Broadband Monopolar Microstrip Patch Antenna With ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

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Broadband Monopolar Microstrip Patch Antenna With Shorting Vias and Coupled Ring Juhua Liu, Member, IEEE, Shaoyong Zheng, Yuanxin Li, Member, IEEE, and Yunliang Long, Senior Member, IEEE

Abstract—A center-fed circular patch antenna with shorting vias and a coupled annular ring is proposed. With a low-profile configuration, the antenna provides a wide bandwidth by merging three resonant modes, including the TM mode of the circular patch, the TM mode generated by the shorting vias, and the TM mode of the coupled annular ring. The antenna operating in these modes would produce an omnidirectional pattern in the horizontal plane similar to that generated by a monopole antenna. A reduced structure is used to simplify the simulation in optimizing the bandwidth of the antenna. Measured results show that the antenna achieves a bandwidth of 27.4% for a profile of 0.029 wavelengths and has a gain of about 6 dBi. Index Terms—Annular ring, circular patch antenna, microstrip antenna, monopole, shorting vias.

I. INTRODUCTION

M

ONOPOLE antennas are widely used for omnidirectional radiation in wireless communications. However, the monopole antenna has a profile of (where is the wavelength in free space), which is too high for some circumstances that need a low profile or conformal radiator. Microstrip antennas have received extensive research in recent decades because they have advantages of low profile, low cost, and can be fabricated on a printed circuit board (PCB). A center-fed circular patch antenna [1] would generate a monopole-like radiation pattern and has a much lower profile compared to the monopole antenna. Unfortunately, the center-fed microstrip circular patch antenna has a narrow bandwidth of 1.5% for a thin substrate with or 5% for a thick substrate of . A wire-patch antenna presented by Delaveaud et al. [2] has a small size and also generates a monopole-like radiation pattern, but it also has a demerit of a narrow bandwidth (only 1.8% for a profile of ). Several useful techniques [3]–[11] adopting air substrate were proposed to achieve a very wide bandwidth for a monopolar circular patch antenna. However, the thickness of the air substrate in the antennas [3]–[11] is mostly larger than , and the feeding structures are complicated. Manuscript received September 04, 2013; revised November 28, 2013; accepted December 16, 2013. Date of publication December 18, 2013; date of current version January 23, 2014. This work was supported by the Nature Science Foundation of China under Grant 61172026 and the Research Project of Guangdong under Grant 2012B091100050. The authors are with the Department of Electronics and Communication Engineering, Sun Yat-sen University, Guangzhou 510125, China, and also with the SYSU-CMU Shunde International Joint Research Institute, Shunde 528300, China (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2013.2295686

Fig. 1. Geometry of the monopolar microstrip circular patch antenna with shorting vias and a coupled annular ring.

In 2009, Al-Zoubi et al. [12] proposed a center-fed circular patch antenna with a coupled annular ring. The antenna has a simple structure and a very low profile and can be easily fabricated on a PCB. It achieves a bandwidth of 12% with a profile of . Later, a center-fed circular patch antenna with shorting vias [13] was proposed for bandwidth enhancement. The circular patch antenna with shorting vias achieves a bandwidth of 18% for a profile of . Dual-resonance behaviors are observed in the antennas [12], [13]. In this letter, we propose a monopolar circular patch antenna loaded with shorting vias and a coupled annular ring to achieve an even wider bandwidth by coupling three modes. The antenna has a very low profile and can be easily fabricated on a PCB. In the optimization of the bandwidth of the presented antenna, a half-sector structure is introduced to save the time in simulation. The effects of the height of the antenna on the bandwidth are discussed. A prototype is fabricated, and measured results show that the antenna achieves a bandwidth of 27.4% for a profile of and has a gain of about 6 dBi. II. GEOMETRIES A. Structure The geometry of the antenna is shown in Fig. 1. The circular patch is fabricated on a ground-plane backed substrate with a relative permittivity of and a thickness of . The ground plane . The circular patch has a radius of . The has a radius of patch antenna is fed at the center by a coaxial probe with a char. In order to generate another acteristic impedance of shorting vias are used to load the patch mode, a series of symmetrically around the -axis. Each sector that includes one . Each shorting via has shorting via has an angle of from the center a radius of and is placed at a distance of

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Fig. 2. Half-sector of the antenna in Fig. 1.

of the patch. To introduce a third mode, a parasitic annular ring with an inner radius of and an outer radius of is also employed to load the patch. B. Half-Sector Structure In order to generate a monopole-like radiation pattern that is omnidirectional in the horizontal plane, the field must be symmetric around the -axis, and the semi-planes (where ) must be perfect magnetic planes (it is assumed that one of the shorting vias is placed at the semi-plane, as shown in Fig. 2). Therefore, in order to save the time in simulation, only a half sector with an angle of shown in Fig. 2 needs to be concerned instead of simulating the full structure shown in Fig. 1. The simulation using the half-sector structure is different from the one used in [13]. The simulation in [13] uses “Eigenmode” to analyze the resonance modes, and therefore the feeding part is not included, while here the simulation uses “Driven Modal” to calculate the reflection coefficient and the feeding part must be employed. The feeding position for the half-sector structure is at the corner, as shown in Fig. 2. When using the half-sector structure in the simulation, care must be taken in calculating the input impedance for its corresponding times full structure. Since the current of the full structure is that of the half-sector structure, the input impedance of the full times that of the half-sector structure. structure is III. ANTENNA DESIGN Simulated result for the reflection coefficient for the proposed antenna is shown in solid line in Fig. 3, in which three resonant modes are observed. The mode relating to the lowest resonant frequency is the TM mode, which is generated by the shorting vias; the mode with the medium resonant frequency is the TM mode of the circular patch; and the mode with the highest resonant frequency among them relates to the TM mode of the annular ring. In order to merge the three modes to yield a wide bandwidth, the resonant frequencies of the three modes must be tuned to be rightly close, and the coupling strength between the circular patch and the parasitic annular ring must be appropriate. In the design of the presented antenna, we need first to find the dimension of the center circular patch [13] because the center frequency is mainly dominated by the patch, or more specifically, the TM mode of the circular patch. Then, to optimize the bandwidth, we need secondly to tune four parameters of the antenna: the number and position of the shorting vias whose effects on bandwidth can be found in [13], and the inner radius and outer radius of the annular ring whose effects on

Fig. 3. Refection coefficients for a circular patch antenna without shorting via or coupled ring (dotted line), a circular patch antenna with only shorting vias (dashed line), a circular patch antenna with only a coupled annular ring (dotdashed line), and a circular patch antenna with both shorting vias and a coupled annular-ring loading (solid line). The parameters of these patch antennas are given in Table I. The results are obtained from simulation using HFSS. TABLE I PARAMETERS FOR THE CIRCULAR PATCH ANTENNAS IN FIG. 3

bandwidth have been discussed in [12]. In the optimization, the half-sector structure shown in Fig. 2 can be used to save the simulation time. IV. COMPARISONS OF FOUR TYPES OF ANTENNAS When the parameters of the presented antenna are properly tuned, an optimized bandwidth can be obtained. As shown in solid line in Fig. 3, the simulated reflection coefficient for the proposed antenna is less than 10 dB in the band from 5.07 to 6.45 GHz, with a fractional bandwidth of 24%. As shown in dotted line in Fig. 3, the reflection coefficient for the antenna without shorting vias or coupled ring is high because its input impedance is not coincidently matched to the characteristic impedance of the coaxial probe (50 ). The 10-dB return-loss band for the circular patch antenna with shorting vias is from 5.25 to 6.1 GHz (as shown in dashed line in Fig. 3), with a fractional bandwidth of 15%. The 10-dB return-loss band for the circular patch antenna with a coupled annular ring is from 5.76 to 6.59 GHz (as shown in dot-dashed line in Fig. 3), with a fractional bandwidth of 13.4%. Hence, the bandwidth of the presented antenna is over 1/2 wider than that of the circular patch antenna with only shorting vias or that of the circular patch antenna with only a coupled annular ring.

LIU et al.: BROADBAND MONOPOLAR MICROSTRIP PATCH ANTENNA WITH SHORTING VIAS AND COUPLED RING

Fig. 4. Refection coefficients for the proposed antennas with different heights. The geometry is shown in Fig. 1, and the parameters are given in Table II. The results are obtained from simulation using HFSS.

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Fig. 5. Reflection coefficients for the fabricated antenna. The photograph is shown in the inset. The geometry of the antenna is shown in Fig. 1, and the dimensions are given in the last column of Table I.

TABLE II PARAMETERS FOR THE ANTENNAS WITH DIFFERENT HEIGHTS IN FIG. 4

V. EFFECTS OF THE THICKNESS OF THE SUBSTRATE When the height of the proposed antenna increases, its bandwidth would also increase in a certain degree. We simulate five mm mm antennas with their heights of mm. Optimized parameters for the five antennas with different heights are given in Table II, and simulated results for their reflection coefficients are shown in Fig. 4. It shows that the bandwidth would increase when the height increases from mm to mm. However, the bandwidth would not increase further along with the increase of the height when the mm because the inheight of the antenna is larger than ductance of the probe would increase and the impedance match of the antenna would be deteriorated. Fig. 4 shows that the bandmm is smaller than width of the antenna with a height of that of the antenna with a height of mm. The fractional bandwidths (BWs) of the antennas are given in the last row in Table II. It shows that this type of antenna could have a bandmm (about ). width of 50% with a profile of VI. EXPERIMENT A prototype of the presented antenna is fabricated, with a sub, a thickness strate which has a relative permittivity of of mm, and a tangent loss of . The profile of the antenna is very close to the one used in [12], which

Fig. 6. Elevation radiation patterns for the fabricated antenna at: (a) 5.25, (c) 5.75, and (e) 6.25 GHz. Azimuth radiation patterns at: (b) 5.25, (d) 5.75, and (f) 6.25 GHz.

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VII. CONCLUSION A center-fed circular patch antenna loaded with shorting vias and a coupled annular ring is presented. Triple-resonance behavior is observed in the antenna, which contributes a wide bandwidth for the antenna with a very low profile. Measured result shows that the antenna has a bandwidth of 27.4% with a profile of and produces a monopole-like radiation pattern with a gain of about 6 dBi. The bandwidth of the proposed antenna is over 1/2 wider than those of the antenna with only shorting vias and the antenna with only a coupled ring. An increase of the height of the antenna would yield a wider bandwidth in a certain degree. Simulated results show that this type of antenna could have a bandwidth of 50% when the height increases to . REFERENCES Fig. 7. Gains and radiation efficiency for the fabricated antenna.

is 1.57 mm. The geometry is shown in Fig. 1, and the dimensions are given in the last column of Table I. The reflection coefficients are shown in Fig. 5. Measured results show that the reflection coefficient is less than 10 dB in the band from 5.05 to 6.65 GHz. The fractional bandwidth is 27.4%. The profile is , with respect to its center frequency 5.85 GHz. Simulated results are reasonably close to the measured results, although a difference is observed. The elevation and azimuth radiation patterns for the antenna working at 5.25, 5.75, and 6.25 GHz are shown in Fig. 6. It shows that the antenna produces a conical radiation pattern in the elevation plane and an omnidirectional radiation pattern in the azimuth plane, similar to those generated by a monopole antenna. Theoretically, the cross polarization should be very low, as observed in the simulated results. Due to fabrication errors, the cross polarization of the fabricated antenna is about 15 dB below the copolarization. The back lobes (below the ground plane) are about 10 dB below the principal lobe. It is found from simulation that more than 88% of the radiated power is into the upper half-space in the band of interest. The maximum gains for the antenna are given in Fig. 7. Measured result shows that the antenna produces a gain of about 6 dBi. The measured gain is a little higher than the simulated one in the band from 5.5 to 5.75 GHz due to measurement error and a difference between the simulated and measured return losses. Realized radiation efficiency (which accounts the return loss) is also shown in Fig. 7. It shows that the antenna has a high radiation efficiency above 90% in the band of interest.

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