544 ANTENNAS AND RF COMPONENTS 



frequency radar systems may become largely a thing of the past for military 

 use. 



Special care is required in the design and selection of antennas and 

 components for broadband systems. Each separate element and component 

 of the system must be individually broadband matched to a high degree of 

 excellence to ensure satisfactory broadband operation of the complete 

 microwave assembly. If the individual elements of a transmission system 

 are not in themselves well matched over a broadband, then the prospect of 

 matching the assembled system over a broadband is poor. Several compo- 

 nents, individually well matched, when cascaded in a microwave system 

 can create an objectionably high standing wave ratio at certain frequencies 

 if the reflection coefficients of the several elements add in phase. ^^ Thus 

 the quality of components for broadband systems, particularly systems 

 employing numerous components, must be exceptionally good to avoid 

 wide fluctuations of reflection coefficient with change in operating fre- 

 quency. In the worst case, however, the highest reflection coefficient for 

 the assembled system will not exceed the arithmetic sum of the individual 

 voltage reflection coefficients. 



Components, to be broadbanded, basically should not involve severe 

 impedance transformation. Where the transformation is large, either a 

 gradual tapered transformation or a well-designed stepped-type transfor- 

 mation is required for broadbanding. Techniques for matching components 

 with irises are well developed, ^^'^^ but as a simple first principle, for best 

 broadband performance, the matching element should be located as close 

 as possible to the component to be matched. 



There are some important principles to follow in the design of broadband 

 systems: (a) The number of transmission line components used should be 

 kept to a minimum. A proposed layout should be critically reviewed 

 to eliminate superfluous bends, twists, transitions, flange joints, etc. 

 (^) Components used should be individually well matched, (c) Whenever 

 possible, the elements of a transmission circuit should be grouped closely 

 together with a minimum of transmission line between components. This 

 will aid in matching the system as a whole, (rt') Subassemblies of groups of 

 elements should be broadband matched before final assembly in the 

 complete system. 



23Richard K. Moore, "The Effects of Reflections from Randomly Spaced Discontinuities 

 in Transmission Lines," IRE Trans. MTT-5, No. 2, 121-126 (April 1957); J. A. Mullen and 

 W. L. Pritchard, "The Statistical Prediction of Voltage Standing-Wave Ratio," IRE Trans. 

 MTT-5, No. 2, 127-130 (April 1957). 



^■IR. G. Fellars and R. T. Weidner, "Broadband Waveguide Admittance Matching by Use 

 of Irises," Proc. IRE 35, 1080-1085 (1947). 



25Peter A. Rizzi, "A Low VSWR Matching Technique," IRE Trans. MTT-4, No. 3, 

 185-186 (July 1956). Supplementary comments in the IRE Trans. MTT-5, No. 2, 163 

 (April 1957). 



