Ridge waveguide used in microstrip transition

It is shown that a ridge waveguide can be used as microstrip-to-waveguide transition that is easy to fabricate and offers excellent loss characteristics. A quartz substrate is used since it is less lossy than the commonly used alumina substrate. The design of the transition is essentially that of an impedance transformer. A four-step Chebyshev quarter-wave transformer is used. After the desired impedance for each step is determined, a computer program is used to determine the ridge-waveguide dimensions. The impedance of the last section is 50 ohms, and the gap in the ridge is 15 mils, the thickness of the substrate. These two parameters dictate the width of the ridge, which is the same (65 mils) for all four sections. Whereas in theory the first three sections should each be a quarter of a wavelength long, the discontinuities in the ridge at each step give rise to junction capacitances that introduce phase angles to the reflection coefficients.