A Tri-Band Bandpass Filter Realized Using Tri-Mode T-Shape Branches

In this paper, we present a tri-band fllter design using tri-mode T-shaped branches connected by ‚=4 transmission lines. By analyzing the input admittance of a T-shape branch with commensurate electrical lengths, three resonant modes with two transmission zeros between are found and design formulas are derived. The fllter can be regarded as a combination of three bandpass fllters with only one set of coupling elements. To realize difierent bandwidths for each, the admittance slope of each resonating mode is set as required. A genetic algorithm is used in solving related equations to obtain the impedance of each line in a T-shape branch, followed by a flnal optimization. A three-pole tri-band fllter having passbands of 0.6{ 0.9, 1.35{1.65 and 2.1{2.4GHz, is designed, fabricated and measured with low passband insertion losses of 60dB between the passband regions. As a generalization, necessary to achieve a tri-band fllter with arbitrary passbands, a non-commensurate version of the T-shape branch is introduced. An example fllter design