Authors: Siti Nailah Mastura Zainarry, David de Haaij
Black Art Technologies, Unit 9/22 Waddikee Rd, Lonsdale SA 5160.
A study of a miniaturized square microstrip antenna design for low-frequency applications is proposed. The rationale of the design is accomplished by adding circular ring slots at the top of the antenna patch to acquire a unique lowest operating frequency range and dimension reduction. Two small parasitic patches are coupled to the edge of the radiating aperture to improve the antenna impedance bandwidth. This design is later adapted to a layer of metasurface superstrate structure loaded over the antenna, which effectively shifts the antenna operating frequency to a lower-level range. The design is optimized for a relative impedance bandwidth of 41.4% extending from 0.46 to 0.7 GHz with a good performance of the realized gain up to 7.31 dBi.
Index Terms: Antenna miniaturization, circular ring slots, metasurface superstrate, low-frequency applications.
Nowadays, the use of small-sized and wide bandwidth antennas is gaining greater attention in many applications because of their flexibility and adjustment options. Numerous studies with advanced technology have been undertaken on antenna miniaturization to reduce the overall physical dimensions without significant performance degradation. Antenna designers together with structure and material, have merged the energy to set up these new excessive payoff technologies.
In practice, the antenna size is not primarily determined by the innovations used for its manufacture but rather by the laws of physics. This means that the antenna size depends on its operating frequency and wavelength, where they are inversely proportional to each other . Therefore, the antenna size is relatively large in modern mobile communications and wireless systems. Numerous approaches and techniques have been implemented to accomplish sophisticated properties of antenna miniaturization (where the size is smaller than its wavelength) while maintaining acceptable matching and operating bandwidth –. However, most of the studies are limited to high-frequency applications.
In this paper, a miniaturized square patch antenna design is proposed. The design goal is to minimize the antenna size-to-wavelength ratio by maintaining the physical dimension (a small antenna design) for low operating frequency applications. This paper presents a parametric study and its possibilities of changing the electrical and magnetic properties of the antenna structure using circular ring slots and a periodic metasurface superstrate technique.
II. Antenna Design and Operation Principle
Figure 1 illustrates the detailed design of a miniaturized patch antenna. The design begins with a conventional square microstrip patch antenna forming on a single-sided copper-clad substrate, backed with an air substrate at a distance hair = 47 mm above a ground plane. Two WavePro dielectric substrates with a relative permittivity of 4.4 and 2.5 (loss tangent = 0.0009) are used, as a substrate and a superstrate of the antenna, respectively. The thickness h1 = 2 mm is selected to create the patch structure. Meanwhile, on the top of the patch, the thickness h2 = 10 mm is selected to create a periodic metasurface structure to perform miniaturization without changing the overall dimension or affecting the antenna performance.
The patch dimensions are derived for a certain angular operating frequency at a center frequency of 0.86 GHz with Lp = Wp = 113 mm. At this stage, circular ring slots with the inner radius ri = 10 mm and the outer radius ro = 55 mm are added to the top of the patch antenna in order to extend the current path within the aperture, thus obtaining a unique lowest operating frequency range. The slot spacing S = 2 mm is chosen. These slots do not increase the complexity of fabrication, yet they devote to the antenna miniaturization structure and provide a good impedance bandwidth over a certain operating frequency range.
Two small parasitic patches with the dimensions of Lpar × Wpar = 55 mm × 3 mm are placed close to the edges of the radiating patch to improve the antenna impedance bandwidth. In addition, the mechanism of the proposed design miniaturization is further analyzed by a 4 × 4 grid-slotted metasurface unit cell elements, printed on the top of the superstrate dielectric. Each unit cell with the dimension of Lm = Wm = 58 mm and an intermediate gap of Sm = 1 mm are selected.
Fig. 2: Reflection coefficient of a conventional square patch antenna.
Fig. 3: Reflection coefficient of the proposed antenna with and without a periodic metasurface superstrate structure.
The antenna is then equipped with two independent capacitive feeding networks (Lf × Wf = 27 mm × 6 mm), providing control to achieve two different far-field radiation pattern polarizations. A vertical coaxial inner pin is attached to each capacitive feed through the substrate, the air gap, and the ground plane to excite the antenna. This arrangement provides advantages on the antenna performance, such as impedance bandwidth and gain level, as well as the antenna efficiency. The antenna size is Lsub × Wsub = 240 mm × 240 mm.
III. Result and Discussion
To clearly validate the superiority of the proposed antenna, the antenna performance comparisons are presented. Figure 2 shows the reflection coefficient of the conventional square patch antenna. As seen in the figure, the resonance frequencies are appropriately operated between 0.73 to 1.0 GHz within the impedance bandwidth of around 31% at the -10 dB level.
To demonstrate the aforementioned analysis of the antenna miniaturization, Fig. 3 illustrates the reflection coefficient of the proposed antenna loaded with and without the periodic metasurface superstrate structure. It is seen that the combination of added corrugation with the loaded metasurface superstrate can shift the antenna operating frequencies to the lowest frequency range. It is visible from 0.46 to 0.70 GHz with its relative impedance bandwidth (|S11| < −10 dB) roughly 41.4% wider, when compared without the loaded metasurface superstrate, due to their particular characteristics of manipulating electromagnetic waves.
Figure 4 shows the realized gain patterns at three selected resonance frequencies, within the operating frequency range. Due to the size of the antenna relative to the wavelength, the side lobe levels becoming higher than -10 dB. Figure 5 illustrates the realized gain of the proposed antenna as a function of the frequency. The results show that the antenna gain is at 4.23 to 7.31 dBi.
A miniaturization of circular ring slot patch antenna loaded with a metasurface superstrate structure has been presented. The proposed antenna configuration has been designed to provide a lowest operating frequency range at 0.46 to 0.7 GHz. The antenna performance comparisons have also been presented, where a bandwidth improvement of around 41.4% and a realized gain from 4.23 to 7.31 dBi were achieved. Future work will further optimize the design towards the practical realization with enhanced performance, in particular towards side lobe level reduction and radar applications. The next step will be the fabrication and measurement of the proposed antenna.
The authors wish to thank Garlock for providing the dielectric substrates used in this study.
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Keywords: Antenna miniaturization, circular ring slots, metasurface superstrate,
low frequency applications, antenna radiation patterns, microstrip antennas, slot antennas, UHF antennas