Design and Microfabrication of a 230-GHz Planar Reflectionless Lens Based on Miniaturized-Element Frequency-Selective Surfaces

In this article, the design and fabrication of a 230-GHz planar and low-mass reflectionless lens antenna are presented. Phase compensation for the spherical phase front of the feed to a planar one emerging from the other side of the lens is accomplished locally using a three-layer spatially varying miniaturized-element frequency-selective surface (MEFSS) structure. The MEFSS structure is designed to present two-pole bandpass filters with different center frequencies across the surface of the lens. These filters make use of only shunt equivalent circuit elements and their substrates' thickness as impedance inverters. The very small feature size of the metallic components that constitute the unit elements of the MEFSS structures necessitates the development of a special and accurate (within
$\pm 1\; \mu$
m tolerance across the extent of the lens) multilayer microfabrication process. This article presents the fabrication process and performance assessment of a 10-cm-diameter lens with
$f/D=1.4$
operating at 230 GHz. This lens is fabricated using two thin (
$\text{193}\; \mu$
m) low-loss prime fused silica wafers on which fine MEFSS structures are patterned. The provision of shallow groves for the metallic patterns on one face of one of the wafers allows for the ease of wafer-to-wafer bonding. The fabricated lens shows a measured beamwidth of
$\text{0.88}^{\circ }$
and a high gain of 44 dBi, while at the same time providing a very low surface reflection coefficient of
$-$
31 dB (at its center frequency) measured at the feed's terminal. The average reflection coefficient over the desired band is 10 dB lower than that of a conventional dielectric lens of the same
$f/D$
and diameter. This lens that weighs only 7 g and has a total thickness of
$\text{387}\; \mu$
m is a key component of a dual-polarized common aperture radar designed for the phenomenological study of traffic scenes.