Advanced Superdirective Antennas

TL;DR

This review discusses advanced superdirective antennas that achieve high directivity from compact structures through coupled currents and modal interference, highlighting practical implementations, challenges, and emerging solutions across various frequency regimes.

Contribution

It provides a comprehensive overview of design strategies, evaluation methods, and innovative approaches to mitigate superdirectivity trade-offs in small antennas.

Findings

Superdirective antennas can be realized using resonant arrays or single-body radiators.

Trade-offs include reduced radiation resistance and narrow bandwidth.

Emerging techniques like low-loss materials and time-varying loading can improve performance.

Abstract

Superdirective (supergain) antennas aim to produce a narrow main beam from radiators that are electrically small compared with the wavelength. Instead of enlarging the physical aperture, they rely on strongly coupled currents, near-field energy storage, and controlled modal interference so that a compact structure radiates with enhanced directivity. This review emphasizes link-relevant evaluation and reporting: realized gain referenced to a stated impedance plane, clearly stated bandwidth definitions (impedance and performance), and robustness to fabrication spread and platform/environmental loading. Two practical implementation routes are surveyed. The first uses resonant, tightly coupled arrays, including fully driven arrays and single-chain designs based on parasitic or reactively loaded elements. The second uses single-body radiators that enforce a targeted mixture of multipoles or resonant/characteristic modes with one or a few feeds, including symmetry-broken dielectric resonators and mixed electric--magnetic designs. Across RF, microwave, and optical regimes, the same penalties recur as superdirectivity is pushed: reduced radiation resistance, rapid impedance variation, narrow usable bandwidth, and strong sensitivity to small perturbations. Beyond geometric synthesis and multi-resonant stacking, the review highlights emerging levers that can shift these trade-offs in specific system contexts: low-loss materials and cryogenic operation to improve efficiency and frequency stability, and time-varying loading and matching (Floquet/parametric approaches) that can relax linear time-invariant bandwidth constraints, at the cost of added control complexity and spectral conversion.