MICROWAVE lowpass filters (LPFs) are one of the fun- damental blocks in wireless communication systems. In modern microwave communication systems, microstrip LPFs with good specifications such as a compact size, sharp roll- off, low insertion loss, and high return loss are in demand and widely used [1]. Hence, various techniques have been studied to achieve a LPF with the mentioned features [2]–[11]. In [2], a LPF using polygonal resonators was introduced, but the struc- ture suffered from a gradual transition band. A structure with triangular patch resonators was proposed in [3], but the LPF had a low suppression factor. In order to reach a better performance, a LPF using a coupled-line hairpin unit was introduced in [4]. In another approach, a LPF with a sharp roll-off and wide stopband was proposed in [5], but the size of the LPF was relatively large and the unsuitable return loss has an adverse impact on the VSWR. In [6], a novel structure was designed to achieve a suitable return loss, but the other specifications still need to be improved. In another study, a configuration using stepped impedance resonators with good features was reported