Stopband-enhanced and size-miniaturized low-pass filters using high-impedance property of offset finite-ground microstrip line

The uniform finite-ground microstrip line (FGMSL) and its constituted circuit elements are thoroughly characterized via self-calibrated method of moments for innovative design of a stopband-enhanced and size-miniaturized low-pass filter (LPF). Firstly, an offset FGMSL is modeled and extracted to quantitatively exhibit its capacity in achieving higher characteristic impedance compared to the infinite-ground microstrip line (MSL) counterpart. Secondly, the finite-extended FGMSL section with equally widened strip/ground or offset narrow strip/ground conductors is studied in terms of an equivalent T- or π-network, thereby constructing the modified MSL shunt capacitive or series inductive element. By making effective use of enlarged series inductance of an offset FGMSL with shorter electrical length, two novel LPF blocks are designed on a basis of cascaded circuit networks. Predicted S-parameters show widened and deepened stopband beyond the low passband. Furthermore, it is confirmed by measurement.