Dual-band aperture coupled antenna with harmonic suppression capability

The paper presents an aperture-coupled dual-band linearly-polarized antenna with harmonic suppression capability, operating at frequency 2.45 GHz and 5.00 GHz. In purpose of improving the directivity of antenna at the operating frequency of 2.45 GHz and 5.00 GHz, a modified inverted π-shaped slot-etched patch on the lower layer of the stacked antenna is introduced alongside the 50 Ω feed line. The harmonic suppression capability is achieved by the introduction of U-slot and asymmetrical left-right-handed stub at the transmission feed line, suppressing unwanted harmonic signals from 6.00 GHz up to 10.00 GHz. The final design of the antenna has produced very good reflection coefficient of -18.87 dB at 2.45 GHz and -19.57 dB at 5.00 GHz with third and higher order harmonic suppression up to -4 dB.


Introduction
The rapid development in telecommunication particularly in the territory of wireless application has made gigantic requests for high performance antenna keeping in mind the end goal to fulfill superior ability without tempering the ecological difficulties to the completion. As indicated by the frequency spectrum as issued by the Malaysian Communications and Multimedia Commissions (MCMC) on December 2014, the point to multipoint spectrum for Wireless Local Area Network (WLAN) comprises of three resounding frequencies; 2.45 GHz (2400-2500 MHz), 5.25 GHz (5125-5325 MHz), and 5.8 GHz (5725-5875 MHz) [1]. The dualband frequency operating at 2.45 GHz and 5.00 GHz are chosen as it is the commonly used band in the modern Wi-Fi router or access points such as from the router Cisco Aironet 3700 series for indoor and Cisco Aironet 1520 series for outdoor deployment [2].
In radio frequency (RF) applications, there are existence of ambient RF signals that can be harvested or also commonly known as energy harvesting [3]. Electromagnetic interference (EMI) resulting from active integrated microstrip antenna systems is not something new in RF technology. The non-linearity characteristics of active components as illustrated in RF energy harvesting block diagrams contributes to the creation of harmonics [4,5]. In order to suppress or minimize the unwanted harmonic signals at higher order, it is crucial to implement the harmonic suppression filter into the system to avoid EMI radiation which easily produced by the non-linear circuit component of the system [6]. Unfortunately, the filtering technique may lead to the increment of the antenna size and disturbance to the matching network of the antenna and power loss at high frequencies [7]. Regardless the limitations due to the active components, there are many other techniques can be implemented to solve this problem [8] such as from the modification vias and stub loaded on the patch element [9], slot and symmetrical arm of stub at the transmission feed line [10], and stub-loaded resonators [11]. The past researcher in [12] also emphasize on the effect of the input impedance which at twice the desired frequency, the input impedance should be near to zero for the second harmonic not to be produced, and this is also been implemented in [13] with the quarter-wave section at the transmission feed line. Apart from technique implemented, there are various types of antenna structures that possess the  [14], dipole antenna [15], microstrip patch antenna [16], and dual-rhombic loop antenna [17]. Different design structures of these antennas are suited to be used for different applications however the ability of energy harvesting is only limited to single frequency ambient signal.
For the past few years, researchers have focus on the antenna design which possess the integrated harmonic suppression capability within an antenna. According to the findings by [18], the antenna overall physical size is reduced using the fractal design and with the introductory of H-shaped defected ground structure (DGS) to suppress the harmonics created from the fractal iteration, thus the complexity of the antenna geometry increased. The implementation of defected ground structure (DGS) in [19] caused the patch size to be reduced to 26.7x30 mm 2 , together with notch-loaded and curvature slots being introduced to suppress higher-order harmonics.
Different techniques implemented such as in [20][21][22][23][24][25], introduced the reflection coefficient of less than -3 dB of higher order harmonics, however the application of the antenna is limited to the functionality of single-band frequency application. The vision of green technology can be seen to be implied where most of the design are much more compact with the same structural physical antenna have been achieved in order to reduce development cost and sustain the environment. The comparison of past research study and the proposed antenna are tabulated in Table 1. Unbalanced slots and dumbbell shape defected ground structure (DGS) Rejection of second and third harmonic up to -2 dB reflection coefficient Proposed U-slot and asymmetrical stub embedded at the feed line Dual-band characteristics with harmonic signals suppressed from frequency range of 6.00 GHz to 10.00 GHz Based upon authors literature review and knowledge findings, the study by previous researchers focused more on either single-band frequency characteristic or, multi-band frequency of that 0.9 GHz and 2.45 GHz and very small amount of study which involved multiband frequency response of that WLAN frequency spectrum. For this paper, a dual-band aperture coupled antenna at frequency 2.45 GHz and 5.00 GHz with the ability of harmonic suppression of the third and higher order is presented. In addition, the proposed design demonstrated the significant findings of asymmetrical stub on the frequency harmonics, achieving dual-band frequency response without having to have temper down significantly the reflection coefficient response of the second harmonic.

Antenna Design
For this project, the antenna is required to be functioning for the WLAN application thus, the resonant frequency of 2.45 GHz and 5.00 GHz are chosen. As for the board, FR4 substrate is chosen and has the dielectric constant, ε r 4.4, loss tangent 0.019, dielectric height, h of 1.6 mm and double sided copper conductor with height, t of 0.035 mm. The dimensions of the antenna can be determined using the formula given in [18], [25].
Length of patch, L = L − 2∆ where; f = resonant frequency = speed of light; 3 10 8 ms Figure 1 shows the geometry and dimension of the proposed inverted π-shaped slot-etched aperture coupled antenna with the capability of dual-band operation, at which it is being excited with 50 Ω microstrip feed line. Figure 2 shows the proposed antenna design with the dimension of 80 mmx80 mmx5.07 mm. The detailed antenna dimensions are illustrated as in Figures 1 (a)-(e).  Figure 2 illustrates the initial geometry and dimension of the transmission feed line of inverted π-shaped slot-etched aperture-coupled antenna with the capability of multi-band operation where it is also being excited using 50 Ω microstrip feed line. Figure 3 shows the proposed feed line design with the harmonic suppression technique as implemented in [16], [21]. The proposed harmonic suppression technique are illustrated as in Figure 3

Results and Analysis
The reflection coefficient parameter of different transmission feed line topology as illustrated as in Figure 2 and Figures 3 (a)-(d) are highlighted in Figure 4. The graphical illustration highlights the differences on the resonant frequency and the reflection coefficient responses. Based on the observation, the initial design of the transmission line as illustrated in Figure 2, produces three resonant frequencies of 2.45 GHz, 5.00 GHz and 9 GHz with reflection coefficient of -30.096 dB, -21.143 dB, and -26.253 dB respectively. The proposed embedded U-slot at the feed line is observed to suppress the reflection coefficient at frequency 5.00 GHz, initially from -21.143 dB to -14.537 dB however with the proposed design, the fourth harmonic is being introduced at 9.45 GHz with the reflection coefficient of -43.439 dB. Since the introductory of U-slot is not sufficient to suppress the harmonics of higher than 5.00 GHz, stub is introduced to the feed line. The combination of U-slot and right-handed stub at the feed line is observed to suppress the higher harmonics of 5.00 GHz, while enhancing the reflection coefficient of the first two harmonics at 2.45 GHz and 5.00 GHz, producing reflection coefficient of -25.953 dB and -25.414 dB respectively. This design still produces third harmonic with reflection coefficient of smaller than -5 dB. Hence, the U-slot with symmetrical left-right-handed stub is analyzed and the reflection coefficient at 2.45 GHz is -18.889 dB and at 5.00 GHz is -19.416 dB. The proposed symmetrical stub is observed to enhance the frequency response of the second resonant frequency however, also enhancing the reflection coefficient at 7.3 GHz from -4.886 dB to -8.587 dB. The current distribution of the feed line is analyzed and from the concentrated electrical length, the feed line with U-slot and asymmetrical left-right-handed stub is proposed. The proposed design is observed to suppress the higher order harmonics generated at 7.3 GHz, 8.69 GHz, 9.00 GHz and 9.45 GHz. The outcome from the asymmetrical stub design has suppressed the unwanted third and higher order harmonics of reflection coefficient greater than -5 dB, ranging from the 6.00 GHz up to 10.00 GHz.The proposed asymmetrical stub design has produced dual-band resonant frequencies of almost similar reflection coefficient response at 2.45 GHz and 5.00 GHz whilst suppressing the third and higher order harmonics unlike the design proposed in [18][19][20], at which only focused on the operation single band frequency.  Figure 6 (a) and 6 (b), the intensity of the current distribution is larger than the intensity of that distribution at 7.3 GHz and 9 GHz. Figure 6 (c) and 6 (d) show that the current distribution is blocked from flowing into the main radiating slot at frequency 7.3 GHz and 9 GHz with the addition of U-slot and asymmetrical left-right-handed stub, completely suppressing the third and fourth harmonics response. These findings are also found in [26] which had revealed that the amount of current distributes into conductive element can be affected by the surface wave acting on it.

Parametric Analysis on Parameter Lsu2
Optimization of the antenna structure is crucial in order to obtain better outcomes in terms of reflection coefficient, bandwidth and even the directivity of antenna. This optimization process can be done by conducting the parametric analysis on certain parameter of the antenna, and for this project mainly focusing on the position of stub towards the frequency response and reflection coefficient. Figure 7 is the outcomes of the parametric analysis on position of stub, Lsu2. Based on the graph in Figure 7, there is slightly frequency shifting can be observed for both frequency bands but much more visible on the second frequency band, where the resonant frequency seems to be shifting more to higher frequency as the length increases by the increment of 1 mm. For this parameter, the value Lsu2=13 mm is chosen as the optimized value. The effect of the position of stub Lsu2 can be observed on the suppression of the third harmonic where the reflection coefficient is greater than -5 dB starting from 6.00 GHz to 10.00 GHz when Lsu2 is 13 mm. The value of Lsu2 at 14 mm is not the optimum value although the frequency response is almost similar to when it is 13 mm, and this is because when the Lsu2 is 14 mm, the reflection coefficient at 10.00 GHz can be observed to be greater than -5 dB and might as well produce the third harmonic at frequency of higher than 10.00 GHz. The reflection coefficient of the antenna from varying the parameter Lsu2 are tabulated in Table 2.

Conclusion
A dual-band antenna as proposed in this paper has been designed and the desired resonant frequencies that applicable for WLAN application are achieved. The antenna design focuses on the effect of the slot and stub towards the current distribution at the transmission feed line for the purpose of harmonic suppression. The introduction of U-slot at the transmission feed line is for suppressing the third harmonic at frequency 9.00 GHz instead, another harmonic is generated at 8.69 GHz and the reflection coefficient response at 5.00 GHz also deteriorates from -21.143 dB to -14.537 dB. Together with the slot, a right-handed stub is introduced and the third and fourth harmonics are observed to be suppressed up to the reflection coefficient greater than -10 dB, however that value still allows current eventhough in small amount, to flow into the main radiating elements. In the same manner, left-right-handed stub design is further analyzed in comparison of symmetrical and asymmetrical structure of the stub on the antenna reflection coefficient, comparatively suppressing the third and higher order harmonics with the asymmetrical stub design. In essence, the asymmetrical stub design diverts the current distribution towards the stub instead of propagating into the main radiator patch thus creates more harmonic. The proposed U-slot with asymmetrical left-right-handed stub showed that harmonic suppression of third and higher order can be suppressed without having the first and second harmonics to be affected by it, where the reflection coefficient at 2.45 GHz and 5.00 GHz is -18.87 dB and -19.57 dB respectively, meanwhile at 7.3 GHz is less than -5 dB. The harmonic suppression is ranging from frequency 6.00 GHz up to 10.00 GHz and therefore is suitable for antenna with dual-band energy harvesting capability especially for the ambient RF energy harvesting from the WLAN applications.