This dissertation mainly discuses various microstrip bandpass filter (BPF) designs. The filter designs include: a coupled line BPF using nonuniform arbitrary image impedances, miniaturized BPF utilizing dumbbell shaped slot resonator (DSSR), BPF employing isosceles triangle shaped patch resonator (ITSPR), BPF with a complimentary split ring resonator (CSRR) and triple-band BPF (TBBPF). In the coupled line BPF designs, a capacitive gap-coupled BPF and parallel coupled line BPF are introduced, where two different arbitrary image impedances are applied for the designs. Based on the proposed equivalent circuit model, the coupled BPF's design equations are derived, and they are validated from comparisons of the calculated and simulated results. For a miniaturized BPF, the DSSR is utilized in the filter design. An equivalent circuit model of the DSSR is also presented and validated through simulations and measurements. The ITSPR is introduced for simple BPF and diplexer designs. The ITSPR's design equations, effective dielectric constant, and fractional bandwidth are discussed, and their validities are demonstrated from electromagnetic (EM) simulations and measurements. The coupled type complementary split ring resonator (CSRR) is introduced for a compact direct-coupled BPF. The proposed unit cell of the resonator consists of two CSRRs, where gaps of outside rings face each other to achieve a strong cross coupling. For an analysis of the coupled CSRR, an equivalent circuit model is discussed and validated through circuit and EM simulations. Based on the coupled CSRR structure, two-/four- pole direct-coupled BPFs are designed, simulated, and measured. The TBBPF design using admittance inverters are presented. In the TBBPF design, the center frequencies and fractional bandwidths (FBW) of each passband can be adjustable. Low cost phased array systems operating from 8 to 12 GHz are introduced. A phased array using a piezoelectric transducer (PET) phase shifter is designed and tested. Compared to the phased array using the PET phase shifter, another phased array utilizing 4-bit monolithic microwave integrated circuit (MMIC) phase shifters is demonstrated. Both phased array systems are simple and easy to fabricate.