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By utilizing two synthesized microstrip lines, a new miniaturized diplexer is proposed and validated in this letter. The synthesized line, formed by series LC tanks and meander line inductors, behaves like a conventional 50 microstrip line in one frequency band but is equivalent to an open circuit in the other band. This property dramatically improves the electrical performances of the diplexer while still keeping a very compact circuit size. The footprint of the diplexer is merely 0.083 × 0.049λ02. At the center frequencies, the transmission losses are <; -0.5 dB, while the reflection coefficients are <; -27 dB. The fractional bandwidths are 16% and 34%, respectively, in the two bands.