Projects:2021s1-13113 Miniaturised metasurface antennas

Abstract here

Introduction

In this modern age, there is an ever growing demand for wireless communication, resulting to an increasing use of antennas. In portable devices, integrated antennas are becoming more common, as their compact size and nature proves ideal to have a compact and elegant solution, removing the need for traditional, external antennas. There is also an evergrowing number of wireless connection protocols, requiring unique antennas to support each protocol. A typical phone will include at minimum, 3G, Wifi, Bluetooth and GPS antennas.
With many designs in technology, there is a reoccuring theme of attempting to shrink designs, whilst maintaining or improving upon performance. Antennas are not neglected from this, with a particular interest in integrated antennas. Antenna design has physical limitations, as the size of the antenna is dependent on the operating frequency. For lower frequency operations, this proves problematic, as there are greater wavelengths, resulting in larger antennas. This is where a technique known as 'metasurfacing' can be used to miniaturise lower frequency integrated antennas, whilst complying with the physical limitations. This technique involves the periodic arrangement of small elements above the dielectric material, which affects the reflection and transmission of electromagnetic waves. The use of this metasurfacing technique will be explored, and attempted to be utilised to miniaturise a patch antenna, whilst maintaining or improving antenna performance such as bandwidth, and directive gain.

Project team

Project students

• Galvin Chuong
• Isaac Do

Supervisors

• Dr Christophe Fumeaux
• Dr Shengjian Chen (Jammy)
• David de Haaij (Black Arts Technology)

Objectives

The objective of this project focuses on two final desigs of the miniaturised metasurface antenna with the operating frequency at 900 MHz. These designs must match or improve the performance of the antenna before miniaturisation. The antennas must satisfy the bandwidth of 900 MHz – 928 MHz. These final designs will include many milestones, mainly revolving around many design processes until the final design is reached. These include a benchmark antenna, recreation of past literature antennas, first design of a metasurface antenna and then the final design before fabrication. The designs are created and simulated using CST Studio Suite, a 3D Electromagnetic simulation and analysis software.

Background

Since patch antennas are being modified to implement metasurfaces to reduce the antenna size, background about patch antennas and their properties are required. Patch antennas are inexpensive, robust low-profile antennas which have been used in this modern age for many applications, ranging from spacecraft applications to mobile and wireless communication. However, their main drawbacks are their lack of efficiency and power, its narrow bandwidth and their physical size when operating at low frequencies.

The patch antenna is composed with a patch, dielectric substrate, ground plane and a feed line. The patch of the antenna is a very thin metallic patch which is placed on top of a ground plane but separated by a dielectric substrate that is a small fraction of a wavelength in height [1]

Figure 1: Edited excerpt from Antenna Theory: Analysis and Design, Labelled diagram of a typical microstrip antenna design

The patch antenna properties can be altered when replacing the singular patch with a periodic arrangement of metallic patches. This is also known as a metasurface patch, which will alter the space waves emitted from the metasurface patches. The radiated space waves will yield different reflection and transmission properties compared to the singular patch. [2] The gaps between the patches also act as radiating and non-radiating slots, which can excite multiple modes. These multiple excited modes can increase the bandwidth if designed correctly, allowing a higher bandwidth which will help in maintaining performance when miniaturising.

Previous Studies

Previous studies of metasurface antennas all attempt in miniaturising antennas using different techniques. Most notably, the main different in most of these metasurface antennas are the shapes of the patches. The metasurface designs in the previous research papers are adaptations of a 4x4 array of patches. These patches are modified in 2 ways:

• Altering the radiating slots
• Altering the patches

All the research papers utilise the same feeding style for the antenna, known as aperture coupling. Thus, all the antennas contain 2 substrates separated by a ground plane with the microstrip feed at the bottom. The ground plane contains an aperture slot, allowing the antenna to be non-directly fed. The microstrip feedline changes for some designs as it allows for better impedance matching and increased gain. [3] The two designs being investigated in this project alter 2 different aspects. One design is inspired from (paper), where radiating slots are altered from horizontal slots to triangular sawtooth slots. [4] The other design draws inspiration from (paper), which alters the patches by implementing square ring patches instead of solid patches. [5]

Method

The design process of this project is to create the design using antenna simulation software. For this project, CST Studio Suite 2021 is used to design the antenna.

To identify if the miniaturisation of the antenna is successful, there must be a comparison between the miniaturised metasurface antenna and a reference antenna. Therefore, a reference antenna must be designed initially at the operating frequency of 900 MHz to be compared against the miniaturised antenna. The parameters which are an important factor to verify the success of the miniaturised metasurface antenna are the size of the antenna, bandwidth, and gain of the antenna.

The approach to designing the miniaturised metasurface antenna is to firstly recreate an identical antenna to a previous designed antenna. This will give an idea of the size of the miniaturized antenna and the S-parameters of the antenna. As the operating frequency is inversely proportional to the size of the antenna, the recreated antenna is scaled to shift the operating frequency to 900 MHz. The scaled antenna at the correct operating frequency allows for optimization to miniaturise the antenna.

Once the simulated antennas are fabricated, their performance is tested in the anechoic chamber in The University of Adelaide. The fabricated antenna’s S-parameters are measured by connecting the antenna to a vector network analyser. The radiation pattern of the antenna is measured by placing it on the turn table inside the anechoic chamber, that spins 360 degrees to record the full radiation pattern. The vector network analyser used in the anechoic chamber is calibrated using an electronic calibrator. The horn antenna used when measuring the radiation pattern is calibrated by using a standard horn antenna to calculate the gain. This gain is compared to the gain corresponding to the metasurface antennas to compensate for the gain from the horn antenna used to produce the radiation pattern.

Designs

One of the metasurface designs consists of triangle-interembedded patch unit cells, produced from horizontal triangular sawtooth radiating slots and straight vertical non-radiating slots. The central horizontal slot is excited at lower frequencies while the sub-central triangular sawtooth slots are excited at higher frequencies. The excitation of these slots produce 2 peaks that merged that greatly increase the bandwidth compared to a standard patch antenna. The antenna consists of 2 substrates, separated by an airgap to improve the gain. The bottom substrate has a ground plane etched on top with an aperture slot, a fork feed is etched on the bottom and the patches are etched onto the top substrate.

Figure 2: Top view of the triangular sawtooth metasurface antenna design

Figure 3: Bottom view of the triangular sawtooth metasurface antenna design

The second design utilises non-periodic square-rings as the Metasurface. The design is also composed of two layers, a lower and upper substrate. These layers are spaced using nylon spacers to introduce an airgap. The upper substrate contains the Metasurface structure in place of a typical patch antenna. The lower substrate also incorporates a coupling aperture as the feeding technique. This involves a bottom microstrip fed line, with an aperture slot in the ground plane. This microstrip feed line is adapted to contain a fork-style extension to improve the bandwidth and antenna matching. The final design has a physical size of 0.48λ_0 x 0.48λ_0 x 0.01λ_0 and is a low profile design.

Figure 4: Top view of the nonperiodic square rings metasurface antenna design

Figure 5: Bottom view of the nonperiodic square rings metasurface antenna design

Results

S-Parameters

The fabricated antennas S-parameters and radiation patterns are illustrated. As seen in the S-parameters for this antenna, the bandwidth is 867 - 1416 MHz, therefore the percentage of bandwidth for the measured antenna is 61%. It is noted that the measured antennas bandwidth is slightly larger than the simulated antenna’s bandwidth. The slightly increased bandwidth from the measured antenna was due to discrepancies during the fabrication of the antenna. As the antenna operates at a low frequency, the environment where the antenna is being tested greatly impacts the measurement due to its high wavelength. This can be seen as the simulation S-parameters are a smoother curve than the measured S-parameters.

The fabricated antennas S-parameters and radiation patterns are illustrated in (figures). As seen in the S-parameters for the fabricated antenna, the bandwidth is 880 - 980 MHz, therefore the percentage of bandwidth for the measured antenna is 11%.

Figure 8: Simulated and Measured S-parameters for Triangular Sawtooth Metasurface Antenna

Figure 9: Simulated and Measured S-parameters for Non-Periodic Square Ring Metasurface Antenna

3D Radiation Patterns

Figure 6 shows the radiation pattern of the simulated design. It can be seen that the antenna is directive in the +Z direction, with a main lobe and a back lobe. The achieved realized gain is approximately 8.6dBi, which would be considerably directive.

Figure 7 shows the radiation pattern of the simulated design. It can be seen that the antenna is directive in the Z direction, however with an equal front lobe and back lobe propagating simultaneously. The achieved realized gain is approximately 4.1dBi, which would not be considered a highly directional antenna. As such, the antenna design is more suited for omnidirectional usage.

Figure 6: 3D Radiation Pattern for the triangular sawtooth metasurface antenna

Figure 7: 3D Radiation Pattern for nonperiodic square ring metasurface antenna

Conclusion

• The triangular sawtooth design achieved a bandwidth of 61% and a stable realized gain of 6.8 dBi. This antenna design exceeds the performance of the reference antenna and the metasurface designs from previous literature.
• The nonperiodic square ring design achieved the bandwidth of 11% and a stable realized gain of 4 dBi. This antenna design exceeds in miniaturizing when compared against metasurface designs from previous literature.

References

[1] C. Balanis, Antenna theory, 4th ed. Hoboken: John Wiley & Sons, 2016, pp. 783-815.

[2] M. Faenzi et al., "Metasurface Antennas: New Models, Applications and Realizations", Scientific Reports, vol. 9, no. 1, pp. 1-2, 2019. Available: 10.1038/s41598-019-46522-z [Accessed 24 May 2021].

[3] W. Liu, Z. N. Chen and X. Qing, "Metamaterial-Based Low-Profile Broadband Aperture-Coupled Grid-Slotted Patch Antenna," in IEEE Transactions on Antennas and Propagation, vol. 63, no. 7, pp. 3325-3329, July 2015, doi: 10.1109/TAP.2015.2429741.

[4] D. Chen, W. Yang, Q. Xue and W. Che, "Miniaturized Wideband Planar Antenna Using Interembedded Metasurface Structure," in IEEE Transactions on Antennas and Propagation, vol. 69, no. 5, pp. 3021-3026, May 2021, doi: 10.1109/TAP.2020.3028245.

[5] D. Chen, W. Yang, W. Che and Q. Xue, "Broadband Stable-Gain Multiresonance Antenna Using Nonperiodic Square-Ring Metasurface," in IEEE Antennas and Wireless Propagation Letters, vol. 18, no. 8, pp. 1537-1541, Aug. 2019, doi: 10.1109/LAWP.2019.2919692.