A compact diamond-shaped dual-band bandpass filter with multiple transmission zeros

In this paper, a compact diamond-shaped dual-band bandpass filter with seven transmission zeros in the stopband and two poles in each passband is designed, analyzed, and implemented. The proposed filter consists of coupled lines and stepped impedance open stubs. Theoretical analysis based on lossless transmission line theory is carried out to verify the transmission zero frequencies. A filter prototype with center frequencies of 1.17 GHz and 2.85 GHz is fabricated to exhibit the feasibility of the design. The prototype is printed on a 0.79 mm Rogers RT Duroid 5880 with a dielectric constant of (is an element of(r) = 2.2) and occupies an area of 0.29 lambda(g) x 0.17 lambda(g), where lambda(g) is the guided wavelength at the first center frequency. The experimental results show a 3-dB fractional bandwidth of 13.65% and 3.5% at low and high center frequencies, respectively. The simulated and experimented frequency responses are consistent with each other.