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An improved mushroom resonator cell is proposed for the design of the compact elliptic-function lowpass filter (LPF). This resonator cell provides sharp skirt and wide stopband for the LPF. As an example, a five-pole elliptic-function LPF with cutoff frequency 2.5 GHz has been designed and fabricated. The measurement results show good agreement with those of simulations. The measured insertion loss in the passband is less than 0.3 dB, and the stopband over 20 dB is from 3 to 10.7 GHz. The dimensions of the LPF are as small as 6 15 mm (i.e. 0.082 0.18 where is the guided wavelength at cutoff frequency).