A Simple Method for Spacing the Adjacent Passbands of a Coupled-Line Filter

A bandpass filter using two coupled transmission lines is considered. Two ports of the coupled-line four port are both short-circuited (or open-circuited); the other two ports are terminated in the characteristic impedances of the lines (e.g., 50 Omega). For a constant coupling along the coupling section l the attenuation poles are fixed at l/lambda = n/2 (n = 0,1,2,...). If, however, the coupling changes along the coupling section in three steps with the coupling factors k₁, k₂, k₃ = k₁, the stopband between two adjacent passbands can be extended for certain values of k₁ and k₂. A simple calculation method for the coupling factors k₁ and k₂ is described. A practical design example shows good agreement with theoretical results.