574 ANTENNAS AND RF COMPONENTS 



The FM, CW, and pulse doppler systems described in Chapter 6 fall in this 

 category. In addition to duplexing problems already mentioned, special 

 problems of phase, amplitude, and frequency stability are encountered. 

 Extraneous modulation of any of these factors by sources other than the 

 radar target of interest -can result in troublesome noise which, if within the 

 passband of interest, will deteriorate system performance. 



Mechanical resonances and vibrations in any part of the microwave 

 plumbing may lead to serious system noise. Thus, components which 

 vibrate or flex under operational stress may introduce phase modulation 

 which adds to system noise. Sections of flexible waveguide, for example, 

 are especially prone to phase modulating when flexing, and unless damped 

 or well secured can be a troublesome source of noise. 



Not the least of the problems in coherent systems is that of duplexing. 

 In CW and FM systems which must rely on a linear duplexer for receiver 

 isolation, even minute variations in antenna reflection coefficient will 

 seriously affect receiver isolation, as can be inferred from Fig. 10-44. 

 Antenna scanning is virtually impossible in such systems because of 

 inevitable modulation of the reflection coefficient with scanning motion. 

 Separate transmitting and receiving antennas are indicated if scanning is 

 required. Separating transmitting and receiving antennas in such a system 

 to obtain the necessary isolation does not, however, solve all the problems 

 of extraneous modulations. 



Mechanical vibrations of the antenna feed or dish also can introduce 

 phase modulation. One especially troublesome area is that surrounding the 

 radome — not only the mechanical vibrations and noise introduced by 

 buffeting of the radome, but also the high-intensity sound waves generated 

 within the radome, which increase seriously as Mach number rises. Added 

 to this are the problems of signal modulation and reflection from the shock 

 wave and turbulence which shrouds a radome traveling at supersonic speed. 

 At hypersonic speeds ionization creates additional modulation problems. 

 Pulsed doppler systems avoid the duplexing problems of other coherent 

 systems, but are faced with many of the same problems of extraneous 

 modulation. 



10-21 SOLID STATE AMPLIFIERS 



Since the original amonia gas MASER work of C. H. Townes and others 

 in 1954, the use of molecular systems to provide low noise amplification at 

 high frequencies has received great attention. In an effort to obtain 

 increased bandwidth with little increase in noise output, suggestions were 

 made in 1956 that solids be used in MASER amplifiers. A further develop- 

 ment was the realization in 1957 that nonlinear reactance MAVAR 

 amplifiers are also capable of providing very low noise amplification. 



