Performance of CRLH-TL-Hilbert Structure Inspired Antenna Loaded with AMC Reflector for Wireless Applications

Summary

This study presents a high-gain CRLH transmission line antenna integrated with a Hilbert fractal EBG and an artificial magnetic conductor reflector for 5G sub-6 GHz applications. The design achieves 20 dBi gain, 2.1 GHz bandwidth, improved front-to-back ratio, and limited optical beam steering using light-dependent resistors for reconfigurability.

Abstract

This study presents a novel reconfigurable antenna design for 5G‑sub6 GHz communication networks, achieving significant gain enhancement and beam‑steering capability through the synergistic integration of a composite right/left‑hand transmission line (CRLH‑TL), a Hilbert‑curve electromagnetic bandgap (EBG) structure, and an artificial magnetic conductor (AMC) reflector. The antenna comprises 17 CRLH TL unit cells coupled to a third-order Hilbert fractal EBG, eliminating conventional via holes and minimizing surface-wave losses. Without the AMC reflector, the antenna achieves a peak gain of 16 dBi at 5.6 GHz. Incorporation of a 7 × 10 hexagonal AMC reflector array exhibiting zero reflection phase enhances forward gain to 20 dBi at 5.6 GHz, representing a 4 dB improvement while suppressing back lobes and producing a unidirectional radiation pattern. Reconfigurability and beam‑scanning are enabled via an optical switching mechanism employing light-dependent resistors (LDRs), achieving gain variation and ±5° beam steering at 5 GHz. The design achieves a 2.1 GHz impedance bandwidth (S11 < –10 dB) and 78% radiation efficiency. Compared to prior CRLH‑TL antennas, this work demonstrates a 4 dB gain improvement and introduces optical reconfigurability without via-based fabrication complexity. Simulation and measurement results exhibit excellent agreement, validating the design methodology. The proposed antenna is suitable for adaptive 5G base stations, satellite communications, and beam-steerable radar systems.

Introduction

The increasing demand for adaptive, high-performance antennas in fifth-generation (5G) wireless communication systems has accelerated research into metamaterial (MTM)-based designs^1,2. Metamaterials offer unique electromagnetic properties, including negative permittivity and permeability, which enable zero-order resonance in composite right/left-hand transmission line (CRLH-TL) structures, facilitating significant antenna miniaturization^3,4. However, conventional MTM antennas frequently suffer from low gain, narrow impedance bandwidth, and fabrication complexity arising from via hole requirements^5,6.

To address these limitations, electromagnetic bandgap (EBG) structures and artificial magnetic conductor (AMC) reflectors have been integrated to suppress surface-wave propagation, improve impedance matching, and enhance forward gain^7,8. Recent studies have demonstrated a gain enhancement of 2–6 dB using AMC reflectors in dual‑ and multi-band configurations⁹⁻¹⁸. For example, Jwair et al.⁹ reported a metasurface antenna achieving 14 dBi gain at 5.5 GHz, while Ali et al.¹² demonstrated a Hilbert fractal MIMO array with 12.5 dBi gain. Despite these advances, existing designs exhibit gain limitations (typically 12–16 dBi), lack reconfigurability, or require complex via‑based fabrication.

This work proposes a novel CRLH-TL antenna integrating a third-order Hilbert-curve EBG and a zero-phase AMC reflector for 5G sub-6 GHz applications. The key innovations include: (i) elimination of conventional vias via Hilbert‑based EBG incorporation, reducing insertion loss and fabrication complexity; (ii) 4 dB gain enhancement over the baseline design, achieving 20 dBi at 5.6 GHz—a significant improvement over prior CRLH‑TL antennas (typically 12–16 dBi); (iii) optical reconfiguration using LDRs, enabling beam steering and gain variation without electrical bias lines; and (iv) direct amplitude modulation capability. These advances collectively address the critical limitations of gain, reconfigurability, and fabrication complexity in existing MTM antenna designs.

Protocol

Antenna design and simulation setup
The antenna was designed using commercial 3D electromagnetic simulation software based on the finite integration technique (FIT)¹⁰. The simulation parameters were configured as follows: (i) solver type: time domain solver with hexahedral mesh; (ii) frequency range: 4–7 GHz; (iii) mesh density: 20 lines per wavelength, adaptive mesh refinement enabled with accuracy threshold of –30 dB; (iv) boundary conditions: open (add space) in all directions with minimum distance of λ/4 from the structure; (v) farfield monitors: configured at 0.1 GHz intervals across the frequency range; (vi) port excitation: waveguide port with 50 Ω impedance. A Rogers RT/duroid 8550 was selected with relative permittivity (εr) of 2.55, thickness of 1.5 mm, and loss tangent (tan δ) of 0.0033. Visual checkpoint: Upon completion of the model geometry, the antenna structure should appear as shown in Figure 1, with clearly defined CRLH unit cells, interdigital capacitors, and T-stub inductors.

CRLH-TL antenna fabrication
The antenna employed an asymmetric coplanar waveguide (CPW) feeding structure. The series capacitance was realized using an interdigital capacitor, and the shunt inductance was implemented using a T-stub inductor, with all geometric dimensions defined according to the layout shown in Figure 2A. The shunt inductance was implemented via a T‑stub inductor (stem width: 0.5 mm, stem length: 15 mm, stub width: 6.0 mm, stub length: 6.0 mm). Instead of conventional via holes, a third‑order Hilbert curve structure (outer dimensions: 7.87 mm × 7.87 mm; trace width: 0.48 mm; gap: 1.19 mm; additional segment lengths of 2.39 mm, 1.43 mm, and 3.10 mm as shown in Figure 2B) was incorporated on the reverse side of the substrate to provide inductive loading while minimizing radiation losses¹¹. Seventeen-unit cells were arranged in a 1 × 17 linear array with inter‑cell spacing of 8.5 mm (approximately λg/4 at 5.6 GHz). The interdigital capacitor and T-stub inductor dimensions were parametrically optimized; the final design corresponds to the configuration that yields maximum gain and minimum S11, as shown in Figure 2.

EBG structure implementation
A 17 × 1 array of third‑order Hilbert curve structures was printed on the back surface of the substrate, aligned concentrically with each T‑stub inductor. Each Hilbert element occupied an area of 7.87 mm × 7.87 mm, with the T‑stub inductor positioned centrally within this area to maximize inductive coupling¹². The EBG array was positioned such that its longitudinal axis coincided with the centerline of the CRLH-TL array, with registration achieved using optical alignment marks during photolithography. The Hilbert EBG suppresses surface wave propagation and improves impedance matching¹³. Visual checkpoint: The third-order Hilbert fractal geometry used for EBG implementation is shown in Figure 2B.

AMC reflector design
A 7 × 10 AMC reflector array was designed using hexagonal unit cells (side length: 5.2 mm; periodicity: 9.0 mm). Each unit cell featured three rectangular slots (length: 3.5 mm; width: 0.4 mm) arranged at 120° intervals on the patch center to achieve zero reflection phase at 5.6 GHz. The array was fabricated on an identical Rogers RT/duroid substrate and positioned 8.0 mm (approximately λ0/6.7 at 5.6 GHz) below the antenna ground plane. Element spacing was maintained at 0.5 mm between adjacent hexagons, yielding a total array footprint of 200 mm × 100 mm. Alignment of the AMC array relative to the antenna was achieved using mechanical fixturing with micrometer‑adjustable stages, ensuring ±0.1 mm positional accuracy. Visual checkpoint: The AMC reflector unit-cell geometry and the complete array layout should correspond to Figure 3.

Optical switch integration
Surface‑mount light‑dependent resistors (LDRs) were integrated into the T‑section of each CRLH unit cell. Each LDR was positioned across a 0.5 mm gap in the T‑stub inductor stem, 3.0 mm from the stub junction. LDR attachment was performed using conductive epoxy cured at 80°C for 30 minutes. An external 635 nm red laser diode (optical power: 5 mW; beam diameter: 3 mm) served as the optical source. The laser beam was directed via a computer‑controlled galvanometer mirror system, enabling selective illumination of individual LDRs or groups of LDRs. For OFF‑state characterization, illumination was blocked using opaque black polyvinyl chloride (PVC) covers (thickness: 1.5 mm; dimensions: 10 mm × 15 mm) affixed over each LDR. Visual checkpoint: The LDR integration points within the T stub structure should be visible, as shown in Figure 1.

Fabrication and measurement process
Antenna fabrication employed a wet chemical etching process. Substrate panels were cleaned with isopropyl alcohol, dried at 60°C for 10 minutes, and laminated with positive photoresist (AZ 4562, spin‑coated at 3000 rpm for 30 seconds, soft‑baked at 100°C for 90 seconds)¹⁴. UV exposure (350 nm, 12 mW/cm², 25 seconds) was performed through a photomask, followed by development in AZ 400K developer (1:4 dilution with deionized water, 45 seconds)¹⁵. Copper etching was conducted using ferric chloride solution (FeCl₃, 40% w/v, 40°C, 8 minutes)¹⁶. Residual photoresist was removed with acetone, followed by rinsing with deionized water and nitrogen drying¹⁷. Visual checkpoint: The fabricated antenna prototype as shown in Figure 4.

Measurements were conducted inside an RF-shielded chamber (dimensions: 4.5 m × 3.0 m × 2.5 m; isolation > 90 dB from 1–12 GHz). The antenna under test (AUT) was mounted on a computer‑controlled azimuth/elevation positioner (rotational accuracy: ±0.1°; angular range: 360° azimuth, ±90° elevation). A wideband dual‑ridge horn antenna (1–12 GHz) served as the reference antenna. S-parameter measurements were performed using a vector network analyzer calibrated with a full two-port SOLT (short-open-load-thru) calibration kit. Far‑field gain was determined using the gain‑transfer method with a standard gain horn of known gain (8.2 dBi at 5.6 GHz). All measurements were conducted under ambient laboratory conditions (temperature: 22 ± 1 °C; relative humidity: 45 ± 5%).

Results

Antenna design and simulation setup
The simulated and measured S11 parameters for the baseline CRLH-TL antenna (without AMC reflector) demonstrated excellent agreement across the 3–6 GHz frequency range. The antenna exhibited a –10 dB impedance bandwidth of 2.1 GHz (3.7–5.8 GHz), with the minimum S11 of –32 dB occurring at 5.6 GHz. The corresponding VSWR was below 1.5 across the entire operational bandwidth. Parametric studies revealed that increasing the number of interdigital capacitor fingers from...

Discussion

This work proposes a novel methodology for achieving high-gain, reconfigurable radiation in a planar metamaterial antenna through the synergistic integration of three complementary technologies: via-less CRLH TL with Hilbert EBG for loss minimization, a zero-phase AMC reflector for gain enhancement, and optical LDR switching for reconfiguration that inspired by Al Naiemy²¹. The approach uniquely eliminates the fabrication complexity associated with conventional via-based CRLH‑TL implementati...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Substrate	Taconic	www.taconic.co.kr/rf-43	Rogers RT/duroid 8550, εr=2.55, h=1.5 mm
Photoresist	MicroChemicals	www.microchemicals.com/products/az-4562	AZ 4562 positive photoresist
Developer	MicroChemicals	www.microchemicals.com/products/az-400k	AZ 400K developer concentrate
Etchant	Sigma-Aldrich	270290-1KG	Ferric chloride, 40% w/v
Solvent	Sigma-Aldrich	270725-1L	Acetone, semiconductor grade
Solvent	Sigma-Aldrich	278475-1L	Isopropyl alcohol, semiconductor grade
Conductive epoxy	CircuitWorks	www.chemtronics.com/cw2400	CW2400 conductive epoxy
LDR	Advanced Photonix	PDV-P8103	Light-dependent resistor, 5-10 kΩ (light) / 1 MΩ (dark)
Laser diode	Thorlabs	L635P5	635 nm, 5 mW, elliptical beam
Galvanometer	Cambridge Technology	6215H	Optical scanning mirror system
VNA	Keysight	37347A	Vector network analyzer, 10 MHz-20 GHz
Horn antenna	ETS-Lindgren	3117	Dual-ridge horn, 1-12 GHz
Positioner	ORBIT/FR	AL-4163-4A	Azimuth/elevation positioning system
RF chamber	ETS-Lindgren	FACT-3	Shielded enclosure, 4.5×3.0×2.5 m
Simulation software	Dassault Systèmes	CST Studio Suite 2023	3D electromagnetic simulator