On the design of novel compact broad-band planar filters

On the basis of impedance steps and coupled-line sections as inverter circuits, novel wide-band and very compact filters are presented. The application of alternately high- and low-impedance lines presented to the connected transmission-line resonators partly reduces their lengths to a quarter-wavelength only. In addition, effective techniques are demonstrated to reduce spurious stopband resonance resulting from a remaining half-wavelength resonator. Both suspended stripline (SSL) and microstrip filters were designed, fabricated, and tested, proving this concept in an excellent way. For the prototype filters, center frequencies around 6 GHz were selected. Bandwidths are between 2.5-3.25 GHz, and insertion-loss amounts to around 0.25 dB for the microstrip filters and 0.5 dB (including the transitions to coaxial line) for the SSL filters, respectively. For the selected center frequency and on a substrate with a dielectric constant of 10.8, the smallest microstrip filter is only 15 mm*5 mm in size.