MICROWAVE RADAR TESTING 457 



to damage and bum-out is inherently low, and added protection results 

 from impedance mismatch during overload. Because of these character- 

 istics thermistors have been far more widely used than other detectors for 

 microwave power measurement. Broad-band thermistor mounts have been 

 designed to match both wave guide and coaxial transmission lines, the latter 

 not only in the microwave range but also down to low frequencies. Some 

 of the test sets specifically intended for power measurement or for combined 

 power and frequency measurement are shown in Fig. 8. 



The change in the thermistor resistance due to RF heatmg current is 

 determined by placing the thermistor in one arm of a d.c. bridge. By 

 noting the d.c. power necessary to balance the bridge with and without RF 

 power in the thermistor, the magnitude of the RF power may be determined. 

 For most purposes, however, a direct reading power meter is preferable. 

 This can be obtained over a moderate range of power levels by employing 

 an unbalanced bridge. The bridge is balanced for d.c. only and the measure- 

 ment consists in noting the meter deflection when RF power is added. 



The resistance of a thermistor is a highly sensitive function not only of 

 electrical heating power but also of ambient temperature. For convenient 

 field measurement, the effect of ambient temperature must be cancelled out 

 in the indicator circuit so that the indication depends only on RF power. 



Water Loads 



A method which has been used in the laboratory and factory for measuring 

 high-level microwave power consists in terminating the RF transmission 

 line in a water load arranged as a continuous flow calorimeter. This method 

 can be made quite accurate but is cumbersome. More recent practice is 

 to terminate the RF line in a solid load of a type described later in this ar- 

 ticle, and to couple a thermistor power meter to the line by means of a 

 directional coupler (described below) of known loss. Ver>^ close correla- 

 tions have been obtained between the two methods over the entire micro- 

 wave band. 



Echo Boxes 



A device unique to radar testing is a high Q resonant cavity, known as an 

 "echo box" or "ring box." The cavity is coupled to the radar transmission 

 line or antenna as indicated in Fig. 11. During the transmitted pulse, 

 microwave energy is stored in the cavity. In the period immediately there- 

 after, energy is returned to the radar over the same path, producing a signal 

 on the radar indicator. The energy in the cavity builds up exponentially 

 to an amplitude dependent on the radar power. .\t the end of the pulse 

 the returned energj- decays exponentially, disappearing into the noise at a 

 point determined by receiver sensitivity. The time interval between the 



