14-4] MAJOR CHARACTERISTICS AND COMPONENTS 737 



one used in the earlier doppler radars largely because of the state of the art 

 of techniques and components at the time. The continuous wave (CW) 

 type of transmission is the simplest and inherently most efficient technique; 

 however, it is still faced with a number of difficulties (primarily transmitter- 

 receiver isolation) which have made the design and installations of such 

 systems somewhat precarious and troublesome and have resulted in 

 double-aperture antenna systems requiring larger antenna cutouts in the 

 airframe. The high-duty cycle (25-50 per cent) pulse and the frequency- 

 modulation-continuous-wave (FM-CW) types of transmissions combine 

 most of the advantages of the other two systems, namely high efficiency, 

 high transmitter-receiver isolation (permitting single antenna operation), 

 and only moderate complexity. 



The coherent type of high-duty cycle pulse system is intrinsically 

 somewhat more efficient than the incoherent type, primarily at high 

 altitudes. However, it tends to be more complex than the incoherent 

 system, since in addition to special modulation techniques it normally 

 requires several klystron tubes as compared with the single magnetron of 

 incoherent systems. Also, at least from certain aspects, the transmitter 

 frequency stability requirements for a coherent pulse system are more 

 critical than for an incoherent pulse system. For the latter system the 

 frequency needs to remain only within the receiver IF bandwidth, and 

 the frequency deviation during a transmitted pulse must not be large 

 compared with the reciprocal of the pulse duration. 



The FM system combines many of the advantages of CW and pulse 

 systems. It resembles a CW system in simplicity, since normally a single 

 klystron is required in the transmitter which needs to be modulated only 

 by a simple sine-wave oscillator. The receiver of such a system is so 

 designed as to accept and use only the doppler shift of a particular sideband 

 of the beat between the received and transmitted signals; thereby the return 

 energy from nearby objects is largely rejected. This is so because the 

 modulation index of the beat, and hence the amplitude of all but the 

 zero-order sidebands, decreases very rapidly with decreasing range, 

 becoming zero at the receiver terminals. Transmitter-receiver isolation 

 and radome reflection problems (prevalent in CW systems) are eliminated 

 here on a frequency basis, much as they are eliminated in pulse systems 

 on a time basis. Since the energy in all other sidebands, except a particular 

 one, is rejected in such a system, a certain spectrum utilization inefficiency 

 results. In certain cases, coherence requirements are similar to those of CW 

 and coherent pulse systems, and the requirement for maintaining the 

 modulation index at a particular value is reasonably critical. In single- 

 antenna FM-CW systems the problem of receiver crystal deterioration 

 caused by transmitter power feed-through can be severe, preventing the use 

 of higher transmitter powers (greater than 1 watt or so) which might be 



