A Millimeter-Wave Multibeam Transparent Transmitarray Antenna at Ka-Band

This letter presents a transmitarray antenna ( TA) in the 27.5-29.5 GHz frequency band. The proposed TA is optically transparent and offers the possibility of steering the beam in the H-plane from -30 degrees to 30 degrees. One can achieve these properties using a novel unit cell, composed of meshed double-circle rings printed on polymethylmethacrylate substrates, a plastic material transparent to visible light. This unit cell provides a phase shift of 300 degrees for an insertion loss lower than 1 dB at 28.5 GHz. In turn, the TA sources consist of 2 x 2-element arrays of aperture-coupled stacked patch antennas. The choice of a patch array over a horn array allows one to reduce the total profile and weight of the TA. We have fabricated and tested a transparent TA with a diameter of 9.9 wavelengths at 28.5 GHz, yielding a broadside gain of 25 dBi and -1 dB gain bandwidth of 1.8 GHz; the OFF-axis 30 degrees beam presents a peak gain of 21.5 dBi and -1 dB gain bandwidth of 1.4 GHz. The measured results are in good agreement with the simulated ones all over the frequency band of interest.