Extended-Bandwidth Microstrip Circular Patch Antenna for Dual Band Applications

Received Oct 11, 2017 Revised Dec 28, 2017 Accepted Jan 10, 2018 This paper presents a new wideband microstrip circular patch antenna (MCPA) fed by proximity-coupled line with double-stub matching to achieve dual-band operation. Bandwidth extension is achieved by exciting higherorder modes in the circular radiating patch, and using two stubs to achieve adequate matching across the obtained two bands. The characteristics of the antenna such as reflection coefficient, impedance bandwidth, gain and radiation pattern are investigated and optimized through parametric studies using the CST Microwave Studio Suite. The antenna achieved a large relative bandwidth of 45.16% at the upper band, while the lower one has 10.3% relative bandwidth. The maximum achieved gain of the dual-band antenna in the 5.8GHz band is 4.62dBi while it is 4.85dBi in the upper band. The antenna has an overall size of 30×30×3.2mm3 corresponding to 0.58λ × 0.58 λ × 0.062 λ at the lower band of 5.8 GHz. The proposed antenna should be useful for WLAN and X-band communication systems. Keyword:


INTRODUCTION
Microstrip patch antennas (MPAs) have various advantages such as low profile, light weight and being conformal to hosting surfaces. Thus, they are used in wide range of applications like mobile communication systems, global positioning systems (GPS), microwave sensors, wireless local area networks (WLAN), etc [1]. However, microstrip antennas suffer from the small working bandwidth of less than about 10%. Therefore, various techniques have been proposed for bandwidth extension of the microstrip antennas. The proximity coupling was found to achieve a 13% relative bandwidth in rectangular patch antenna [2]. This technique has other benefits such as ease of matching and fabrication compared to the other feeding schemes. Another proposed solution was the use of a parasitic resonator coupled to the patch forming a wideband stacked microstrip patch antenna [3]. In [4] a special shape antenna consisting of a rectangular patch tapered at two corners and loaded with three notches and one slot was presented. A proper selection of dimensions and positions of the slot and notch have led to 30.5% relative bandwidth.
The realization of a dual-band operation can be considered as another scheme to achieve wider bandwidth. A popular method for designing a single-fed dual-band antenna is to stack two resonating structures of proper dimensions to excite two fundamental modes corresponding to the desired bands [5], [6]. The height between the resonators needs optimization to ensure impedance matching and adequate bandwidth of the antenna. The achieved bandwidths in [5] were 8.3% and 7% for the lower and upper bands respectively. Slightly larger bandwidths of 10.64% and 8.82% for the L 1 and L 2 GPS bands respectively were obtained in [6]. However, these acquired benefits are on the account of size increase. In another design with inserting a slot on the ground plane and a stacked patch supported by a wall, the bandwidth was increased up to 25% [7]. A staked patch antenna based on aperture coupling and asymmetry feeding technique was presented in [8]. A cross-shape aperture imbedded in the ground plane with asymmetry feeding structure provided a relative wideband electromagnetic coupling. The experimental results showed that an impendence bandwidth of 35.3% was achieved [8]. In [9] a dual-band GPS antenna incorporating a proximity-coupled feed was proposed. The radiating element is a circular patch with a cross-slot and a ring-slot to excite the TM 11 mode at the GPS L 1 and L 2 bands, and the measured bandwidths were 6.1% and 3.7% respectively. A novel feeding system for microstrip radiators which is based on an electromagnetic coupling was presented in [10]. The single feed line is ended by two arms which hug the patch near the edge in a symmetrical manner with a capacitive gap to the two adjacent sides of the rectangular patch in order to initiate circular polarization. The result of the developed prototype was a 3dB axial ratio bandwidth of 3.9%. The achieved impedance bandwidth was 4.9%. A broadband proximity-coupled dual-polarized microstrip antenna with an L-shape backed cavity was presented [11]. The L-shape cavity enhances the feed coupling and thus broadens the bandwidth. A prototype antenna with optimized parameters showed relative bandwidth of more than 30% (8.2-11.4 GHz) and the two port isolation is larger than 20 dB. The antenna has three substrates and a thick ground plane leading to dimensions of 28X28X9.4 mm 3 . Slots and rings in the circular microstrip patch were also used to achieve multiband operation [12], [13]. A compact wideband dual-frequency microstrip antenna is proposed in [14]. The antenna has an offset microstrip-fed line and a strip close to the radiating edges in the circular slot patch, and achieved bandwidths of 26.2% and 22.2% for the two bands. A stacked dual-layer circular patch antenna with enhanced bandwidth is recently proposed in [15]. The antenna has a slotted circular patch, a parasitic circular patch on the top layer and showed a 25% relative bandwidth. In [16] using probe-fed stacked circular patches and optimization of the dielectric constants of the two substrates it was found that bandwidths up to 30% can be achieved. The lower patch is probe-fed with high dielectric constant substrate while the upper one is capacitively coupled through. However, bandwidth expansion is achieved on the account of increased thickness. The antenna in [16] has thickness of 31% of the circular patch diameter leading to a large antenna volume.
This paper presents a new wideband microstrip circular patch antenna (MCPA) fed by proximitycoupled line with double-stub matching to achieve dual-band operation. The proposed antenna is analyzed and optimized using the CST Microwave Studio Suite. The proposed antenna is then developed to exhibit dual-band of WLAN and X-Band operation. The higher band is achieved by exciting higher modes in the circular radiating patch. To furnish proper matching for the higher band, two stubs are connected to both sides of the feeding line. The two-stub technique can offer matching across a wide band that covers more than one resonating mode of the circular patch antenna. Section 2 discusses the theoretical aspects of the circular patch antenna. Section 3 demonstrates the parametric study of the antenna performance, and the optimized Antenna-I is presented in Section 4. The development of Antenna-I to a dual-band antenna II is discussed in Section 5. Finally, the conclusions are given in Section 6. Figure 1 shows the configuration of the proximity-coupled microstrip circular patch antenna (Antenna-I) with detailed dimensions and parameters. The antenna consists of a grounded substrate of dimensions (30 × 30) mm 2 where a microstrip feed line is printed. The microstrip line is W f = 3.1mm wide and L f = 16mm long and is located at a distance of x 1 = -2.4mm from the center of the circular patch. The substrate of this layer is FR4 (glass epoxy) with dielectric constant of  r = 4.3 and thickness of h = 1.6 mm.

ANTENNA GEOMETRY
Above this layer, there is another dielectric laminate of the same material and thickness as the first layer with a microstrip patch etched on its top surface. The end of the feed line is shifted from the center of the circular patch by a distance of y = 1.5mm. A copper film of thickness 0.05mm was used as the ground plane of the antenna. The electromagnetic coupling between the feed line and the patch results in power transfer, as opposed to a direct contact [17].
The patch used in this paper is of a circular shape whose resonance frequency f nm can be calculated using the cavity model as [18], [19]: (1) where χ nm is the m th root of the Bessel function derivative J' n (ka), ε r is the relative permittivity of the substrate, and c is the speed of light. The effective radius a e of the radiating circular patch is to compensate for the fringing field effect along the edge of the circular disk. For the TM 11 mode, it was suggested that a e can be given by [18,19]: where h is the height of substrate and a is the actual radius of the circular patch. Thus using Equation (1) and Equation (2), it was found that a resonant frequency of 5.8 GHz is obtained for a 6mm patch radius.

PARAMETRIC STUDY AND OPTIMIZATION
In this section, the antenna structure is analyzed and optimized using the CST Microwave Studio Suite 2011, which is an electromagnetic simulator based on the finite integration technique [20]. The analysis of the antenna for different parameter values has been carried out by varying one parameter while the other ones are kept constant. The optimized dimensions of Antenna-I are shown in Table 1. Figure 2 shows the simulated reflection coefficient of the optimized Antenna-I as a function of frequency. It is clear from this figure that the antenna has a relative impedance bandwidth of 9.5% or 0.55GHz for reflection coefficient  -10dB. Figure 3 shows the simulated realized gain of the optimized antenna as a function of frequency. The antenna has a maximum gain of 4.62dB i across the band of interest.   Figure 3. Simulated realized gain of optimized Antenna-I as a function of frequency

BANDWIDTH EXTENSION
The proposed Antenna-I analyzed in the former sections was developed to obtain a dual-band operation with extended bandwidth. For this purpose, the higher resonance modes of the circular patch have been utilized. However, to excite these modes properly matching of the proximity coupled circular disk needs to be improved. Therefore, a double-stub matching network was added to the feed line of Antenna-I to form a proposed Antenna-II whose geometry is shown in Figure 4.  Figure 6 shows the simulated reflection coefficient of Antenna-II as a function of frequency for various lengths of the upper stub (x 2 ). It is clear from this figure that the matching is improved and the bandwidth is increased for the upper band with increasing the length of upper stub from 1mm to 2mm. After 2mm i.e. at x 2 = 2.5mm, the situation gets worse in terms of matching and bandwidth. For the lower band, the figure shows that the resonance frequency shifts towards left and the matching degrades slightly with increasing in x 2 . Thus, a length of x 2 = 2mm for the upper stub was found to give the best result. During this simulation, the length of the lower stub was kept equal to x 3 = 1mm at a distance of y 3 = 8.4mm from the upper end of the feed line and the distance of the upper stub is y 2 = 4mm from the upper end of the feed line.   Figure 8 shows the simulated reflection coefficient of Antenna-II as a function of frequency for various lengths of the lower stub (x 3 ). The figure shows that for the upper band the matching gets worse and the bandwidth decreases with increasing the length of the lower stub, while there is a lithe effect on the lower band. The best results were obtained for lower stub length of x 3 = 1mm. The length of the upper stub was kept equal to x 2 = 2mm at a distance of y 2 = 4mm from the upper end of the feed line and the distance of the lower stub is y 3 = 8.4mm from the upper end of the feed line. The optimum parameters for Antenna-II are listed in Table 2. Figure 9 shows a comparison between the simulated reflection coefficients of the optimized Antenna-I and Antenna-II. This figure shows that adding the impedance matching network to Antenna-I (to obtain Antenna-II) has improved its characteristics in terms of getting an upper band with a wider bandwidth. Antenna-II has a relative bandwidth of 10.3% for the lower band which is larger than that for Antenna-I. The upper band of Antenna-II covers the whole X-Band corresponding to a much larger relative bandwidth of (45.16%).  The achievement of the second band can be explained through the following. The resonance frequencies f nm of the circular patch (a = 6mm) for the first 3 modes were calculated using Equation (1) and Equation (2), and the obtained results are shown in Table 3. Figure 9 clearly shows that before the addition of the two stubs (Antenna-I) the lowest resonance mode TM 11 has a frequency of 5.80 GHz. The other two higher modes (TM 21 and TM 02 ) should appear at frequency of 9.518 GHz and 11.941 GHz according to Equation (1) and Equation (2). The simulated results of Figure 9 show that, for Antenna-I, these two modes have appeared at 9.77GHz and 13.21 GHz. They look partially matched as seen from the shallow dips (around -5dB) in the reflection coefficient response. However, after adding the two stubs (Antenna-II), the upper two modes have appeared at 8.80GHz and 12.37GHz. The two modes have merged together to form a joined band covering a wide range extending from 8.4 GHz to 13.4 GHz.
It should be noted that these calculations are based on the assumption that the resonating circular patch is not accompanied with the feed line (the cavity model). However, the addition of the feed line for coupling will affect the values of the resonating frequency. In this design, the microstrip feed line is partially inserted between the circular patch and the ground plane. Table 3 shows that the simulated frequencies for Antenna-I are within less than 7% of those estimated by Equation (1) and Equation (2). These results show that Equation (1) and Equation (2) can give good estimates of the resonance frequencies. In the design process, better tuning of the frequencies to the desired values can be obtained through the analysis of the CST Microwave Studio suite [18].  Figure 10 shows a comparison between the realized gains of the optimized Antenna-I and Antenna-II. The addition of the two stubs has slightly increased the gain at the lower band, while has led to an upper band of a minimum gain of 2 dB i across the X-Band. It is clear from this figure that Antenna-I has a maximum gain of 4.62dB i at 5.79GHz while Antenna-II has a maximum gain of 4.88dB i the 5.72GHz lower band and a maximum gain of 3.74dB i at 11GHz for the upper wider band. In order to validate the results obtained by CST MWS the performance of the optimized antenna II was simulated using EM simulator based on HFSS. Figure 11 compares the obtained reflection coefficient of the optimized antenna II from the two simulation suits. Table 4 depicts a quantitative comparison of the two frequency responses. From Figure 11 and Table 3, a good agreement between two simulation results is observed. A slight difference between the two results can be attributed to different numerical techniques employed by the two simulators.  The simulated realized gain against frequency obtained from the CST and HFSS software packages are shown in Figure 12 and Table 5 summarizes the result obtained from this figure. Figure 13 shows the 3-D radiation patterns of Antenna-II for four selected frequencies (5.72GHz, 8.81GHz, 10.14GHz and 12.4GHz) which satisfy the lowest reflection coefficient across the two satisfied bands. The far field has a maximum along the normal direction to the radiating disk with good coverage across the half plane. The antenna shows a good front to back ratio of better than 20 dB.

COMPARISON WITH OTHER WORKS
The obtained results of the proposed Antenna-I and Antenna-II are compared with those of the antennas presented in [1][2][3][4][5][6][7][8][9][10][11], [14][15][16], [21]. The comparison takes into consideration the total substrate dimensions and the volume of the antenna, frequency range, bandwidth, gain, as shown in Table 4. The dielectric constant of the substrate is also listed to show fair comparison. At a given operating frequency, lower dielectric constant of the substrate leads to larger bandwidth and larger size. From Table 4, it can be noticed that the dimensions and volume of the proposed Antenna-I and Antenna-II are much smaller than those presented in [1][2][3][4][5][6][7][8][9][10][11][12][13][14], but slightly larger than that in [21]. The listed dimensions in [21] are those of the printed patch and the dimensions of the ground plane are not given in [21] but they are obviously larger. The bandwidth of the proposed antenna2 is much better than most of the listed antennas. The gain of Antenna-I and Antenna-II are competitive with those of the other antennas as regards to its small size.

CONCLUSION
In this paper, a proximity-fed microstrip circular patch antenna is proposed to achieve dual-band operation with a wider bandwidth. The antenna has been designed to operate in more than one resonating mode and the bandwidth enhancement is achieved by using a two-stub matching network on the microstrip feed line. Simulation results showed that the proposed antenna covers two bands of frequency. The frequency range of the first band is from 5.43 GHz to 6.02 GHz (10.3% relative bandwidth) with a minimum reflection coefficient of -20.45dB at 5.72GHz. The second band covers a wide frequency range from 8.4GHz to 13.3GHz (45.16% relative bandwidth). The double-stub matching has facilitated the utilization of two highermodes for bandwidth extension of the circular microstrip antenna that is known by its narrow bandwidth. The maximum achievable gain of Antenna-II in the first band is 4.62dB i while it is 4.85dB i in the second band. The antenna offers low profile dual-band, high gain and compact size which are favorable features for Wi-Fi and X-band broadband applications.