Ultra-wideband, true-time-delay, metamaterial-based reflectarray antenna

Summary form only given. With the recent progress in the areas of periodic structures and metamaterials, numerous innovative designs of reflectarray antenna systems have been reported as potential replacements of parabolic antennas. Reflectarrays have also found applications in at phased arrays that are used for many high-gain antenna applications such as satellite communications and radar systems. Reflectarray antennas are generally composed of locally-periodic structures with unit cells that act as spatial phase shifters or spatial time-delay units. These unit cells are commonly implemented from resonant building blocks (e.g. patch antennas). Such reflectarrays suffer from relatively small bandwidths and tend to be highly dispersive. Moreover, in situations where reflectarrays are fed with multiple feed antennas to achieve a multi-beam aperture, the sensitivity of the responses of these spatial phase shifters to the angle of incidence of the electromagnetic wave can seriously deteriorate the response of the structure. Hence, in applications where signals with instantaneously broad bandwidths are used, reflectarray antennas free of chromatic aberration must be employed. In this paper, we propose a new technique for designing low-profile, ultrawideband, and true-time-delay (TTD) reflectarray antennas. Such an antenna is composed of numerous spatial time delay units (TDU) distributed over a planar surface that provides a desired time delay over a wide frequency range. Each spatial TDU is a unit cell of appropriately designed miniaturized-element frequency selective surfaces (MEFSSs) composed entirely of non-resonant constituting elements. A prototype of the proposed TTD reflectarray with focal length to aperture diameter ratio (f/D) of 1 is designed to operate at a center frequency of 10 GHz. The designed structure is a low-profile structure with an overall thickness of 4.78 mm (or equivalently 0.16λ_o where λ_o is the free space - avelength at the center frequency of operation), and uses spatial TDU with the dimensions of 6.5 mm x 6.5 mm (≈ 0.22λ_o x 0.22λ_o). The antenna gain at the center frequency is 28.4 dB, and the frequency range within which the gain does not vary more than 4 dB is 7-13 GHz (a relative gain bandwidth of 60%). The side lobe level (SLL) remains below -12 dB across the band when it is illuminated under normal incidence angle. Also, because of the sub-wavelength feature of each TDU as well as its small overall thickness, the reflectarray demonstrate a scanning performance in a wide field of view of ±45^o, which comes at the expense of slight degradation of the SLL. Details of the design of the structure as well as the measurement results of the fabricated prototype will be presented and discussed at the symposium.