Linear-phase transmission-line filters with increased selectivity

A new class of transfer functions approximating a constant group delay that can be realized as transmission factors of doubly loaded transmission-line filters of commensurate line lengths is introduced. The magnitude response obtained is more general than the analogous response of filters having a maximally flat type of approximation to a constant delay. This is due to an additional parameter that may be used to obtain a magnitude response having either better passband performance or sharper cutoff than the maximally fiat type of delay approximation. Explicit formulas for the evaluation of the coefficients of transmission factors of any order n are derived.