Bandwidth enhancement and miniaturization of circular-shaped microstrip antenna based on beleved half-cut structure for MIMO 22 application

Teguh Firmansyah, Supriyanto Praptodiyono, Herudin, Didik Aribowo, Syah Alam, Dian Widi Astuti, Muchamad Yunus Department of Electrical and Engineering, Universitas Sultan Ageng Tirtayasa, Indonesia Department of Electrical Engineering Vocational Education, Universitas Sultan Ageng Tirtayasa, Indonesia Department of Electrical Engineering, Universitas 17 Agustus 1945 Jakarta, Indonesia Department of Electrical Engineering, University of Mercu Buana, Indonesia Department of Electrical Engineering, University of Pakuan, Indonesia


INTRODUCTION
In recent years, microstrip patch antenna technology is widely used. The microstrip patch antenna has advantages such as low fabrication cost, light in weight, capable of supporting multiple frequency bands, easily etched on any PCB and integrated them with MICs or MMICs [1][2][3]. However, it has disadvantages such as low gain, large PCB structure, and narrow bandwidth due to conductor losses, surface wave losses, and dielectric losses [4,5]. Several studies investigating bandwidth enhancement of microstrip antenna have been carried out by [6,7]. The proposed methods include defected ground structure (DGS) [6], electromagnetic band gap (EBG) [7,8], parasitic patch [9,10], metamaterial [11], metamaterial bilayer substrates (MBS) [12], monopole slot [13], T-shaped slot [14], cylindrical dielectric slot (CDS) [15], polymeric grid [16], spiral split ring (SSR) [17], Jerusalem cross-shaped [18], and characteristic modes [19]. The DGS method was proposed by Marotkar [6], it is realized by etching the ground plane so the current distribution in the ground plane is disturbed. As the results, the antenna has a wide bandwidth of 302 MHz with center frequency of 2.4 GHz, and reflection coefficient of -23.26 dB. Furthermore, Gupta [7] and Hadarig [8] investigated bandwidth enhancement of microstrip patch antennas by implementing EBG structures. This proposed antenna has a center frequency of 10 GHz and 2.4 GHz for X-band Radar and VHF RFID, respectively. Then, to reduce the size of the antenna, Rothwell and Raoul O [21] proposed a metamaterial structure. The metamaterial microstrip structure has advantages such as compact size and broadband. However, the structure has complex geometry, and it is difficult to fabricate. Then, H. Mosallaei and K. Sarabandi [22] proposed bandwidth enhancement by using a reactive impedance substrate (RIS). This method succeeds to increase the bandwidth of the antenna and reduce antenna size.
Moreover, a fascinating method was proposed by Mohamadi [18]. It investigated the bandwidth enhancement of antenna for Long Term Evolution (LTE) technology with multiple input multiple output (MIMO) application. He introduced the basic modes method, this method successfully to enhance the bandwidth of the antenna, but it was still a drawback such as complex microstrip structure. Other methods include, G-shaped band-notched antenna [23], dielectric resonator antenna (DRA) [24], and U-shaped slot antenna [25]. The DRA antenna that is proposed by [24] has good bandwidth. However, the antenna structure is still large.
As the novelty, to enhance bandwidth and reduce antenna size, beleved half-cut microstrip structure is proposed in this paper. Further, this proposed antenna structure will be applied to multiple input multiple output (MIMO) antenna 22. Therefore, this research was investigated conventional circular shape antenna (CCSA), circular shaped beleved antenna (CSBA), and MIMO circular shaped beleved antenna (MIMO-CBSA) as Model 1, Model 2, and Model 3, respectively. An FR4 substrate with er=4.4, thickness h=1.6 mm, and tan d=0.0265 was used. In brief, Table 1. provides the research position of this paper compare to another research of bandwidth enhancement and miniaturization of the antenna. This rest of this paper is detailed as follows. In Section 2, the proposed circular shaped beleved antenna and MMO circular shaped beleved antenna are presented. The detail of numerical simulation was also described in Section 2. Furthermore, the measurement results of the fabricated antenna and the comparison with simulation result was explained in Section 3. Finally, Section 4 concludes this research.

RESEARCH METHOD
In this section, the proposed circular shaped beleved antenna and MMO circular shaped beleved antenna were designed. Figure 1  The radius of circular-shaped microstip patch antenna is formulated by [26], [27]: with h=thickness of substrate (cm), r=permittivity of substrate, and =resonant frequency (Hz). A direct feeding method was used in this paper. Moreover, the impedance characteristic (Z0) can be determined by the ratio of a thickness of substrate (h) and its width (W) [28]. When 0 √ > 89.91, W/h ratio is given by [29], [30]: when 0 √ ≤ 89.91, W/h ratio is given by [29], [30]: Furthermore, the conductivity loss (∝ ) of microstrip transmission line feeding is given by [29], [30]; with = √ 0 , = resistivity of the conductor, f = frequency (Hz), and 0 = 4π × 10 −7 N·A −2 is the magnetic permeability of free space, The numerical simulation of the antenna parameters has been conducted by using Advanced Design System (ADS). The FR4 substrate with r=4.4, thickness h=1.6 mm, and tan d=0.0265 was used. Figure 2(a) shows the extracted reflection coefficient with varied R. The data shows that by modifying the radius (R), the reflection coefficient can be tuned. However, for R=22 mm and R=24 mm, the antenna is not resonance. Furthermore, Figure 2(b) shows the voltage standing wave ratio (VSWR) value by varied radius (R). The simulation result shows that VSWR value is better than 2 at R=28 mm and R=30 mm.  Figure 3(a), we can see that the WG is essential parameters to make the antenna resonate. The Figure 3(a) shows that by increasing the WG (mm), the antenna will be more resonate. Moreover, Figure 3(b) shows clearly trend that the VSWR of the antenna is better than 2 (two) for WG is longer than 28 mm. However, the dimension of this antenna is large. The next step explains the miniaturization process. In this paper, the bandwidth enhancement and miniaturization of the antenna is obtained by the beleved method as shown in Figure 1(b) and Figure 1(c). The beleved method was applied by cut one side of the antenna, partially. Furthermore, the size of the antenna will be reduced by d (mm). Moreover, the result of numerical simulation based on the beleved method is depicted in Figure 4(a) and Figure 4(b). Figure 4(a) illustrates the extracted reflection coefficient with varied d (mm). Base on Figure 4(a), the data shows that at d=10 mm produce large bandwidth. However for d=10 mm at the frequency of 2.8 GHz, the reflection coefficient is higher than -10 dB at frequency of 2.8 GHz. Therefore, the value d=10 mm is not chosen because the reflection coefficient is too high. Therefore, the data shows that the largest bandwidth is generated at d=8 mm. This result is also indicated in Figure 4 Figure 1(e). Furthermore, Figure 5(a) shows the extracted reflection coefficient with different WSMB (mm) and Figure 5(b) illustrates the extracted voltage standing wave ratio (VSWR) with varied WSMB (mm). Figure 5(a) shows a clear illustration that the reflection coefficient is stable for the different length of WSMB and it shows that the reflection coefficient values are lower than -10 dB. However, the reflection coefficient for WSMB=180 mm generates lower bandwidth than others. Furthermore, it appears from Figure 5(b) that the VSWR values are still lower than 2 (two). This data shows that the antenna is working properly with good performance. The numerical simulation result of mutual coupling MIMO antenna is shown in Figure 6(a) and Figure 6(b). Figure 6(a) exhibits the extr acted mutual coupling (S21) with varied WSMB (mm) and Figure 6(b) illustrates the extracted mutual coupling (S31) with varied WSMB (mm). The mutual coupling value of S21 (dB) and S31 (dB) demonstrate the coupling between Antena 1 to Antenna 2 and Antena 1 to Antenna 3, respectively. The coupling coefficient is lower than -15 dB almost over the whole band which shows a good isolation performance. However, the coupling coefficient for WSMB=180 mm is higher than -15 dB at the frequency of 2.8 GHz. So, the WSMB=180 mm cannot be chosen. The distace between antenna effects on mutual coupling. The mutual coupling can be decreased by increasing the distance between the MIMO antennas. However, the size of the antennas cannot be made too large.

RESULTS AND ANALYSIS
To verify the simulation result, the measurement of the antenna prototype must be carried out. The photograph of the fabricated proposed antenna is depicted in Figure 7 Figure 9 Figure 10(a)-(h). In conclusion, all of these advantages make it particularly valuable in multistandard antenna applications design such as GSM950, WCDMA1800, LTE2300, and WLAN2400.

CONCLUSION
In order to reduce the antena size and enhance the bandwidth of antena, this paper was proposed the beleved half-cut microstrip structure. Moreover, this research was investigated conventional circular shape antenna (CCSA), circular shaped beleved antenna (CSBA), and MIMO circular shaped beleved antenna (MIMO-CBSA) as Model 1, Model 2, and Model 3, respectively. This antenna was fabricated on FR4 substrate with er=4.4, thickness h=1.6 mm, and tan d=0.0265. The numerical simulation has been conducted using Advanced Design System (ADS). The measured result showed that proposed antenna CSBA [Model 2] has wider-bandwidth of 58,2% and smaller-size of 18.2% compared to CCSA [Model 1] antenna. Then, after CSBA [Model 2] structure was applied to MIMO 22 [Model 3], the MIMO antenna obtain very good mutual coupling (<-15dB). Moreover, the measured results are good agreement with the simulated results.
In conclusion, all of these advantages make it particularly valuable in multistandard antenna applications design such as GSM950, WCDMA1800, LTE2300, and WLAN2400.