The base antenna in the study is an electromagnetic band-gap resonator antenna, or ERA, a class of high-gain antenna that has become an attractive alternative to conventional 16 to 25 dB designs built around bulky structures and lossy feeding networks. A classical ERA consists of a feed source, a metal ground plane, and a partially reflecting surface suspended roughly half a free-space wavelength above it. The resonance created between these two planes boosts the far-field radiation performance, and because the antenna is planar with a simple structure and an easily implementable feeding system, it suits millimeter-wave communication, satellite links and radar applications. The prototype considered in this work operates at 12 GHz, with a ground plane and partially reflecting surface of identical lateral dimensions of 100.8 by 100.8 millimeters, built on commercially available Rogers TMM4 substrate with a dielectric constant of 4.7 and a loss tangent of 0.002.

The performance of a classical ERA, however, is fundamentally limited by the non-uniform phase distribution of its dominant electric field. When waves leave the aperture, they arrive with phases that vary across the surface, and this phase error prevents the antenna from combining its energy into a single, sharply focused beam. The team’s solution exploits the principle of near-field phase transformation, or NFPT, in which a metastructure is loaded directly on top of the radiative aperture to manipulate the near-field phase distribution. The approach creates a passive, low-loss and low-profile structure that can significantly improve antenna performance, and it can in principle be applied to any aperture-type antenna, including horn, slot and patch antennas.

The design process begins with a hypothetical plane placed 5 millimeters above the antenna aperture, where the dominant electric field phase is recorded. This input phase plane is divided into a square grid of cells 8.4 millimeters on a side, and the phases are probed at the center of each cell. The recorded phase pattern follows a concentric circular arrangement, so six concentric circular arrays of cells were used to develop the proposed phase-transforming metastructure, or PTM. From the probed data, the required phase delay for each ring of cells is calculated so that all phases above the aperture become nearly uniform. The probed phases for the six cell types range from 140 degrees at the center to minus 43 degrees at the outermost ring, requiring phase delays of 60 to 243 degrees to achieve the uniform reference output.

The heart of the innovation lies in the unit cells themselves. The metastructure comprises two categories of cells made of dielectric material with a permittivity of 4.5 and copper, because a single cell type cannot cover a wide phase range while maintaining a transmission coefficient greater than minus 1 dB. Category-I cells consist of three identical metal layers placed at the top, bottom and middle of the cell at equal distances from each other, with the gap between the inner arms varied from 3 to 4.5 millimeters to generate the necessary transmission phase delay. Category-II cells contain a single metal layer inserted in the middle of the cell along with a cuboid air hole, whose size is varied from 3 to 5 millimeters to produce different delays. All cells share the same height of 6.25 millimeters and lateral dimensions of 8.4 millimeters, which makes the metasurface planar and symmetric and free from the shadowing loss that afflicts non-planar structures. The metal layers are 0.5 millimeters thick and 1 millimeter wide.

The transmission characteristics of these cells are what set the design apart. At the design frequency of 12 GHz, the reflection coefficient of every cell is less than minus 10 dB, indicating wide operational bandwidth, and the transmission coefficient magnitude is greater than minus 0.77 dB for all cells. This exceptional transparency means almost all of the energy passing through the phase-correcting structure is preserved rather than absorbed or reflected, which directly increases the aperture efficiency compared with previous designs. Because the wideband behavior of the unit cells mirrors the wideband characteristics of the base antenna, the complete system operates effectively across a broad frequency range rather than at a single frequency. The metastructure is also fully passive and polarization-independent, removing two drawbacks commonly associated with phase-transforming structures of the all-metal or multi-layer printed variety, which tend to suffer from heavy weight, high profile, polarization dependence, high cost or high loss at high frequencies.

When the full three-dimensional structure of the ERA with the proposed PTM was simulated using the time-domain solver of CST software with open boundary conditions, the results were striking. Without the metastructure, the phase error across the aperture, the difference between maximum and minimum phase, was substantial. With the PTM in place, the phase error was notably minimized, and the far-field performance improved dramatically. The peak directivity of the bare ERA is 11.4 dBi, but loading the PTM raises it to 21.4 dBi, a gain of 10 dBi attributed to the improved phase uniformity in the near-field region. The peak directivity occurs at 12.4 GHz, with a slight shift from the design frequency due to the common loading effect of the phase-transforming structure. The peak gain reaches 20.7 dBi at the same frequency.

The wideband capability of the system is equally impressive. The 3-dB directivity bandwidth spans from 10.9 GHz to 13.66 GHz, equivalent to 22.48 percent, and the peak aperture efficiency reaches 54.65 percent. The radiation pattern remains stable across the operating range: in the E-plane, side-lobe levels stay at least 9 dB below the main beam, with cross-polar components lower than minus 15 dBi, while in the H-plane the side lobes are at least 10 dBi below the main beam peak and cross-polar components fall below minus 18 dBi. The ERA with the PTM also demonstrates effective impedance matching across the operational frequency range, with a maximum return loss of minus 12.5 dB and a minus 10-dB return-loss matching bandwidth greater than 40 percent. To validate the simulation data, the team cross-checked the prototype using HFSS, another commercial RF software, and found good agreement between the two solvers, with only minor discrepancies attributable to their different numerical methods.

Benchmarking against state-of-the-art phase-transforming structures reported in the literature reveals why the low profile matters. The proposed metastructure stands just 6.25 millimeters tall, equivalent to a quarter of the free-space wavelength, comparable only to a metal-dielectric design with lower performance and a printed circuit board structure that is costly to fabricate. Fully metallic designs reach 13.5 millimeters, all-dielectric counterparts climb to 21 or even 39 millimeters, and while those structures achieve directivities of 20 to 22 dB, their aperture efficiencies range from 24 to 51 percent and their 3-dB directivity bandwidths from 6 to 16.5 percent, all below the 54.65 percent efficiency and 22.48 percent bandwidth of this work. The lower height also means less material consumption during fabrication and reduced manufacturing cost.

The path from simulation to physical hardware runs through additive manufacturing. Conventional subtractive methods such as computer numerical control machining are costly, wasteful and unable to realize certain customized structures, whereas three-dimensional printing offers rapid prototyping, intricate customization, minimal waste and the ability to catch design errors before final production. Common printing filaments such as ABS, PLA, ASA and TPU are limited to a permittivity range of roughly 2.0 to 2.8 and suffer high loss, so the team proposes exploiting RF-graded PREPERM ABS filament, a thermoplastic formulated for high-frequency wireless devices that offers dielectric constants from 2.0 to 22.0 with ultra-low RF loss and stable operation over a wide frequency range. By incorporating metal and air holes within the host dielectric material, continuous phase variation is achieved without relying on commercially available materials that cannot provide a continuous permittivity range. The researchers note that the resulting EBG resonator antenna could be realized in applications spanning 5G wireless networks, satellite communications and radar, bringing a design that is printable, passive, polarization-independent and highly transparent one step closer to the antennas that will carry the next generation of wireless traffic.

Subject of Research: A metal-dielectric 3D-Printable metastructure for the radiation enhancement of electromagnetic band-gap resonator antennas

Article Title: A metal-dielectric 3D-Printable metastructure for the radiation enhancement of electromagnetic band-gap resonator antennas

Article References: Ali, M. Y., Lalbakhsh, A., Singh, K., Taheri, S. H., & Mukhopadhyay, S. (2025). A metal-dielectric 3D-Printable metastructure for the radiation enhancement of electromagnetic band-gap resonator antennas. Frontiers in Antennas and Propagation, 3, Article 1585028. https://doi.org/10.3389/fanpr.2025.1585028

Image Credits: AI Generated

DOI: 10.3389/fanpr.2025.1585028

Keywords: metal-dielectric, D-Printable, metastructure, radiation, enhancement, electromagnetic, band-gap, resonator, antennas, scientific research