional couplers developed for association with operating 

 radars are shown in Fig. 21. 



Because they are more convenient than slotted lines and can be made more 

 accurate, directional couplers have been extensively used for SWR measure- 

 ment in the laboratory. A number of special arrangements have been 

 devised to improve both accuracy and convenience. A directional coupler 

 arrangement which has been provided for field measurement of SWR in the 

 vicinity of 25,000 mc is also illustrated in Fig. 21. In this the direct power 

 is brought to equality with the reflected power by an attenuator whose dial 

 is calibrated directly in SWR. A wave guide switch facilitates the power 

 comparison. 



Auxiliaries and Components 

 RF Loads 



An RF load (or dummy antenna) which will absorb the radar power in 

 an impedance which matches the transmission line is very useful in radar 

 work. Such a device permits testing the radar in operating condition with- 

 out actual radiation which might give information to the enemy or interfere 

 with other radars. It also makes it possible to test the radar in locations 

 where reflections from the ground or nearby objects would otherwise hamper 

 or prevent a test. RF loads for microwave work usually consist of a section 

 of transmission line (either coaxial or wave guide, depending on wavelength) 

 containing a high-loss dielectric. The impedance of such a load is neces- 

 sarily low and must be matched to the radar line by tapering the dielectric 

 over a distance of several wave lengths.^" Moreover, if the line is to handle 

 high power, tapering over a considerable length is necessary to distribute 

 the heat. 



A coaxial load is preferably tapered from outer conductor to inner con- 

 ductor, since this both reduces the voltage gradient and facilitates heat 

 dissipation. A dielectric consisting of a mixture of bakelite, silica and 

 graphite, molded in place, has been found satisfactory. For wave guides a 

 ceramic containing carbon may be preformed, with taper in one or two 

 dimensions, and cemented in place. 



Figure 22 shows a number of RF loads developed for different frequency 

 bands. One of these, TS-235/UP, provides an excellent impedance match 

 over the frequency range from 500 mc to above 3,000 mc. When equipped 

 with a blower designed for uniform transverse ventilation, it will handle a 

 peak power of the order of 750 kw with a duty cycle of about .001. 



Microwave Attenuators and Pads 



RF attenuators and pads are cornerstones of microwave testing. At- 

 tenuators are used to adjust unknown signals to levels suitable for measure- 



