Design of parallel coupled microstrip bandpass filter for C band TV satellites

In this study, a parallel coupled bandpass filter operating in the 4.0-4.2 GHz frequency range, vulnerable to electromagnetic interference from adjacent bands, has been designed for C-band TV satellites. Since microwave frequency filter design is to be performed, an inductor-capacitor circuit providing the bandpass characteristic designed with lumped elements method was implemented using microstrip lines from planar transmission lines. Due to the frequency spectrum density in the application, the element values of a Chebyshev low-pass filter prototype with a cutoff frequency decreasing rapidly and a 0.2 dB ripple level in the passband were utilized. During the transition from the low-pass filter prototype to the bandpass filter prototype, the element that is an inductor is replaced with a series inductor and capacitor, and the element that is a capacitor is replaced with a parallel inductor and capacitor. Admittance inverter constants were calculated, obtaining even and odd mode impedance values. After theoretical examination of the filter design stages, the physical parameters of the parallel-coupled microstrip bandpass filter were calculated using analytical methods. Simulations were performed using AWR (Applied Wave Research) Design Environment and CST (Computer Simulation Technology) Studio Suite 3D programs. The design, simulation, and optimization were conducted on three different substrate materials. Substrates with RO4835 model having a dielectric constant εr=3.48, respectively, with dielectric thickness h = 1.52 mm, 0.76 mm, 0.25 mm, and conductor thickness = 17.5 μm, 35 μm, 17.5 μm were used. Subsequently, physical parameters were re-obtained using AWR Design Environment, and the values were chosen based on observed superior scattering parameters return loss S11 and insertion loss S21 performance according to analytical methods. Using CST Studio Suite 3D, which enables three-dimensional design, electric field and magnetic field distributions of the parallel-coupled microstrip bandpass filter were obtained, and the corner-coupled operating mode was revealed. An optimal designs with minimum insertion loss, maximum return loss, and sharp transition at cutoff frequencies was produced using the LPKF ProtoMat S63 device. The scattering parameters, S11 and S21 of the produced parallel-coupled microstrip bandpass filters were measured using a network analyzer.