Aperture - limited near field superstrates for gain restoration in miniaturized helical antennas

Abstract Miniaturization of axial-mode helical antennas often leads to a significant degradation of gain and directivity due to the reduced effective aperture and disturbed current distribution. In this work, a compact and lightweight approach for compensating these drawbacks is proposed using Fresnel-inspired dielectric superstrates fabricated by 3D printing techniques. Unlike classical Fresnel lenses, the presented superstrates are designed to operate in close proximity to the antenna aperture. Several superstrate geometries based on the discretized Fresnel zone construction were developed, with the geometric parameters initially derived from analytical formulations. In the near-field regime adopted here, where the superstrate is positioned at only 0.15λ from the antenna aperture, the gain enhancement arises from a redistribution of the radiating field across the effective aperture rather than from classical phase correction. The influence of key design parameters was systematically investigated. Simulation results demonstrate that all proposed superstrates enhance the gain of the miniaturized helical antenna by 1.1–3.3 dB. When compared with the full-size results, only S3 exceeds its gain at all three simulated frequencies by 0.25–0.98 dB. Measurements show a gain enhancement of 1.2–4.4 dB relative to miniaturized antenna for all tested frequencies. In comparison with the full-size antenna, all four systems with superstrates exceed the full-size antenna gain at 2.6 GHz. Only S1, S3 and S4 provide higher gain at 3.0 GHz. None of those superstrate-antenna systems exceeds the reference at 3.3 GHz. These results show that the gain recovery depends on the particular combination of the superstrate geometry and operating frequency. The main contribution of this work is therefore the trade-off between system compactness and recovery of the gain lost through antenna miniaturization rather than a universal gain advantage over the full-size antenna. Among the tested designs, compact superstrates provide a favorable trade-off between size, weight, and radiation performance, while larger structures offer maximum gain improvement. Transmission measurements further confirm improved performance across a wide bandwidth. The presented design enables a 36% reduction in antenna height with only a modest, frequency-dependent decrease in gain, making it attractive for satellite communication systems, compact RF terminals, and space-constrained wireless applications.