TABLE OF CONTENTS The planar waveguide model was devised in the 1970s to tackle the problem of the observed frequency dependence of discontinuities and junctions, such as microstrip impedance steps and T-junctions. At that time microstrip dispersion was under intensive investigation, and a rigorous field solution of the quasi-TEM mode was not known. This applies in particular to the higher-order modes that are the real cause for the frequency dependence of the scattering parameters of the devices under consideration. The introduction of a closed ideal parallel plate model for the open microstrip line with electrical walls at the top and bottom, and magnetic walls on both sides, cannot be truly justified from the theoretical point of view. The boundary conditions of both structures are totally different. However, in engineering work, any model may be acceptable under the condition that it delivers the expected results and passes the experimental test. Even for the strongly dispersive hollow waveguide, a characteristic impedance is known as an auxiliary design variable.
Meanwhile, the waveguide model has been proven to be a worthful analysis tool for a variety of microstrip discontinuites and junctions. The writer himself has treated thoroughly multistep discontinuities, such as quarter-wavelength transformers and tapered microstrip transitions, calculated with both the waveguide model and advanced 3D EM simulators. It seems that the waveguide model concept has still its advantage concerning speed of calculation.
It was shown in Chapter 1 that discontinuities and junctions are widely found in common microstrip circuits, such as amplifiers, oscillators, and multipliers.
