Novel Polarization-Agile Annular Waveguide Slot Antennas

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Slot array antennas using rectangular waveguides were widely used in past decades based on their various important capabilities in microwave telecommunications. Having a low profile, ability to implement different current distributions and high power handling are among the advantages of such antennas. On the other hand, there exist some restrictions in optimum and efficient performance of rectangular slotted waveguide antennas due to the use of rectangular waveguides and slots in a circular boundary (Elliott, 2003). These limitations are mainly caused by the disharmony between the rectangular geometry of radiating elements and the circular boundary of the antenna. This has been the inspiration for exploring novel alternative structures with improved geometrical balance with the circular boundary (Fusco et al., 2003, Noghanian et al., 2001, Takahashi et al., 1995, Kelly et al., 1961). Annular Waveguide Slot Antennas (AWSA) were introduced in (Ebadi et al., a, 2007) intending the application in circular boundary planar slot array antennas. Resonant characteristic along with linear polarization perpendicular to the slot direction were proved the single slot case and it was shown that the AWSA could be modeled as a shunt conductance in the annular line. As the first step to the study of an AWSA, the annular waveguide was compared with the rectangular waveguide (Ebadi et al., b, 2008) and the design procedure was provided in (Ebadi et al., c &d , 2008). It was shown that such waveguide bend could be considered as a transmission line along the cylindrical φ direction. Sinusoidal field distribution along the waveguide cross section and single mode transmission were also provided. As the second step, the Green’s functions required for the analysis of an AWSA by the method of moment were derived and presented in (Ebadi et al., e, 2009). These functions will be used in analyzing the AWSA to derive exact field distribution along the slot and the variation of slot conductance in different frequencies. In an AWSA, the waveguide and the slots are all circularly oriented to fit the boundary. Due to its similarity with the rectangular slotted waveguide case, the circuit model for the AWSA was proposed to be a shunt admittance. Fig. 1 shows a sample AWSA structure. The transmission line is supposed to be the H-plane waveguide bend which is also called azimuthal waveguide. The waveguide is fed from one end and is short-ended at the other 6