Capacitive loading for miniaturization and performance improvement of transmission lines

Loading transmission lines by capacitors is an effective way of miniaturization. The general equation of the quality factor of loaded transmission line section is derived. A careful comparison is, then, carried out between loaded transmission lines and unloaded equivalent ones realized on low-cost substrate and on a good-quality substrate. Results show that the Q-factor of transmission lines can be increased by capacitive loading, with a trade-off between the quality factor and the miniaturization. These conclusions are confirmed by experimental results on a miniaturized DBR filter. The measurements show that the low-cost substrate performs better than the good-quality high-cost one.