A Compact High-Gain Coplanar Waveguide-Fed Antenna for Military RADAR Applications

College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China Department of Information Security and Communication Technology, Norwegian University of Science and Technology, 2815 Gjøvik, Norway Computer and Information Science Department, Linkoping University, Linkoping, SE-58183, Sweden Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen (SIAT, CAS), Shenzhen, Guangdong 518000, China


Introduction
e immense technological advancement in the wireless communication systems has switched focus to the development of compact, high gain, and radiation efficient antennas. Such antenna systems can be useful for various wireless communication applications including radar (radio detection and ranging), satellite, intercommunication, and navigational system [1].
Federal communication commission (FCC) under the third generation partnership project (3GPP) has allocated frequency ranges for different wireless communication applications. For instance, military airborne, land, and naval radars operate in X band 8-12 GHz, ISM, and wireless fidelity (WiFi) on 2.4 GHz frequency band spectrum, and worldwide interoperability over microwave access (WiMAX) operates on a 3.5-5.8 GHz frequency range [2,3]. erefore, it is essential to develop the high-performance antennas which can be utilized with an efficient wireless communication system, particularly for airborne, land, and naval radar applications. e conventional compact planar antennas (CPA) have low gain, limited radiation efficiency, and narrow bandwidth (BW) performance [4]. So, to improve CPA performance, studies have used different techniques.
In past studies, researchers tried to enhance the key performance parameters of the CPA by incorporating the various approaches [5][6][7][8]. Besides, several designs based on different shapes of slots [9][10][11][12], radiators [13][14][15][16], and ground plane [17,18] have been proposed to attain the substantial peak realized gain, good radiation efficiency, and acceptable impedance BW along with perfect matching performance. e investigated antennas have complex designs and analyzed results were optimal with a trade-off between the BW, peak gain, radiation efficiency, and compact dimensions. e wideband bowtie dielectric resonator antenna was realized on Rogers TM10i and obtained a peak gain of 8 dBi [19]. Another study on the bowtie antenna applied the complex structure of the artificial magnetic conductor ground plane and obtained the peak gain near 8 dBi [20]. Xu et al. proposed two antenna models [21]. ey achieved the peak realized gain 5.1-5.5 dBi and the radiation efficiency 71.5-77.2%. Huang et al. proposed a multilayer high-gain antenna [22]. eir designed antenna obtained peak realized gain of 8.0-9.0 dBi and 70-80% radiation efficiency. Li et al. suggested a method to achieve a high gain of 7 dBi with larger dimensions of 22 × 22 cm 2 [23]. Iqbal et al. proposed a design with additional features to filter out the multiple bands and obtained the peak realized gain 3.4-4.6 dBi [24].
Tawk et al. [25]; presented the bowtie antenna and obtained the maximum gain of 4.5 dBi. Sun et al. [26] have been proposed the new structure of the antenna with moderate gain performance of 1.7-5.8 dBi. Jalilvand et al. presented and investigated antenna based on a new 3D microwave tomography approach [27]. Mansoul et al. presented a compact antenna with 8 × 3 cm 2 space and obtained the enhanced peak gain performance of 4.5-5.8 dBi [28]. Wu et al. [29] reported work on the slotted bowtie antenna with compact size (6 × 10 cm 2 ) and achieved a fair gain performance of 3.75 dBi, 3.56 dBi, and 3.93 dBi. A new design of the bowtie slot antenna was investigated by Sallam et al. [30]. e antenna was realized on an 8 × 6 cm 2 compact size substrate and obtained the maximum gain of 6.3 dBi. Moreover, the wideband antenna using the altered T-shaped slot to the antenna was reported by Park et al. [31]. e proposed antenna has the dimensions of 15 × 15 cm 2 and obtained a high gain of 7.5-9 dBi.
Recently, several new approaches have been employed to improve the gain of compact antennas. Khalili et al. merged the traditional concept of bowtie antenna design with new metamaterial technology [32]. e antenna exhibited a peak realized gain of 10.21 dBi. e Fabry-Perot and slot antenna were reported in [33,34]. e authors achieve a high gain by employing the concept of loaded superstrate and increase the number of radiators with a 3D printed approach. e authors in [35] investigated the antenna array with complex methodology and obtained the gain higher than 8 dBi. It is worthy noted that the results analyzed and explained in [25][26][27][28][29][30][31] the researchers have not paid attention to the radiation efficiency of the antennas; also, the suggested methods were complex. However, the proposed antenna presented in this paper employed an alternative method that outperforms the maximum gain, radiation efficiency, and compact dimensions in comparison with most of the previously reported work.
Likewise, many other studies also improved the gain and efficiency of antenna systems using various techniques. However, such techniques were complex and time-taking. erefore, we propose a novel method that enhances the gain and efficiency of an antenna. e employed method differs from other techniques, which generally altered radiation patch and added slots in the ground plane. Our multielement method applies to the simple geometry of the single bowtie arm radiator. e method consists of rectangular open-circuit stubs loaded extended ground plane (EGP) and circular disc-shaped stubs engraved on the upper left and right edge of the bowtie arm patch. Compared to previous work, our proposed method is a simpler one to achieve high gain and optimum radiation efficiency. e proposed approach is validated by applying the loaded stubs step by step to the proposed antenna lattice. It is verified that the antenna design structure using the EGP and extended bowtie arm patch is an effective and efficient way to attain the high gain, optimal radiation efficiency, and good impedance BW.
For the simulation and design of an intended radiator, a 3D electromagnetic high-frequency structure simulator (HFSS) virtual design prototyping research and development (R&D) software is employed. As such, this study contributes to the academic research as follows: firstly, it designs and fabricates an antenna with miniaturized dimensions of 0.195λ × 0.163λ × 0.0052λ on a new substrate material (WangLing TP-2); it presents a framework and optimal numerical results of the designed antenna, and it provides a comparative analysis with an existing state-ofthe-artwork. e remainder of the manuscript proceeds as Section 2 illustrates the proposed approach for designed antenna and analyses of simulated results; Section 3 offers experimentally verified results; Section 4 elucidates comparative analysis; and Section 5 explains the conclusion of the study.

The Proposed Approach for Antenna Design
is section mainly focuses on the antenna framework design and analyses of simulation results. e cross-sectional view of the proposed compact, high gain, and radiation efficient antenna along with the fabricated sample are depicted in Figures 1(a) and 1(b). e simple design of the antenna composed of bowtie arm triangular-shaped radiator and ground plane fed by a standard 50 Ω CPW feed line as elucidated in Figure 1 is enhancement in the metallic ground plane is realized with loaded open-circuit rectangular stubs. e main purpose of this step-by-step alteration in the antenna structure is to tune resonances and attain the matching performance. Finally, two open-circuit circular disc-shaped loaded stubs etched on the upper right and left edge of the bowtie arm to form the proposed antenna prototype as displayed in Figure 1(a). e designed antenna is tuned by selecting the optimum values of important five variables, such as the width of the radiator (Y/Z), length of the radiator (X), the length, and width of a substrate (L s /W s ). Table 1 offers the utilized variables and their optimized values used in the designed antenna. Figure 2 illustrates the return loss (S 11 ) performance of the proposed antenna design structure. We notice that the proposed antenna achieves the broad BW of 9.78-10.97 GHz at 10 dB return loss, which constitutes relative impedance BW of 11.48%. Additionally, two resonances centered at 9.9 GHz and 10.8 GHz are observed across the operational frequency band. Also, the proposed antenna impedance bandwidth is increased by 37.8% as compared to [21]. e proposed antenna dimensions are very compact and decreased by 74.2, 82.24, 93.8, and 86.6% as compared to recently reported work [21][22][23]31].  [25,26] and 61.2, 37.9, and 11.3% [29][30][31] in results as compared to the proposed highperformance antenna simulated results. Figure 3(b) delineates the associated numerical result of radiation efficiency over the specified frequency. One can observe that the proposed antenna model achieves an excellent performance of 99.8% at 10.23 GHz resonance. e antenna attains an efficiency of 96.7-98.6% across the operational band (9.78-10.97 GHz). e radiation efficiency results presented in the literature have shown the decrement of 18.37 and 20% [21,22] as compared to the proposed antenna results.
It is important to investigate the concentration of current along the surface of antenna. is study demonstrates the current flow mechanism across the proposed structure. Figure 4(a) shows a simulation result of surface current distribution (Jsurf ) at 9.9 GHz. A strong current density along the quarter length CPW feed line, bowtie arm radiator, and open-circuit rectangular stub loaded metallic ground plane is observed. Figure 4(b) illustrates the current distribution at 10.8 GHz. One can analyze that high current intensity is concentrated along with the structure of the proposed antenna.

Experimental Verified Results
is section covers the analyses of numerical and experimental verified outcomes of the return loss (dB), maximum    parasitic inductance and capacitance losses which may lead to a discrepancy among the tested and simulated results. Moreover, this inaccuracy in results can be controlled by utilizing efficient substrate, high precision feeding connector, and good quality cables. Also, this small error in the measured results ensures the realization of the fabricated prototype.

Peak Realized Gain (dBi) and Radiation Efficiency (%).
e peak realized gain of a fabricated prototype was calculated by employing the standard Friis transmission formula approach. e value of gain is obtained using the following equation [36]: where S 11 and S 22 are the reflection coefficients of feed antenna and antenna under test (AUT). Also, G T and G R are gain of the feed antenna and AUT [37]. S 21 is the insertion loss, denoting the power ratio at port2 when feeding to port 1. e antenna is placed at a certain distance R on the other side at a particular frequency f. Considering the above condition, two identical horn antennas with a known gain are placed in the line of sight (LOS) transmission as depicted in Figure 6(a). Likewise, an AUT (the sample of the antenna) is located at a certain distance under the same circumstances as delineated in Figures 6(b) and 6(c). Two physical samples were linked up to the transmitting and receiving ports of the network analyzer. e measurements were performed inside the anechoic chamber at a fixed distance (which fulfills the requirements of far-field transmission) of 4 m and 1 m high above the ground. Figure 7 delineates the graphical representation of overall measurement setup. e tested and simulated peak realized gain of an antenna over the operable frequency range are illustrated and compared in Figures 8(a) and 8(b). It can be clearly   International Journal of Antennas and Propagation seen that the optimal gain of 10.15 dBi (simulated) and 9 dBi (measured) is observed at 10.07 GHz and 10.09 GHz. Likewise, the minimum gain of 7.6 dBi (simulated) and 7.1 dBi (measured) is observed at 9.5 GHz. e calculated and simulation results are acquiescent and show the discrepancy of 1.15 dBi. e radiation efficiency is a paramount parameter of the antenna which is defined as the ratio of radiated and accepted power. A simple, precise, and expeditious approach predicated on the directivity/gain information is employed to compute the efficiency of the fabricated archetype [38]. e measured efficiency is extracted from the following equation: where η m and G m denote the measured efficiency and gain associated with fabricated prototype and D s represents the simulated directivity related to designed prototype.

Co-Polar and Cross-Polar Patterns.
e co-polar and cross-polar two-dimensional (2D) patterns in standardplanes were measured inside the chamber room under farfield condition. e standard horn antenna was connected with a cable to the transmission signal anchorage of a VNA along with a tunable signal generator, whereas the fabricated sample of the antenna was kept fixed on a positioner, which acts as a far-field receiver at a 4 m distance as shown graphically in Figure 6. e positioner is rotated around 360°t o measure the radiation patterns in orthogonal planes, elevation (E-plane (ϕ � 0°)), and azimuth (H-plane (ϕ � 90°)), respectively. Figures 9(a) and 9(b) provide the far-field two-dimensional (2D) normalized radiation pattern in orthogonal planes at 9.9 GHz. It is evident from Figure 9(a) that the antenna exhibits the omnidirectional radiation pattern in the elevation plane. Likewise, the measured results in the same plane are stable and symmetrical and show higher directivity around 200°and 300°as compared to simulated results. Figure 9(b) shows the antenna patterns in H-plane. e result shows that the antenna radiates equally in all directions. Moreover, the normalized radiation patterns in orthogonal planes at 10.8 GHz are displayed in Figures 10(a)  and 10(b). It is observed from Figure 10(a) that the radiating element exhibits the stable end-fire radiation pattern at a 90°b eam angle and maneuvering in the E-plane. Also, it can be noticed from Figure 10(b) that the proposed antenna radiated omnidirectionally in the azimuth plane.
In fact, the compact antennas have a good polarization level particularly in H-plane caused by the dominant TM 10 mode. As can be observed from Figures 9 and 10, the crosspolarization level is less than −25 dB. It is worthy to note that the X-polar level increases at certain points due to the excitation of TM 02 mode. Also, the discrepancy in measured results is observed owing to the fact while the antenna rotation particularly in H-plane; the metallic surface of the antenna could not be covered by radio absorbing material (RAM) and power meter sensitivity in both standard planes.
Furthermore, it is noticed that in both principle planes the cross-polarization is lower than co-polarization owing to a conviction that the co-polarization level must be higher than the cross-polarization level especially for compact antennas. e proposed antenna has proven the above facts and figures about the radiation pattern performance. Hence, it has been confirmed that the proposed antenna received the maximum power in co-polar orientation and received less power in cross-polar orientation.

Comparison Analysis
e proposed model of antenna exhibits excellent performance results of peak realized gain and efficiency with miniaturized dimensions. A comparative analysis of the antenna key features with state-of-the-artwork is demarcated in Table 2. e high-gain and radiation efficient antennas were investigated in [21,22] which occupied an ample space. In [23,31], the proposed antenna achieved only high gain with ample space. Besides, it is noted that, in the presented work in [23,31], the authors have not investigated the radiation efficiency of the antennas. e methods employed in the reported works were complex and timeconsuming. In this study, an alternative multielement approach is applied and excellent results are achieved as compared to most of the presented works. Moreover, with the deep analysis and owing to the trade-off among the antenna key features of the presented and analyzed results  reported in the open literature, it is estimated that the performance analysis results may not be better than the obtained results of the antenna presented in this paper. erefore, it is concluded that the utilized approach in the designed antenna can exhibit high gain, optimum radiation efficiency performance, good impedance BW, stable far-field patterns, and strong flow of current across the radiator space.

Conclusion
A new structural design of a compact, high-gain CPW-fed antenna has been presented. e model comprised of a metallic single bowtie arm radiator, ground plane, and standard 50 Ω CPW feeding line. e antenna elements were engraved on the new WangLing TP-2 thick substrate material. e novel multielement approach was employed to realize optimal performance features. e proposed structure possessed a compact size of 0.195λ × 0.163λ × 0.0052λ at the lowest resonance of 9.78 GHz. e antenna model has been fabricated and results are verified. e proposed archetype exhibited a good relative BW of 11.48%, an optimal realized gain better than 9.0dBi, and excellent radiation efficiency better than 87.6%. e symmetrical stable radiation patterns and strong density of current were observed. Moreover, the proposed antenna key characteristics have been compared with state-of-the-artwork. e antenna exhibited the optimal results of realized gain and radiation efficiency with a great advantage of small size. An acceptable agreement among the simulated and tested results has been observed that makes it a competitive choice for military airborne, land, and naval radar applications.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this manuscript.