Performance Analysis of Novel Multi-band Monopole Antenna for Various Broadband Wireless Applications

This study investigates a novel concept of using slot and Coplanar Waveguide fed antenna to obtain multiband operation. The compact antenna includes a star monopole patch with a slit gap and a Star-shape slot on the patch as the radiating part and Dual Square shaped slots on the material as the ground plane. The presentation of techniques in the star-shaped patch and ground plane expands the total length of the current path. This increasing makes the multiband antenna to operate at 4.9 (WLAN), 5.9 (upper WiMAX), 8.2 (satellite TV), 11.8 (X-band) and 17.7 GHz (Ku-band), respectively. The antenna has a physical dimension of $$0.32\lambda_{0} \times 0.24\lambda_{0} \times 0.01\lambda_{0} = (20 \times 15 \times 1)$$0.32λ0×0.24λ0×0.01λ0=(20×15×1) mm3, at the minimum frequency of 4.9 GHz. The proposed configuration operates at 4.9, 5.9, 8.2, 11.8 and 17.7 GHz with bandwidth (S11 < −10 dB) of about 90%, 74.7%, 53.8%, 8.4% and 4.7%, respectively for simulation part. The antenna exhibits measured bandwidth (S11 < −10 dB) of about 108%, 84%, 64%, 9%, and 7% at resonant frequencies of 4.69, 6.03, 7.99, 11.88, and 17.53 GHz, respectively. The stability of the radiation features, VSWR < 1.5 and good agreement impedance are noticed across the operating bandwidth of the multiband antenna.


Introduction
Nowadays, there are many demands for the new generation of wireless communication systems such as multi-frequency antennas, wide broadband antennas, and small size antennas for mobile and satellite communication systems. Therefore, multi-band antennas consist of various band frequencies that can be applied for more than applications of wireless communication like radar, mobile, and satellite communications [1]. Multi broadband wireless antennas are usually designed by adding fractals [2], multi-split strips, feeding methods [3][4][5][6], print slots [7][8][9], etc. These technologies face some critical points for instance low gain, narrow bandwidth and less efficiency characteristics. However, many researchers attempt to control 1 3 and resolve their parameters by creating some modifications like multi dielectric methods, slot cut and using different shapes [10,11].
The main aim of this work designs a small size antenna with multiband frequencies for various applications. Therefore, the technique of slots has been used to design the multiband antenna. Since, slot methods make some discontinuities in the path of electric current which lead to adding extra band frequencies. As a result, a multiple operating frequency is achieved with the aid of various slots [12,13]. The features of using slots make the antenna more eligible for mobile and wireless devices. The proposed antenna is compared with designs of many researchers who are recently working on multiband wireless devices and different sorts of the antennas examined in writing [14][15][16][17][18][19][20][21][22][23], in terms of the size, operational frequencies, bandwidth achieved, gain, and number of operational bands, as represented in Table 1.
A designed low profile monopole antenna works for multi devices like; super WiMAX, wireless computer networks, satellite communication, radar, and Ku-band devices. The overall obtained results of the fabricated and simulated design are in good matching. The proposed planar configuration has small size, multi resonating bands, wide bandwidth, good gain and stable radiation pattern for wireless technologies as compared with those examined in writing [14][15][16][17][18][19][20][21][22][23]. In Sect. 2, the design of the monopole antenna and the different steps of the iteration process have been mentioned in detail to achieve the proposed antenna.

Antenna Geometry
The star patch design is combined from the iteration of the triangle patch shape. Therefore, the equations below have been applied to compute the resonant frequency of this star monopole antenna [24], where S 1 is the length of the triangle side in (mm) as revealed in Fig. 1. All major dimensional parameters of the novel structure are listed in Table 2.
As demonstrated from Fig. 2, the dimensions of the proposed design are 20 × 15 mm 2 which is printed on the cheap dielectric substrate (FR4) having thickness (h) of 1 mm with loss tangent ( ) and dielectric constant ( r ) of 0.02 and 4.4 separately. The compact configuration consists of a star patch with slit etched on the center of the radiating patch. A coplanar waveguide (CPW) feeding transmission line (50 ) is used to excite the compact antenna with a strip line of width 1.8 mm and length 8.3 mm. Several antenna parameters, in particular the antenna reflection characteristics can be controlled by the proper impedance of the feed line [25]. The radiating patch is bordered by a U-shaped line of width 0.1 mm which connects both the ground planes on the substrate material. There is a gap distance of 0.3 mm between both radiating materials. The main idea behind of the adding dual square-shape slots (1 × 1 mm 2 ) on the ground plane, in order to produce a new resonant frequency. The copper material with a thickness of 0.035 has been used to design the patch and ground plane of the antenna. Consequently, a star shaped slot with a radius of 3 mm is designed on the center of the radiating patch.
(1) Resonance frequency (fr) fr = 2c 3S eff √ eff  Fig. 1 a Basic structure of triangular patch and b first iteration of the triangular patch

Design Stages of Proposed Antenna
Form Fig. 3, the design stages of the novel five-band antenna are depicted and described step by steps. First of all, a star monopole antenna with the slot method is created as illustrated in the design "A1" of Fig. 3. As can be observed in Fig. 4, this antenna operates only at two bands with less value of reflection parameter S 11 . A slot star is etched on the radiating patch to improve the parameters of the antenna "A1", as indicated in configuration "A2" of Fig. 3. So that, the antenna "A2" operates at extra bands, the U-shaped copper material is printed on the border of star monopole as presented in configuration "A3" of Fig. 4. The introduction of U-shaped material leads the antenna to resonate at 11.8 GHz for wireless computer networks and International Telecommunication Union (ITU) band, as revealed in Fig. 4. To insert another operating band for the antenna "A3", dual square shaped slots are designed on the ground conductive surface of the star monopole structure as demonstrated in configuration "A4" and "A5" of Fig. 3. The effects of the dual square shaped slots make the antenna to operate at 17.7 GHz (Ku-band), as revealed in Fig. 4. A strip-shaped slot is etched on the top star monopole to further increase the operating skills of antenna "A5", as indicated in configuration "A6" of Fig. 4. A discontinuity in the radiating element of the monopole antenna is created due to the proposal of strip shaped slot. Because of the discontinuity, the long path of the surface electrical current rises [26]. This increase in the length of current path impacts the input impedance of the star monopole. As a result of this effect, the antenna adds extra resonant frequency at 4.9 GHz for a

Antenna Parametric Studies
The effect of parametric analysis of the antenna is observed to investigate the operation bands of the monopole antenna with the variant of the physical dimension. The optimised parametric study is fulfilled in two circumstances which are (1)  Finally for the second condition, the split gap (S) width observes on the star patch of the compact design. The effects of S width on the parameters of the proposed design are presented where its value is also varied from 0.1 to 0.4 mm. The study reveals that, varied slot width S changes the first band into two bands. As shown from Fig. 5b, the return loss value at bands 1, 2, and 3 enhances. In addition, the S 11 shifts at band 1 for all S values and its magnitude slightly changes for band 3 and 5. The optimum result is achieved at S = 0.3 mm for the monopole configuration.

Result and Discussion
The novel multiband antenna has been analyzed with assistance of electromagnetic simulator software of High-Frequency Structure Simulator HFSS v.15.0 as depicted in Fig. 2. The monopole antenna depicted in Fig. 6 is fabricated using the same optimized size of the simulated design. The compact dimension of the monopole antenna is 0.32 0 × 0.24 0 × 0.01 0 (20 mm × 15 mm × 1 mm), at the minimum resonance of 4.9 GHz.

Return Loss (S 11 )
The result of the theoretical S 11 of the design is obtained using Ansoft HFSS and CST software. The electronic device of (VNA) Vector Network Analyzer (N9918A) is used to measure this simulated S 11 . This study shows that the antenna resonates at 4.9 (WLAN (802.11j)), 5.

Voltage Standing Wave Ratio (VSWR)
In addition, the simulated values of the VSWR is achieved which is about 1.04, 1.01, 1.15, 1.10 and 1.26 at 4.9, 5.9, 8.2, 11.8 and 17.7 GHz, and about 1.02, 1.04, 1.19, 1.19 and 1.12 at 4.69, 6.03, 7.99, 11.88, and 17.53 GHz in the measured values, respectively. The screenshots of the VSWR and the return loss of the fabricated multiband monopole antenna are measured using analyzers as appeared in Fig. 7.
The comparison between the obtained studies of the measurement and simulation of S 11 and VSWR parameters of the novel antenna is further demonstrated in Fig. 8. A good matching in the obtained results of the theory and experiment is observed for the proposed design. The slight difference may be because of the use of the VNA cable, fabrication tolerance, and point connection of SMA connector and cables applied for the measurement of the configuration. However, the obtained bandwidth of the novel design is enough to fulfill the necessities of WLAN, WiMAX, Satellite communication, ITU-band, and Ku-band applications.

Surface Current Distribution
The results of surface current distribution was also simulated in HFSS at all resonant frequencies of the proposed multiband antenna as illustrated in Fig. 9 respectively. From the figure has been revealed that current is uniformly distributed on the radiating patch and ground planes at all the resonance bands of the proposed antenna. The surface current has a maximum value at resonant frequency of 11.8 GHz and a minimum magnitude at resonance band of 5.9 GHz for the antenna loaded with U-shape material.

Radiation Patterns and Gain
Theoretical and experimental far-field radiation characteristics of the monopole star-shape patch antenna are illustrated in Fig. 10. The radiation pattern of the fabricated prototype model is measured from 0 to 360 degree with step of 30 degree, at the resonance bands 4.69, 6.03, 7.99, 11.88, and 17.53 GHz, both in E-plane (co and cross) and H-plane (co and cross), respectively. As can be clearly revealed from Fig. 10, the proposed design has an almost omnidirectional pattern in E-plane (Y-Z) and bi-directional pattern in H-plane (X-Z). For both the cases, the achieved results of the measurement and simulation are in close agreement with a slight deviation because of alignment errors, measurement, and electromagnetic interference from the nearby devices. The experimental and simulated peak gain along the operational frequencies for the compact structure has been depicted in Fig. 10. The figure shows that fluctuated gain along the desired bands is obtained. The simulated results of achieved gain of each operating frequency at 4.9, 5.9, 8.2, 11.8 and 17.7 GHz are about 1.8, 2.4, 3.6, 4.7 and 4.9 dBi, and The HFSS software has been used to find the radiation efficiency characteristics of the multiband antenna. The operating band efficiencies are about 94, 94, 84, 87 and 77% at the simulated operating bands of 4.9, 5.9, 8.2, 11.8 and 17.7 GHz, respectively. The star monopole antenna acts as a transmission line instead of radiating part at higher frequencies. Therefore, the magnitude of radiation efficiency decreases at higher resonances as revealed in simulations and Fig. 11. The summarization of the antenna results is listed in Table 3.

Conclusion
A compact multiband monopole antenna design loaded with the star-shape slot is studied and with the dimensions of (20 × 15 × 1) mm 3  Simulation and measurement parts are applied to investigate the results of the compact antenna with respect to the antenna parameters of S 11 , gain, VSWR, and radiation pattern. The experimental results reveal that the novel planar antenna has five frequencies at about 4.69, 6.03, 7.99, 11.88, and 17.53 GHz, with stable radiation characteristics, VSWR < 1.5 and good matching impedance. Due to the mentioned purpose, the proposed design can