MICROWAVE RADAR TESTING 455 



type of scale sometimes makes accurate reading difficult in a small compact 

 meter. Scale mechanisms which have been used include counter types, 

 clock face or expanded drum types with gearing between vernier and coarse 

 scales, and a single direct reading scale with divisions arranged in a spiral 

 for compactness. 



Effect of Temperature and Humidity 



To minimize the effects of temperature on the accuracy of readings, invar 

 has been employed for elements whose dimensions affect the wavelength. 

 For accurate work, the scale reading must be corrected for temperature. A 

 rough approximation is that the scale reading varies in accordance with the 

 coefficient of expansion of the metal. 



Water vapor included in the air dielectric of a wavemeter has an appre- 

 ciable effect on the dielectric constant and hence on the resonant frequency. 

 Thus, for example, in going at sea level from 25° C, and 60% humidity to 

 50° C, and 90% humidity, the scale reading should be reduced by .03%. 

 Correction can be made by means of a chart, a convenient form of which 

 has been prepared by Radiation Laboratory. 



Calibration 



Frequency meters are calibrated against sub-standards which in turn are 

 calibrated against a multiplier from lower frequencies for which a high order 

 of accuracy can be obtained. Such multipliers have been made available 

 at Radiation Laboratory and the National Bureau of Standards. The 

 accuracy obtained at interpolation frequencies is of course less than at exact 

 multiples of the base frequency. In the microwave range the accuracy is 

 believed to be of the order of one part in 100,000. 



Power Measurement 



There are two needs for power measurement in radar maintenance, namely 

 (1) in evaluating transmitter performance and (2) in standardizing test 

 signals. Power output is, of course, only one factor in transmitter perform- 

 ance, others being (a) frequency and (b) spectral distribution or shape of 

 RF envelope. Ability to measure absolute power is desirable to permit 

 interchangeable use of test sets in the field. 



Measurement of Pulse Power 



The transmitter power as used in Formula (1) is the average power during 

 the pulse. The relationship of pulse to (long) average power is 



Zi^ = Tfr (7) 



Pt pulse 



