10-13] 



BROADBAND SYSTEM DESIGN 



543 



between the two different transmission lines. The transformer is simply 

 a quarter wavelength of transmission line having a characteristic impedance 

 which is the geometric mean of the characteristic impedances of the two 

 transmission lines to be joined. Thus Zj = -^Z^Z^, where Zi and Z2 are 

 the characteristic impedances of the two lines to be joined, and Zt is the 

 characteristic impedance of the quarter-wavelength section of line used for 

 th6 transformer. A simple quarter-wave transformer can provide a perfect 

 match at a single frequency, but is not broadband. A series of quarter-wave 

 transforming sections, successively differing in impedance according to 

 formula, are often used to obtain greatly increased bandwidth. ^^ 



In addition to the various stubs 

 and impedance transformers em- 

 ployed in matching waveguide sys- 

 tems, an assortment of obstacles 

 have been adapted to this purpose. 

 Such devices as ball bearings, dents, 

 screws, pins, posts, irises, and win- 

 dows are being used. While all of 

 these elements are reactive and loss- 

 less, the inductive iris illustrated in 

 Fig. 10-17 is generally preferred, 

 since it does not reduce breakdown 



power of the line. For low-power applications the flexibility of the capaci- 

 tive tuning screw is convenient. 



Matching elements, discontinuities in themselves, are so designed and so 

 positioned on the transmission line as to set up a reflected wave of such 

 phase and magnitude as to cancel the wave reflected from the offending 

 discontinuity. The result is the elimination of standing waves on the 

 generator side of the matching element. A standing wave will still exist 

 between the matching element and the offending discontinuity, however. 

 Best bandwidth will be realized when the matching element is placed as 

 close as possible to the mismatch. 



Fig. 10-17 Symmetrical Inductive Iris 

 in Rectangular Waveguide. 



10-13 BROADBAND SYSTEM DESIGN 



Increased concern over radar countermeasures, and the need for greater 

 flexibility of airborne radar systems as the number of equipments operating 

 in a given band increases, is forcing a continual extension of the frequency 

 band over which systems must be able to operate. Advances in microwave 

 power generators, both mechanically and electronically tuned (see Chapter 

 11) are paving the way for improved system bandwidth capability. Fixed- 



22Seymour B. Cohn, "Optimum Design of Stepped Transmission Line Transformers," 

 IRE Trans., MTT-3, No. 3, 16-21 (April 1955). 



