Effect of waveguide dimensions of feed guide on overall admittance characteristics of slot coupled inclined armed T-junction

Abstract

Slot radiators are popular because of their compactness. However, design of slot arrays for a given application is highly involved. In addition, design of slot arrays, which do not result in EMI either in radar or in communication, is much more complicated. It is well known that for the sake of radiation the slots in the narrow wall of standard rectangular waveguide are made to incline in order produce horizontally polarized radiated fields. However, the resonant slot length cannot be accommodated due to dimensional considerations of the waveguide. Hence the slots are extended into the broad wall to accommodate resonant length. These two aspects are: i) extra length in the broad wall, ii) inclination of the slot in the narrow wall, create EMI due to the presence of cross polarized field components. It is possible to suppress such undesirable field components to escape from the presence of EMI by connecting a T-arm so that slot couples power into the T-arm so that slot couples power into the T-arm instead of free space. The T-arm has to be carefully designed with appropriate orientation to meet the requirements of polarization. E and H plane junction are conventionally used and they are analyzed by several authors. However, to design planar arrays consisting of slot coupled waveguide junctions, there is a need to twist the T-arm by an appropriate angle. That means it results in a waveguide junction in which both slot and T-arm are inclined. In the present work, the waveguide junction, which is suitable for the design of planar structures, is considered and the data on the effects of waveguide dimensions on the overall admittance characteristics is presented. The overall admittance is numerically evaluated from the derived analytical expressions as a function of dimensions of feed waveguide. The change in waveguide dimensions for the feed waveguide has become necessary in order to make the overall structure resonate. An expression for the admittance is derived from the knowledge of self-reaction and discontinuity in modal current. The self-reactions in the feed and coupled guide are separately obtained from TE and TM mode considerations. In the evaluation of admittance the energy stored in the primary guide is also considered. The data on the variation of normalized conductance and susceptance as a function of frequency for different feed guide dimensions and different slot and coupled arm inclinations are presented in this work.