Sub-Wavelength Unit-Cell for Ultra-low F/D Ratio Transmitarray Antenna

In this work a novel sub-wavelength transmitarray antenna (TA) unit-cell based on Substrate Integrated Waveguide (SIW) cavity integrated frequency selective surface is proposed. The unit-cell is implemented with a single Printed Circuit Board (PCB) featuring four metal layers. Two patch antennas are used as receiving and transmitting elements and they are coupled through a SIW cavity placed between them. The unit-cell operates at 12 GHz and its periodicity is 0.22 λo×0.22λo. The high miniaturization results from the combination of the SIW cavity features and the Frequency Selecting Surface (FSS) formed by aperture coupled resonators. Specifically, the SIW cavity is used to achieve a dual-configuration unit-cell, each one with 180 degrees phase range. Moreover, the passband transmission response results from the combination of the coupling resonance and the patch antenna resonance. As a result, the proposed unit-cell exhibits an extremely limited sensitivity to oblique incidence, allowing for a low-profile TA design. The unit-cell achieves a 280-degree phase range within a 3 dB insertion loss at a 70-degree incidence angle. This capability enables a reduction of the F/D ratio to 0.13 without compromising overall TA performance. To validate the proposed design, a 400-element TA was prototyped and tested. The measured realized peak gain is 19.58 dBi, corresponding to an aperture efficiency of 36%. The measured -1 dB and -3 dB gain bandwidths are 7.5% and 13%, respectively. Furthermore, a scanning range of ± 27 degrees has been achieved with 3 dB scan loss while the steering is implemented by mechanically displacing the feed along the H-plane.