This paper presents a novel microstrip quad-channel diplexer based on stub loaded U-shape resonators, which are coupled to the step impedance feed lines. The stubs are loaded inside the U-shape cells creating extra channels without increasing the size of diplexer. The proposed diplexer is miniaturized with an overall size of 0.029 λ g 2 . It operates at 1.67, 2.54, 3.45 and 4.57 GHz for GPS, wireless and WiMAX applications. Due to its narrowband channels, it is appropriate for the modern long-range communication systems, which are widely accepted by the industry. The proposed diplexer has high performance in terms of low insertion and return losses and wide stopband. The insertion losses at the resonance frequencies are 0.5, 0.38, 0.53 and 0.58 while the common port return losses are better than − 20 dB at all channels. In order to verify the simulation results, we fabricated and measured the designed diplexer. A good agreement between both results is obtained.
