A New Technique to Design 1-D Dual-Band EBG Resonator Antennas Basit Ali Zeb, Yuehe Ge, and Karu P. Esselle* Center for Electromagnetics and Antenna Engineering Department of Electronic Engineering, Macquarie University Sydney, NSW 2109, Australia e-mail:
[email protected] Abstract—This paper describes a novel technique to design a dual-band electromagnetic band gap (EBG) resonator antenna by engineering the reflection phase characteristics of a simple superstrate composed of two uniform dielectric slabs. The working principle is explained using the unit-cell analysis and a composite antenna design is presented with a simple EBG superstrate made out of two low-cost FR4 sheets. Antenna simulations and computed radiation patterns confirm the directivity enhancement and dual-band operation of this simple 1-D EBG resonator antenna.
depending upon the type of the resonant element employed, e.g., dipole, patch, ring patch and square patch etc. However, in all these designs at least two resonant inclusions are required to achieve the dual-band operation. In a recent effort, a novel method to design a dual-band EBG resonator antenna is presented, where a single-resonant single-layer FSS is used [10-11] to obtain a dual-band antenna. By using the locally inverted gradient of the reflection phase curve of slot-array FSS superstrate, dual-band directivity is achieved using singleresonant inclusion only.
Keywords- electromangetic bandgap structures, superstrate, EBG resonator antenna, dual-band directivity enhancement, Fabry-Perot cavity, reflection phase
In this paper, an extension of the technique in [11] is presented, to design a simple uniform one dimensional (1-D) EBG structure that is used as a superstrate to make a dual-band EBG resonator antenna. This presents a novel and simple approach to enhance dual-band antenna directivity as compared to other techniques presented in the literature [5-6]. This paper is divided in two sections. In the first section, working principle of the dual-band EBG structure is presented based on the analysis of the unit-cell model. In the second section, the design of a dual-band EBG resonator antenna is discussed and the computed radiation patterns are presented. CST Microwave Studio commercial software has been used for the analysis and design of the dual-band EBG resonator antenna. The CST results are verified through simulations in Ansoft HFSS software as well.
I.
INTRODUCTION
Recently, the application of Electromagnetic Band Gap (EBG) technology in antenna designs has become an exciting topic for antenna engineers and researchers. The main impetus behind this research is to improve the antenna performance owing to the two distinct properties of EBG structures: (1) to block the propagation of an electromagnetic wave within a frequency bandgap, and (2) to display localized frequency windows within the bandgap when the structure periodicity is broken due to the presence of some defects. The later property is extremely useful to realize compact high gain antennas by utilizing the focusing effect of EBG superstrates. The use of EBG structures as antenna superstrates is a promising solution for enhancing the directivity of various antennas [1]. Such class of antennas are called EBG resonator antennas or Fabry-Perot Resonator antennas and have the main advantages of design simplicity and low complexity as compared to the conventional planar antenna arrays. A large variety of such antennas have been developed to enhance the directivity at single, multiple and broad frequency bands, based on the fundamental principles of the Fabry-Perot cavity model [2-3] and EBG defect-mode transmission model [4]. In the recent past, EBG structures and printed frequency selective surfaces (FSS) have been applied to design dual-band EBG resonator antennas. These antenna configurations are based on either the introduction of a defect dielectric layer or printed resonant elements on single or double sided FSS layer [5-9]. Directivity enhancement can be achieved in two bands
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II.
DESIGN OF DUAL-BAND EBG SUPERSTRATE
The configuration of the 1-D EBG resonator antenna is shown in Fig. 1. The working principle is based on the concept presented previously [11]. The two dielectric slabs make a 1-D EBG structure and are used to form a resonant cavity.
Figure 1. 1-D EBG resonator antenna.
AP-S/URSI 2011
Around this cavity resonance frequency, the reflection phase of the EBG structure varies significantly and this profile of the reflection phase verses frequency curve gives the possibility to design a dual-band EBG resonator antenna. Together with the PEC ground plane, the reflection phase helps satisfy the cavity resonance condition between the superstrate and the ground at two distinct frequencies. A. Unit-Cell Model In order to understand the physical phenomenon of the proposed dual-band EBG structure, analysis of the unit-cell geometry is presented with the help of reflection characteristics. The proposed EBG unit-cell model is shown in Fig. 2. The EBG structure consists of two dielectric slabs with equal thickness and similar dielectric constant. The unit-cell model is surrounded by the periodic boundary conditions and excited by two waveguide ports to study its reflection characteristics.
(a)
(b)
Figure 2. (a) Unit-cell model of the EBG structure (b) a cavity model of the unit-cell and its image.
frequency bands. In the following section, a detailed analysis of the unit-cell is presented based on the study of reflection characteristics and electric field distributions. B. Unit-Cell Analysis A normal incidence plane wave is applied to port 1 and port 2 of the unit-cell model. The reflection coefficient phase of the proposed unit-cell cavity model is presented in Fig. 3. The reflection phase initially decreases with the frequency but increases rapidly at the inclusion resonance frequency around 11.5 GHz. At this frequency, the resonance is achieved by the inclusion, which is the secondary cavity formed between dielectric slabs 1 and 2. For a fixed value of ‘h’, the reflection phase of an ideal superstrate for a broadband EBG resonator antenna (that should increase linearly with frequency), which can maintain the cavity resonance condition [11] over a wide bandwidth, is also plotted (dotted line in Fig. 3). Two distinct frequency points, one above and one below the inclusion resonance frequency, determined by the two points of intersection between the two curves, are noted. Cavity resonance condition is satisfied at these two frequencies, referred as ‘f1’ and ‘f3’, using only single resonant inclusion. It is also evident by analysing the transmission coefficient magnitude, which is plotted in Fig. 4. The solid-line presents the transmission through the secondary cavity only, i.e., without images. It resonates at the inclusion resonance frequency, ‘f2’, around 11.5 GHz. The dotted-line represents the resonance of the unit-cell with its image. It is seen that two distinct resonance frequencies are achieved at 10.6 GHz and 13.2 GHz. At the inclusion resonance frequency f2 = 11.5 GHz, the reflection coefficient magnitude of the superstrate is quite low and hence at this frequency the EBG resonator antenna does not have a significant boost in directivity. On the other hand, at the other two resonance frequencies (10.6 and 13.2 GHz), reflection coefficient magnitude is sufficient to provide a good directivity and gain enhancement. The plots in Fig. 3 and Fig. 4 are obtained by setting the parameters ‘h’ and ‘h1’ to 12.1 mm and 12.8 mm, respectively, to achieve the antenna operating frequencies, ‘f1’ and ‘f3’, around 10.6 GHz and 13.2 GHz respectively.
Assuming normal incidence, periodic boundary conditions can be replaced by perfect electric conductor (PEC) and perfect magnetic conductor (PMC) walls [11]. For the example design, the two dielectric slabs are considered to be made out of FR4 material (İr = 4.4), and are separated by an air gap, which is approximately half-wavelength. This creates a resonant cavity that can be made to resonate at a pre-determined frequency, by appropriately adjusting the distance between the two slabs ‘h1’ and thickness of the dielectric material ‘t’. Image theory is then used to remove the ground plane and form a cavity that is composed of the unit-cell of the EBG structure and its image, as shown in Fig. 2(b). This cavity thus resonates at the same frequency as the antenna, i.e., when the half-wavelength is equal to “2h”. The reduction of the analysis to a unit-cell and its image not only significantly saves the computational time and processing memory, but also predicts accurately the working of EBG resonator antenna at dual
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Figure 3. Reflection phase.
III.
Figure 4. Multiple resonances of the proposed EBG structure.
This concept is further illustrated when the field distributions in the cavity are analysed at these frequencies. The necessary and sufficient conditions for dual-band directivity enhancement are that only those frequencies are useful where the electric field distribution is symmetric and vanishing at the structure’s symmetry plane. This is illustrated in Fig. 5, which presents the amplitude of the electric field distribution across the unit-cell cavity model at the three resonant frequencies. It can be seen from Fig. 5 that only frequencies ‘f1’ and ‘f3’ satisfy the necessary criteria, and hence are useful for the directivity enhancement. Based on the unit-cell analysis, it is clear that by controlling the location of two dielectric slabs with respect to the ground plane, dual-band operation can be achieved at the desired frequency bands. This is a remarkably simple technique which eliminates the need to introduce a defect dielectric structure to achieve multi-frequency operation [5-6].
DUAL-BAND EBG RESONATOR ANTENNA
A. Design It can be seen from Fig. 5 that the two frequencies, f1 and f3 have zero tangential electric fields at the structure’s symmetry plane, where a conducting ground plane can be positioned, and hence are useful to implement a dual-band EBG resonator antenna. The proposed configuration of the dual-band 1-D EBG resonator antenna is shown in Fig. 1. For the antenna simulations, the dielectric slabs and ground plane both have overall dimensions of 110 × 110 mm2 (Fig. 1). The thickness of the dielectric slabs, made out of FR4 epoxy, is set to t = 3.2 mm. In simulations, the EBG resonator antenna is excited by a horizontal electric dipole (HED) located 0.8 mm above the ground plane for simplicity and numerical efficiency. Since the antenna is designed for single linear polarization, either xpolarized or y-polarized HED excitation can be used. The desired radiation patterns can be achieved using the HED excitation which is a broadband feed. Because of the symmetry, the antenna can also be excited by a circularly polarised source to obtain CP radiation, without added complexity. B. Simulations Antenna simulations have been performed using CST Microwave Studio and computed radiation patterns for the composite antenna in E- and H- planes are presented in Fig. 6. The antenna has maximum directivity in the main beam direction (ș = 00) with symmetric radiation pattern owing to the symmetry in the antenna structure. The side lobe levels in the two planes are below í20 dB at 10.6 GHz. In the upper band, side lobe levels are below í25 dB in E-plane and below í19 dB in H-plane at 13.2 GHz. The computed peak directivity within the simulated frequency range is shown in Fig. 7. For comparison, the directivity of HED feed is also plotted. Maximum directivities of 17 dBi and 17.2 dBi are predicted at the two frequencies of 10.6 GHz and 13.2 GHz respectively. The 3dB directivity bandwidth of the EBG resonator antenna at the two frequency bands are approximately 8.8% and 7.1%, respectively, with ‘f1’ and ‘f3’as the centre frequencies. Finally, it is worth mentioning that the choice of higher permittivity superstrate material can result in increased directivity level, because of the increase in the quality factor of the resonating cavity. IV.
CONCLUSIONS
A new method to design a dual-band 1-D EBG resonator antenna is presented. The gradient of the reflection coefficient curve verses frequency is engineered to obtain two resonance frequencies. A simple 1-D periodic structure, made out of only two uniform slabs with similar value of dielectric constants, is used as the superstrate. Simulation results have validated the design method and indicate that dual-band directivity enhancement can be achieved, thus avoiding the need to use more complex superstrate structures. Peak computed directivities are 17 dBi and 17.2 dBi at the two operating frequency bands around 10.6 GHz and 13.2 GHz, respectively.
Figure 5. Electric field distribution across the unit cell.
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These directivities can further be enhanced by selecting higher permittivity superstrate material with larger lateral dimensions.
(a)
(b) Figure 6. Computed radiation patterns: (a) at 10.6 GHz (b) at 13.2 GHz.
Figure 7. Directivity verses frequency.
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