Hai-Yan Tian, Xiao-Lin Li, and Lin-Yan Xia

1.Introduction

The great success in the mobile communication industry has fostered the development of various wireless communication systems, which require complex antenna systems to achieve high quality performance.For this, the multiple antenna approach has received much attention in both antenna and wireless communications sectors.The mobile equipment, especially tablet PC and notebook computers, of which the position will not change in slightly long time, have put forward the demand to the radiation reconfigurable antenna.The antennas that can change the beam direction (beam-reconfigurable antenna) have been developed, for example, for direction finding of arriving signals or better communications toward specific directions[1]-[4].

Recently, pattern reconfigurable antennas have received much attention because of their interesting performance in reconfiguring radiation patterns with a single antenna.Traditionally, a phased array is used for this purpose.However, a phased array has large volume and weight,which limits its application in some cases.Recently, large number of methods can be found in literature for designing switch-beam antennas.For instance, diode and MEMS(micro-electromechanical systems) switches can be used to design antennas that can reconfigure their radiation pattern and/or polarization[5]-[9]; a conventional phased antenna array is a promising solution for beam switching and beam steering[10]-[12]; however, both array switch-beam and switches antennas employ a power distribution network and phase shifters, which increase the size, price, and complexity of the design.Thus their applications are limited.

In this design, an active square structure is used around double dipoles, which is used as an excitation source.The square structure is divided into four sectors using metallic sheets in all around of the structure.As a result, the direction of the radiation pattern of the antenna can be switched in different angles.By switching the PIN (positive-intrinsic negative) diodes of each sector, the main-beam with 90°beamwidth switches toward the desired direction with ability of steering over the entire 360°azimuth plane in four steps.In this design, we use a short active square structure to decrease the number of active elements and ease the maintenance of the antenna.Enhancing of directivity is achieved by increasing the height of metallic planes.The proposed antenna can be used in the base station of wireless communication systems.

2.Designing of Antenna and Geometry

To design a switch-beam antenna based on a square structure, a unit cell must be designed to have reconfigurable transmission and reflection coefficients.To do so, the proposed unit cell, presented in Fig.1 (b) and (c),is composed of a metallic discontinuous strip with PIN diodes in its discontinuity.

The square structure consists of array of discontinuous strips with PIN diodes placed in their discontinuities.In each step, diodes in one sector are on and others are off.The sector with off-state diodes has a high transmission coefficient and almost transparent for incident EM(electromagnetic) waves radiated from dipoles and the other sectors with on-state PIN diodes provide a high reflection coefficient; therefore, the antenna directs the main beam back the selected on-state sector.Indeed, PIN-diodes enable us to reconfigure the transmission and reflection coefficients of the periodic structure[13].In this design, the dipoles are chosen to be excited by the PIN-diodes 1-4, and the inverter,which can provide a high reflection coefficient, is chosen by the PIN-diodes 5 to 8.

The final dimensions of the antenna, which is achieved by using comprehensive parametric studies on the directivity,matching, and 3 dB beamwidth of the antenna, are listed in Table 1.The states of the diodes are listed in Table 2.

3.Antenna Operation Mechanism

3.1 One of the Cases

Case A of the plane dipole structure is adopted for the proposed antenna, and Fig.2 shows the configuration and dimensions of the antenna.The antenna is etched on a square grounded dielectric substrate with loss tangent of 0.02, relative permittivity of 4.4, and thickness of 1 mm (h).

Fig.1.Dimension of a proposed antenna: (a) ensemble, (b) center,and (c) external.

Table 1: Antenna dimensions (millimeters)

Table 2: Pin diodes states with different cases

Fig.2.Geometry of reconfigurable antenna.

Fig.3.Electrical field distribution of radiation mode: (a) t=0 and(b) t=T/2.

Fig.4.Radiation patterns of the antenna in Case A.

In Case A, the X-direction magnetoelectric radiation pieces are excited in phase, and an approximately horizontal constant current loop is produced.This can generate a rotationally asymmetric conical beam.Fig.3 (a) and (b)illustrate the current distribution of the antenna at time t=0 and t=T/2, respectively.It is found that an electric dipoles mode is strongly excited when the electric field intensity on the surfaces of the dextral horizontal plates of the dextral dipoles is very high.The excitation of a dextral electric dipole mode provides wideband impedance matching and a stable radiation pattern across the operating band.

The radiation patterns of the antenna when the operating frequency is 5.2 GHz in Case A is shown in Fig.4.The antenna has directive radiation in the E-plane.The 3 dB beamwidth in the E-plane is about 90°and in the H-plane is about 120°.

There are remarkable discrepancies between the simulated and the measured XOY/YOZ pattern results,especially in the main-beams.The real controlling systems for the diodes both in the feeding regions and in the parasitical element regions (namely, the systems which control “on” or “off” states of the diodes) might have some effects on the performance of the antenna.The solder is the joints where the diode meets the copper sheet.Diodes and solders do, however, greatly affect the implementation of those radiation patterns of the antenna.Especially, the diode 5 and 7, in Case A, are placed closer with the end of the active radiation piece.Thus, there are remarkable discrepancies between the simulated and the measured in the main-beams.

The radiation patterns of the antenna at different frequencies are shown in Fig.5.As it is illustrated, the antenna radiation pattern is stable and almost the same from 5.0 GHz to 5.4 GHz.

Fig.5.Normalized radiation pattern of the antenna at different frequencies: (a) E-plane and (b) H-plane.

3.2 All Patterns of the Cases

Fig.6 and Fig.7 show the beam-switching property of the antenna at 5.2 GHz for four steps corresponding to excitation re-configurations presented in Case A.As it is illustrated, the antenna has a beamwidth of 90°and it can scan the entire 360°azimuth plane in four steps by changing the position of the excitation.

Fig.6.3-D radiation patterns of the antenna with different cases.

The S11of the antenna is shown in Fig.8.The proposed antenna has a matching bandwidth of 13.6% (S11<-10 dB).The radiation efficiency and gain of the antenna is shown in Fig.9.The antenna has a gain of 6.3 dBi and radiation efficiency of about 92% when the operating frequency is 5.2 GHz.

Fig.7.Radiation patterns of the antenna in the azimuth plane: (a)E-plane and (b) H-plane.

Fig.8.Return losses of antenna.

Fig.9.Radiation efficiency and gain of the antenna.

4.Conclusions

In this paper, a novel method for designing a beam-switching antenna based on an active square structure has been presented.The proposed antenna operates from 4.8 GHz to 5.5 GHz with a gain of 6.3 dBi and F/B of 13.2 dBi when the operating frequency is 5.2 GHz.The antenna shows beamwidth of 90°in the elevation and a beamwidth of 120°in the azimuth plane, and it has the ability of scanning the entire azimuth plane in four steps by switching diodes in on- and off-states.With these characteristics, the antenna could be used in the base station of the wireless communication systems.

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