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The bandstop behaviour of a stepped impedance microstrip ring is studied. The ring is composed of four sections of microstrip line, which are equal in electrical length, but different in impedance. When the impedances of the lines meet the condition of rotational symmetry, a bandstop response could be obtained. At the central frequency of the stopband, the electrical length of the lines is 90°. By tuning the lines' impedances, the bandwidth could be finely adjusted, typically from 0.5 to 40%. Prototypes are designed, fabricated and measured. Good agreements are observed between the measured and the simulated data. The measured data shows that the 3 dB bandwidth is 24%, with a rejection level of −32 dB at the central frequency 1.99 GHz. In addition, the measured matching within the passband is good, as predicted.

A compact ultra-wideband filter is presented. The filter consists of a section of coupled microstrip-line and a short-ended stub. It has a simple structure and small size (4.7×6.2 mm2), but exhibits good performance. The filter has been investigated numerically using the simulator IE3D, and samples were fabricated and measured. Measured results show that the filter has a fractional bandwidth of 103.2% (from 3.05 to 9.55 GHz) and minimal insertion loss of −0.4 dB. The measured in-band return loss is better than −15.0 dB.