Quintuple Band Antenna for Wireless Applications with Small Form Factor

A coplanar waveguide-fed quintuple band antenna with a slotted circular-shaped radiator for wireless applications with a high isolation between adjacent bands is presented in this paper. The proposed antenna resonates at multiple frequencies with corresponding center frequencies of 2.35, 4.92, 5.75, 6.52, and 8.46 GHz. The intended functionality is achieved by introducing a circular disc radiator with five slots and a U-shaped slot in the feed. The proposed antenna exhibits coverage of the maximum set of wireless applications, such as satellite communication, worldwide interoperability for microwave access, wireless local area network (WLAN), long-distance radio telecommunications, and X-band/Satcom wireless applications. The simulation and measurement results of the proposed fabricated antenna demonstrate the high isolation between adjacent bands. A stable realized gain with an advantageous radiation pattern is achieved at the operating frequency bands. The proposed simple design, compact structure, and simple feeding technique make this antenna suitable for integration in several wireless communication applications, where the portability of devices is a significant concern. The proposed antenna is anticipated to be an appropriate candidate for WLAN, long-term evolution, and fifth-generation mobile communication because of its multi-operational bands and compact size for handheld devices.


Introduction
In recent years, a prominent trend in antenna design is miniaturization including the reduction in the complications involved in the fabrication of such antennas while maintaining feed network connectivity. To achieve this, many designers have demonstrated a significant preference for multiband antennas over conventional ones with a single band. The multiband functionalities can be added to standard microstrip antennas by introducing specific alterations in the antenna geometry, such as creating slots of various shapes and cutting patches of different sizes. Various multiband design approaches have been reported in the literature. Among these are modifying the main radiating element of the antenna by integrating a metamaterial-based split ring resonator [1] and slot insertion [2]; multiple frequency bands are also achieved through the use of defective ground planes [3]. However, the main deficiency of the foregoing changes in conventional microstrip antennas is that these modifications tend to limit the antenna to a narrow operational bandwidth.
To overcome this bandwidth limitation, the monopole antenna, with its wide operational bandwidth, has emerged as the primary alternative because of its simple design, facile fabrication, light weight, compact geometry, and significant efficiency compared with conventional antennas. To achieve high data transmission and reception, the monopole antenna may be incorporated into various portable communication devices. In Karim et al. [4], several types of multiband antennas that can satisfy the abovementioned requisites are identified. In a wireless communication system utilizing such antennas, designing a small-sized multiband antenna while maintaining adequate isolation among all operating bands is crucial. Moreover, coplanar waveguide (CPW)-fed antennas have gained considerable acceptance in several applications because they afford further advantages, such as facile integration, simple structured metallic layer, and lesser coupling [5].
To achieve the characteristic performance of multi-functional transceivers, simultaneous multiband antenna operations are required. Two types of antennas are usually considered: multiband and reconfigurable antennas. For various applications where more than one operating band is necessary, multiple antennas may be used to achieve a better device performance. This may be gained at the cost of complexity and coupling as a result of the diminutive antenna space occupied. For combining multiple applications, a multiband antenna can satisfy certain requisites, such as high efficiency, low cost, and miniaturized circuit size while maintaining the necessary transmission parameters including various operating bands with acceptable gains and wide bandwidths. Currently, few techniques for the development of multiband antennas include the insertion of slots and addition of metamaterial-based structures inside radiating elements. Multiple frequency bands can also be achieved using defective ground structures, fractal geometry, split-ring resonators, etc. [6][7][8][9].
Complementary split-ring resonator-based multiple band achievement has been formulated and verified in Rahman et al. [6] via the distribution and lumped model. It has been demonstrated that a wideband antenna can be designed using a cylindrical radiator and can be switched to multiple bands using integrating capacitors. One of the significant benefits of ultra-wideband antennas is that they include the operating bands of commercial applications, such as wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX); however, there exists a tradeoff between the multiband operation of desired applications and the risk of interplay with other working bands [7]. Ahmad et al. [8] presented the smallest form factor multiband antenna for WLAN and WiMAX applications with the corresponding sophisticated shape and simple geometry. A triple-band microstrip patch antenna with a complex geometric structure and large overall size for WLAN/WiMAX applications is reported in Gautam et al. [9]. A similar type of antenna with multiple bands for wideband applications is also presented in [10][11][12]. In Haider et al. [10], an antenna with the capability of switching between the wideband and narrowband behaviors while being utilized for microwave breast imaging application was designed. Moreover, in Rahman et al. [11], a wideband antenna with a corresponding equivalent circuit and its transformation to different wireless band applications utilizing the Babinet principle is designed. In Rahman et al. [12], similar techniques have been adopted, and a wideband antenna having five notch bands with corresponding distributed and equivalent models has been achieved.
Recently, certain antennas have been designed for dual-band applications and simultaneously integrated to operate at long-term evolution (LTE) and 5G communication. In this regard, Chen et al. [13] developed a microstrip patch antenna operating at 28 GHz with a corresponding impedance bandwidth of approximately 26%. The antenna is capable of operating at a single band for 5G applications. Similarly, in Naqvi et al. [14], an integrated multiple-input multiple-output antenna operating at 5.29-6.12 GHz and 26-29.5 GHz for the corresponding LTE and millimeter wave applications was designed, respectively.
The design of quadruple and pentaband antennas has also gained considerable interest. In this regard, a quad-rectangular shaped microstrip antenna operating at frequencies of 1.074, 3.119, 4.089, 5.683, and 6.514 GHz is presented in Saroj et al. [15]. Moreover, in Ali et al. [16], a novel quad-band antenna operating at the global navigation satellite system, lower WiMAX, upper WLAN, and X-band for satellite communication using FR4 material is developed. A metamaterial-based hepta-band antenna with a reduced specific absorption rate is presented in Saraswat et al. [17]. The proposed structure possesses the capability of simultaneously operating at WLAN (2.4, 5.0, and 5.8 GHz), radio frequency identification (3 GHz), WiMAX (3.5 GHz), and lower Ku band. Similarly, a triple band antenna couple set-fed rectangular-shaped antenna with corresponding M-shaped branches for wireless appliances was developed in Naidu et al. [18]. Moreover, Albawri et al. [19] attempted to enhance the gain and radiation pattern of the quadruple band metamaterial-based antenna for wireless communication. A low-profile horizontal patch stacked antenna with corresponding corner feeding for pentaband wireless applications is presented in Singh et al. [20]. In Park et al. [21], the authors successfully demonstrated a novel technique to reduce mutual coupling between two closely packed antennas.
This paper presents a coplanar waveguide-fed quintuple band antenna with a slotted circular-shaped radiator for wireless applications. The proposed structure has the capability of high isolation between adjacent frequency bands. It simultaneously resonates at multiple frequencies with corresponding center frequencies at 2.35, 4.92, 5.75, 6.52, and 8.46 GHz. The presented structure is demonstrated and formulated with a corresponding validation in the finite element method solver Ansoft high-frequency structure simulator (HFSS). For the utilization of the antenna in practical scenarios, it is also fabricated, and its results are measured. The vector network analyzer (VNA) is used for S-parameter measurement, and the radiation patterns and antenna gain are obtained from anechoic chamber measurements. The simulated results are compared with the corresponding measurements in each scenario.
The rest of this manuscript is organized as follows. Section 2 presents the antenna design and configuration, mainly focusing on antenna development and geometrical dimension selection. Moreover, the step-by-step design methodology is briefly discussed in this section. The simulation and measurement results for the proposed antenna design, such as antenna gain, reflection coefficient, radiation patterns, and surface current distributions, are elaborated in Section 3. The comparison between the proposed quintuple band antenna with other recent state-of-the-art designs is presented in Section 4 to demonstrate the superiority of the former. Section 5 summarizes the conclusion of the study.

Antenna Geometry
The geometric configuration of the proposed quintuple band antenna is shown in Fig. 1a. Its design is implemented on the 1.6 mm thick FR4 substrate. The dielectric constant value and loss tangent are 4.4 and 0.02, respectively; the overall size of the antenna is 40 × 60 mm 2 . Its primary structure is a circular disk that is designed to resonate on a single frequency band, i.e., WiMAX band, according to Eqs. (1) and (2): where l r ¼ 1; a e is the effective radius; t is the substrate thickness; a is the actual radius.
The fabricated prototype of the proposed antenna is shown in Fig. 1b. To verify its performance characteristics, the proposed antenna is fabricated according to the prototype antenna. The VNA is utilized to measure its corresponding reflection coefficient. A through-reflect-line calibration is performed before the measurement to avoid undesirable losses. For the radiation pattern, the VNA along with two horn antennas are employed to obtain the measurements in the anechoic chamber environment. This chamber can detect antennas in the frequency range 0.8-30 GHz. It is equipped with a near-field planner scanner and far-field tower to measure the radiation pattern of a given antenna under test. To overcome the effects of fabrication disparity, the measured results are validated by testing five different prototypes of the proposed antenna; the prototype whose results agree with the simulated results is selected.
The actual radius (a = 24 mm) is calculated using Eqs. (1) and (2); this radius results in the resonation of the antenna at the lowest operating frequency (2.4 GHz) in the first step (antenna a). In the second step (antenna b), with the insertion of three slots perpendicular to each other, three operating frequencies are achieved for the proposed antenna: 2.45, 4.9, and 6.1 GHz; consequently, the radius of the circular disc can be reduced to 14 mm. In the third step, two more slots are inserted into the lower portion of the circular disc, which results in the inclusion of another frequency band (8.4 GHz) along with the previous three resonating bands (antenna c). The final design (antenna d) is realized by introducing a U-shaped slot in the feedline. Through this approach, the resulting antenna structure starts to resonate at an additional frequency band (i.e., 6.52 GHz), as shown in Fig. 2. Furthermore, this newly introduced slot in the feedline only slightly influences the existing bands.
To excite the proposed antenna, a 1-mm wide CPW feedline is used with a characteristic impedance of 50 Ω. The CPW feedline value is estimated by employing Eqs. (3)-(8) [15].
sinh pb = 2h (5) where S c is the width of the central conductor; W is the separation distance between the ground plane and central conductor; h and ε r denote the substrate thickness and relative dielectric constant used for the proposed antenna, respectively; ε eff denotes the effective dielectric constant; K(k) represents a first-order integral having an argument (k) or complementary argument (k΄). To achieve a wide operation bandwidth, improve efficiency, and afford ease in integration, a CPW-fed ground plane is employed.
The inclusion of slots variates the current distributions, causing them to radiate at other frequencies. Accordingly, the slots are inserted, thus creating multiple passbands. As depicted in Fig. 2, the introduction of the slots in the radiator creates passbands at 6.52 and 8.46 GHz. Without a slot, this triple band antenna can only resonate near the fundamental frequency, i.e., 2.35 GHz. Moreover, the symmetrical insertion of the slots eliminates the distortion in the radiation pattern. The design of the proposed antenna is further optimized by employing a high-frequency structure simulator (ANSYS HFSS). Tab. 1 summarizes the parameters of the resultant antenna.   The simulated realized gain (dBi) of the proposed antenna at different operating resonance frequencies is illustrated in Fig. 7. A satisfactory gain is observed at the center of each passband as well as on the corresponding right and left operating edges of the passbands. As a result, any shift in the design parameter will still maintain the realized gain within an appropriate range. Tab. 2 summarizes the results of all the operating frequency bands of the proposed quintuple band antenna.

Conclusion
A coplanar waveguide-fed quintuple band antenna with a slotted circular-shaped radiator for wireless applications with a high isolation between adjacent bands is presented. The proposed antenna is designed, simulated, fabricated, and measured. The antenna resonates at five different frequency bands, and this is attributed to the inclusion of slots and the antenna's simple structure. The simulated and measured results of the proposed fabricated antenna exhibit a high isolation between adjacent bands. The stable realized gain with an advantageous radiation pattern is also achieved at the operating frequency bands. The proposed antenna is anticipated to be a suitable candidate for wireless and mobile communication systems.  Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.