A Simple Ultra-Wideband Magneto-Electric Dipole Antenna With High Gain

Frequenz 2018; 72(1-2): 27 32 Chen-yang Shuai and Guang-ming Wang* A Simple Ultra-Wideband Magneto-Electric Dipole Antenna With High Gain DOI 10.1515/freq-2016-0321 Received vember 2, 2016 Abstract: A simple ultra-wideband magneto-electric dipole antenna utilizing a differential-fed structure is designed. The antenna mainly comprises three parts, including a novel circular horned reflector, two vertical semicircular shorted patches as a magnetic dipole, and a horizontal U-shaped semicircular electric dipole. A differential feeding structure working as a perfect balun excites the designed antenna. The results of simulation have a good match with the ones of measurement. Results indicate that the designed antenna achieves a wide frequency bandwidth of 107 % which is 3.19~10.61 GHz, when VSWR is below 2. Via introducing the circular horned reflector, the designed antenna attains a steady and high gain of 12 ± 1.5dBi. Moreover, settled broadside direction main beam, high front-to-back ratio, low cross polarization, and the symmetrical and relatively stable radiation patterns in the E-and H-plane are gotten in the impedance bandwidth range. In the practical applications, the proposed antenna that is dc grounded and has a simple structure satisfies the requirement of many outdoor antennas. Keywords: differential-fed, ultra-wideband, magnetoelectric dipole, circular horned reflector 1 Introduction Recently, ultra-wideband (UWB) radio communication technology has become a hot research field because of its many advantages, such as insensitivity to the channel fading, low cost and so on. After the Federal Communications Commission distributed the frequency band from 3.1 GHz to 10.6 GHz for civil use in 2002 [1], many applications have been proposed based on UWB including radar location, imaging system, intelligent transportation system, and medical treatment. In general, UWB antennas can be *Corresponding author: Guang-ming Wang, Air-Defense and Anti- Missile College, Air force Engineering University, Xi an 710051, China, E-mail: wgming01@sina.com Chen-yang Shuai, Air-Defense and Anti-Missile College, Air force Engineering University, Xi an 710051, China, E-mail: 13980377569@163.com classified into two types, omni-directional antennas and unidirectional antennas, according to their radiation patterns. These antennas can be utilized for impulse radiation or work as multi-band antennas [2]. The radiation patterns of them is mainly the conical radiation patterns [3], [4], so they can not satisfy some significant demands in many applications. On the other hand, the unidirectional antennas are more popular. However, many disadvantages among most of the unidirectional antennas including unstable gain, not steady radiation patterns, and their large size limit the applications in practice [5] [7]. On the other hand, in recent years, there is an increasing concentration on the magneto-electric (ME) dipole antennas. Luk and Wong proposed the ME dipole antenna in [8], [9]. The antenna contains two parts, vertical quarterwave shorted patches as a magnetic dipole and a horizontal patch as an electric dipole. This type of antenna have lots of advantages such as a simple structure, low back lobe, wide bandwidth, low cross-polarization, and steady and same radiation patterns of E- and H- plane. What s more, its gain is steady in the operating frequency bandwidth. Because of these advantages, ME dipole antennas are widely used in lots of wireless communication systems. Based on the simple magneto-electric dipole antenna proposed by Luk [8], many novel UWB antennas were also designed. In [10], [11], the proposed UWB magneto-electric dipole antennas realize better performances than conventional UWB antennas. They have both the mentioned advantages of ME dipole antenna and UWB antenna. But their drawbacks are also obvious. Due to dielectric loading and intricate structure, the cost of manufacturing and fabrication are higher. So this reason limits the applications for UWB. In order to solve this problem [12], proposed novel UWB ME dipole antenna without dielectric loading. Though the antenna attains high gain, as a cost, it loses the stability of gain and radiation patterns. In this paper, a novel and simple ultra-wideband magneto-electric dipole antenna with high gain is designed. A semicircular radiation structure and a differential feeding structure are utilized for achieving the ultra-wide impedance bandwidth. In order to improving the low frequency performance of the designed antenna, a U-shaped tapered structure is used. In addition, for attaining the steady and high gain and symmetrical

28 C.-Y. Shuai and G.-M. Wang: Ultra-Wideband Magneto-Electric Dipole Antenna With High Gain radiation patterns, a circular horned reflector is employed. The designed antenna with a simple structure realizes a wide impedance bandwidth, a steady and high gain and other great electrical characteristics. The simulation and measurement have a good match. 2 Antenna principle As is known to us, the magneto-electric dipole is composed of the electric dipole and the magnetic dipole. The combination of the two kinds of dipoles can be explained through the Figure 1. From the Figure 1, it can be clearly seen that the radiation patterns of the ME dipole is a compound of the patterns of the electric dipole and the magnetic dipole. Due to the complementation of the two dipoles, the forward radiation of the ME dipole is strengthened, whereas the back radiation is offset. Therefore, a heart-shaped radiation pattern is achieved and the ME dipole realizes low back lobe level. On the other hand, the equivalent circuit of the ME dipole is shown in Figure 2. Figure 1: Synthesis of the patterns of the ME dipole. Figure 2: Equivalent circuit of the ME dipole. According to the circuit, the following formula can be gotten. 1 ImðY in Þ =Im R m + jwc m + j 1 wl m + R e + jwl e + j wce ðwc m 1 wl m Þ wl e 1 wc e In this formula, Y in is input admittance. R m,c m, and L m represent resistance, and capacitance, and inductance of the magnetic dipole. R e, C e, and L e are resistance, capacitance, and inductance of the electric dipole. If Im (Y in )=0, L m C m = L e C e and R 2 e = C e /L m. In other words, the electric dipole and magnetic dipole achieve resonance oscillation at the same frequency. So the superposition of the resonance frequency band widens the impedance bandwidth. This explains the reason why ME dipole antenna has a wide impedance bandwidth. 3 Antenna description and design geometry The proposed UWB antenna s geometrical shape is depicted in Figure 3. All the final dimensions of the antenna were decided by a parametric optimization in the Ansoft HFSS 13.0. The designed antenna shown in Figure 3(a) consists of a differential feeding structure, a novel ME dipole and a circular horned reflector without dielectric loading. In Figure 3(b), we can see that two small identical semi-ellipses are introduced and they are together with the semicirculars to compose U-shaped radiation structure, on the purpose of improving the low frequency performance. In fact, the pair of U-shaped patches is equivalent to an electric dipole. And the U-shaped patches are shorted to the ground plane via two vertically-oriented semicircular patches. And as a matter of fact, the two vertical semicircular patches work as a magnetic dipole. The differential feeding structure shown in the Figure 3 (c) has two sections including two parallel vertical metal patches and a horizontal rectangular patch. We can analyze the structure from the standpoint of the equivalent model. The two vertical parallel patches play a role of a balanced transmission line, which take charge of propagating the differential signal to the horizontal portion of the feeding structure. The horizontal plate works for coupling the differential signal to the U-shaped radiation structure, so it can tune impedance matching by changing the size. The two SMA ports which are shown in the Figure 3(d) are fabricated under the ground plane as the differential input ports to excite the antenna. 1 R 2 e

C.-Y. Shuai and G.-M. Wang: Ultra-Wideband Magneto-Electric Dipole Antenna With High Gain (a) 3-D view 29 (b) Radiation structure (c) Differential-fed structure (d) Reflector Figure 3: Geometry of the proposed antenna (a) 3-D view (b) Radiation structure (c) Differential-fed structure (d) Reflector. Figure 3(d) shows the geometry of the circular horned reflector. Compared with the conventional planar reflector and box-shaped reflector, the circular horned reflector has a greater effect on improving the gain of the antenna and radiation patterns. All the detailed dimensions of the proposed antenna is indicated in the Table 1. Table 1: The proposed antenna s dimensions. Parameters w d p l r Values/mm. (. λ) g (. λ). (. λ) h. (. λ). (. λ) D (. λ). (. λ) D (. λ). (. λ) H (. λ) Parameters Values/mm te: λ refers to the beginning frequency of the proposed antenna. 4 Antenna performance and analysis A photograph of the proposed antenna is shown in the Figure 4. And the prototype was fabricated and tested. The simulation results of the gain, radiation patterns and VSWR were attained by using Ansoft HFSS 13.0. And the measurement results of the antenna gain, radiation Figure 4: Photograph of the proposed antenna. patterns and VSWR were gotten via Agilent Vector Network Analyzers and microwave anechoic chamber. In order to depict the operating principle of the proposed antenna, we simulated the surface current distribution at 7 GHz shown in the Figure 5. T represents the time period of the antenna. When time t = 0, the current mainly are converged on the surface of U-shaped horizontal radiation patches, whereas the current on the vertically shorted patches is small. In fact, at this time, the electric dipole is functioning. At t = T/4, the current on the vertically shorted patches reaches the maximum, whereas the current on the horizontal patches is negligible. Therefore, the magnetic

30 C.-Y. Shuai and G.-M. Wang: Ultra-Wideband Magneto-Electric Dipole Antenna With High Gain Figure 5: Current distribution for the proposed antenna at 7 GHz, (a) t = 0, (b) t = T/4, (c) t = T/2, (d) t = 3T/4. dipole is excited at this time. When t = T/2 and 3T/4, the current distribution is similar with t = 0 and t = T/4, but the direction of the current is inverse. Hence, the whole process completes one cycle. To better understand the effect of the reflector on gain and VSWR, a simple comparison was performed by using HFSS 13.0. Figure 6 shows the effects on the simulated gain and VSWR of the designed antenna with a circular horned reflector and a planar plane reflector. We can obviously see that the antenna with a circular horned reflector attains higher and more stable gain of 12 ± 1.5dBi, and the gain with a planar reflector is from 3.6dBi to 9.2dBi. Moreover, the antenna with a circular horned reflector covers lower frequency band. Figure 7 depicts the gains and VSWRs of simulation and measurement of the proposed antenna with the variation of the operating frequency. It is seen obviously that the impedance bandwidth (VSWR 2) of the simulation reaches 107 % from 3.19 GHz to 10.61 GHz, whereas the Figure 6: The VSWRs and gains of simulation with a circular horn and a plane. Figure 7: The VSWRs and gains of simulation and measurement.. impedance band-width of the measurement is 106 % (VSWR 2) from 3.29 GHz to 10.70 GHz. The results of simulation and measurement show a good agreement. And both of them achieve the UWB. On the other hand, the gain of simulation attained is about 12dBi with variation of 1.5dBi in operating frequency band and the measured results gotten are from 10.5 to 14.4dBi. The antenna realizes relatively stable and higher gain compared with most of the UWB ME dipole antennas depicted in Table 2. However, there exist some little discrepancies between simulated and measured gain, mainly due to measurement error, imperfect soldering, and fabrication tolerance. Table 2: Comparison. Paper [ ] [ ] [ ] [ ] [ ] [ ] [ ] proposed Bandwidth (GHz) Gain (dbi). ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~.. ~. Dielectric loading Yes Yes Yes The radiation patterns of the simulation and measurement at 4 GHz, 7 GHz, and 10 GHz are shown in Figure 8 (a) 8(c). We can see that the good match is obtained. Within the whole operating frequency band, the symmetrical and relatively stable radiation patterns of E- and Hplane is attained and the main beams are mainly fixed to the broadside direction. Moreover, the cross polarization levels of simulation are below 43 db. However, due to some cross polarization levels in E- and H-planes that are lower than 50 db, we can t see them in many pictures.

C.-Y. Shuai and G.-M. Wang: Ultra-Wideband Magneto-Electric Dipole Antenna With High Gain 31 the measured results. According to the simulated results, the proposed antenna achieves impedance bandwidth of 107 %, from 3.19 GHz to 10.61 GHz. By adding a circular horned reflector, the steady and high gain of 12 ± 1.5dBi is realized in the operating frequency range in comparison with most of the UWB ME dipole antennas. What s more, the low cross polarization, high front-to-back ratio, and the symmetrical and relatively stable radiation patterns of E-and H-plane are also obtained. Simultaneously, the antenna without dielectric loading is easily manufactured and fabricated for relatively low cost. With above these advantages, the antenna is well suitable for the UWB applications. On the other hand, the antenna is dc grounded, so it also satisfies the requirement of many outdoor antennas. Funding: The research was supported by grants from the National Natural Science Foundation of China (.61372034). References Figure 8: The patterns of simulation and measurement at (a) 4 GHz, (b) 7 GHz, and (c) 10 GHz. Whereas the cross polarization of the measurement is less than 27 db. And the antenna obtains stable 3 db beamwidth of E-plane and high front-to-back ratio over the operating frequency band. However, with the frequency increasing, the 3 db beamwidth of the H-plane decreases rapidly and the side lobe starts to appear mainly from the effect of the high order mode. 5 Conclusion A novel and simple ultra-wideband magneto-electric dipole antenna with a circular horned reflector and a differential feeding structure has been designed, made, and measured. The results of simulation agree well with [1] H. Schantz, The Art and Science of Ultrawideband Antennas. rwood, MA: Artech House, 2005. [2] G. Adamiuk, T. Zwick, and W. Wiesbeck, UWB antennas for communication systems, Proc. IEEE, vol. 100, pp. 2308 2321, 2012. [3] A. Mehdipour, K. Mohammadpour-Aghdam, R. Faraji-Dana, and M.-R. Kashani-Khatib, A novel coplanar waveguide-fed slot antenna for ultrawideband applications, IEEE Trans. Antennas Propag., vol. 56, pp. 3857 3862, 2008. [4] R. Eshtiaghi, J. urinia, and C. Ghobadi, Electromagnetically coupled band-notched elliptical monopole antenna for UWB applications, IEEE Trans. Antennas Propag., vol. 58, pp. 1397 1402, 2010. [5] B. Wu and K. M. Luk, A UWB unidirectional antenna with dualpo-larization, IEEE Trans. Antennas Propag., vol. 59, pp. 4033 4040, 2011. [6] S. W. Qu, C. H. Chan, and Q. Xue, Ultrawideband composite cavity backed folded sectorial bowtie antenna with stable pattern and high gain, IEEE Trans. Antennas Propag., vol. 57, pp. 2478 2483, 2009. [7] Q. Wu, R. Jin, J. Geng, and D. Su, On the performance of printed dipole antenna with novel composite corrugatedreflectors for low-pro-file ultrawideband applications, IEEE Trans. Antennas Propag., vol. 58, pp. 3839 3846, 2010. [8] K. M. Luk and H. Wong, A new wideband unidirectional antenna elemen, Int. J. Microw. Opt. Technol., vol. 1, pp. 35 442, 2006. [9] K.-M. Luk and H. Wong, Complementary wideband antenna, US Patent7,843,389, vember 30 2010.

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