[Paper Review] Multilayer Minkowski Reflectarray Antenna with Improved Phase Performance

Ender Öztürk, Birsen Saka|arXiv (Cornell University)|Aug 21, 2020

Advanced Antenna and Metasurface Technologies26 references31 citations

TL;DR

This paper proposes a multilayer Minkowski fractal reflectarray unit cell with aperture-coupled phasing stubs to achieve over two cycles of phase range (740°) while reducing patch surface area by 21.9% and maintaining reflection loss below 0.14 dB. The design uses a two-cycle phase response via stub length variation, enabling a full-scale 221-element reflectarray fabricated and measured at 10 GHz, showing strong agreement between simulation and experiment across beamwidth, sidelobe levels, and gain (26.8 dBi).

ABSTRACT

We propose a multi-layer unit cell consisting of Minkowski fractal shaped reflector with aperture coupled phasing stubs to obtain a broad phase range for the reflectarray antenna with smaller unit cell size and inter-element spacing compared to some other studies in the literature. In the study, simulations are conducted using HFSS(TM) program with Floquet Port incidence. A unit cell with two cycles of phase range and very low reflection loss is designed. 21.9% shrinkage is achieved in patch surface area using Minkowski fractals. Subsequently, proposed unit cell is used to design and fabricate a full scale 221 element reflectarray antenna. At 10 GHz, simulation results are compared with measured data and good agreement is observed.

Motivation & Objective

• . To address the limited phase range in single-layer Minkowski fractal reflectarrays, which typically offer less than one cycle.
• . To overcome the miniaturization-efficiency trade-off by relocating phasing stubs below the main reflector using aperture coupling.
• . To achieve a broader phase range with reduced unit cell size and lower reflection loss compared to prior designs.
• . To demonstrate the feasibility and performance of the proposed unit cell in a full-scale 221-element reflectarray fabricated and measured at 10 GHz.

Proposed method

• . Designs a multilayer unit cell with a Minkowski fractal-shaped main reflector on the top dielectric layer (D2), separated by an air gap (D3) from a lower dielectric layer (D4).
• . Implements aperture-coupled phasing stubs beneath the main reflector: one matching stub along the y-axis and one phase-controlling stub along the x-axis, both connected at the center.
• . Uses HFSS with Floquet port excitation to simulate the unit cell’s phase and amplitude response under periodic boundary conditions.
• . Optimizes the stub length to achieve a total phase range of 740° (over two cycles), with reflection loss below 0.14 dB across the range.
• . Fabricates a 221-element reflectarray using the optimized unit cell, with phase distribution calculated via the method in prior work.
• . Measures the full-scale antenna in an anechoic chamber and compares results with HFSS simulations to validate performance.

Experimental results

Research questions

• RQ1 . Can a multilayer Minkowski fractal reflectarray with aperture-coupled stubs achieve a phase range exceeding two cycles while minimizing unit cell size?
• RQ2 . How does relocating the phasing stubs below the main reflector affect phase range, reflection loss, and surface area compared to coplanar stubs?
• RQ3 . To what extent does the use of Minkowski fractal geometry reduce patch surface area without degrading phase performance or increasing loss?
• RQ4 . How well do simulation results using the Floquet model match measured performance in a full-scale 221-element reflectarray?
• RQ5 . What is the impact of aperture coupling on mutual coupling and cross-polarization levels in a multilayer Minkowski reflectarray?

Key findings

• . The proposed unit cell achieves a total phase range of 740°, exceeding two full cycles, with a 21.9% reduction in patch surface area compared to a conventional square patch.
• . Reflection loss is maintained below 0.14 dB across the entire phase range, with an average loss of only 0.08 dB.
• . The 221-element full-scale reflectarray achieves a maximum directive gain of 26.8 dBi, closely matching simulated results.
• . Half-power beamwidth (HPBW) is 9.6° in both simulation and measurement, with side lobe level (SLL) of -14.2 dB measured versus -15.1 dB simulated.
• . Cross-polarization levels are below -45 dB in simulation, well below the dynamic range of the measurement chamber, indicating negligible cross-polarization degradation.
• . The measured and simulated E- and H-plane radiation patterns show strong agreement, especially in the main lobe, with minor discrepancies near the horizontal plane due to feed blockage and spatial reflection effects.