Broadband True-Time-Delay Microwave Lenses Based on Miniaturized Element Frequency Selective Surfaces

We present a new technique for designing low-profile, ultrawideband, true-time-delay (TTD) equivalent microwave lenses. Such a lens is composed of numerous spatial time-delay units (TDUs) distributed over a planar surface. Each spatial TDU is the unit cell of an appropriately designed miniaturized-element frequency selective surface and provides a frequency-independent time delay within the frequency band of interest. Two TTD lens prototypes with focal length to aperture dimension (f/D) ratios of 1 and 1.6 are designed, fabricated, and experimentally characterized at X-band. The 3-dB gain bandwidths of these lenses are respectively 7.5-11.6 and 7.8-11.5 GHz. Each fabricated lens has an overall thickness of 4.76 mm, which corresponds to ~ 0.150λ
_0,
where λ
₀
is the free-space wavelength at the center frequency of operation. Each lens uses spatial TDUs with physical dimensions of 6 × 6 mm
²
, or ~ 0.19λ
₀
× 0.19λ
₀
. Both lenses have a system fidelity factor close to 1, when excited with a broadband pulse. Furthermore, due to their true-time-delay equivalent behavior, the fabricated lenses do not suffer from chromatic aberration within their operational bands. When used in a beam-scanning antenna system, each lens demonstrates an excellent scanning performance in a field of view of ± 60°.