Broadband and high gain circularly polarised truncated corner square patch metasurface antenna using aperture CPW feed

Broadband and the high gain circularly polarised antenna are designed in a single-layered planar substrate. An array of 3 × 3 truncated corner square patches are designed as a metasurface antenna on one side of the substrate and the metasurface antenna is fed by a stub-loaded uniform aperture CPW feed on another side of the substrate. In the metasurface antenna, the size of the central patch is increased for improving the bandwidth of impedance bandwidth and 3 dB axial ratio. Furthermore, the amount of truncation is reduced in the patches surrounding the central patch to enhance the bandwidth of impedance bandwidth and 3 dB axial ratio bandwidth. The designed metasurface antenna has achieved an impedance bandwidth of 38.2% (4.5–6.63 GHz) with an axial ratio bandwidth of 13% (5.0–5.7 GHz). Moreover, the antenna has a peak gain of 10.36dBic. The simulation and measurement results are compared.

Keywords: CPW fed antennas; metasurface; high gain and broadband; circular polarization; truncated corner square patch

Introduction

The circularly polarised antennas are useful for signal propagation regardless of the transmitter and receiver antenna orientation. Circularly polarised (CP) waves are more difficult to generate than linearly polarised (LP) waves because CP waves require the creation of orthogonal fields of equal magnitude. For CP generation, dual/multiple feed approaches are used. Nowadays, single feed techniques are possible with truncated corners-/slits-/slots to the patch for generating the CP waves. But the broadband circular polarisation with truncated corners to the patch is obtained in the presence of a metasurface in another substrate layer [[1], [3], [5]], an air gap as another substrate layer [[7], [9]] and parasitic elements to the probe fed driven patch [[11], [13]].

Several publications on the design of broadband and high gain circularly polarised antennas with metasurface in an additional substrate layer. The truncated corner patch is employed as a radiating element in between the ground plane and metasurface for generating circularly polarised antennas [[1]]. The corner truncated square patch is rotated by 45° and it is laid on the RIS structure [[2]], the slot-loaded patch is laid on the 45° rotated RIS structure [[3]] to create circular polarisation with broad bandwidth. The 45° rotated rectangular loops are placed as a metasurface and they are excited by the microstrip line [[4]]. The metasurface is developed with an array of square patches and a slot in the shape of L is designed in the ground plane in [[5]], the metasurface is formed with an array of truncated corner square patches and a uniform slot is used in the ground plane in [[6]] to obtain broadband circularly polarised antenna. The above-described approaches use an additional substrate layer for the metasurface to design a high gain and broadband circular polarisation.

Some papers have published broadband circularly polarised antennas using an air gap as a substrate layer. The parasitic patches are used around the truncated corner patch and the air gap as a substrate is used to generate circularly polarised antennas [[7]]. The truncated corner patch is used as a feed element and stacked patches are used above the air gap to generate circularly polarised antennas [[8]]. These techniques use air gap as a substrate. The authors have published broadband and high gain circularly polarised antennas using reactive impedance surface as a substrate and frequency selective surface as a superstrate [[9]]. These techniques use multiple layers with an air gap.

Some papers have published broadband circularly polarised antennas in single layer planar substrates using parasitic elements to the probe fed radiating patch [[11], [13]]. The central patch is driven by probe feed and parasitic patches are placed in proximity to the driven patch. These structures require holes in the radiating patch and substrate to insert the probe feed. Soldering to the patch is required to feed the radiating patch. The novel structure is proposed in [[14], [16]] to avoid holes and soldering problems. In this structure, the patch is placed on the top plane of the substrate and the excitation to the patch is provided by aperture CPW feed on the opposite side. The patch is replaced with metasurface to develop high gain and broadband linearly polarised antennas in single layer planar substrates [[17]]. In this structure, holes in the substrate and patch can be avoided for probe insertion. It is also avoiding the soldering problems to the truncated patch. The extra substrate layer and extra air gap can also be avoided.

In this research, a single-layered planar substrate is used to achieve high gain and broad circular polarisation. The antenna is initially created by printing a truncated corner square patch metasurface antenna on the top face of the substrate, which is then driven by an aperture coupled CPW feed on the opposite face. The broad bandwidth in impedance bandwidth and 3 dB axial ratio can be achieved by changing the dimensions of the central patch in the metasurface. Furthermore, by reducing truncation in patches surrounding the central patch, the bandwidth of impedance bandwidth and 3 dB axial ratio can be increased.

Antenna design

Figure 1 depicts the proposed structure's geometry. The antenna's modelling is done on a Rogers RT/Duroid 5880 substrate with a dimension of W × W × H. The antenna is made up of a 3 × 3 array of truncated corner square patches on the substrate's top face. The central patch's size is selected as L with the amount of truncation p on two diagonal corners. The surrounding patch's size is selected as M with the amount of truncation q on two diagonal corners. On the opposite side of the substrate, a stub-loaded CPW fed aperture is designed. The slot width of W1 is extended on both sides with a length of L1. The stubs of dimension Ls × Ws are arranged in the slot. The coplanar feed line of width Wf is connected across the slot. The gap between the coplanar feed line and the ground plane is g. The following section has a detailed analysis.

PHOTO (COLOR): Figure 1. Geometry of proposed antenna (a) Top view and side view (b) Bottom view.

Design steps

The design steps of each antenna are depicted in Figure 2. The respective impedance bandwidth (S11), gain, and axial ratio (AR) are shown in Figure 3. The dimensions of all the patches are the same in antenna 1 and antenna 2, whereas the central patch and surrounding patch dimensions are different in antenna 3 and the proposed antenna. Optimised parameter values are displayed in Table 1.

PHOTO (COLOR): Figure 2. Evolutions of the proposed antenna (a) Top plane view of antenna 1, antenna 2 (b) Top plane view of antenna 3, proposed (c) Bottom plane view of antenna 1 (d) Bottom plane view of antenna 2, antenna 3, proposed.

PHOTO (COLOR): Figure 3. Simulated results of four designs (a) S11 (b) Gain (c) AR.

Table 1. Optimised parameter values.

Parameter	Value (mm)
W	58
W1	1.5
L1	14
Ls	3
Ws	0.5
Wf	2.1
g	0.3
L	16.25
M	13.75
D	2
p	5.3
q	4.85
H	3.175

The metasurface is designed with patches on one side and CPW fed aperture on another side of the substrate in [[17]] to design wideband and high gain linearly polarised antennas. Antenna 1 is composed of square patches with a truncated corner metasurface antenna on the top plane of the substrate and the metasurface antenna is driven by aperture CPW feed on the opposite side. The impedance bandwidth has multiple resonant modes apart from each other and 3 dB axial ratio bandwidth is nil for antenna 1. In antenna 2, the stubs are inserted across the slot [[17]] as displayed in Figure 2. Here, the stubs are helpful to get resonance modes close to each other. When all the truncated corner square patches are the same size for antenna 2, the impedance bandwidth has resonance modes close to each other as shown in Figure 3.

In antenna 3, the central patch dimension is chosen larger than the surrounding patches. When the central patch's size is increased for antenna 3, the first and second resonance frequencies overlap with each other. Hence, the impedance bandwidth and 3 dB axial ratio bandwidth are improved as displayed in Figure 3. In the proposed antenna, the same truncation is maintained in a central patch and the truncation in surrounding patches is reduced. The upper resonance frequency and lower resonance frequencies overlap with each other to further enhance the impedance bandwidth and 3 dB axial ratio as shown in Figure 3. These all antennas are maintaining gain at around 10dBic because of the metasurface. The variations are explained in detail in the following section.

Consider the values of 'p' and 'q' as 5.3 mm, and varying the dimension 'L'

In antenna 2, the size of all truncated corner square patches is the same i.e. 'L' and 'M' are taken as 13.75 mm. When the size of central patch 'L' is increased, the resonant frequency of the central patch is shifted to lower frequencies. The resonant frequency of surrounding patches is maintained in the same frequency range as there is no change in the dimensions. Further, the resonance frequency range of the central patch and surrounding patches are overlapping with each other for 'L' = 16.25 mm and impedance bandwidth is enhanced as shown in Figure 4(a). The axial ratio (AR) of the antenna gradually increases as the 'L' value increases, finally, the broadband axial ratio bandwidth is obtained at 'L' considered as 16.25 mm as shown in Figure 4(b).

PHOTO (COLOR): Figure 4. Simulated results with respect to L (a) S11 (b) AR.

Considering the dimension 'L' as 16.25 mm, 'M' as 13.75 mm, 'p' as 5.3 mm, and varying the di...

In antenna 3, the size of the central patch is chosen as 16.25 mm, the size of the surrounding patches is chosen as 13.75 mm and the truncation in the central patch is maintained at 5.3 mm. When the truncation in surrounding patches is reduced, the size of surrounding patches is increased. As a result, the resonant frequency of surrounding patches shifts to lower frequencies and the resonant frequency of the central patch is maintained at the same frequency range. Hence, the upper and lower resonance frequencies overlap with each other and the impedance bandwidth is further enhanced as the truncation in the surrounding patches is reduced as shown in Figure 5(a).

PHOTO (COLOR): Figure 5. Simulated results with respect to q (a) S11 (b) AR.

When the uniform truncation (p = q = 5.3 mm) is maintained in all patches in antenna 3, the orthogonal fields are generated for circular polarisation. When the non-uniform truncation is provided (p ≠ q), the orthogonal fields are spread in other frequencies also. Hence, the axial ratio bandwidth is further increased. The truncation in surrounding patches 'q' is chosen to get maximum 3 dB axial ratio bandwidth as shown in Figure 5(b). Therefore in the proposed antenna, the dimension 'L' of a central patch is taken as 16.25 mm, dimension 'M' of surrounding patches is taken as 13.75 mm and truncation 'p' of a central patch is selected as 5.3 mm. The truncation 'q' of the surrounding patch is considered as 4.85 mm to achieve the broad bandwidth in impedance bandwidth and axial ratio as depicted in Figure 5.

Variation of Ls

The parametric study is performed by varying the length of the stubs in the slot. It is observed from Figure 6 that the proposed antenna has shown a maximum 3 dB axial ratio bandwidth when the dimension 'Ls' is 3 mm and results are represented for the other values also.

PHOTO (COLOR): Figure 6. Simulated AR results with respect to Ls.

The simulation results for the orthogonal fields (Ex and Ey) of proposed antennas are displayed in Figure 7. It can be noticed that the amplitude of orthogonal fields (Ex and Ey) is the same (less than 3 dB difference) and the phase difference is 90° attained throughout broadband frequency ranges (5.0G–5.7 GHz). Hence the axial ratio below 3 dB is achieved over a broadband frequency range (5.0G–5.7 GHz). The simulation results for the left hand circularly polarised (LHCP) gain and right hand circularly polarised (RHCP) gain are displayed in Figure 8. Over a resonant frequency range, the RHCP gain is observed to be high and the LHCP gain is observed to be quite low. As a result, the proposed antenna is a right hand circularly polarised antenna. Table 2 shows a comparison of proposed antenna simulation results with published literature. It can be noticed that the published literature used a dual substrate layer, multilayer substrates, and air gap to attain broadband circular polarisation. In recent works, broadband circular polarisation is achieved in single layer substrates using parasitic elements to the radiating patch. But the holes are required for probe fed radiating patch and soldering problems to the radiating patch. Such problems are avoided in some of the works to generate broadband linear polarisation in single layer substrate. In this paper, the above-noticed problems are avoided to create a broadband and high gain circularly polarised antenna in single layer substrates.

PHOTO (COLOR): Figure 7. Orthogonal fields for the proposed antenna.

PHOTO (COLOR): Figure 8. Simulated results for the LHCP gain and RHCP gain.

Table 2. The proposed antenna results are compared with published literature.

S.No	Ref	Substrate layers	Air gap	Probe insertion	Size (λ3)	Frequency (GHz)	IBW (%)	Gain (dBic)	AR (%)
1	[1]	2	No	Yes	0.58λ ×0.58λ×0.056λ	4.70–7.48	45.6	7-7.6	23.4
2	[2]	2	No	Yes	0.72λ×0.74λ×0.088λ	4.52–7.42	48.6	6.6	20.4
3	[3]	2	No	Yes	7.69 λ×7.69 λ×0.06λ	2.2–2.4	14	2.5–5.7	15.3
4	[4]	2	No	No	0.76 λ×0.76 λ×0.06λ	5–6.05	19	7.4–8.5	11.3
5	[5]	2	No	No	0.93λ×0.93λ×0.054λ	4.66–8.23	55.4	7.3–8.1	23.6
6	[6]	2	No	No	1.60λ×1.60λ×0.065 λ	7.88–12	41.45	13.5	23.16
7	[7]	2	Yes	Yes	1.13λ×1.13λ×0.13λ	1.87–3.11	49.8	8	24
8	[8]	2	Yes	Yes	0.8 λ×0.8 λ×0.09λ	5.1–7.08	31.5	8.6	20.7
9	[9]	4	Yes	Yes	λ×λ×0.5λ	4.93–5.89	17.72	12.48	2.4
10	[10]	3	Yes	Yes	λ×λ×0.5λ	4.81–6.63	31.8	10	18.9
11	[11]	1	No	Yes	0.86 λ×0.86 λ×0.04λ	5.05–6.03	17.7	8.4	12.8
12	[12]	1	No	Yes	0.60 λ×0.60 λ×0.05λ	5.1–6.2 GHz	19.7	6.9	19.7
13	[13]	1	No	Yes	1.18 λ×1.18 λ×0.05λ	5.05–5.85	14.7	10.8	14.7
14	[17]	1	No	No	1.28λ×1.28λ×0.09λ	4.81–9.69	67.3	9.18 dBi	–
15	[18]	1	No	No	λ×λ×0.053λ	9.07–11.36	22.1	7.5 dBi	–
16	Proposed	1	No	No	λ×λ×0.052λ	4.5–6.63	38.2	10.37	13%

The proposed antenna has been fabricated and is shown in Figure 9. The measurements for the proposed antenna have been performed. The measured results seem to be very similar to the simulation results. Due to fabrication errors, there is a slight variation. A vector network analyzer is used to calculate the reflection coefficient, which is presented in Figure 10(a). In an anechoic chamber, the antenna gain is measured and plotted in Figure 10(b). The patterns are used to calculate the axial ratio, which is presented in Figure 10(c). In an anechoic chamber, the radiation patterns are measured and plotted in the X-Z and Y-Z planes, as illustrated in Figure 11.

PHOTO (COLOR): Figure 9. Fabricated Prototype model for the proposed antenna (a) top plane view, (b) bottom plane view.

PHOTO (COLOR): Figure 10. Simulated and measured results are compared for the proposed antenna (a) S11, (b) Gain, and (c) AR.

PHOTO (COLOR): Figure 11. Normalised gain patterns for the proposed antennas at X-Z plane(left) and Y-Z plane (right) (a) 5.3 GHz (b) 5.5 GHz(c) 5.7 GHz.

Conclusion

A truncated corner square patch metasurface antenna is driven by an aperture CPW feed. The metasurface consists of a 3 × 3 array of truncated corner square patches. In addition to that stubs are inserted in the uniform slot. Moreover, the central patch size in the metasurface is increased for enhancing the bandwidth of impedance bandwidth and 3 dB axial ratio. Furthermore, truncation in surrounding patches is reduced to further enhance the bandwidth of impedance bandwidth and 3 dB axial ratio. The antenna designed with the above aspects achieves a broadband and high gain circularly polarisation in single layer planar substrate at 5 GHz used for Wi-Fi and Wi-Max applications.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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By Srinivas Guthi and Vakula Damera

Reported by Author; Author

Srinivas Guthi received his bachelor's degree in Electronics and Communication Engineering from Jawaharlal Nehru Technical University, Andhra Pradesh, India in 2011 and his master's degree in Communications Systems from the National Institute of Technology Allahabad, Uttar Pradesh, India in 2014 with a focus on MIMO antenna respectively. From 2014 to 2017, he worked as an assistant professor at Guru Nanak Institute of Technical campus, Telangana, India. Since Dec 2017, he is pursuing a Ph.D. degree in the area of high gain and wideband antennas at National Institute of Technology Warangal, Telangana, India. His research interests include high gain and broadband antennas, circularly polarised antennas, CPW fed aperture antennas, metasurface antennas, MIMO antennas, and FPC antennas.

Vakula Damera received the bachelor's degree in electronics and communication engineering from Nagarjuna University, Andhra Pradesh, India, and the master's degree in tech from the Birla Institute of Technology, Mesra, India, with a focus on microwave specialisation in 1992 and 1994, respectively, and the Ph.D. degree in fault diagnostics of antenna arrays from the National Institute of Technology, Warangal, India, in 2010. She has been an Assistant Professor with the National Institute of Technology since 2006. She has authored 31 papers in international conferences and journals. Her areas of interests include phase array antennas, ultra wideband antennas, multiband antennas, fault diagnostics, and neural network.