Impedance loading is a common technique traditionally used in the RF to enhance the performance of an antenna, but its application in the optical regime is not as rigorously studied. This is mainly due to a lack of exact analytical expressions that can be used to rapidly predict the radiation properties of loaded nanoantennas. This paper will derive a set of useful analytical expressions for the far-field radiation properties of loop antennas loaded with an arbitrary number of lumped impedances that are valid from the RF to optical regimes. The analytical expressions will be validated with full-wave solvers and can be evaluated more than 100x faster. The ability to perform such rapid evaluations enables, for the first time, large-scale single-and multi-objective optimizations. A series of optimizations reveal that electrically small super-directive antennas can be achieved at a variety of far field angles through capacitive loading, paving the way for a pattern reconfigurable antenna. In addition, gains of greater than 3 dB can be achieved for electrically small antennas over a fractional bandwidth of 28%. Finally, it is shown that impedance loading can be used to achieve circularly polarized radiation from a single loop.
Loop antennas are among one of the simplest antennas to construct. Yet, despite this fact, the mathematical complexity of the associated radiation integrals has complicated the understanding and modelling of such structures. Recently, a full analytical theory for the radiation properties of loop antennas has been derived, which is able to bridge this gap and provide a basis for better understanding the behavior of these antennas across all frequency ranges. Further, these models have been extended to include the effects of loading and coupling, as well as have revealed interesting properties such as superdirectivity by enabling rapid parametric studies and optimizations to be carried out.
The loading of antennas greatly expands the design space by making otherwise challenging performance goals more easily realizable. It is demonstrated that the pairing of the analytical theory of loop antennas with a powerful global optimizer can achieve designs that offer significant radiation pattern shaping in both the RF and optical regimes.
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