Microstrip Lines for Microwave Integrated Circuits

Transmission lines and passive lumped or distributed circuit elements, which are manufactured and assembled from planar metal conductors or conducting stripes on insulating substrates, are essential basic elements in microwave and millimeter wave hybrid integrated circuits. The metal strips or microstrips are deposited by thin-film or thick-film technology on dielectric substrates; the processing steps are substantially different compared to conventional coaxial and waveguide circuit technology. Circuits built with microstrip transmission lines or microstrip components have three important advantages: (i) The complete conductor pattern can be deposited and processed on a single dielectric substrate which is supported by a single metal 1421 1422 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , M A Y - J U N E 1969 ground plane. Such a circuit can be fabricated at a substantially lower cost than waveguide or coaxial circuit configurations. (ii) Beam-leaded active and passive devices can be bonded directly to metal stripes on the dielectric substrate. (Hi) Devices and components incorporated into hybrid integrated circuits are accessible for probing and circuit measurements (with some limitations imposed by external shielding requirements). The purpose of this paper is to derive formulas for the electric parameters which are the impedance, attenuation, propagation constant, and unloaded Q of the microstrip transmission line. In addition to the electrical design data, attenuation measurements at 30 GHz are presented because: (i) The attenuation is the most important electrical parameter of a microstrip because it determines the circuit losses of microwave and millimeter wave hybrid integrated circuits.