BANDPASS-FILTER AND DIRECTIONAL-COUPLER TECHNIQUES FOR MILLIMETER-WAVE, DIELECTRIC-WAVEGUIDE AND MICROSTRIP CIRCUITS

Dielectric waveguide (DW) is attractive for use at the high millimeter-wave (mm) frequencies. However, conventional filter techniques are unsuitable for most DWs because of the strong radiation of DWs at discontinuities. New techniques for DW bandpass filters were investigated, extending the previous work of Park and Matthaei. These filters utilize parallel-coupled and uncoupled DW gratings. Theory was obtained with which they can be designed for a prescribed Chebyshev (or other) passband. In their stopbands the parallel-coupled gratings provide strong and broadband, dissipative attenuation.;The design theory of simple symmetrical 3-dB DW couplers (hybrids) is well known. However, such couplers tend to be narrowband. It was found that certain asymmetrical designs give much wider bandwidths. The broadbanding is achieved by properly taking advantage of the dispersion and the frequency sensitivity of coupling between dissimilar DWs. An experimental slab-type DW coupler achieved 30 percent bandwidth for 1-dB unbalance tolerance. An image-guide coupler also gave good results. These couplers can approximate either "Magic-T" or quadrature performance.;Use of "forward coupling" for microstrip hybrids was also investigated. These couplers are unusual because forward coupling in microstrip is generally believed to be too weak for this application. It was found that such couplers easily achieve low radiation losses and high directivity on even electrically thick substrates where conventional microstrip hybrids tend to have poor performance. Microstrip forward couplers look promising for mm-wave applications where their large electrical length is easily accommodated. An experimental asymmetrical microstrip hybrid achieved 57 percent bandwidth for 1-dB unbalance with 40 dB or more of isolation. As for DW couplers, either quadrature or "Magic-T" performance can be approximated.;For the lower mm-wave ranges microstrip lines are still quite useful, but use of relatively thick substrates would be desirable in order to minimize losses. On such substrates the usual types of microstrip bandpass filters tend to radiate strongly, giving poor performance. It was found in this work that the DW grating theory can be adapted for microstrip, with some modifications. The grating technique yields narrowband mm-wave microstrip filters with litte radiation and strong filter characteristics.