Comparison of Inductive and Capacitive End Couplings in the Design of a Combline Microwave Cavity Filter for the E1 Galileo Band

Inductive and capacitive end couplings in the design of combline microwave filters are compared. For instance, a combline microwave cavity filter for the E1 Galileo band (1559-1591 MHz, 2 % fractional bandwidth) is considered. The inductive end coupling is composed of an L-shaped pin in galvanic contact with the end resonator, while the capacitive end coupling is realized by a straight pin parallel to the end resonator's axis. Although both end couplings can realize the desired external quality factor, the capacitive end coupling is easier to manufacture, while the inductive end coupling is less sensitive to the design parameter. An excellent agreement between synthesized (5th-order Chebyshev mask with 0.05 dB ripple) and measured responses is observed for the realized prototype of the filter. The insertion losses at the center frequency are 1.72 and 1.53 dB for the inductive and capacitive end couplings, respectively. The spurious-free ranges are up to 8.23 and 8.92 GHz for inductive and capacitive end couplings, respectively.