Metamaterial superstrate is a significant method to obtain high directivity of one or a few antennas. In this paper, the characteristics of directivity enhancement using different metamaterial structures as antenna superstrates, such as electromagnetic bandgap (EBG) structures, frequency selective surface (FSS), and left-handed material (LHM), are unifiedly studied by applying the theory of Fabry-Perot (F-P) resonant cavity. Focusing on the analysis of reflection phase and magnitude of superstrates in presently proposed designs, the essential reason for high-directivity antenna with different superstrates can be revealed in terms of the F-P resonant theory. Furthermore, a new design of the optimum reflection coefficient of superstrates for the maximum antenna directivity is proposed and validated. The optimum location of the LHM superstrate which is based on a refractive lens model can be determined by the F-P resonant distance.

Artificial electromagnetic materials, such as electromagnetic band gap (EBG) structures, frequency selective surface (FSS), and left-handed material (LHM), are broadly classified as metamaterials. They have attracted significant research interest in recent years due to their special electromagnetic properties, which are applicable to a wide range of electromagnetic devices. Array antennas had been widely used for applications requiring high directivity. However, array antennas require a complicated feeding network which makes difficulty in design and fabrication of array antennas.

Recent papers have proposed the use of metamaterials as superstrates for antennas to achieve gain enhancement [

In this paper, based on the theory of Fabry-Perot (F-P) resonant cavity, we further explain the unified mechanism of directivity enhancement using three different metamaterials as antennas’ superstrates. By analyzing the reflection phase and magnitude of superstrates, we can design high-directivity antennas with different types of superstrates according to F-P resonant theory. The organization of the paper is as follows. First, the theory of F-P resonat or antenna is briefly described, and the ideal optimum reflection magnitude of superstrate related to antenna maximum directivity is given. Second, taking EBG, FSS, and LHM superstrates proposed by previous references [

A geometrical optics model has been applied to describe the theory of F-P resonator antenna [

F-P resonant cavity composed of a ground plane and a superstrate.

It is shown that the resonant cavity height

Furthermore, we assume that an original planar antenna without superstrate has a directivity of

Taking into account three kinds of different metamaterial superstrates proposed by [

A typical design example using one-dimension periodicity dielectric EBG superstrate proposed by [

(a) Dielectric EBG structure, (b) circular waveguide antenna with EBG superstrate, (c) radiation patterns of

Here, we will provide an insight into the above phenomenon in terms of the reflection phase and magnitude of the EBG superstrate based on the theory of F-P resonant cavity. A full-wave EM simulation was performed by using finite-element analysis based on HFSS [

Simulated phase and magnitude of reflection coefficients (

The directivities of the antenna with the EBG superstrate versus different actual

(a) Simulated directivity of the antenna with EBG superstrates versus different

FSS structures can also be used as antenna superstrate to enhance antenna directivity. A FSS structure of square patches proposed by [

(a) Unit cell of the FSS and (b) patch antenna with FSS superstrate.

Reflection coefficient (

Analyzing the reflection magnitude of the FSS superstrate, we can see that the actual

It can be predicted from F-P cavity theory that the original FSS design is not optimal, because its reflection magnitude

(a) Directivity of the patch antenna with FSS superstrates versus different

Comparison of directivities patterns of different FSS superstrates, (a)

Generally, the analysis of LHM superstrate for directivity enhancement of an antenna is based on the effective near-zero refractive index of metamaterials. However, it is still difficult to determine the optimum distance between LHM superstrate and antenna aperture. In this section, we give an insight into the mechanism of the LHM superstrate by combining the near-zero refraction theory with Fabry-Perot resonant cavity theory.

The LHM structure proposed by [

Geometry of LHM structure.

Effective refractive index of the LHM structure.

(a) Patch antenna with LHM superstrate, (b)

Directivities of the patch antenna with LHM superstrate located at different distance.

From another point of view, we consider the antenna system with LHM superstrate as the F-P resonant cavity antenna approximately. We first analyze the reflection phase

Reflection phase and magnitude of the LHM superstrate.

It is revealed that the maximum directivity of the antenna system can be achieved by setting the LHM superstrate at the resonant height of the F-P resonant cavity. When the LHM superstrate is placed at another position, the directivity enhancement would be less than that at the resonant position, which is only contributed from the near-zero refraction index response. Therefore, it is shown that we can obtain an optimum design of the LHM superstrate by combining the F-P resonant cavity theory with the near-zero refractive index of metamaterials. In addition, it is found that the directivity enhancement of the LHM superstrate is weak compared with the EBG and FSS superstrates. The reason for this is that the LHM superstrate has a small reflection coefficient magnitude at the working frequency, that is,

In this paper, the mechanism of directivity enhancement using different metamaterial structures as antenna superstrates was studied by unifiedly applying the theory of Fabry-Perot (F-P) resonant cavity. An improved design of superstrates for the maximum antenna directivity enhancement is proposed and validated. Based on the analysis of the reflection magnitude and phase of superstrates, three kinds of metamaterial superstrates, such as EBG structure, FSS, and LHM, are proved to satisfy the theory of Fabry-Perot resonant cavity. It has been shown that if the reflection magnitude

This work is supported partly by the Program for New Century Excellent Talents in University of China, and partly supported by the National Natural Science Foundation of China under Contract no. 61072017 and no. 61072021, National Key Laboratory Foundation, the Fundamental Research Funds for the Central Universities, and Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Shaanxi Province.