The Methods for Generating Radiation Null in Filtering Antennas

Filtering antennas have been achieved more attention for the advantages of reducing the requirement of the filter in wireless communication systems. The radiation nulls of filtering antenna can further improve the frequency selectivity and decrease the mutual coupling between the closely placed antennas operating at different frequencies so that the placement distance of the antennas can be decreased. Therefore, various methods for generating radiation null have been investigated. One is to use cross-coupling or electromagnetic hybrid coupling of filtering theory. The other is to generate transmission zero by feed structure, which is a radiation null for the whole antenna. The last one is to achieve the radiation null by the radiator structure, which will produce a field that cancels each other out. In this paper, two filtering antennas with radiation nulls are reviewed to introduce the methods for radiation nulls. One is the dual-polarized filtering patch antenna fed by substrate integrated cavity (SIC). The second-order filtering response is achieved by using one SIC resonator and one slotted square patch while the radiation nulls can be produced by connecting the coupled lines with feeding lines in parallel. The other is a circular dense dielectric (DD) patch antenna realized by stacking an upper DD patch on the lower DD patch. The two reflection zeros are achieved because the dual DD patches are both operating at TM₁₁ mode. The two radiation nulls near the lower band-edge are produced by the opposite E-field distribution inside the upper DD patch and the quarter-wavelength open-circuit stub, while the two radiation nulls near the upper band-edge are produced by the opposite E-field distribution inside the lower DD patch and the opposite E-field distribution at the edges of the two DD patches. The performances of the two designs are given.