A Low Proﬁle Dual-Band High Gain Directional Antenna for Anti-Interference WLAN Station Applications

: This paper presents a low-proﬁle dual-band antenna with directional radiation characteristics for wireless local area network (WLAN) applications. The proposed directional antenna is composed of a coupling microstrip line, two F-shaped strips, two rectangular strips, and a defected ground plane. The measured impedance bandwidth of the proposed antenna is 180 MHz (2.33–2.51 GHz) and 830 MHz (5.09–5.92 GHz), which can cover Institute of Electrical and Electronic Engineers (IEEE) 802.11 a/b/g frequency bands. The dual-band antenna exhibits a desirable directional radiation patterns in the vertical and horizontal planes with the peak gain of 6.55 dBi in the lower frequency band and 8.1 dBi in the higher frequency band. The measured antenna efﬁciency is 70% at 2.4 GHz and 84.5% at 5.5 GHz. The proposed dual-band WLAN station antenna is designed on a FR4 substrate with overall dimensions of 69 mm × 50 mm × 1.6 mm.


Introduction
Nowadays, almost all wireless local area network (WLAN) station antennas show omnidirectional radiation patterns. However, for some applications, antennas with high gain and directional radiation characteristics are required, such as Radio Frequency Identification (RFID) reader [1], microwave imaging systems [2] and public areas with numerous WLAN antennas from different mobile operators. Although the structure of the directional antennas is complicated, it has vital practical significance due to its reduced sensitivity to environmental interference.
In order to obtain the directional radiation pattern, there are many methods used in antenna design [3,4]. In [3], a patch is used as the last-order resonator of the network and the radiating element to realize a directional patch antenna. In [4], double reflecting floors are introduced in the dipole antenna to obtain stable radiation patterns and relatively high gains in the upper frequency band. For the needs of specific systems, some antennas have the characteristics of being low profile, compact and wideband, and have stably directional radiation [5][6][7]. A monopole antenna with a fork-shaped slot electromagnetic bandgap (EBG) structure as a reflector is proposed in [5]. The fork-shaped EBG eliminates the adverse effect of the metal sheet on the performance of the monopole antenna. The bended structure can be used to realize a low profile and compact directional antenna. In [6], the antenna is based on a modified Vivaldi antenna and characterized by a three-dimensionally bended structure, which performs a stable directional radiation pattern on wideband and is of compact size. In [7], four bent diamond-shaped half-loops are placed on the ground plane to form a directional antenna, and half-loops are fed by a feed network to realize a cardioid directional radiation pattern.
For directional antennas, high gain performance becomes an important consideration [8][9][10][11][12][13][14][15]. The classic Yagi antenna [8,9] is a good solution for designing high-gain directional antennas, but the disadvantages are large size and narrow bandwidth. As a narrowband antenna, it is time-consuming to design a dual-band Wi-Fi Yagi array. Quasi-Yagi antennas [10][11][12] can increase the impedance bandwidth, but the gain performance is not as good as that of the Yagi antenna. In general, it is a challenge to design a wideband and compact Yagi antenna. Slot antennas are often used as high-gain directional antennas. A multiband antenna with directional radiation can be obtained by loading slots on the radiator. Currently, slot antennas have been widely studied as directional antennas [13,14]. A directional slot antenna with a squared arch structure is presented to achieve impedance matching, high gain, and vertical polarization in dual band Wi-Fi unidirectional antenna design [13]. A slot in a ground plane with a complimentary microstrip stub was exploited to design a high-gain directional antenna with high radiation efficiency [14]. Based on the characteristics of the slot antenna, a slot-loop antenna, equivalent to a dipole array, is proposed, showing high-efficiency directional characteristics [15]. The size of the slot loop antenna is smaller than that of the Yagi antenna, and it is a suitable candidate as a high gain directional antenna.
In this paper, a dual-band antenna with directional radiation patterns is presented. Two symmetrical rectangular strips and F-shaped strips are introduced into the ground layer to form good impedance matching, thereby realizing a dual-band antenna. A large ground plane and a microstrip line are combined to form a high-gain directional antenna. The size of the dual-band antenna is 50 × 69 × 1.6 mm 3 with a gain of 6.55 dBi and 8.1 dBi at two resonant frequencies, respectively. The proposed dual-band antenna exhibits good directional radiation characteristics for wireless local area network (WLAN) applications.

Antenna Geometry and Design
The geometry of the proposed antenna is shown in Figure 1. The presented antenna consists of a coupling microstrip line, two F-shaped strips, two rectangular strips and a defected ground plane. The coupling microstrip line is printed on the top side of the substrate. The two F-shaped strips, two rectangular strips and the defected ground plane are printed on the bottom side of the substrate. The coupling microstrip line is fed by a coaxial SubMiniature version A (SMA) connector through the ground plane. The proposed antenna is fabricated on a 50 mm × 69 mm FR4 substrate with relative permittivity εr of 4.4, loss tan δ of 0.02, and thickness of 1.6 mm. The defected ground plane is connected to two symmetrical rectangular strips and two symmetrical F-shaped strips. A rectangular slit is etched in the middle of the ground plane to enhance the impedance matching and realize a high gain of the directional antenna. In addition, the coupling between the rectangular microstrip line and rectangular slit results in good directional radiation characteristics. In order to achieve the dual-band characteristics of the presented antenna, two symmetrical F-shaped strips are introduced into the ground plane near the upper part of the rectangular slit. A rectangular strip with a length of L 7 and a width of W 8 is added to the F-shaped structure to provide impedance matching at the desired frequency bands. The two rectangular strips and F-shaped strips are used to obtain good impedance matching in the lower and higher frequency bands, respectively.
A photograph of the manufactured antenna is shown in Figure 2. The design and optimization of the entire antenna are completed with the 3D full-90 wave high frequency structural simulator (HFSS). In order to further study the influence 91 of the ground plane on the proposed antenna, the current distribution of the antenna at 92 different frequency bands has been investigated. The simulated current distribution at 2.4 93 GHz and 5.5 GHz is shown in Figure 3. As shown in Figure 3(a), the surface currents are 94 concentrated on the edge of the rectangular strips at 2.4 GHz. The current mainly flows 95 along the two F-shaped strips at 5.5 GHz, as shown in Figure 3(b). Therefore, we can adjust 96 the dimension of the rectangular strips and the F-shaped strips to generate a lower and 97 higher resonant frequency, respectively. In Figure 3, it can be seen that most of the current 98 The design and optimization of the entire antenna are completed with the 3D full-wave high frequency structural simulator (HFSS). In order to further study the influence of the ground plane on the proposed antenna, the current distribution of the antenna at different frequency bands has been investigated. The simulated current distribution at 2.4 GHz and 5.5 GHz is shown in Figure 3. As shown in Figure 3a, the surface currents are concentrated on the edge of the rectangular strips at 2.4 GHz. The current mainly flows along the two F-shaped strips at 5.5 GHz, as shown in Figure 3b. Therefore, we can adjust the dimension of the rectangular strips and the F-shaped strips to generate a lower and higher resonant frequency, respectively. In Figure 3, it can be seen that most of the current is concentrated around the rectangular slit. The current in the vertical direction is in the opposite direction, which can cancel part of the radiation. The current in the horizontal direction is almost in the same direction, which can enhance part of the radiation. Therefore, the characteristics of directional radiation can be realized.
According to the results of the current distribution diagram, the antenna can be optimized by adjusting the dimension of the F-shaped strips and the rectangular strips. During the optimization process, one geometric parameter is adjusted, and the other parameters remain fixed. Figure 4a,b illustrate the reflection coefficients of the presented antenna with different values of L 7 and L 4 , respectively. As shown in Figure 4a, the value of L 7 mainly affects the lower frequency band. It can be seen that when L 7 is 18 mm, the proposed antenna operates at the desired lower frequency band. Figure 4b shows the effect of different values of L 4 on the higher frequency band. When L 4 is 6.5 mm, the desired higher frequency band is obtained. The final parameters of the proposed antenna are listed in Table 1.    Figure 5 shows the simulated and measured reflection coefficients (S 11 ) of the proposed antenna. It can be seen that the measured S 11 are in good agreement with simulation. From the measurement results, we can see that the bandwidth of −10 dB reflection coefficient is 180 MHz (2.33-2.51 GHz) at the lower frequency band and 830 MHz (5.09-5.92 GHz) at the higher frequency band. Therefore, the two operating frequency bands can cover IEEE 802.11 a/b/g (WLAN, 2.4-2.4835 GHz, 5.  GHz) frequency bands.  Figure 6 shows the normalized measured radiation patterns of the proposed antenna at 2.4 GHz and 5.5 GHz. The solid and dash lines represent co-polarization and cross-polarization, respectively. The proposed antenna exhibits unidirectional radiation performance, and no obvious sidelobe and split the beam in the main radiation direction at the desired two frequencies. The cross-polarization for both the E-plane (xoz plane) and the H-plane (yoz plane) are less than −20 dB. The antenna exhibits lower cross-polarization levels at the lower and the higher frequency bands. It can be seen from Figure 6 that the beamwidth of the E-plane is narrower than that of H-plane. The beamwidth of the E-plane is about 60 • , while the beamwidth of the H-plane is about 120 • . The narrower beamwidth in the E-plane can facilitate the separation of desired signals from interference. The results in Figure 6 indicate that the radiation patterns demonstrate the desired directional performance at 2.4 GHz and 5.5 GHz.

Results and Discussion
The measured gain and total efficiency are shown in Figure 7. The measured gain is 6.55 dBi at 2.4 GHz and 8.1 dBi at 5.5 GHz. The measured antenna efficiency is 70% and 84.5% at the lower and the higher frequency band, respectively. The measured gain and efficiency of the proposed directional antenna demonstrate an upward trend, with larger values at the higher frequency bands.

Conclusions
In this paper, a dual-band directional antenna has been designed and fabricated. Two symmetrical rectangular strips and two symmetrical F-shaped strips are introduced to obtain the lower and higher frequency bands, respectively. The printed antenna is excited by a coupling rectangular microstrip. Simulated and measured S 11 agree very well at the desired frequency bands. The directional antenna exhibits a stable directional radiation pattern with measured gain of 6.55 dBi at the lower frequency bands and 8.1 dBi at the higher frequency bands. The proposed dual-band directional antenna is a promising candidate for WLAN stations with characteristics of anti-interference, high radiation efficiency and high gain.