MICROSTRIP
Microstrip transmission lines are quasi-TEM structures. They behave similarly to coaxial lines which are true TEM structures. The quasi-TEM nature is a consequence of the open physical structure which results in capacitive fringing field effects. This fact complicates the design of microstrip. Microstrip characteristic impedance is a function of the width to stubstrate height ratio and the effective dielectric constant. The effective dielectric constant is in turn a function of the characteristic impedance and the sub- strate dielectric constant.The analysis situation is straight forward, i.e., given the physical parameters of line width, substrate height, and substrate dielec- tric constant, the resulting characteristic impedance is readily calculated. The synthesis case is more complicated and requires an iterative procedure. For this application it is necessary to start with a definite value for the characteristic impedance, 50 ohms, and derive the corresponding width to height ratio.
The following analysis relations apply:
The following synthesis relations apply:
The synthesis situation is germane for these purposes since the substrate parameters of dielectric constant and thickness are input and a target value of Z0 must be realized.