Novel low cost compact size planar low pass filters with deep skirt selectivity and wide stopband rejection

Low pass filters with ultra-wide band rejection and compact size by using radial stub and proposed radial stub with transformer are investigated. The stopband skirt selectivity and stopband width of lowpass filter are improved through generation of multiple zero points using proposed new structures. The implemented low pass filters with cutoff frequency f_c of 3 GHz demonstrates stopband rejection more than 35 dB up to 8 f_c and insertion loss better than 1 dB. The size of the low pass filters are only 0.4 λg × 0.155 λg and 0.31 λg × 0.24 λg, (λg is the guide wavelength at center cutoff frequency).