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 fc of 3GHz demonstrates stopband rejection more than 35dB up to 8fc and insertion loss better than 1dB. The size of the low pass filters are only 0.4λg × 0.155λg and 0.31λg × 0.24λg, (λg is the guide wavelength at center cutoff frequency) .