Nowadays wireless communications have been developed for entertainment, education, economic, health and industry. Mobile technology is the most popular wireless communication and has progressed rapidly from 1 G to 4 G. The development of antennas with new performances becomes currently imperatively essential for the new services and network of telecommunication. Therefore, the purpose of this paper is to design a high gain antenna for a mobile base station; also, the antenna requirements are as follows [1] :

・ Sector antenna gain: 13 - 16 dB;

・ Sector antenna half power beamwidth: 60 - 65 degree or 90 degree;

・ Impedance: 50 Ohm.

To achieve requirements, there is an interest in the dipole antenna in wireless communication systems because of its broadband characteristics, elementary structure and simple concept [2] -[4] . Presently, the dipole antenna extensively used in practical is S-shaped dipole antenna [5] . Moreover, the dipole is bent into a V-shape to achieve the maximum gain when it is properly aligned with the curvature of the earth [6] . Then, V-shape is compared with the upside bell dipole antenna, and it seems that the upside bell dipole can reduce the side lobe of the dipole [7] [8] . However, the dipole antenna has low gain and its radiation pattern is an omni-direction. This argues that, if we can design the antenna to increase a predefined directive gain, it will have more efficiency for field radiating.

Many new technologies have appeared in the current antenna design and one exciting discovery is the development of Electromagnetic Band Gap (EBG) structures [9] . The EBG structure applications in the antenna designs have become a thrilling topic for antenna engineering. It’s a matter of the EBG structure technology, a new technology for the improvement of the antenna performances applicable for frequency spectrum of which extremely wide covered from the acoustic until the optical frequencies. Besides, EBG structures known as photonic crystals are also used to improve the performance of the antenna. The EBG can be applied to grating, frequency selective surface (FSS) and so on. Moreover, the EBG is not only used to a reflector plane [10] -[12] , but also adapted for a superstrate of the primary radiator with reflector plane [13] -[16] . When EBG is used to superstrate on excitation feed with reflector plane, the advantage of EBG resonator antenna is the gain enhancement and can be increased the radiation efficiency [17] [18] . The other important feature of the primary radiator with EBG is its ability to generate linearly-polarized waves to radiate a circularly polarized (CP) wave required in many communication environments, such as the dipole antenna placed on EBG reflector plane at a 45˚ angle [19] [20] and the microstrip antenna with dual feed in the cavity wall [21] [22] . As mentioned above, when a dipole antenna is oriented along direction on EBG reflector plane, it seems that the directive gain is not suitable for a cellular system. In other words, if we use a microstrip antenna with dual feeding, it has more complexity. Therefore, this paper presents a curved strip dipole antenna placed on the U-shaped conductor plane at a 45˚ angle. In this proposed antenna, the directive gain is increased in x and y axises by using metallic rod EBG and the circular polarization is generated by using the EBG polarizer.

The rest of this paper is as follows. After brief introduction, the configuration geometry of a curved strip dipole and the proposed antenna is shown in Section 2. Then, the analysis of the resonator is presented in Section 3. Next, the simulated and measured results of the proposed antenna are discussed in Section 4. Finally, the conclusions are given in Section 5.

The detailed geometry and parameters of resonator antenna using curved strip dipole antenna with metallic rod EBG are illustrated in Figure 1 and Table 1, respectively. First, an upturned curved strip dipole antenna was designed to resonate around 2100 MHz and it is constructed of a copper plate (a 1 mm thickness). Next, it is mounted over an inexpensive curved polyvinyl chloride (PVC) with the dielectric constant of 3.4. The feed center of this antenna is connected at. The half-wavelength of curved strip dipole antenna is. Then, U-shaped reflector plane with the size of 300 mm × 950 mm is added behind the 45˚ oriented curved strip dipole to control the radiation pattern to be directional pattern and to reduce the side and back lobe. For the gain enhancement and the polarization development, the multi-layers metallic rod EBG structure is added in this design to perform as the superstrate. The configuration of three EBG layers consist of EBG polar H (in -axis), EBG polar V (in -axis), and EBG polarizer. For design studied here, the lower EBG polar V layer is made up of five aluminium rods of 3.85 × 950 × 6 mm³ periodicity. The upper EBG polar H layer is composed of nineteen aluminium rods of 300 × 13.26 × 1 mm³ periodicity. Furthermore, the polarizer layer is constituted of five aluminium rods of 3.85 × 950 × 2.4 mm³ periodicity; it has been located on the top layer. When, the superstrate is

placed over a radiating element with the gap of 63 mm, the circularly polarized resonator antenna is defined. It is possible to generate a 90 degree phase shift between the electric field in the and axis.

This section shows the resonator antenna, which consists of three components. There are a superstrate, a reflector plane, and a curved strip dipole antenna for the excitation source. The superstrate forms a 1-D EBG structure where periodic conducting pattern are metallic rods. It behaves as the EBG array at the antenna operating frequencies. A curved strip dipole antenna is placed between PEC reflector plane and 1-D EBG as shown in Figure 2. The radiation of feeding antenna has a center point at P, whereas the radiation pattern of a curved strip dipole antenna is. In addition, the distance between the PEC reflector plane and EBG is h, and reflection coefficient of EBG is.

Assume that the transmission is lossless, the amplitude of the direct wave 1 is, the amplitude of the once-reflected wave 2 is, in the same way the amplitude of the twice-reflected wave 3 is, etc. Therefore, summation of electric field is examined from

where, is the phase variations. When the phase difference between wave 2 and wave 1 is

the phase difference between wave 3 and wave 1 is

If the direct wave has number, so the phase difference is

When, we obtain

Therefore, we can represent the electric field as follows:

and the power pattern is

However, the amplitude and phase of the EBG reflection coefficient are a function of the angle. Maximum power in the normal direction is obtained, when

So, the distance between PEC reflector plane and EBG is

where,.

In a previous article, an antenna design of high directive gain using a curved strip dipole on electromagnetic band gap (EBG) was studied [10] . The resonant EBG technology has been used to be a reflector for directive gain increment by utilizing the good performance of the mushroom-like EBG structure, which is capable of providing a constructive image current within a certain frequency band. Therefore, when wide beam curved strip dipole is appropriated located horizontally on a resonant EBG reflector, high directive gain can be obtained of 7.6 dBi for RFID reader. The purpose of this paper is the antenna for mobile base station design, so the antenna performance is developed to perform the resonator antenna by using EBG structure as the superstrate. In this design, the EBG structure can partially reflect wave of a primary radiator. This section proposes the design and analysis of the proposed antenna.

The design of a circularly polarized resonator antenna using a curved strip dipole with electromagnetic band gap

material for mobile communications was presented. The antenna was analyzed by using the CST software. From the simulation results, we can conclude that a 45˚ oriented curved strip dipole is fed between the mutilayers EBG and the reflector plane to produce the circularly polarization. Furthermore, the sectoral radiation pattern (HPBW around 60˚) can be obtained by reducing the size of the antenna in xz plane. The antenna prototype was fabricated on aluminium to verify the antenna performance. The measured maximum gain is 15.11 dB. Good agreement between simulated and measured results is obtained.

The authors gratefully acknowledge financial support for this research project from the Telecommunications

Research and Industrial and Development Institute (TRIDI), National Telecommunications Commission (NTC) Fund, Thailand. Moreover, this work was supported by Suranaree University of Technology (SUT) and by the Office of the Higher Education under NRU project of Thailand.