10-17] MICROWAVE MATERIALS 565 



In a radar, the isolator is inserted near the magnetron to "isolate" the 

 magnetron from any load mismatch by largely absorbing any power 

 reflected back toward the magnetron. The effectiveness of an isolator in 

 reducing load mismatch is shown in Fig. 10-39, where the reflection coeffi- 

 cient presented to the magnetron is plotted against the load reflection 

 coefficient, for various amounts of "isolation." In general, however, the 

 reflection coefficient presented to the magnetron is not likely to be better 

 than the input reflection coefficient of the isolator. Although resonance 

 absorption isolators are generally rated for a maximum load VSWR of 2 : 1, 

 higher VSWR's are permissible with reduced power. Benefits to be derived 

 from use of an isolatoi include improved spectrum, improved AFC perform- 

 ance, and reduction or elimination of long-line effect^^ and magnetron 

 pulling, moding, skipping, and sparking. 



Many other ferrite devices and applications are now available to the 

 microwave engineer and system designer, who is referred to the numerous 

 current works and references on the subject. 



10-17 MICROWAVE DIELECTRIC, MAGNETIC, AND 

 ABSORBENT MATERIALS 



The role of dielectric, magnetic, and absorbent materials in microwave 

 applications is expanding at a rapid rate. Dielectric synthesis, or the custom 

 tailoring of difelectric materials for specialized microwave applications, is 

 becoming commonplace. Numerous liquid, solid, and gaseous dielectric 

 materials having favorable microwave characteristics are available for 

 application by the design engineer. Teflon,* for example, noted for its zero 

 moisture absorption and low loss-factor and for its frequency-independent 

 dielectric constant, has exceptional microwave properties. Dielectric foams 

 are finding wide use as filling and supporting structures within transmission 

 line components where low loss and low dielectric constant are important. 

 Relatively low loss, high dielectric constant materials ("high-K" materials) 

 having a wide selection of electrical and physical properties are also available. 



The subject of dielectrics has received extensive treatment^^'^" and the 

 dielectric properties of numerous materials have been studied and tabu- 

 lated. ^°'®^'^^^ Among the newer dielectric materials of particular importance 



^J. F. Hull et al., "How Long-Line Effect Impairs Tunable Radar," Electronics 27, No. 2, 

 168-173 (February 1954). 



*Teflon is a trademark of the DuPont Company, Wilmington, Delaware. 



59A. R. von Hippel, Dielectric and Waves, John Wiley & Sons, Inc., New York, 1954. 



fiOA. R. von Hippel et al., Dielectric Materials and Applications, John Wiley & Sons, Inc. 

 New York, 1954. 



61A. R. von Hippel et al., "Tables of Dielectric Materials," Laboratory for Insulation 

 Research, Technical Report 119, MIT, April 1957, Vol. 5. 



6i''"Tables of Dielectric Materials," Laboratory for Insulation Research, Technical Report I2(i 

 MIT, June 1958, Vol. 6. 



