A novel microstrip low‐pass filter is presented to achieve an ultra‐wide stopband with 11 harmonic suppression and very sharp skirt characteristics. The filter is composed of a modified U‐shaped resonator (which creates two fully adjustable transmission zeroes), a T‐shaped resonator (which determines a cut‐off frequency), and four radial stubs (which provide a wider stopband). The operating mechanism of the filter is investigated based on a proposed equivalent‐circuit model, and the role of each section of the proposed filter in creating null points is theoretically discussed in detail. The presented filter with 3 dB cut‐off frequency (fc=2.35 GHz) has been fabricated and measured. Results show that a relative stopband bandwidth of 164% (referred to as a 22 dB suppression) is obtained while achieving a high figure‐of‐merit of 15,221.

A new type of lowpass filter with an ultra-wide stopband, sharp transition and compact size is proposed. To reduce the circuit size, a bended transmission line is adopted in the stepped impedance resonators. To achieve a sharp roll-off, a pendulum-shaped resonator is implemented in the filter. The transition band is from 1.30 to 1.45 GHz with −3 and −20 dB, respectively. The stopband with an attenuation level better than 20 dB is from 1.45 up to 23.4 GHz, hence an ultra-wide stopband with 18th-harmonic suppression is achieved. Furthermore, the proposed filter exhibits an extremely small size of 0.09λ g × 0.11λ g where λ g is the guided wavelength at the cutoff frequency.

In this paper a new lowpass filter (LPF) with curved line resonators and fountain shaped resonators is proposed. The proposed filter has achieved wide stopband, compact size, sharp roll-off rate and low return loss. Two tapered resonators are used to produce transmission zeros and widen the stopband. Also, a fountain shaped resonator is proposed using curved transmission lines to obtain good sharpness in the filter response. The cut-off frequency of the designed LPF is 1.74 GHz and the overall size of the designed LPF is 0.135 λg × 0.132 λg where λg is the guided wavelength at the cut-off frequency. The results show that the overall size of the lowpass filter could be reduced using curved line resonators and curved transmission lines.