A Butterfly-Shaped Wideband Microstrip Patch Antenna for Wireless Communication

A novel butterfly-shaped patch antenna for wireless communication is introduced in this paper. The antenna is designed for wideband wireless communications and radio-frequency identification (RFID) systems. Two symmetrical quasi-circular arms and two symmetrical round holes are incorporated into the patch of a microstrip antenna to expand its bandwidth. The diameter and position of the circular slots are optimized to achieve a wide bandwidth. The validity of the design concept is demonstrated by means of a prototype having a bandwidth of about 40.1%. The return loss of the butterfly-shaped antenna is greater than 10 dB between 4.15 and 6.36GHz. The antenna can serve simultaneously most of the modern wireless communication standards.


Introduction
Microstrip patch antennas have found many applications in wireless communication systems because of their properties as low profile, light weight, low cost, and easy fabrication [1].These applications include UMTS (Universal Mobile Telephone System), synthetic aperture radar (SAR), and radiofrequency identification (RFID) systems [2][3][4].However, typical microstrip patch antennas present a narrow bandwidth.Various methods have been accomplished to improve the bandwidth of microstrip antennas.These methods include the adoption of thick substrates, the employment of parasitic elements, either in coplanar or stack configuration, the shaping of the radiant patch, or the inclusion of suitable slots.This last approach is particularly attractive because it can provide excellent bandwidth improvement maintaining a single-layer radiating structure to preserve the antenna's low characteristic profile.The successful examples include Eshaped patch antennas [5][6][7], U-slot patch antennas [8][9][10], V-slot patch antennas [11,12], and L-probe patch antennas [13][14][15].By using these methods, the bandwidth of these microstrip antennas could exceed 30%.
Recently, high-speed wireless computer networks have attracted the attention of researchers, especially in the 5-6 GHz band.This band can cover the frequencies of the high-speed wireless computer networks (e.g., IEEE 802.11a) [16] and the RFID UHF band in North America [17].Such networks have the ability to provide high-speed connectivity (>50 Mb/s) among notebook computers, PCs, personal organizers, and other wireless digital appliances.Although current 5 GHz wireless computer network systems operate in the 5. 15-5.35GHz band, future systems may make use of the 5.725-5.825GHz band in addition to the 5.15-5.35GHz band, for even faster data rates.Furthermore, an antenna with a wider operating band is more suitable to operate in complex environments, such as buildings, factories, hospitals, railway stations, and airports, where the field propagation is dominated by many scattering processes due to obstacles (furniture, walls, openings, etc.) [18].Therefore, further enhancement of the performance of microstrip antennas to cover the demanding bandwidth is necessary.
Hence, this work presents a novel single-patch wideband microstrip antenna with air gap for the requirement of        RFID systems (5.8 GHz) and high-speed wireless computer networks (5 GHz).The patch geometry is cut into a butterfly shape which offers satisfactory performances by only adjusting the diameter of the radiant arms and the position of the circular slots.In addition, the frequency band and the return loss can be adjusted by these parameters independently.The numerical simulations are performed using HFSS software.A comprehensive parametric study has been carried out to understand the effects that the different geometrical parameters have on the electromagnetic performance of the proposed antenna.Using the parametric study, a wideband   butterfly-shaped antenna with 40.1% bandwidth, having a return loss greater than 10 dB in the frequency band between 4.15 and 6.36 GHz, has been designed.These characteristics are very desirable for RFID systems and high-speed wireless computer networks.
The paper is organized as follows.Section 2 details the structure and the characteristics of the proposed butterflyshaped patch antenna.In Section 3, numerical and experimental results, which demonstrate the performance advantages of the proposed butterfly-shaped patch antenna, are presented.Finally, the conclusion is drawn in Section 4, which also summarizes the main contributions of this work.

Antenna Geometry
The geometry of the proposed patch antenna is shown in Figure 1.The antenna consists of a butterfly-shaped metallic patch suspended, by means of a 5.5 mm thick air gap (  = 1), over an 80 × 80 mm 2 ground plane.The patch is centered at the middle of the ground plane.The antenna is fed with a coaxial probe at position   from the geometric center of the microstrip line.The feed point is welded at the 50 Ω SMA coaxial connector.The butterfly-shaped patch consists of two symmetric small loops and of 3/4 loops linked together (see Figure 1).Moreover, the width  and the length  of the microstrip line exciting the antenna, the inner radii  in1 ,  in2 , and the outer radii  out1 ,  out2 of both metallic loops forming the radiating arms have to be optimized.An interconnecting line of length  and width , excited at a distance   from the origin, is employed to match the antenna input  to the coaxial feeding line, resulting in a return loss greater than 10 dB in a wide frequency band (over 5-6 GHz).The geometrical dimensions of the antenna (see Figure 1) are  = 2.35 mm,  = 21.79 mm,   = 3.85 mm,  in1 = 15.29 mm,  out1 = 20.22 mm,  in2 = 8.70 mm, and  out2 = 10.09mm.

Results and Discussion
In this section, a comprehensive parametric study and a comparison between the numerical and experimental results, including the antenna radiation patterns, are provided and discussed in detail.

Parametric Study.
For a better understanding of the electromagnetic behavior of the proposed butterfly-shaped patch antenna, a parametric analysis with respect to some antenna geometrical parameters has been performed.To do that, the High Frequency Structure Simulator (HFSS) software was employed.Figures 2-8 show the frequency behavior of the magnitude of the reflection coefficients as a function of the length , the width , the outer diameters  out1 and  out2 , the inner diameters  in1 and  in2 , and the feed position   , respectively.As shown in Figures 2, 3, and 4, the length , the width , and the feed position   can reasonably modify the impedance matching.Among the four parameters of the inner and outer diameters, namely,  in1 ,  in2 ,  out1 , and  out2 ,  in1 and  out2 play a vital role in determining the resonant frequency as well as the bandwidth of the antenna, as shown in Figures 5 and 8, while the parameters  out1 and  in2 can effectively adjust the return loss, as depicted in Figures 6 and 7.
Figure 9 depicts the surface current distribution excited on the butterfly-shaped antenna at the frequencies of 4.4 GHz, 5.5 GHz, and 6.2 GHz, respectively.In particular, from Figure 9(a), it appears that, at 4.4 GHz, the current distribution concentrates on the radiating arms close to the exciting microstrip line.This current behavior is due to the increment of the current path caused by the removal of the inner circles of the 3/4 loops forming the radiating arms while, at 5.5 GHz, a typical /2 resonant antenna current distribution is excited in the radiating arms (see Figure 9(b)).Finally, at 6.6 GHz all the metallic surfaces of the antenna are affected by the surface current as it appears from Figure 9(c).

Measurement Results.
The photograph of the antenna prototype is shown in Figure 10.The butterfly-shaped patch and the extended ground plane have been manufactured using conductive copper tape with 1 mm thickness.The simulated and measured reflection coefficients of the antenna are shown in Figure 11.The measurements are performed using an Agilent E5071C network analyzer.Considering a bandwidth of −10 dB, the experimental value of the relative bandwidth to the center frequency (5.5 GHz) is 40.1%, while the corresponding value obtained by numerical simulation is of about 41.7%.A good agreement between the simulated and experimental results is observed.It addition, it can be noticed that the antenna has sufficient bandwidth to cover the requirement of the RFID systems and the wireless computer networks.

Antenna Radiation Pattern.
The tridimensional radiation pattern of the proposed antenna, computed at 5.8 GHz, is shown in Figure 12.The antenna radiates nearly unidirectionally from the top side of the ground plane.Figure 13 shows the simulated antenna gain for frequencies across the operating bands.The range of the antenna gain is 4.0-8.6 dBi.
In particular, at 5.8 GHz the corresponding gain is 8. while at 5.0 GHz it is 4.4 dBi.Therefore, the antenna has a good performance in this frequency band.The measured and simulated radiation patterns in the -plane (-plane) and -plane (-plane) are shown in Figures 14 and 15.It can be seen that there is a good agreement between the measured and simulated results.The measured and simulated radiation patterns in these three frequencies within the bandwidth are approximately stable.

Conclusion
A wideband butterfly-shaped microstrip patch antenna for RFID systems and high-speed wireless computer networks has been presented.The return loss is greater than 10 dB from 4.15 to 6.36 GHz (40.1% bandwidth).The performance is more than that required to meet the demanding bandwidth specifications useful to cover the 5-6 GHz frequency band.At the same time, using an air gap, the antenna offers a wide band and presents limited dielectric losses.All these features are very useful for worldwide portability of wireless communication equipment.The comprehensive parametric study provides a good insight into the effects that the geometrical parameters have on the behavior of the proposed antenna.This analysis can provide the guidance on the design and optimization of the novel butterfly-shaped microstrip patch antenna, whose bandwidth can be easily tuned by
mm P x = 3.65 mm P x = 3.75 mm P x = 3.95 mm P x = 4.05 mm

Figure 10 :
Figure 10: Photograph of the proposed antenna.

Figure 11 :
Figure 11: Measured and simulated frequency behavior of the magnitude of the reflection coefficient.

Figure 14 :
Figure 14: Simulated and measured radiation patterns of the antenna at 4.4 GHz, 5.5 GHz, and 6.2 GHz in the -planes.

Figure 15 :
Figure 15: Simulated and measured radiation patterns of the antenna at 4.4 GHz, 5.5 GHz, and 6.2 GHz in the -planes.