566 ANTENNAS AND RF COMPONENTS 



for microwave applications are the aluminas, the ferrites, and the special 

 glass-derivative crystalline materials of the Pyroceram] family. The 

 aluminas and the Pyroceram materials are characterized by unusual 

 electrical, mechanical, and thermal properties. Pyroceram possesses the 

 important advantage that it can be fabricated to shape by conventional 

 glass-forming techniques. Heat treatment after forming converts it to a 

 hard, non-porous, fine-grained crystalline material having a density slightly 

 less than that of aluminum. A most important application of the material 

 is in the manufacture of precision radomes for high-speed airborne systems 

 as was mentioned in Paragraph 10-9. 



Low-loss ferrites have found their principal microwave application in the 

 various reciprocal and nonreciprocal microwave ferrite devices which 

 exploit the gyromagnetic resonance properties of the materials. The low 

 dielectric loss and the high dielectric constant (e^ typically 13) of the 

 microwave ferrites make them potentially valuable as special-purpose 

 dielectrics. In an interesting application to dielectric rod antennas,^" both 

 the dielectric and the magnetic properties of ferrite have been exploited in a 

 single element. The high dielectric constant of the ferrite permits the rod 

 antenna to be miniaturized, while the magnetic properties are utilized in 

 an integral Faraday rotator for electronic lobe switching or control of 

 polarization. Ingenious application of dielectric and magnetic materials 

 in microwave systems can lead to simplification, size reduction, and 

 Improvement in performance. 



Numerous microwave absorbent materials, some still in a classified 

 category, have been developed for a wide range of applications. Lossy 

 plastic materials, readily molded or machined, are used in such applications 

 as dummy loads and attenuators. Thick blocks of impregnated plastic 

 form, or mattress-like slabs of treated hair fiber,^^ some with surfaces 

 elaborately carved to minimize surface reflections, are used as camouflage 

 materials and for lining microwave "darkrooms. "^^ Many of these thicker 

 absorbent coverings reflect less than 1 per cent of the incident power, over 

 wide frequency ranges. For narrow-band applications, absorbers can be had 

 in rather thin sheet form. 



10-18 THE DUPLEXING PROBLEM 



It is common practice in radar systems to use a single antenna for both 

 transmitting and receiving whenever possible. This is done not only to 



tPyroceram is a trademark of the Corning Glass Works, Corning, New York. 



62F. Reggia et al., "Ferrod Radiator System," Proc. IRE 45, 344-352 (1957). 



63W. H. Emerson et al., "Broadband Absorbing Materials," Tele-Tech 14, No. 11, 74 

 (November 1955). 



«4A. J. Simmons et al., "Anechoic Chamber for Microwaves," Tele-Tech 12, No. 7, 47 

 (July 1953). 



