Dielectric Waveguides for Millimeter Waves

Abstract

In this thesis, we analyze high-permittivity dielectric waveguides for use as guiding structures of millimeter waves. Two basic geometries are considered: the circular and rectangular guides. In Part I, we describe the theory of round fibers surrounded by an infinite cladding. Millimeter wavelengths are comparable to the physical dimensions of the guide. Therefore, a large difference in permittivity between the core and the cladding is required in order to provide for a tight confinement of the fields. We present the results of computations of the propagation characteristics and losses of fibers of very high permittivity. We note that the distribution of the electromagnetic power between the core and the cladding can be deduced from the dispersion curves. Finally, we consider the feasibility of a dielectric fiber made of thallium bromide-iodide (KRS-5) for the long distance transmission of W-band signals (94 GHz). Using our measurements of the dielectric parameters of KRS-5, we find that the losses are several orders of magnitude higher than the losses of conventional metallic waveguides. In Part II, we analyze rectangular dielectric guides made of high-permittivity materials such as GaAs that would permit the fabrication of active devices directly into the transmission line. We present a new numerical technique base on finite-differences for computing the of dielectric guiding structures. This method is simple and efficient in computer storage and computational time. We use it to compute the of a rectangular dielectric waveguide and compare the numerical results to those obtained from Marcatili's closed-form solution. We find that this latter one is a good approximation for the dominant mode of a rectangular guide even when the permittivity of the guide is large compared to the outer medium. For higher order modes, Marcatili's solution predicts incorrect propagation curves. We have also observed the presence in our numerical solution of spurious modes that are thought to be due to the mathematical indefinitiveness of the problem. In Part III, we present a waveguide technique for the measurement of complex dielectric constants at millimiter wave frequencies: the shorted-waveguide method. Waveguide methods have been extensively used at lower frequencies but this is the first application at 94 GHZ. We use a novel sample preparation technique that allows for an accurate and gap-free positionment of a ductile dielectric material inside a metallic waveguide. We note that the correct choice of sample lengths is critical to the accuracy of the measurement of the loss tangent. Finally, we summarize the results of our measurement of the dielectric constant and loss tangent of thallium bromide-iodide (KRS-5) and thallium bromide-chloride (KRS-6).