This paper proposes a new compact quad-channel diplexer (2.45/4.2 GHz and 3.5/5.2 GHz) using defected stepped impedance resonators (DSIRs). The proposed quad-channel diplexer is composed of one common input feeding line, sixteen folded DSIRs, and two output feeding lines. Every four DSIRs are designed to determine passband characteristics of one individual channel, and two passbands are filtered out eventually at each output port. The distributed coupling technique featured by small loading effect is introduced to eliminate the necessity of extra impedance matching networks, which consequently results in a reduced circuit size. A diplexer prototype operated at 2.45/4.2 GHz and 3.5/5.2 GHz bands with measured 3-dB fractional bandwidths of 12.5%, 7.2%, 6.4%, and 5.0% has been implemented, showing a high isolation of larger than 33 dB between the two output ports. Experimental results coincide well with the theoretical predictions and simulation results.
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