Transmission line filter

Claim 1

In a coaxial transmission line low pass filter of the type having an outer conductor and a coaxial inner conductor, said filter having a plurality of cylindrical conductors coaxial with and spaced along said inner conductor, the diameter of each of said cylindrical conductors being greater than the outer diameter of said inner conductor and less than the inner diameter of said outer conductor, said cylindrical conductors cooperating with said outer conductor to provide distributed capacitance sections of relatively low characteristic impedance and wherein lengths of transmission line comprising portions of said inner and outer conductors extending between adjacent cylindrical conductors provide distributed inductance sections of relatively high characteristic impedance, the improvement wherein: said cylindrical conductors are open at each end thereof, said cylindrical conductors being of equal given diameter and wherein means, centrally located within said cylindrical conductors, are provided for electrically short circuiting each of said cylindrical conductors to said inner conductor, adjacent cylindrical conductors having various different predetermined lengths and various longitudinal predetermined spacings therebetween along said inner conductor, said predetermined cylindrical conductor lengths being determined such that the average length of said cylindrical conductors is substantially a half wavelength at a given frequency wherein the average length of the corresponding portion of said cylinders between said centrally located short circuiting means and one end of said cylinder is substantially a quarter wavelength long at said given frequency for providing substantial elimination of at least one spurious pass band in the frequency response of said filter.