Radial Loaded Transformed Radial Stub for Lpf Stopband Extension

A low pass filter with ultra-wide band rejection and compact size using a proposed radial loaded transformed radial stubs is introduced and investigated. The implemented unite cell low pass filter with 1-dB cutoff frequency of 3.2 GHz demonstrates stopband rejection up to 11.8 i.e. 38 GHz. The design is further extended to the high-order LPF through cell cascading. The implemented four-cell LPF with of 3.6 GHz demonstrated 35dB rejection from 4.8GHz to 39GHz, 50 dB rejection from 5.4 GHz to 26 GHz (7.2 ) and 20dB rejection from 4.4 GHz to 50 GHz (13.8 ). The measured passband insertion loss is less than 1.7 dB, and group delay is from 0.45~0.8 nS. The size of the four-cell low pass filter is only 0.33λg × 0.135λg, (λg is the guide wavelength at center cutoff frequency) without using any lumped elements.