THE RADAR RECEIVER 803 



to be considered important for military equipment may result in a frequency 

 shift of 20 mc from the time the equipment is turned on until thermal equi- 

 librium is established. The magnetron frequency is extremely sensitive 

 to its terminating load impedance. This termination in a radar equipment 

 is composed of the antenna, the interconnecting RF transmission line, and 

 the duplexing devices. The typical radar antenna system employs rotating 

 joints or connecting devices to enable transfer of RF power to the antenna 

 proper while it is mechanically operated over its scanning cycle. These 

 connections cannot be made to present an entirely uniform impedance over 

 their entire mechanical operating range and thus introduce variable im- 

 pedance irregularities to the magnetron generator. The frequency of these 

 impedance variations may range from a fraction of a cycle per second to 

 perhaps 60 cps. The input impedance of the antenna proper is dependent 

 on the extent and character of nearby obstructions in the radiation path. 

 The characteristics and form of the radome employed to protect the antenna 

 contribute to the variable impedance characteristics of the antenna and 

 thus influence the magnetron frequency. An additional instability in mag- 

 netron operation which is introduced through power supply variations within 

 the modulator and transmitter portion of the system must also be considered 

 in the detailed design of the AFC system. 



The receiver itself is responsible for a major contribution to the frequency 

 instability characteristics of the radar system. The local beat oscillator 

 frequency is critically dependent on the physical dimensions of its oscillatory 

 structure and on the supply voltages. The effects of temperature and at- 

 mospheric pressure on the frequency of a reflex oscillator of the types pre- 

 viously described is considerable. For example, a thermal coefficient of 

 0.25 mc per degree Centigrade, typical of the 10,000-mc tubes, will produce a 

 total excursion of perhaps 25 mc over the range of ambients experienced in 

 military equipment. In the case of supply voltage variations, a 5-mc fre- 

 quency shift will result for a 1% change in anode and repeller potential for a 

 typical 10,000-mc reflex oscillator. Another source of receiver frequency 

 instability is associated with the shift of the IF amplifier frequency selectiv- 

 ity characteristic with tube aging and operating conditions. 



If the operating requirements for an AFC system are now reviewed from 

 a consideration of these factors, it will be observed that for a radar system 

 operating at the higher frequencies a total effective frequency change of 

 perhaps as much as 50 mc may be encountered whose rate of change, in 

 general, will be relatively slow and may be classified generally as effects 

 due to "warm up". In addition fast variations of frequency will be present 

 whose rates of frequency change may extend from 1 mc per second per 

 second to 1000 mc per second per second. At the lower radar frequencies 



