The X-band microstrip filter design

A fifth-order hairpin filter and a seventh-order hairpin filter are designed in this paper. The center frequency is 10.2GHz and the bandwidth is 1.9GHz. For the fifth-order hairpin filter, quarter-wavelength open-circuit stubs are added in the tapped lines to inhibit the 2nd harmonic. A microstrip line is added between the non-adjacent resonators to improve the selectivity of the filter. Progressive tapped lines avoid the mismatch of joints. As for the second and the fourth resonance units, their self-coupling spacing is widened to increase the frequency distance between their transmission poles and the rest units' transmission poles. As a result, the possibility of achieving wider bandwidth is raised. The insertion loss and the return loss are better than 1.39dB and 19.26dB, respectively. 19.76dB suppression within 6–8.85GHz and 40.40dB within 12–14GHz are achieved. 40dB suppression of 2nd harmonic(2f0) and 20dB suppression up to 2.4f0 are obtained as well. The seventh-order hairpin filter is designed by changing the parameters of the symmetrical resonators and coupling different resonators. It has been proved in this paper that coupling different resonators can be an easier way to acquire good performance in the target frequency band, compared with coupling the same resonators. The simulation results show that the insertion loss and the return loss are better than 1.48dB and 18.12dB, respectively. The suppression within 6–8GHz and 12–17GHz are greater than 50dB. 16.8dB suppression up to 2.4f0 are obtained. Compared with the filter in [5], two filters in this article acquire better suppression at about 2GHz away from the center frequency.