Extremely narrow microstrip filters using periodic couplers

A novel class of microstrip bandpass filter using periodic couplers for use at frequencies above 10 GHz has been proposed. This type of filter can be easily realized by varying the strip conductor to give a bandwidth narrower than 1%. One type of periodic coupler which used quarter-wavelength step-c hange geometry was employed to demonstrate the principle. The measured freq uency response of a microstrip filter which used two three-section periodic couplers on either side of a half-wave resonator exercise at 9.6 GHz showe d 25 dB rejection at 10 GHz and a bandwidth of 1%. Simulations have shown t hat this type of filter can be realized at millimeter-wave frequencies for low-phase-noise oscillators and narrowband selective filters. (C) 1999 John Wiley & Sons, Inc.