540 



ANTENNAS AND RF COMPONENTS 



In many applications, square waveguide, employing orthogonal TEio 

 modes, can be used instead of circular waveguide, with the advantage of 

 greater bandwidth and reduced problems in mode coupling and ellipticity. 

 It should be appreciated that circularly polarized waves can propagate in 

 square waveguide. Faraday rotation-type microwave ferrite devices, for 

 example, which depend on circularly polarized waves for their operation, 

 are often designed in either circular or square waveguide. These devices 

 are described in more detail later in this chapter. 



Strip Lines. Strip-type transmission lines offer many possibilities 

 for reduction in size, weight, and cost of microwave assemblies. Two types 

 of strip lines are illustrated in Fig. 10-15. In other types the strip conductor 



Conducting Strip 



Ground Plane 



Conducting Planes Conducting Strip 



(a) (b) 



Fig. 10-15 Two Types of Strip Transmission Lines. 



may be principally air supported, or supported by a thin dielectric sheet in 

 the plane of the strip conductor. Certain types are especially adaptable to 

 printed-circuit production techniques like photoetching. 



In general, strip lines have higher loss and lower power-handling capabil- 

 ities than conventional waveguides and generally require a shielding 

 enclosure to prevent leakage and crosstalk. Current applications are to 

 microwave filters, receiver heads, printed antenna arrays, and in miniatur- 

 ized microwave assemblies. Strip lines are undergoing intensive research,^^ 

 while present applications require custom development of the product. 



10-12 STANDING WAVES AND IMPEDANCE MATCHING 



The problem of standing waves universally arises in any dealing with 

 practical RF transmission line systems. This is particularly true in the 

 complex antenna and plumbing assemblies of modern radar systems, where 

 numerous circuit elements are involved. The presence of standing waves in 

 a transmission line system is generally undesirable and often harmful to the 

 proper operation of the system. Standing waves, caused by reflections from 

 impedance discontinuities somewhere along the transmission line, are a 

 simple example of interference phenomena between two coherent waves 



2i"Symposium on Microwave Strip Circuits," IRE Trans. MTT-4, No. 2 (March 1956). 



