A novel computed aided technique for designing reflectarray antennas is presented. The developed approach automatically generates the geometrical model of reflectarray antennas taking into account some input parameters, such as, the unit cell type and dimensions, frequency, focal length, periodicity, dielectric materials, and desired main beam radiating direction. The characteristics of the reflecting elements are selected considering the spatial phase delay at each unit cell to achieve a progressive phase shift. The implemented procedure also provides the phase characteristic of the unit element, which is rapidly computed by using a parallelized Moment Method (MoM) approach. The MoM is also used to obtain the radiation pattern of the full reflectarray antenna. In order to evaluate the new technique, a dual-interface prototype has been designed and simulated showing high-performance capability.

Reflectarray antennas [

Reflectarray antennas are composed of one or more layers of metallic elements and dielectric substrates over a ground plane. They are typically fed by a horn antenna and their operation consists of converting a spherical wavefront into a desired scattered wavefront using a phase shift mechanism. Although it has been demonstrated that reflectarray antennas can be used to generate contoured beams [

Diagram of the reflectarray antenna.

Hence, the resonant size, periodicity, orientation, and substrate’s properties of every cell must be properly selected to provide the phase shift given by (

Due to the radiating elements must reflect the incident electromagnetic field with a progressive phase distribution, the most challenging aspect when designing reflectarray antennas is to find the adequate phase shift to compensate the phase delay caused by the distance from the feed to each unit cell. Thus, the requirement of having a wide database that associates the properties of the cells and their correspondent phase shifts is crucial. In fact, many works proposing different techniques to achieve a wide phase range of the reflection coefficient have been recently published. In the literature, mainly four mechanisms can be found. The first one is the easiest to manufacture and more used [

Other important aspect is the adequate choice of the radiating element’s shape, since the scatter of the incident field strongly depends on its geometry. Hence, an essential goal is to obtain the type of cell that provides a wide reflection coefficient phase range when varying its properties. In fact, it is well-known that a reduced phase range lower than 360° implies a decreasing in directivity. On the other hand, it has been demonstrated [

Normally, the reflection coefficient phase as a function of the patch size is calculated to determine whether the phase range is wide enough and the curve slope is smooth enough. Thus, a lot of effort is usually required to find a unit cell able to provide the mentioned phase characteristic. The traditional shapes of microstrip patches have been crosses, squares, rectangles, rings, and circles. In this paper, the double square ring has been chosen to prove the efficiency of the developed computer tool.

A new method to automatically design reflectarray antennas is proposed. The properties of the radiating elements, including their resonant dimensions and substrate’s characteristics, are computed by means of an efficient process that compensates the phase shift introduced by each elementary cell. Although several works have been recently carried out to investigate the phase characteristics of the cell elements, most of them do not report the reflectarray design once the phase curve has been properly achieved. The contribution of this paper also relies on the final design of the reflectarray, besides implementing the new approach and obtaining the widely used phase curve of the elementary cell. The proposed approach has been included in the computer tool NEWFASANT [

The paper is organized as follows. The procedure to obtain the geometrical model of the reflectarray is described in Section

The basic idea of the new technique is the automatic design of reflectarray antennas by varying the parameters of the unit cells. The variation may include the resonant size of the cells, their position, orientation, stub’s length, periodicity, number of layers, dielectric materials, and so forth, so that the phase shift given by every element is compensated according to the phase of the reflection coefficient.

First, a database which stores the values of the modified parameters and its correspondent reflection coefficient phase is created at a given frequency and for a determined periodicity. It is important to highlight that the analysis can be conducted by varying one or more parameters. Each entry in the database that associates pairs of parameter values and reflection phases is obtained by analyzing the behavior of a quasi-infinite array of identical elements by applying a full wave method based on the MoM [

Then, the phase delay caused by the path difference from the feed point is calculated taking into account the position of each unit cell. Once all phase delays are obtained, the system searches in the database the proper dimensions to compensate the required phase at every element. Hence, each cell can properly adjust their phase delay, introducing no errors and obtaining the maximum directivity. The phase shift needed in each cell when using several incident angles is obtained from one of those databases depending on its position and the feed location. The results provided considering several incident angles are slightly more accurate than only considering normal incidence. The next step consists in creating the geometrical model of the reflectarray, according to the values provided by the database. Finally, the ground plane is correctly located under the cells conforming to the dielectric thicknesses. By default, the reflectarray is created in the

The flexibility when designing reflectarray antennas by using the presented tool is enormous, since many combinations of different shapes, dimensions, diverse substrates, incidences, and so forth can be used to design the radiating elements. In addition, the configuration of the unit cell is not constrained to a single layer. In fact, the tool provides a great versatility and allows defining various layers of metallic patches and interfaces of different substrates. Moreover, in order to facilitate the use of the implemented tool, several databases that contain correspondences between dimensions and reflection coefficient phases of many elementary cells (rectangular, circular and square patches, square and circular rings, metallic crosses, patches with holes, etc.) are available.

The module includes three techniques to model the feed antenna. The first one allows importing the radiation pattern of a real horn antenna. The second one allows importing the geometrical model of the feed antenna in several CAD formats, and the third one allows creating the physical model of the feed antenna.

In order to validate the performance of the presented method, the specifications of an existing design recently published [

Top view of the reflectarray antenna.

Figure

Phase characteristic of the unit cell varying the length of the external square ring.

As in [

Radiation pattern of the feed antenna at 20 GHz.

On the other hand, Figure

Radiation pattern in 3D at 20 GHz.

Normalized radiation pattern. Cut

Finally, Figure

Gain. Cut

A powerful tool to automatically design reflectarray antennas has been presented. The reported technique is able to generate the geometrical model of the antenna considering some input parameters, such as, the unit cell type, the operating frequency, the focal length, the periodicity, and the desired main beam radiating direction. The tool also computes and provides the phase curve of the unit cell. To evaluate the performance of the new method, a recently published reflectarray has been redesigned. Excellent results in terms of gain and polarization purity have been achieved. The developed tool can be very helpful for researchers.

This work has been supported, in part by the Comunidad de Madrid Project S-2009/TIC1485 and by the Castilla-La Mancha Project PPII10-0192-0083, by the Spanish Department of Science, Technology Projects TEC 2010-15706 and CONSOLIDER-INGENIO No CSD-2008-0068.