Novel compact bandpass filters with wide controllable fractional bandwidth

In this letter, a novel compact structure is proposed for bandpass filter (BPF) design. The structure is realized using only one resonator. The resonator is excited by two open microstrip stubs. Further, ground plane aperture technique is used to obtain wide bandwidth. 3-dB fractional bandwidth can be varied between 1.0 and 90.0% with good in band as well as out of band filter characteristics. The filter has the advantages of compactness, wide bandwidth, low insertion loss (IL), and ease of fabrication. A prototype fabricated filter of FBW 45.1% and passband center frequency f₀ = 3.795 GHz shows a maximum passband IL of 2.0 dB. The basic BPF occupying area is less than 0.22 λ_g × 0.23 λ_g, where λ_g is the guided wavelength at f₀. A simple stub modification extends its upper stopband at least three times the upper cutoff frequency. Good agreement between measurements and simulations are obtained. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 29–31, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22039