210 BELL SYSTEM TECHNICAL JOURNAL 



employ an artificial dielectric material which duplicates, on a much larger 

 scale, processes occurring in a true dielectric. This involves arranging 

 conducting elements in a three dimensional array or lattice structure to 

 simulate the crystalline lattice of the true dielectric. Such an array responds 

 to radio waves just as a molecular lattice responds to Hght waves, and if the 

 spacing and size of the elements is small compared to the wavelength, the 

 index of refraction is substantially constant, so that lenses made of this 

 material are effective over large wavelength bands. 



A lens employing conducting spheres as the lattice elements is sketched 

 in Fig. III-6. A more convenient structure for large lenses is shown in 

 Fig. III-7 and III-8; it uses thin metalUc strips, with the width dimension 

 parallel to the electric vector. Slotted polystyrene foam sheets support 

 the strips and they are stacked up to form the lens. A quarter wavelength 

 step in the lens causes the reflections from the lens surfaces to cancel at the 

 feed point, which, in the drawing, is the apex of the horn shield. 



Over a 10% wavelength band, a 6 foot square shielded lens antenna of 

 this type exhibited an efficiency of better than 60% and the impedance mis- 

 match due to the lens produces only a 0.2 db standing wave ratio in the 

 feed line. This antenna thus retains the dimensional tolerance, weight, 

 size and crosstalk advantages of the shielded lens over the shielded reflector, 

 and has the advantage of higher gain and broader band performance over 

 the shielded metal plate lens, 



IV. Filter Research* 



Frequency space for common carrier radio relay systems is available in 

 blocks several hundred megacycles wide. Where heavy traffic is to be 

 carried such bands must be efficiently exploited. This may in time be 

 accomplished by using extremely wide band amplifiers, for example traveling 

 wave tubes; however, more immediate success is offered by the possibility 

 of operating a number of narrower band circuits of different frequencies. 

 This could be done by using a separate transmitting and receiving antenna 

 for each circuit. But each antenna, for sound technical reasons, must be 

 large and expensive and in addition requires adequate tower support. 

 Consequently there is a need for filters which can connect a number of 

 individual radio channels to a common antenna. 



The design22 of these radio frequency branching filters must be coor- 

 dinated with the design of the relay system as a whole. At lower frequencies 

 where little or no antenna crosstalk protection can be counted on it is natural 



* This section prepared by W. D. Lewis and L. C. Tillotson who were responsible for 

 a large part of the research on filters. 



« For more detailed discussion see, W. D. Lewis and L. C. Tillotson "A Constant 

 Resistance Branching Filter for Microwaves," B. S. T. J., vol. 27, pp. 83-95, Jan. 1948. 



