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In this letter, a compact diplexer is implemented by using slotline hairpin stepped impedance resonator (SIR) in ground plane. Two sets of slotline SIR with different dimensions are designed to resonate at different frequencies for the diplexer operation. Also, the SIR can provide a degree of freedom to improve the out-of-band rejection by properly tuning its geometrical parameter ratio. Besides, microstrip-slotline transition is used to split the signal to two channels without extra matching circuits and thereby economizes the circuit size. Finally, good agreement between measurement and simulation is achieved.