A compact CPW-fed wideband slot antenna with reflector for wireless communication

A compact CPW-fed slot antenna is proposed here. The antenna has a pentagonal patch nested within an asymmetrical slot along with a coplanar waveguide feed. The suggested antenna also has a circular polarization over a wide range. For achieving better performance and to increase the gain of the antenna a single aluminum reflector is used. Results with reflectors of different materials are studied and simulated. The reflector helped to achieve a higher gain, stable radiation pattern, and a good impedance match. The antenna covers an impedance bandwidth of 10.5 GHz and an axial ratio bandwidth of 1.3 GHz allied with a fractional bandwidth of 118%. The simulated result complies with the bandwidth requirements of IEEE 802.11a (5.15–5.35 GHz/5.47–5.725 GHz) for wireless applications.

Keywords: CPW-fed; aluminum reflector; circularly polarized

Introduction

In today's world, wireless communication is outstretching into every sphere of people's daily life, be it homes, offices, transports, and other public environments. There is a continuous increase in consumer demand for wireless transfer of multimedia and signals with a much larger throughput. Therefore, an antenna which is operational in a wide range of bandwidth with low power requirement is the need of the hour. Various types of microstrip patch antennas and a number of procedures have been undertaken by many authors to come up with a very compact size wideband antenna.

Asymmetrical slotted antennas with CPW feed for UWB operation for application in the wireless communication systems have been proposed in (Kushwaha [11]; Raj, Naidu, and Kamble [15]) having omnidirectional radiation pattern on FR4 substrate. Reference (Nasimuddin, Chen, and Qing [14]) was the first to propose a single feed circularly polarized rectangular shaped slotted patch antenna along with meta-surface for bandwidth enhancement. A 7 × 7 rectangular-ring unit cells based meta-surface is used with an antenna size of 60 mm x 88 mm x 4.8 mm. Efforts are also being made for the design development of indoor wireless application along with WLAN and WiMAX bands with defected ground and stub integrated structure in reference (Manouare et al. [12]).

In reference (Bhobe et al. [3]), wideband slot antennas with CPW technique of feeding are depicted. The design and development of CPW-fed hybrid slot antennas (HSA) with impedance bandwidths (VSWR < 2) up to 57% is portrayed. CPW-fed log periodic slot antenna and half wave CPW-fed slot antenna is described here. The substrate used here is with dielectric 4.3 with height 1.58 mm and dielectric 12.5 with height 1.27 mm. Again, CPW-fed slotted antenna and stacked patch type co-axial-fed antenna is developed (Nasimuddin, Esselle, and Verma [13]; Shanmuganantham, Balamanikandan, and Raghavan [17]) for indoor wireless communication and satellite communication applications. The stacked patch and the slots helped in achieving higher gain and wider bandwidth but at the cost of complicated antenna design. A multi-layered CPW loop fed with parasitic patch-incorporated microstrip antenna, which is integrable with other various types of microwave devices, has been developed (Chen [5]) for 3.12 to 4.41 GHz and 3.18 to 4.47 GHz bandwidth. The footprint of the antenna specified is 60 mm x 60 mm x 4.572 mm. A half elliptical microstrip fed antenna for UWB operation is used in (Koley and Pant [10]) with a length of 135 mm and width 46.5 mm. A folded slot CPW-fed antenna is proposed in (Shanmuganantham and Raghavan [1]) for 5.15 GHz to 5.35 GHz application. The feeding method of CPW, where the central strip is trimmed at the lower edge of the folded slot, is coupled into the folded-slot antenna for indoor applications. A patch array antenna for non-linear junction detector system application, which is multiband in operation, is presented in (Kim and Min [9]). A design of circularly polarized disc patch antenna with three 450 inclined slots is presented with two grooves and truncated corners of the ground plane edge. To improve the gain linear array of antennas was considered. In (Huang [8]), the zero reflection in the CPW-fed slotted antenna is backed by the fact that the reflection coefficient is 1/3 in two-port network along with theoretical method to deduce slotted antenna dimensions is presented. In (Chaimool, Chung, and Akkaraekthalin [4]; Encinar [6]), metamaterial-based reflective surface with square array of patches is used to enhance bandwidth and gain and also a progressive phase distribution is achieved in (Encinar [6]). An uwb operation is obtained by using an FSS reflector along with a three-stepped narrow rectangular shaped slot antenna which is excited using a three-stepped microstrip feed line thus using a multi-layered structure in (Ram Krishna and Kumar [16]). Reference (Ghorbani and Waterhouse [7]) presented an eight layered structure of patch antenna with cross-shaped reflector in multiple layers and feeding mechanism through multiple layers to achieve a bandwidth of 52% and a gain of 7.4 dBi along with the front-to-back ratio of greater than 20 dB. A custom-made high dielectric bio-plastic substrate (ε = 15) is utilized for fabricating the prototype of meta-surface reflector-loaded patch antenna in (Ahsan et al. [2]). A three-layer structure with a slotted circular patch and a meta-surface reflector is studied to have a result of triple-band operation and an average gain of 6 dBi.

In the proposed article, an effort has been made to design a simple single-feed patch antenna with a metallic reflector having circular polarization and a wideband characteristic. The use of metallic reflector aids in the increase of the gain of the antenna. The antenna has a co-planar waveguide feed and a pentagonal patch nested within an asymmetrical pentagonal slot on the ground plane. A considerable 3 dB axial ratio bandwidth is achieved along with a good overall gain of 6.64 dBi in the desired frequency band. The results are simulated and studied in HFSS to minimize the errors by ensuring that the mesh used was sufficiently fine.

Analytical study

The proposed structure was compared with the reference antennas and a detailed study is depicted in Table 1, which shows that some of the limitations of other antennas have been overcome by the proposed model in terms of dimensions, Fractional Bandwidth (FBW), Gain, Return loss.

Table 1. Comparison of antennas (in mm).

Sl. No.	Type of antenna	Dimension	Overall BW	Fractional BW	Max gain	Max Return loss	Applicable bands
Ref. 1	Slot antenna	36 x 30 × 1.6	11.85 GHz	134%	7dBi	– – -	Wireless Communication
Ref. 2	CPW antenna	34 x 30 × 1.58	200 MHz & 12 GHz	– –	7 dBi	−15 dB	UWB and Bluetooth
Ref. 3	Slotted meta-surface antenna	60 x 88 x 4.8	1.2 GHz	36%	2.5–5.7 dBi	−14 dB	Wideband wireless systems
Ref. 4	CPW patch antenna	20 x 37x 0.508	2.085–2.72, 2.89–4.35, & 4.59 − 7.055 GHz	– – –	4.39 dBi	−48 dB	WLAN and WiMAX
Ref. 5	CPW slot antenna	25 x 20.5 x 3.175	3.31 GHz	40%	4.39 dBi	−45 dB	Wideband Applications
Ref. 10	CPW folded slot antenna	18.8 x 12.3 x 3.175	200 MHz	5%	3.9 dBi	−43.69 dB	Wireless Application
Ref. 11	CPW array antenna	164 x 81 x 1.6	2.44, 4.88,7.32 GHz	– –	1 dBi	−35 dB	Wireless Application
Proposed Antenna	CPW slot antenna with reflector	25 x 20 x 1.6	10.5 GHz	118%	9.64 dBi	−33 dB	Very Wideband Applications

Implementation of slot in the ground plane would definitely enhance the impedance bandwidth of the antenna but at the price of increased back lobe radiation, which is due to the lack of a perfect electric conductor ground plane. The utilization of reflector structure eventually reflects back the radiated wave, which in turn amalgamate with the fundamental wave giving a rise in the overall bandwidth. To get the best performance of the proposed model the gap between the patch and the reflector is optimized and then physically designed.

Antenna design

The evolution of the proposed antenna design is shown in Figure 1. At first, the geometrical shape of the patch structure is chosen to be a pentagonal patch as shown in Figure 1(a). A defected ground plane with pentagonal slot with the same patch dimensions is tested which shown in Figure 1(b). The feed position in Figure 1(b) is in the middle of the substrate, and the gap between patch and ground is symmetrical. Now, the proposed antenna is optimized to a smaller size and the gap between the patch and ground is asymmetrical also a metallic reflector placed on the rear side. The feed position is modified to perform as a wideband antenna with a CPW feed which is shown in Figure 1(c), i.e. proposed Antenna. The proposed antenna is nested on FR-4 substrate having dielectric constant Ɛr = 4.4 and loss tangent, δ = 0.02. The patch length is taken as L1 = 7 mm with a width W2 = 5.1 mm. W1 = 18.5 mm which is the asymmetrical pentagonal slot width and gap between the feed line and the ground plane is g1 = 0.5 mm.

Graph: Figure 1. Geometry of the proposed antenna.

The wideband characteristic and low radiation leakage are facilitated by the chosen CPW feed. The slot of the proposed antenna is excited by the side of the pentagonal patch capacitively. The length of the side of the patch antenna significantly affects the overall impedance bandwidth. The structure of the slot was initially chosen to be rectangular but was optimized in a step by step procedure by examining the reflection coefficient to achieve a better impedance matching. The pentagonal slot proves to be a good choice to get a better return loss performance. A better impedance matching over a wide range of frequency bandwidth is brought about by the asymmetry within the slot which reduces the variation in impedances. The feed line asymmetry excites several new modes in the slot and the ground plane which in turn results in impedance matching over a wider range of bandwidth.

The metallic reflector is placed at a distance of 11 mm from the substrate at rear side of the antenna model. The size of the reflector is chosen to be the same as the size of the patch antenna where it behaves like an inductive reflector which enhances the performance of the excited patch. Here, the reflector acts as the inductive metal while the patch acts as an excited driven element. There is an effect of the air gap distance of the reflector from the proposed model of the antenna as shown in Figure 13. In comparison with the conventional antenna of Figure 1(a), it is clear that there is an increment in the gain along the distance range from air = 0 mm to air = 11 mm in Figure 1(c). The use of this reflector enhances the radiation performances, gain and the circular polarization bandwidth is improved. Equation 2 maintains the relation between g2 and g3 slots.

(1)

Graph

${\epsilon }_{reff}=\left({\epsilon }_r+1\right)/2=\left(4.4+1\right)/2=2$

gap, g2 = 3 mm; Small gap, g3 = 0.4 mm

Thus,

(2)

Graph

$g2=7.5\ast g3$

Again,

(3)

Graph

$g2=0.09\ast \lambda candg3=0.012\ast \lambda c$

λc is taken from the center frequency of the bandwidth 4 to 14 GHz.

The antenna is then simulated in HFSS to have the desired result. The parameters along with its dimensions are shown in Table 3.

Table 2. Comparison of different types of reflector antennas.

Sl. No. (References)	Type of reflector & antenna	Dimension(mm)	Gain	Bandwidth (GHz)
Ref. 13	Metamaterial reflector array of 4 × 4 square rings	30 x 30	8.5dBi	2.31–2.66
Ref. 14	Circular two layer reflect arrays using variable patch sizes	406 mm diameter	31 dBi	11 – 13
Ref. 15	Frequency selective surface with two layer array of rectangular patches	30 x 60	8.2 dBi	3.2 – 12
Ref. 16	Structure of 8 layers is used consisting of multiple layers of patches, feed points and reflectors	105 x 105	7.4 dBi	1.5–2.5
Ref. 17	Three layered structure with meta-surface reflector array of square patches	51 x 45	6.09 dBi	0.5–0.6 0.46–0.6 0.75–1.06
Proposed Antenna	Single layer patch antenna with a single aluminum reflector	25 x 20	9.64 dBi	4 – 14.5

Table 3. Parametric dimensions of proposed antenna (in mm).

Parameters	g1		w1	w2	g3	g4	g5	h	W	L	M
g2	g3
Dimension (mm)	0.5	3	0.4	18.5	7	0.4	9.5	7.5	1.6	20	25	11

Results and discussion

The proposed antenna model is simulated and studied in HFSS tool. The experimented results are the measured in a VNA network analyzer. Here in Figure 2, the return loss and the bandwidth of the proposed structure is compared with other similar structures. The patch of the proposed model is capacitively coupled with the slot in the ground plane. The combined use of asymmetrical patch and the ground slot have aided the enhancement of the impedance bandwidth. A large bandwidth of about 10.5 GHz is achieved by the proposed antenna and the slot contributes to catering a better return loss. The asymmetry of the slot helps in reduction in the impedance variations and thus anticipate in impedance matching over a wider range of frequency bandwidth.

PHOTO (COLOR): Figure 2. S11 (dB) vs frequency (GHz) plot for different slots.

Figure 3 shows the circular polarization of the antenna with a 3-dB axial ratio bandwidth of 1.18 GHz (5.11 GHz to 6.29 GHz). Circularly polarized antennas send and receive signal in all planes and so the strength of the signal is not lost, but it is transferred to a different plane and are still thereby utilized. The problems of absorption, multipath fading, and inclement weather are solved to a great extent due to circular polarization. Figure 4 depicts the plots of the peak gains of the proposed antenna. The gain is between 2 and 6 dBi in the lower band frequency region. The maximum gain is achieved at 9.5–10 GHz frequency.

Graph: Figure 3. Axial ratio (dB) vs frequency (GHz) plot.

PHOTO (COLOR): Figure 4. Measured and simulated peak gain comparison of the proposed antenna.

The slot cutting technique on the patch is effectively used in the reduction of patch antenna size and simultaneously creating multiple frequency bands of operation within a desired range of frequency by sensibly selecting the dimension and orientation and position of the slot. It is worthy here to mention that the shape and size of the radiating patch also play an important role for the frequency of operation of a particular antenna. In the proposed structure, the shape of the patch is asymmetrically pentagonal which contributes to its operable frequency range. The inclusion of the outer asymmetrical slot consequently affects the overall surface current distribution and hence augmenting the realizable bandwidth of the proposed antenna. The arrangements of the slot and shape of the antenna undoubtedly help to streamline the flowing current for the fundamental mode of resonant frequencies and thus lowering down the frequencies in comparison to the reference antenna without slots.

Comparison of the radiation patterns at 3.55 GHz, 5.25 GHz, 5.85 GHz, 6 GHz, 6.5 GHz, and 10.45 GHz in the E-plane is given in Figure 5. From this figure, it is evident that the proposed antenna model operates as a left hand circularly polarized antenna (LHCP). Also, the radiation pattern is slightly tilted due to the asymmetrical feeding technique means the different gap introduce between feed line and asymmetric slot.

PHOTO (COLOR): Figure 5. Radiation pattern comparison of the proposed antenna at different resonance frequencies.

Figure 6 shows the field distribution at 00 phase angle. The feed and patch is carefully placed at the maximum field current position in this configuration. The electric field distribution of the Antenna at 5.85 GHz is obtained during characteristic modal analysis. It is observed that in the left side of the feed line, where the ground plane length is higher, the field is concentrated there and at the two corners of the slot at 0° phase angle. This is due to the shape and asymmetry of the feed structure. At 90° phase angle at 5.85 GHz resonant frequency, the field distribution is seen over the patch and the ground plane. In general, square slotted/slit/truncated-patch and other nearly square patches and unit cells of meta-surface antennas have been used to generate a narrow band CP radiation. The introduction of asymmetry introduces an inductance into the structure as the reduced metallization in the right forces a better current density near the edges. The inductance so introduced compensates the inherent capacitive reactance offered by the wide pentagonal slot which in turn brings the imaginary part of the impedance closer to 0 ohm at the resonant frequencies. The improvement in the impedance bandwidth can be said due to the introduction of several modes into the patch and the ground plane because of the asymmetry and the shape of both the ground slot and the patch.

PHOTO (COLOR): Figure 6. Simulation results – electric field distribution at 5.85 GHz.

Simulated and measured results

The fabrication of the proposed antenna model was done using FR4 substrate by photolithographic technique. The performance of the model was tested using a vector network analyzer. On the basis of various design formulations and design processes, the most suitable dimensions of the radiating patch, size, and position of the slots, measurements of the ground plane are obtained from parametric investigations. Considering the prior knowledge of the fact that the size of the antenna and the feed position have a considerable effect on the microstrip patch antenna performance, these are not directly considered during parametric investigations. Figure 7 shows the fabricated prototype along with measured reflection coefficient, and reflector. Figure 7 depicts the actual front view, back view and side view of the proposed antenna. This figure illustrates that a better agreement is achieved in measured S11 result with the proposed structure.

PHOTO (COLOR): Figure 7. Fabricated prototype and measured result of proposed antenna.

Parametric study

Parametric studies are performed by simulation with one parameter at a time. Parametric studies show the variation of different parameters and their effect on the presented model. All the asymmetrical slots of the proposed model are studied in this section.

Variation of fedline gap

The gap, g2 between the patch and the ground plane on the right side of the antenna is varied from 1.5 mm to 3 mm keeping g3 constant. The different return loss vs. frequency graph is plotted for each of the values of g2 in Figure 8. The gap, g3 between the patch and the ground plane on the left side of the antenna is varied from 0.4 mm to 3 mm keeping g2 constant. S11 for each value is depicted in Figure 9. The proposed antenna has unequal g2 and g3 values.

PHOTO (COLOR): Figure 8. Effect of different values of g2 when g3 kept constant (0.4 mm).

PHOTO (COLOR): Figure 9. Effect of different values of g3 when g2 kept constant (3 mm).

Substrate parameter

Figure 10 depicts the variation of the reflection coefficient with change in the height of the substrate. Substrate is an important parameter for the performance and mechanical strength of the antenna. It is used for depraved electrical properties as the surface waves generated on the dielectric part is a portion of total power for space waves. The actual height of the substrate is given as h = 1.6 mm. Figure 11 shows the return loss plot of the proposed antenna by using five different substrates commonly used in the market. However, the estimated bandwidth (4 − 14.5 GHz) has been obtained by using FR4 as a substrate having a dielectric constant of 4.4.

PHOTO (COLOR): Figure 10. Effect of substrate height in the proposed structure.

PHOTO (COLOR): Figure 11. Effect of different substrates in the structure.

Variation of gap g2 and g3 simultaneously

In the first case, the CPW feed line is inserted toward one side which makes the two halves of the ground plane to have equal dimension that is, g2 = g3 = 1.5 mm. In the second case, the asymmetry slot structure is introduced to make two halves of the ground plane to have unequal dimensions which are shown in the antenna model, where g3 = 0.4 mm and g2 = 3 mm have unequal distance. Here g2 and g3 are introduced for the gap between patch and ground plane. Return loss vs frequency plot reflected in Figure 12 with the variation of g2 and g3 together.

PHOTO (COLOR): Figure 12. Effect of g2 and g3 equal and unequal.

Variation of the position of the reflector

The reflector is placed behind the proposed antenna model. The correct placement of the metallic reflector acts as an inductive plane, while the gap between the antenna and the reflector behaves as a capacitive part. As a result of the surface current and the combined effect of the inductive and capacitive parts, more in-phase current is produced in the patch antenna and hence more directive radiation occurs. Thus, this fact makes the metallic reflector to eventually reduce the back radiation and so increase the overall bandwidth and radiation efficiency of the antenna. Figure 13 shows plots of frequency vs return loss for different positions of the reflector. The position of the reflector behind the antenna is varied from 0 mm to 11 mm. The 3-dB axial ratio which is shown in Figure 3 is achieved using the metal reflector. Thus, circular polarization is achieved for the proposed antenna model.

PHOTO (COLOR): Figure 13. S11dB vs frequency (GHz) plot for different position of reflector.

Conclusion

The design is simple because the return loss and AR requirements are achieved without any structural complexities. Reflector position and the material of the reflector are analyzed. The presence of aluminum reflector which has a relative permittivity of 1 enhances the property of circular polarization of the antenna. Circular polarization is also achieved through single feed only. This provides a better reflectivity, better signal transmission strength, robust to phasing issues as signals which are circularly polarized are much better at penetrating and deviating around obstructions because of the reflected signal return in the opposite orientation. The antenna with a size of 20 × 25 mm with the metallic aluminum reflector of the same size is presented. A 3-dB axial ratio bandwidth of 1.18 GHz and impedance bandwidth of 10 GHz is achieved. The gain and radiation patterns of the antenna have been presented. Further optimization of the thickness of the substrate and the aspect ratio of the patch may contribute to wider axial ratio bandwidth and gain. The antenna will be suitable for wireless broadband, medical imaging, radar applications, and other portable wireless applications.

1 Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uemg. Shanmuganantham, T., and S. Raghavan. 2008. Design of CPW - Fed folded-slot antenna for wireless applications. 10th International Conference on Electromagnetic Interference & Compatibility, IEEE. 2 Ahsan, M. R., M. T. Islam, U. Mohammad Habib, M. J. Singh, and M. T. Ali. may 27 2015. Metasurface reflector (MSR) loading for high performance small microstrip antenna design. PloS One 10 : e0127185. doi: 10.1371/journal.pone.0127185. 3 Bhobe, A. U., C. L. Holloway, M. Piket-May, and R. Hall. october 2004. Wide-band slot antennas with cpw feed lines: Hybrid and log-periodic designs. IEEE Transactions on Antennas and Propagation 52 (10): 2545 – 54. doi: 10.1109/TAP.2004.834425. 4 Chaimool, S., K. L. Chung, and P. Akkaraekthalin. 2010. Simultaneous gain and bandwidths enhancement of a single-feed circularly polarized microstrip patch antenna using a metamaterial reflective surface. Progress In Electromagnetics Research B 22 : 23 – 37. doi: 10.2528/PIERB10031901. 5 Chen, N. Z. N. 2009. Wideband multilayered microstrip antennas fed by coplanar waveguide-loop with and without via combinations. IET Microwaves, Antennas & Propagation 3 (1): 85 – 91. doi: 10.1049/iet-map:20080065. 6 Encinar, J. A. October 2001. Design of two-layer printed reflect-arrays using patches of variable size. IEEE Transactions on Antennas and Propagation 49 (10): 1403 – 10. doi: 10.1109/8.954929. 7 Ghorbani, K., and R. B. Waterhouse. August 2004. Dual polarized wide-band aperture stacked patch antennas. IEEE Transactions on Antennas and Propagation 52 (8): 2171 – 74. doi: 10.1109/TAP.2004.832484. 8 Huang, J.-F. 2000. A simple model of designing cpw-fed slot antenna. 2nd International Conference on Microwave and Millimeter Wave Technology Proceedings, Beijing, China, IEEE, pp. 383 – 86. 9 Kim, I.-H., and K.-S. Min. 2014. Circularly polarized high gain multiband antenna for non-linear junction detector system. The 2014 International Workshop on Antenna Technology, Sydney, NSW, Australia, pp. 224 – 27. Koley, S., and H. Pant. 2018. Half-elliptical uwb planar monopole with reconfigurable slot antenna for cognitive radio front-end. Journal of Circuits, Systems, and Computers 27 : 1 – 13. doi: 10.1142/S0218126618502134. Kushwaha, N. R. Kumar, and R. V. S. Ram Krishna. August 2014. Design of cpw-fed asymmetric slot uwb antenna for wireless application. Journal of Electronics (china) 31 (4): 341 – 45. doi: 10.1007/s11767-014-4004-y. Manouare, A. Z., S. Ibnyaich, A. E. L. Idrissi, and A. Ghammaz 2016. Miniaturized triple wideband CPW-Fed patch antenna with a defected ground structure for WLAN/WiMAX applications. Journal of Microwaves, Optoelectronics and Electromagnetic Applications 15 (3): 157 – 69. July/Sept. doi: 10.1590/2179-10742016v15i3497. Nasimuddin, K., P. Esselle, and A. K. Verma. 2007. Wideband circularly polarized stacked microstrip antennas. IEEE Antennas and Wireless Propagation Letters 6 : 21 – 24. doi: 10.1109/LAWP.2006.890749. Nasimuddin, N., Z. N. Chen, and X. Qing. 2016. Bandwidth enhancement of single-feed circularly polarized antenna using meta-surface. IEEE Antennas & Propagation Magazine 2 – 9. doi: 10.1109/MAP.2016.2520257. Raj, K., and V. P. Naidu and V. Kamble. 2014. Design of asymmetric slot antenna with meandered narrow rectangular slit for dual band applications. Progress In Electromagnetics Research B 60 : 111 – 23. doi: 10.2528/PIERB14042205. Ram Krishna, R. V. S., and R. Kumar. April 16 2015. Slotted ground microstrip antenna with FSS reflector for high-gain horizontal polarization. Electronics Letters 51 8 : 599 – 600.doi: 10.1049/el.2015.0339 Shanmuganantham, T., K. Balamanikandan, and S. Raghavan. 2008. CPW-Fed slot antenna for wideband applications. International Journal of Antennas and Propagation 2008 : 1 – 4. doi: 10.1155/2008/379247.

By Amrita Gorai; Bappadittya Roy and G.K. Mahanti

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