A proposed rectangular-patch antenna for transmitting or receiving microwave circularly polarized (CP) radiation would be fed via a cross-shaped aperture that would have unequal arms and would, in turn, be fed at a single point via a microstrip transmission line. As a consequence of the unequal-arm aperture design, the antenna could be made smaller in one dimension, relative to a typical prior CP antenna that comprises a nearly square patch fed via an equal-crossed-arm aperture. Hence, in designing a phased array of such antennas, the antennas could be packed together more closely along one dimension, making it possible to scan the beam radiated by the antenna over a wider angular range in a plane that includes that dimension. Alternatively or in addition, one could lay out transmission lines in the extra spaces created by the shortening in one dimension

In designing a rectangular-patch CP antenna, one chooses the dimensions of the patch and of the crossed arms of the aperture to introduce a 90° phase shift between the two orthogonal electromagnetic modes of the patch. In the case of a conventional “nearly square” design, the aperture arms are constrained to have equal dimensions, it is assumed that the value of the resonance quality factor (Q) is the same for both modes, and the effects of the electromagnetic modes of the crossed aperture arms are neglected. In designing an antenna according to the proposal, one allows the aperture arms to have unequal dimensions and does not assume equal Q values. Instead, one explicitly includes the aperture modes in the calculations and assesses the overall effects (including possibly unequal Q values) on the patch modes and the characteristics of the radiation.

The decision to allow unequal dimensions of both the patch and the aperture arms gives the designer more degrees of freedom than could be utilized to optimize the antenna. Among other things that one seeks to do in optimization is to obtain an axial ratio as close to 1 as possible and an axial-ratio bandwidth as large as possible. (As used here, “axial ratio” signifies the ratio between the radiated power polarized along one axis of symmetry of the antenna and the radiated power polarized along the other, perpendicular axis of symmetry of the antenna. An axial ratio of exactly 1 corresponds to perfectly circular polarization. “Axial-ratio bandwidth” signifies the width of the frequency band over which the axial ratio is acceptably close to 1.)

In designing an antenna as proposed, to compensate for an axial ratio that deviates from 1 by an unacceptably large amount, the dimensions of the aperture arms along one direction could be adjusted in tandem with the corresponding orthogonal patch dimension. For example, if the vertical dimension of the patch were modified, then the horizontal dimensions of the aperture arms would be adjusted accordingly. Consequently, an additional benefit of the proposed design approach would be that the axial ratio of an antenna would be closer to 1, and theaxial-ratio bandwidth would be greater, relative to the corresponding parameters of the prior “nearly square” antenna.

This work was done by Larry Epp of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.


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Patch Antenna Fed via Unequal-Crossed-Arm Aperture

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This article first appeared in the June, 2005 issue of NASA Tech Briefs Magazine.

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