AbstractIn this work, a compact microstrip lowpass‐bandpass diplexer using radial stubs (RSs) is proposed. By controlling the input impedance of the RS resonators, it is possible to obtain a selective lowpass filter (LPF) and a narrow bandpass filter (BPF) with a fractional bandwidth of 5%, without increasing the insertion losses (ILs) of both filters. The design methodology is explained in details and verified through the design of a diplexer achieving very low IL, 0.15 dB for the LPF and 1.2 dB for the BPF. Moreover, the isolation between the ports is 25 dB making the proposed diplexer useful for designing mixer circuits. Experimental results exhibit high correlation with EM simulation data validating the proposed methodology.