CPW-Fed Super-Wideband Antenna With Modified Vertical Bow-Tie-Shaped Patch for Wireless Sensor Networks

In this paper a coplanar waveguide-fed super-wideband antenna is presented for wireless sensor networks. The studied low-profile design is comprised of a modified bow-tie-shaped vertical patch and two asymmetrical ground planes and has been prototyped on a single-sided FR4 microwave substrate. The anticipated antenna has an overall size of $0.25\lambda \times 0.20\lambda $ at 3.035 GHz, the lowest frequency of the operating band. The vertical radiator coupled well with the two coplanar ground planes which enabled the studied antenna to achieve an operating band of 3.035–17.39 GHz (140.56%). The presented antenna demonstrates almost omnidirectional radiation patterns over the entire operating with an average gain of 4.56 dBi and average efficiency of 76.62%. The antenna also features with high fidelity factor, flat group delay, small phase distortion, and good response of transfer function. All these characteristics of the studied antenna make it a robust contender for wireless sensor node applications.


I. INTRODUCTION
Wireless sensor network (WSN) is one of the most promising and rapidly evolving wireless technologies that comprised of a group of spatially spread nodes. The potential applications of WSNs are environmental sensing, healthcare monitoring, area monitoring, air pollution monitoring, water quality monitoring, industrial monitoring, forest fire detection, landslide detection, battlefield surveillance, combat monitoring, and intruder detection [1]- [3]. The nodes in the sensor network sense the surrounding perform the processing and the node antenna wirelessly transmits the data to other nodes and/or to the base station. The nodes in the WSNs have to operate with limited sources of energy which is normally supplied by the battery. The node antenna is considered to be one of The associate editor coordinating the review of this manuscript and approving it for publication was Guido Valerio . the major factors that control the consumption of power by a transceiver. For transceiving the signals over a long distance high power source is necessary. The design of a suitable node antenna that establishes an efficient low powered communication system in sensor network is a challenging task. As the antenna in the node occupies the most area of the whole node and it is, therefore, necessary to miniature the antenna to lessen the overall node size without compromising its performances. Moreover, to transceive the data in all directions, the radiation patterns of the node antenna should be omnidirectional.
In recent years, the super-wideband (SWB) and ultrawideband (UWB) antennas have attracted more attention due to their numerous advantages including very wide operating band, low-profile, light-weight, inexpensive, high data transmission rate, ease of integration with MICs or MMICs. Hence, these antennas are the most favorite choice to be used as node antenna in wireless sensor network applications. However, the design of an appropriate SWB/UWB node antenna is still a difficult task. A suitable SWB/UWB node antenna must be small, planar, inexpensive, and should work over a wide range of frequencies with omnidirectional radiation characteristics. At the same time, they should exhibit good time-domain characteristics to minimize signal distortions.
Recently good numbers of SWB/UWB patch antennas have been reported [4]- [23]. Different methods such as the use of the thick substrate and substrate with low permittivity, incorporation of slit/slot in the radiator/ground plane, and optimization-based procedures have been reported to design and enlarge the bandwidth of microstrip patch antennas [24]- [25]. Microstrip line feeding [5], [7], [8], [10]- [14], [16], [18], [20], [23] and coplanar waveguide (CPW) [4], [6], [9], [15], [17], [19], [21], [22] feeding techniques are also employed to achieve super/ultra/wide operating band. For example, in [5], a symmetric open slot antenna was designed for UWB application. Using the U-shaped feedline, the reported design attained a working band ranging from 3.1-13.2 GHz. The antenna reported in [5] demonstrates better miniaturization with an overall footprint of 580 mm 2 when compared to the patch antenna with modified partial ground plane [7], a diamond-shaped radiating patch with partial multi-slotted ground plane [8], bird face-shaped patch with a single slotted limited ground plane [10], modified slotted U-shaped antenna [11], hibiscus petal-shaped radiator and small trapezoid ground plane [12], modified annular ring-shaped radiator with rectangular semi ground [13], corner truncated rectangular patch with single slotted ground plane [14], the circular patch with iterations of a hexagonal slot [16], annular ring-shaped radiator with small ground plane [18], circular radiator with hexagonal-square-shaped fractal geometry and partial ground plane [20], and semi-circular radiator with small trapezoid ground plane [23]. However, in microstrip line-fed antennas, the space available on both sides of the substrate is not entirely used resulting in elevated fabrication cost during mass production.
In contrary to microstrip line-feeding, CPW-fed antennas utilize most of the space around the patch [17]. In the CPWfed antenna, the patch and the ground plane are printed on the same side of the substrate. CPW technique is mainly useful for fabricating antenna, due to the presence of the center radiator and the proximity of the ground planes. CPW-fed antennas demonstrate higher operating bandwidth, less dispersion, and lower radiation leakage than microstrip line-fed antennas. Moreover, it is easy to prototype the CPW-fed antenna as etching is done on one side of the microwave substrate and it does not require any via holes. In [4], a CPW-fed UWB monopole antenna was reported. Using two L-shaped coplanar open ground stubs, the antenna attained a working band of 3.1-13.2 GHz. A CPW-fed antenna with the polygon-shaped radiator was presented in [6]. To enhance the bandwidth, in this design small fractal elements have been added to the patch that helps it to achieve a bandwidth of 7.85 GHz. In [9], fractal geometry was adopted in the patch to achieve an ultra-wide operating band of 2.5-10 GHz. A CPW-fed umbrella-shaped antenna was reported in [15]. Having a total area of 30 × 35 mm 2 , the designed antenna achieved a relative bandwidth of 123%. In [17], a wideband antenna on a defected crown-shaped FR4 substrate was designed. Using an extended U-shaped patch and two coplanar ground planes this antenna attained an operating band of 4.5-13.5 GHz. A CPW-fed UWB monopole antenna with a triangular-shaped radiator was presented in [19]. To enhance the working band, a top-cross-loop is used to connect the two coplanar ground planes. However, it possesses an overall size of 1650 mm 2 . In [21], a spiral antenna with coplanar strips was reported. But its larger size makes it difficult to integrate within the sensor node. A CPW fed patch antenna that comprised of the semi-circular patch with slots and two rectangular coplanar ground planes was presented in [22]. However, it possesses a dimension of 40 × 53.3 mm 2 and not suitable for sensor applications. Though most of the above-mentioned antennas successfully achieved a super/ultra/wide operating band, some of them possess a large footprint and not suitable for wireless sensor nodes.
A small SWB antenna for wireless sensor nodes is presented and prototyped in this paper. The compactness of the proposed low-profile antenna is achieved by the effective coupling of two asymmetrical coplanar ground planes and a CPW-fed modified bow-tie-shaped vertical patch. With a total footprint of 490 mm 2 , the studied antenna is much smaller than many reported antennas and achieved good gain, efficiency, and demonstrations almost omnidirectional radiation patterns over the entire working band of 3.035-17.35 GHz. The proposed antenna features with advantages of low fabrication cost, small, easy prototyping, and easy integration with RF circuitry is suitable for wireless sensor network applications.

II. ANTENNA DESIGN
The foot-print of the anticipated antenna is displayed in Fig. 1. The antenna is designed only on the top side of the FR4 microwave substrate with dielectric constant 4.6, thickness 1.6 mm, and loss tangent 0.02. The metallization used for the proposed antenna is 0.035 mm copper cladding which is commercially available in the FR4 substrate and there is no copper on the backside of the substrate which provides an advantage for portable communication devices due to low manufacturing cost. The antenna is comprised of a vertical bow-tie-shaped patch with unequal wings and is fed by a CPW strip line of width w 1 . On both sides of the feed line, a gap g is kept between the feed line and the coplanar ground plane. The width of the feed line and gap are so chosen that the characteristics impedance of the presented antenna becomes equal to 50 ohms for the used microwave substrate. The width and length of the tapered transitional wing of the bow-tie-shaped radiator are respectively w 2 and l 2 while the width and length of the upper wing are respectively w 3 and l 3 . The total length of the vertical radiator is l 2 + l 3 and its lower end is connected to the CPW feed line of length l 1 . Instead of using the complex matching network to attain a very wide operating band, the presented asymmetric antenna is fed by an SMA connector which acts as a self-resonator with a very compact dimension [26].
In the studied design, two partial coplanar ground planes are used which not only act as an elementary source of radiation but also improves the impedance matching. An L-shaped ground plane is placed at the left side of the bow-tie-shaped patch that is comprised of one vertical arm of size (L 1 − L 3 ) × t 1 and one horizontal arm having two parts of dimensions (W 2 − W 4 ) × L 3 and W 4 × L 4 . On the right side of the patch, a modified inverted U-shaped ground plane is positioned that consists of four parts of dimensions t 2 × L 2 , W 3 × t 2 , t 2 × L 5 , and W 5 × L 6 . The determined optimal parameters of the anticipated antenna having a compact volumetric size of 24.5 × 20 × 1.6 mm 3 are listed in Table 1.

A. ANTENNA EVOLUTION
To comprehend the working mechanism, six evolution stages of the studied antenna are depicted in Fig. 2 and the corresponding S-parameter and input impedance of these stages are respectively presented in Fig. 3. In Fig. 2(a) Ant # 1 is depicted which is consists of a CPW-fed vertical bowtie-shaped patch, one L-shaped coplanar ground plane, and one inverted L-shaped coplanar ground plane placed on the left and right sides of the patch respectively. A rectangular strip line of dimension W 3 × t 2 is horizontally added to the top edge of the vertical arm of the inverted U-shaped coplanar ground plane to form the Ant # 2 as shown in Fig. 2(b). As in Fig. 2(c), Ant # 3 is formed by vertically adding another strip line of size t 2 × L 2 to the last end of the inverted U-shaped ground plane of Ant # 2. A small rectangle of dimension W 4 × (L 4 − L 3 ) is added to the last end of the L-shaped coplanar ground plane to form Ant # 4 as shown in Fig. 2(d). To form Ant-5, a triangular portion is added to the left lower end of the bow-tie-shaped radiator. Finally, another triangular portion is added to the right lower end of the bow-tie-shaped radiator to form the proposed antenna of Fig. 2 The simulated S 11 presented in Fig. 3shows that Ant #1 achieved a −10 dB operating band of 5.12-7.30 GHz where the input impedance is very much close to 50 characteristics impedance line. The addition of horizontal strip-line to the inverted L-shaped ground plane of Ant # 1 (i. e. Ant # 2 excited an additional resonance at around 9.17 GHz and the first resonance moved to the lower band resulting in the exhibition of dual operating bands of 4.15-6.99 GHz and 9.05-9.68 GHz. The impedances in these bands are also better than that of Ant # 1. When the vertical strip-line is added  Fig. 3. Moreover, the second, third, and fourth resonances exhibit better performances in-terms of S 11 due to proper impedance matching. When another triangular portion is added to the right lower end of the radiator of Ant # 5 (the proposed antenna), a significant improvement of impedance matching over the 3-18 GHz band has been observed. As in Fig. 3 In the scale, the blue color region represents the current null and the red color indicates the strongest current. As shown in the first one of Fig. 4, at 3.25 GHz, the current is stronger in vertical (Y -direction) and horizontal (X -direction) arms of L-shaped ground and upper horizontal arm of the inverted U-shaped ground plane. As shown in the second one of Fig. 4, at 6.0 GHz stronger current flows in the lower horizontal arms and two outer vertical arms of the two ground planes, and the directions of current in corresponding arms in two ground planes are opposite to each other. Moreover, stronger flows of current have also been observed in the CPW-fed patch and feedline. As the frequency increases the current concentration in horizontal directions of the antenna gradually decreases. This low concentration of current develops a high cross-polarized component especially in the H -plane pattern which has been reported in [26]. Furthermore, the concentration of current in the modified bow-tie-shaped radiator and the CPW-feedline are increasing with the frequency which plays a vital role in the flow of vertical (Y -direction) current. Because of these higher vertical currents, the co-polarized component significantly enhances the radiated field patterns. From the current distributions, it is observed that at low frequencies the currents are stronger in the vertical arms of the coplanar ground planes while at higher frequencies strongest currents have been observed at lower horizontal arms of the ground planes and bow-tie-shaped radiator. As conclusions, using a modified bow-tie-shaped radiator and two asymmetric ground planes, the studied antenna achieved the desired super-wide operating band.

C. PARAMETRIC STUDIES
To reveal the behavior of the proposed antenna, the effects of L 1 , L 2 , L 7 , W 3 , l 1 , and l 3 are analyzed using IE3D software and presented in Fig. 5. The simulated S 11 with different values of L 1 , the length of the vertical arm of the L-shaped coplanar ground plane is shown in Fig. 5(a). When the value of 5346 VOLUME 9, 2021 L 1 increases, the lower cutoff frequency shifts to some lower value due to the increased current path. Moreover, as the value of L 1 moves away from the optimized one, dual notch bands are generated at the two edge frequencies of the operating band resulting in the decrement of overall bandwidth. Trading off the bandwidth and the matching, L 1 = 20 mm is taken as the optimized value. Fig. 5(b) depicted the S 11 responses for different values of L 2 , the length left vertical arm of the inverted U-shaped ground plane. As the value of L 2 increases or decreases from the optimized value of 11 mm, the antenna performances become poorer in-terms of S 11 . The S 11 responses with different values of L 7 , the length of the right vertical arm of the inverted U-shaped ground plane are shown in Fig. 5(c). It is seen in the plot that a lower value of L 7 gives a narrower operating band whereas a value higher than the final value (20 mm) exhibits poor matching at around 7.41 GHz. The higher cutoff frequency also shifted to the lower band as the value of L 7 becomes smaller or larger than 20 mm. Fig. 5(d) depicts the simulated S 11 for various values of W 3 , the length of the upper horizontal arm of the inverted U-shaped coplanar ground plane. It can be seen that for values of W 3 lower and higher than the optimized value, the impedance matching in between 3 to 4 GHz become worse resulting in the narrowing of the operating band. Moreover, the increment of W 3 moves the lower cutoff frequency towards the higher band. In this design, the value of W 3 is taken as 19.5 mm to prototype the anticipated antenna. The effects of l 1 (length of CPW strip-line) and l 3 (length of the vertical radiator) are respectively plotted in Fig. 5(e) and 5(f). From figures, it is seen that if the values of l 1 and l 3 increase from the optimized ones (6 mm and 5 mm respectively), poor impedance matching have been observed at around 4 GHz, 11/12 GHz and 16 GHz results in the exhibition of three distinct operating bands. Moreover, the higher cutoff frequency significantly sifted towards the lower band as the values of l 1 and l 3 increases from their final values. From the above parametric studies, it can be noted that the coplanar ground planes play an important role in lower cutoff frequency while the CPW-fed modified bow-tie-shaped radiator controls the higher cutoff frequency of the proposed antenna which has demonstrated in Fig. 4.

A. FREQUENCY-DOMAIN CHARACTERISTICS
The design of the antenna presented in the paper has been prototyped for the substantiation of the results obtained from IE3D and CST MWS software with optimal parameters and is shown in the inset of Fig. 6. The input impedance characteristic of the antenna is measured using N5227A PNA microwave network analyzer in the frequency spectrum of 10 MHz -67 GHz. The comparison between the simulated and measured S 11 is presented in Fig. 6. Good agreements have been observed between the simulated and measured results. It is evident from the plot that, the measured results offer an operating bandwidth of 14.36 GHz from 3.035 GHz to 17.39 GHz which is equivalent to a fractional bandwidth of 140.56%. The differences between the two simulated results are mainly due to the unequal mesh sizes. The discrepancies between the simulated and measured results might be arises due to the prototyping and measurement inaccuracies. Moreover, in simulation, the antenna is fed by a lumped port which is assumed to be loss-free and of 50 characteristics impedance for the entire frequency spectrum of 2-18 GHz which is very tough to achieve during the prototyping, especially on low-cost FR4 substrate. The radiation characteristics of the studied super-wideband antenna are measured in an anechoic chamber using near field antenna measurement system StarLab from MVG. The peak gain in the bore-sight (+Z ) direction is illustrated in Fig. 7(a). As predicted, the peak gain increases with frequencies. A good matching has been seen between the simulated and measured values. The antenna achieved a maximum gain of 6.16 dBi at 16.80 GHz and an average of 4.56 dBi. As the antenna is suitable for wireless sensor nodes, the achieved gain is within the acceptable limit. The gain of the presented antenna can be enhanced using expansive Rogers/Duroid substrate with low loss tangent. The simulated and measured radiation efficiency is displayed in Fig. 7(b). It is clear to see that the average radiation efficiency of the studied antenna is 76.62% and the maximum efficiency is 89.36%. In the efficiency curve, a null has been observed which maybe due to the limitations of measurement.
Moreover, at the null frequency, the SMA connector has a degenerative effect on the small antennas which has been reported in many CPW-fed antennas [27]- [28]. The measured 2D normalized E-and H -field radiation patterns of the studied antenna at 3.25 GHz, 6.0 GHz, 8.19 GHz,8.99 GHz,14.31 GHz,and 16.75 GHz are respectively depicted in Fig. 8 and Fig. 9. In the figures, the solid black line represents the co-polarized component, E θ while the dashed red line represents the co-polarized component, E ϕ . It is clear to see in the figures that the anticipated antenna exhibit slightly deviated omnidirectional radiation patterns. Despite the asymmetrical shape, the radiation patterns of the studied antenna over the entire operating band are almost symmetrical with some nulls. In comparison to the other asymmetric CPW-fed antennas [27]- [30], the studied antenna demonstrates stable and ripple-free radiation patterns even at higher frequencies of 14.31 GHz and 16.75 GHz which is mainly due to the concentration of maximum currents along with the vertical bow-tie-shaped patch. In the E-plane pattern as shown in Fig. 8, the cross-polarized (E ϕ ) component is remarkably small and the main beams are positioned towards the Z -direction (90 • , 270 • ) which makes the proposed antenna suitable for the handheld sensor network devices as it can help to avoid the interferences with other EM radiations. In the H -plane pattern as shown in Fig. 9, the cross-polarization component is higher, especially at lower frequencies. This is maybe due to the strong horizontal components of the surface currents. The contribution to the antenna radiation is mainly from vertical components (in Y -direction) of the surface currents. Instead of contributing to the co-polarized radiation in the H -plane pattern, the currents along the horizontal arms of the L-shaped and inverted U-shaped ground planes enhance the cross-polarized component [26]. However, at higher frequencies, the cross-polarization component decreases as the currents in horizontal arms of the coplanar ground planes decreases and the size of the antenna becomes larger in comparison to the wavelength.

B. TIME-DOMAIN CHARACTERISTICS
Since SWB as well as UWB systems, directly transmit narrow pulses rather than continuous wave, the time domain characteristic of the SWB/UWB antenna is very vital. In contrast to the narrow-band counterpart, good time-domain behavior is a basic requisite of SWB/UWB antennas. The antenna features can be optimized to avoid the undesired distortions. In the VOLUME 9, 2021 time domain analysis, the transmitting and receiving antennas are placed at a distance of 240 mm apart in face to face and side by side orientations.
The transmitting antenna is excited by a Gaussian pulse and the transmitted impulse is received by the second antenna. The normalized transmitted and received signals in both orientations are displayed in Fig. 10 (a). Using the normalized transmitted and received pulses, the fidelity factor (FF) that defines the correlation between the transmitted and received pulses can be calculated using [31], [32] where S T (t) and S R (t) are respectively the transmitted and received pulses and τ is the group delay. The calculated fidelity factor in the face to face and side by side orientations are respectively 94.3% and 96.6% which confirm that in side by side orientation the correlation is more than that in the face to face orientation. This finding suggests that in side by side orientation the studied antenna transmits the exciting pulse with less distortion in comparison to the other orientation. The group delay is defined as the negative derivative of the phase response with respect to frequency. It indicates the time delay of an impulse signal at different frequencies. The simulated and measured group delay of the anticipated antenna in face-to-face orientation is shown in Fig. 10(b) which indicates that over the entire operating band the measured group delay varies between 0.52 ns and 1.35 ns with an average of 0.87 ns. These limits of group delay ensure the phase linearity over the operating band which is a vital requisite of sensor network applications [33]. The simulated and measured amplitude of transfer function, S 21 is shown in Fig. 10(c). The ripples in the measured result are mostly attributed to the noise in the measurement which can be minimized by implementing the averaging algorithm. It is clear to see in Fig. 10 (c) that the value of S 21 is better than −45 dB over the entire working band. This higher value of S 21 suggests that the transfer of signal between two identical radiators is not correlated [34]. Fig. 10(d) depicts the simulated and measured phase variation of S 21 . It is seen that the phase is almost linear across the operating band which indicates that the variation of the antenna does not add any pernicious phase distortion to the incoming/outgoing signals.
To highlight the advantages, the performance of the studied antenna in-terms of size, electrical dimensions, operating band, bandwidth, bandwidth dimension ratio (BDR), and peak gain have been compared with recently reported SWB/UWB antennas and presented in Table 2. BDR indicates that how much operating bandwidth in percentage can be achieved by per unit electrical area of an antenna and its value should be high to substantiate the advantages of the proposed design over other existing designs. The footprint and electrical dimension of the proposed antenna are smaller than those reported in [4]- [22] while achieving higher operating bandwidth, and larger BDR which makes it an larger footprint than the proposed one and not suitable for wireless sensor node.

IV. CONCLUSION
A low-profile CPW-fed antenna is proposed for wireless sensor network which is the most standard services used in industrial, commercial, and military applications. The antic-ipated antenna is made up of a modified vertical bow-tie-shaped radiator and one L-shaped and one inverted U-shaped coplanar ground planes. Measured results show that the antenna exhibits seven resonant modes and merging of these nearby resonances help the antenna to achieve a super-wide operating band ranging from 3.035 to 17.39 GHz (14.36 GHz). The antenna also realized good gain and efficiency and exhibits almost omnidirectional radiation patterns both in E-and H -planes. Moreover, it demonstrates good timedomain behavior over the entire operating band. As the SWB/UWB technology characterize by high data transmission rate, low power, and low cost, the proposed asymmetric SWB antenna with small size and simple structure is a very promising candidate for wireless sensor nodes.

REZAUL AZIM (Member, IEEE) is currently a
Professor with the Department of Physics, University of Chittagong, Bangladesh. He has been very promising as a researcher with the achievement of several medals and awards for his research and innovation. He has filed one patent application. He has authored or coauthored 51 journal articles, 35 conference papers, and two book chapters. His articles have been cited more than 1234 times with an H-index of 18  HASLINA ARSHAD received the B.Sc. degree in computer science from the University of Bridgeport, USA, the M.Sc. degree in IT for manufacture from Coventry University, Coventry, U.K., and the Ph.D. degree in manufacturing system (virtual system) from University Putra Malaysia. She is currently a Professor and the Director of the Institute of IR4.0, Universiti Kebangsaan Malaysia (UKM). She had been working as an Analyst Programmer and System Analyst at IBM, before joining Universiti Kebangsaan Malaysia. Her research interests include augmented reality and virtual reality.
MD. MOTTAHIR ALAM (Member, IEEE) is currently a Faculty Member of the Department of Electrical and Computer Engineering, King Abdulaziz University, Jeddah, Saudi Arabia. Before joining academics, he worked as a Software Engineer (Quality) for six years for leading software multinationals, where he worked on projects for companies, like Pearson and Reader's Digest. He is also an ISTQB Certified Software Tester. He has published several research journal articles in leading international journals and conferences. His current research interests include wireless communications, microstrip and ultra-wideband antennas, machine learning, software reusability, nanomaterial synthesis, and characterization.
NEBRAS SOBAHI (Member, IEEE) received the B.Sc. and M.Sc. degrees in electrical engineering from King Abdulaziz University, and the second M.Sc. and Ph.D. degrees in electrical engineering from Texas A&M University, USA. He is currently an Assistant Professor with the Department of Electrical and Computer Engineering, King Abdulaziz University, Saudi Arabia. His research interests include nano/microfabrication, MEMS, microfluidics, BioMEMS, and signal and image processing.
ASIF IRSHAD KHAN (Member, IEEE) is currently working as an Assistant Professor with the Computer Science Department, Faculty of Computing and Information Technology, King Abdulaziz University, Saudi Arabia. His areas of research interests include software engineering, software security, cybersecurity, smart technology, and machine learning. He has been working on various projects related to software security and machine learning. His research is financially supported by several grants. He has coauthored more than 45 high-quality research papers in SCI-indexed journals and international conferences. He has 20 years of experience, which is an amalgam of teaching as well as corporate. He has been recognized for excellence in graduate and undergraduate teaching. He is a regular member of IEEE and is currently serving as a reviewer for several well-known journals, including IEEE ACCESS and International Journal of Information Management. He has been involved in conferences and workshops at various capacities, such as the chair and a technical program committee member. VOLUME 9, 2021