A Novel Multiband Fractal Antenna for Wireless Application

0is paper proposes a novel multiband antenna using circle and triangle fractals for wireless application. By cutting a triangle slot in the circular monopole, a novel fractal method of the circular nested triangle structure is presented. 0e above structure is iterated four times, which forms the proposed fractal antenna. 0e antenna adopts the microstrip feeding method. In order to improve out band rejection and expand bandwidth, a ring resonator is designed on the back of the dielectric plate. 0e designed antenna covers 1.8GHz–2.9GHz applied to Bluetooth, TD-SCDMA, WCDMA, CDMA2000, and LTE33-41, 3.4 GHz–4.6GHz applied to LTE 42/43 and WiMAX, and 5GHz–5.6GHz applied to WLAN. 0e substrate is FR4 with a dielectric constant of 4.4 and a loss tangent of 0.02. 0e size of the fabricated antenna is 87.5× 61× 1.6mm. 0e measured pick gain achieves 2.98 dBi, 2.58 dBi, and 3.34 dBi at 2.6GHz, 3.8GHz, and 5.3GHz, respectively. 0e measurement and simulation results are in good agreement, which verifies the rationality of the design.


Introduction
With the advancement of wireless communication technology, multiband antenna has played a very crucial role in the wireless personal area network [1][2][3][4][5][6][7][8][9]. Mobile devices, such as hand-held computers and smart phones, are widely using wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX) [10][11][12][13][14]. e WLAN/WiMAX module, used to avail of these environments, is capable of operating at multiple frequency bands. e typical frequency bands required for a single antenna are therefore 2.4, 5.2, 5.4, and 5.8 GHz for WLAN applications, 3.3-3.8 GHz for WiMAX, and 2.4-2.484 GHz for Bluetooth applications [15][16][17][18]. e use of printed technology is the best choice since it allows to design low-cost planar antennas with high performances and compact size. Moreover, a printed antenna can be easily integrated into front-end circuits, providing low profile and a relatively simple fabrication [3,[19][20][21]. erefore, a number of printed antennas with different geometries have been experimentally characterized to enhance the bandwidth for wireless applications. In the work by Gautam et al. [22], a low-profile planar triple-band microstrip antenna was used for WLAN/WiMAX applications. e proposed antenna consists of F-shaped slot radiators and a defected ground plane. In the work by Augustin and Denidni [23], a multiband trapezoidal monopole antenna is proposed, which is fed by coplanar waveguide (CPW). Linear and circular polarizations are realized in a single substrate layer. In the work by Li and Mao [24], a novel Koch-like fractal curve is proposed. A small isosceles triangle is cut off from center of each side of the initial isosceles triangle; then, the procedure iterates along the sides like Koch curve does, forming the Koch-like fractal bowtie geometry. In the study by Liu et al. [25], an optically controlled aperture ultrawideband antenna is presented, which is able to generate reconfigurable multiband notches. Two independent narrow-band complementary split ring resonators are controlled by two optically controlled microwave switches. In the work by Kunwar et al. [26], inverted L-slot patch with a defected ground plane is used for tripleband operation. A frequency-agile triple-band microstrip antenna using the defected ground structure is presented in [27]. MTM-inspired reactive loading and metamaterial transmission lines are used in [28,29]. All the reported antennas exhibit multiband characteristic. However, bandwidths of these antennas are not wide enough to cover many applications, and the structures are complex. e proposed antenna in this paper occupies the wider bandwidth and has simpler geometry, while ensuring the gain and efficiency performance. e proposed antenna structure is simple. It is easy to br fabricated. Moreover, the antenna model is extensible. On the basis of the proposed structure, the antenna can be fractal more times inward for higher frequency and can be fractal outward for lower frequency. Performance comparison of the proposed antenna with other antennas is shown in Table 1.
In this paper, a multiband antenna using a novel circle and triangle structures fractals is proposed and designed for wireless communication systems that can support Bluetooth/WCDMA/CDMA2000/LTE3341WLAN/WiMAX applications. e proposed antenna consists of a main fractal radiator, a 50 Ω microstrip feed line, a ring resonator, and a partial ground plane on the back of the substrate. e proposed antenna shows three distinct resonances with impedance bandwidths of 1.8-2.9 GHz (1.1 GHz), 3.4-4.6 GHz (1.2 GHz), and 5.0-5.6 GHz (0.6 GHz), respectively. e antenna configuration has a compact size of 87.5 mm × 61 mm. e antenna design and studies are described in Section 2. Section 3 includes results and discussion, which are followed by the conclusion in Section 4.

Antenna Design and Analysis
Firstly, a triangular gap is nested in a circular radiation patch.
is basic geometry is called first-order fractal, as shown in Figure 1(a). e second step is to make an inscribed circle on the triangle gap and nest a triangular gap in the inscribed circle. e rest can be done in the same manner; the antenna radiator adopts four iterations of the square nested triangle slot fractal structure. e final configuration and size parameters of the proposed antenna are shown in Figure 2 and Table 2. In order to make the current flowing through the antenna surface longer, a certain distance d is kept between the circle and the top of the triangle.
ere is a certain proportion between the dimensions: By optimizing the antenna radiating slot, the current direction on the metal surface of the microstrip antenna can be changed to achieve multiband coverage. In order to achieve multiband, while improving out band rejection and expanding bandwidth, a resonator ring is designed on the back side of the dielectric board. e current will be coupled to the resonant ring when it flows through the radiator, resulting in disturbing of the current direction and generating multiple resonance frequency points. Figure 3 shows the reflection coefficient S 11 of the antenna with different iterations, respectively. For the first and second iteration, S 11 at the resonance point becomes smaller with the increase of iteration times. For the third iteration, reflection loss characteristics are not improved, but another resonance frequency appears at 4.6 GHz. S 11 becomes further deeper for the fourth fractal. e antenna property tends to be stable and the best matching status. Compared with the antenna with no resonant ring, the introduction of the resonant ring can improve the reflection coefficient characteristics, increase the resonance points, and effectively expand the bandwidth. e black solid line presents the simulated reflection coefficient S 11 of the final proposed antenna. e antenna covers three different frequency bands. At the center frequency, S 11 is −26 dB at 2.6 GHz, −30 dB at 3.8 GHz, and −15 dB at 5.3 GHz, respectively. e simulated impedance bands with S 11 ≤ −10 dB are 46.2% from 1.8 GHz to 3.2 GHz, 31.6% from 3.4 GHz to 4.6 GHz, and 15.1% from 4.9 GHz to 5.7 GHz. ese bands can be applied for 2G, 3G, 4G-LTE, Bluetooth, GPS, WLAN, COMPASS, GLONASS, and ISM2.4G.
As shown in Figure 4, current amplitude distribution of the antenna is presented, which illustrates the relationship between resonant frequencies and structure of the antenna. e red part represents strong current, and the blue and green parts represent weak current. Figures 4(a)-4(c) show the surface current at operation frequencies of 2.6 GHz, 3.8 GHz, and 5.3 GHz successively. In Figure 4(a), the current is mainly concentrated in the outer area of the radiator. e color of the middle area and the innermost area is mostly blue. erefore, it can be understood that the 2.6 GHz resonance occurs due to the first fractal. In Figure 4(b), except for the part red area in the outer, there is also a lot of red in the middle. It is observed that the resonance occurs due to second fractal. e third resonant mode of the antenna, by considering the current distribution of the antenna as shown in Figure 4(c), arises from the currents in the inner area owing to the third fractal. Based on the above analysis, the current flows from outside to inside with the frequency increasing. According to f � c/λ, it can be known that the small size of the antenna produces high radiation frequency. erefore, the resonance frequency generated by the inner ring is higher than the resonance frequency generated by the outer ring.
For the first resonance at 2.6 GHz, the maxima of the current occur in the outer ring. e length of the radiating patch responsible for first resonance can be calculated as At the resonance, the length would be half of the wavelength. L 1 should be λ/2. For the printed antenna, half of it is on the media plate and the other half is in the air. e calculated wavelength should be between the medium wavelength and the free space wavelength. We take the intermediate value as the estimation temporarily to verify the rationality of the fractal design. erefore, For the second resonance at 3.8 GHz, the maxima of the current occur in the middle ring. e length of the radiating patch responsible for second resonance can be calculated as 2 International Journal of Antennas and Propagation   International Journal of Antennas and Propagation For the third resonance at 5.3 GHz, the maxima of the current occur in the inner ring. e length of the radiating patch responsible for third resonance can be calculated as e 3D pattern of the designed antenna is shown in Figure 5, and the E/H plane polarization is shown in Figure 6. e blue solid line represents coplanar polarization, and the red dotted line represents cross polarization. It can be seen from the figure that the three-dimensional spatial radiation field at three resonance points has certain changes. With the increase of the resonance frequency, the radiation directivity of the antenna becomes stronger. e main lobe becomes smaller, weak side lobes appear, and directivity becomes complicated. e maximum gain can achieve 3.33 dBi, 3.82 dBi, and 5.77 dBi at central frequencies of 2.6 GHz, 3.8 GHz, and 5.3 GHz, respectively.

Results and Discussion
e proposed multiband antenna is manufactured on a 1.6 mm thick FR4 substrate with a dielectric constant of 4.4 and a loss tangent of 0.02. e copper with 0.03 mm thick is laid over the patch and ground plane. e prototype of the fabricated antenna is shown in Figure 7.
e Keysight Technologies FieldFox Handheld Microwave Analyzer N9917A is used to measure the reflection coefficient of the fabricated antenna. Figure 8 illustrates the comparison of simulated and measured results. e measured −10 dB bandwidth is 42.5% from 1.8 GHz to 2.9 GHz, 30% from 3.4 GHz to 4.6 GHz, and 5.3% from 5 GHz to 5.6 GHz, which is in good agreement with the simulation result. e commercial band coverage is shown in Table 3. Slight discrepancies between measurement and simulation results may be caused by fabrication precision, interface error, and test environment. e substrate thickness of the antenna is only 1.6 mm, and the influence of the dielectric thickness change during the processing on the performance of the antenna cannot be ignored. e connection method of SMA between the feeder and the signal is solder connection. e solder increases the thickness of the dielectric board, which has a certain impact on the test results. e error is within the allowable range. Figure 9 displays the measured 3D radiation patterns at central frequency of 2.5 GHz, 3.8 GHz, and 5.3 GHz, respectively. e pick gain can achieve 2.98 dBi, 2.58 dBi, and 3.34 dBi at frequencies of 2.5 GHz, 3.8 GHz, and 5.3 GHz, respectively. e measured radiation patterns are nearly omnidirectional. With the frequency increasing, the generation of higher order modes increases the side lobes.
Gain and efficiency variation with the frequency is shown in Figure 10. It is found that the gain varies from 1.9 to 4.1 dBi in I band, from 0.5 to 4.0 dBi in II band, and from 1.2 to 3.3 dBi in III band. e radiation efficiency of the antenna can achieve 92.5%, 75.5%, and 95.0% at central frequency of 2.5 GHz, 3.8 GHz, and 5.3 GHz. e test results are consistent with the simulation results basically. However, when testing the antenna pattern in the dark room, it is inevitable to connect the antenna to the test equipment with radio frequency cables and connectors. ese RF cables and connectors are very close to the antenna, and their radiation will affect the pattern test.

Conclusion
In this paper, an optimal microstrip-fed printed antenna with a novel circular nested triangle slotting multiband fractal is successfully proposed, simulated, and fabricated. e antenna consists of four parts, including a main fractal radiator, a 50 Ω microstrip feed line, a ring resonator, and a partial ground plane on the back of the substrate. e antenna is designed on a single-layer FR4 and covers the frequency of 1.8 GHz-2.9 GHz, 3.4 GHz-4.6 GHz, and 5 GHz-5.6 GHz. Experiments are carried out to validate the design concept and method, showing good agreement between simulations and measurements. At all frequency bands, the proposed antenna features stable radiation performance indicating that it can be a good candidate for Bluetooth, TD-SCDMA, WCDMA, CDMA2000, LTE33-41, WLAN, and WiMAX applications.

Data Availability
e data used to support the findings of this study are included within the article.   International Journal of Antennas and Propagation