^{1}

^{2}

^{1}

^{1}

^{1}

^{2}

Reflectarray antennas (RAs) are nowadays a quite popular technology, used in several applications, due to a significant number of attractive properties, such as low cost, low weight, conformal deployment, and the possibility of introducing suitable reconfigurable capabilities. Unfortunately, they present also some intrinsic limitations and drawbacks compared with other solutions and, in particular, a relatively narrow bandwidth; that of course could be enlarged, but generally with a drastic increase of the structure complexity. The objective of this work is the design of a single-layer passive reflectarray, in which the reradiated elements have no conventional shape and enough degrees of freedom to compensate both the spatial and frequency phase variation of the reradiated field. In particular, here we focus on a reradiating element consisting in two concentric square rings in which two different and quite independent geometric parameters are varied.

As it is well known, reflectarray (RA) antennas consist of one or more feed antennas illuminating a usually flat reflecting surface, whose electromagnetic reflecting performances have to be suitably designed in order to obtain the required performances of the whole radiating system. Reflectarray antennas have been first proposed in 1963 by Berry et al. [

Probably this is the reason why for more than a decade this strange solution, without apparent advantages but with evident drawbacks compared, for example, with parabolic reflectors, has not been considered again, till 1975, when the feasibility of a reflectarray with scanning possibilities has been claimed in a US patent [

The real breakthrough in reflectarray technology came when the evolution of printed circuit technology and high-frequency laminates synthesis allowed low-profile, light-weight implementations. In fact, even if the first reflectarray patent introducing a microstrip patch antenna-based reflecting surface has been published in 1977 [

Furthermore, in order to achieve good antenna performances, a very large array of patches has to be suitably designed exploiting in the proper way all the possible geometrical free parameters, requiring the adoption of numerical electromagnetic solvers, sophisticated numerical optimization tools, and in any case a significant numerical effort.

These, that is, the technology enablement and the numerical modeling tool availability, are the reasons why only nowadays printed reflectarrays technology became well assessed, and in the last years it substituted other technologies in many fields of applications, in particular where it is of paramount importance to fulfill constraints such as high gain, narrow beam with low side lobes, light weight and smaller volume, easiness of deployment, and foldability.

The main limitation now to a complete diffusion of this kind of solutions is due to the fact that the most recent antenna systems require a very large bandwidth, typically even the multiband operability or the possibility of beam steering, features that are still difficult goals to be achieved with a printed reflectarray.

In fact, for what concerns the bandwidth, it is intrinsically limited for two different orders of reasons: the poor bandwidth of printed radiating elements themselves, usually no larger than the 3–6%, and, most important, the frequency dependence of the phase delay of the incident field. In particular this second aspect is quite critical and becomes dominant in large RAs [

The usual assessed way to enhance the RA bandwidth is that of using radiating elements that consist in two or more stacked printed single radiators (see, for instance, [

Recently, alternative solutions have been proposed, in which the RA elements are single-layer printed patches of nonconventional shape [

These are the reasons why, in the framework of this paper, we consider as radiating element a concentric double-square ring configuration, in which at least two geometrical parameters are varied. In this way, it is possible to compensate with one parameter the spatial phase shift and with the other the frequency variation of the incident field phase, so that the reradiated field remains almost the same in the whole bandwidth, overcoming the previously considered limitations. In other words, it means that each element of the array has to provide a phase contribution to the reradiated field that varies both with the element position and with the frequency. With the aim of validating the effectiveness of the such reradiating element, we considered different RAs with increasing size: the results of their full-wave analysis show that the proposed reradiating element is a good candidate for single-layer, large-bandwidth reflectarrays.

The RA reradiating element considered in this work is of the type sketched in Figure

Geometry of the considered reradiating element.

In order to achieve this result, for all of them the variation of the reradiated field with a suitable set of different couples of geometrical parameters has been computed, considering the element embedded in an infinite periodic lattice and adopting a full-wave MoM approach. An example of the kind of obtained results is reported in Figure

Phase variation provided at 10.75 GHz as a function of two geometrical parameters.

Figures

Phase variation provided a as a function of

Phase variation provided as a function

The design of each reradiating element in a RA, which implies the optimal choice of

Difference between the phase at the central frequency and at one extreme of the band as a function of

In order to experimentally validate the synthesis design concepts previously detailed and, in particular, to prove the real possibility to enhance the bandwidth with the use of the introduced double parameters reradiating element, two reflectarray configurations of different size have been considered. Both of them are offset fed, since in that case the distances between the feeder and the lower and upper sides of the reflector are quite different and the frequency compensation of the introduced delays is more complex to achieve. The planar reflectors have been designed in such a way that the direction of maximum radiation is slanted with respect to the broadside: in this way it is possible to better control if the phase compensation introduced by the reradiating elements is effective at the different frequency, checking if the direction of maximum radiation remains constant. The first RA we considered is the configuration depicted in Figure

View of the designed 16

The structure has been designed using the two degrees of freedom of the double-ring reradiating elements to obtain the maximum reradiation in a direction tilted of

Radiation patterns of the 16

The entire RA has been analyzed using a commercial full-wave simulator [

The second considered configuration is the 32

View of the designed 32

In Figure

3D radiation patterns of the 32

Finally, in Figure

Radiation patterns of the 32

In this paper we present the design of planar reflectarray antennas using concentric square rings as reradiating elements. In opposite to what already published, here two geometric parameters have been varied in order to better control both the spatial and frequency phase variation of the reradiated field and to overcome one of the main drawbacks of the RAs, that is, their narrow bandwidth. The full-wave numerical analyses, carried out on two sample antennas of 256 and 1024 elements, respectively, show a 1 dB gain bandwidth of almost the 17% and confirm the promising characteristics of such radiating elements.

This work was partially supported by the Italian Ministry of Education in the frame of the National Interest Research Project “Active reflectarray antenna for mobile terminal” (2008N3B2LP_001).