456 BELL SYSTEM TECHNICAL JOURNAL 



T is the pulse duration in seconds and fr is the pulse recurrence frequency 

 (P. R. F.) in cycles per second. The product T/^ is the duty cycle. (Some- 

 times the reciprocal of this number is referred to as the duty cycle. The 

 magnitude is usually such that no ambiguity arises.) 



During the early days of radar it was the practice to measure pulse power. 

 The test equipment was coupled to the radar by a path of known loss. The 

 RF envelope was derived by means of a crystal rectifier and applied to an 

 oscilloscope. With the aid of an RF attenuator the level applied to the 

 crystal rectifier and oscilloscope was held constant. Calibration was ob- 

 tained by using a signal generator whose output was standardized, prior to 

 pulsing, with an averaging type of power meter. The procedure was rather 

 involved, with several sources of possible error. Since it is much simpler 

 to measure (long) average power, field measurement of pulse power was 

 soon abandoned. Though the pulse power can be computed from average 

 power if the pulse width, pulse shape and repetition rate are known, it soon 

 became the practice to specify field performance requirements in terms of 

 average power. 



Thermistor Power Meters 



A number of devices have been used for measuring average power in the 

 microwave range. Those suitable for handling the small amounts of power 

 normally involved in field tests include (1) thermistors, (2) platinum wires 

 and (3) thermocouples. In each case the RF power to be measured is 

 absorbed in the measuring element. The measurement consists in observing 

 the resistance change m the thermistor or platinum wire, or the thermo- 

 electric voltage from the thermocouple. By analogy with devices used for 

 measuring minute quantities of radiant heat, either a thermistor or a 

 platinum wire instrument is sometimes referred to as a bolometer. The 

 platinum wire device has also been termed a barretter. 



A thermistor for microwave power measurement is a tiny bead (about 5 

 mils in diameter) composed of a mixture of oxides of manganese, cobalt, 

 nickel and copper, constituting a resistor with a very high negative tem- 

 perature coefficient. 



The thermistor has a number of advantages for microwave work, namely: 

 (1) resistance is highly sensitive to change of heating power, which obviates 

 any need for amplification or a super-sensitive meter, and makes it possible 

 to use a rugged d.c. meter; (2) reactance is low compared with RF resistance, 

 which makes it possible to incorporate the thermistor in a power absorbing 

 termination which matches the impedance of a microwave transmission line 

 over a wide band; (3) resistance change is the same function of electrical 

 heating power at any frequency, which permits direct comparison of the 

 unknown microwave power with easily measurable d.c. power; (4) sensitivity 



