A transmitarray antenna (or just transmitarray or called as layered lens antenna) is a phase-shifting surface (PSS), a structure capable of focusing electromagnetic radiation from a source antenna to produce a high-gain beam. Transmitarrays consist of an array of unit cells placed above a source (feeding) antenna. Phase shifts are applied to the unit cells, between elements on the receive and transmit surfaces, to focus the incident wavefronts from the feeding antenna. These thin surfaces can be used instead of a dielectric lens. Unlike phased arrays, transmitarrays do not require a feed network, so losses can be greatly reduced. Similarly, they have an advantage over reflectarrays in that feed blockage is avoided.
It is worth clarifying that transmitarrays can be used in both transmit and receive modes: the waves are transmitted through the structure in either direction. An important parameter in transmitarray design is the ratio, which determines the aperture efficiency. is the focal length and is the diameter of the transmitarray. The projected area of the feeding antenna determines the illumination efficiency of a transmitarray panel. Provided that the insertion loss of each unit cell is minimised, an aperture area appropriate to the feed radiation pattern can efficiently focus the wavefronts from the feed.
Overview of techniques
Transmitarrays can be split into two types: fixed and reconfigurable. As described earlier, a transmitarray is a phase-shifting surface consisting of an array of unit cells. These focus the wavefronts from a feeding antenna into a narrower beamwidth. By applying a progressive phase shift across the aperture of the transmitarray, the beam can be focused and steered towards a direction away from boresight (0° angles).
Fixed transmitarrays
First, consider fixed transmitarrays. At each location on the surface of the structure, the unit cells are physically scaled or rotated in order to obtain the required amplitude and phase distribution. Thus, only one focusing direction is available. The aim is to approximate the ideal phase distribution, such as for a feed located at , which can be achieved by discretising the surface of the transmitarray into several Fresnel zones. High aperture efficiency (55%) can be achieved at oblique angles of incidence using precision-machined double split ring slot unit cells. A switched-beam transmitarray covering the 57 – 66 GHz band has been reported. Three different types of unit cells were used, based on patches and coupling slots. Similarly, a 60 GHz design used unit cells with a 2-bit phase resolution and selected an optimal ratio to widen the bandwidth. When = 0.5, a scan loss of 2.2 dB was achieved at a 30° steering angle.
Different types of unit cells have been used within the same transmitarray. In, slot elements were placed near the centre of the transmitarray, as their polarisation performance is better at normal incidence, whereas double square ring slot elements were used at the edges, as they perform better at oblique incidence angles. This enabled the subtended (flare) angle of the feed horn to be increased, and hence the length of the horn, and the overall antenna size, to be reduced. Unit cells were not required at the centre of the transmitarray, where the phase shift was 0°. This reduced the insertion loss to around 1 dB at 105 GHz, as the majority of the beam amplitude was in the central region. In a different design, substrate integrated waveguide (SIW) aperture coupling was employed to reduce insertion losses and widen the bandwidth of a transmitarray operating at 140 GHz. Due to the large number of vias required, this performance improvement was at the expense of a more complex and costly fabrication.
It has been shown that transmitarray implementation can be divided into two approaches: layered-scatterer and guided-wave. The first approach uses multiple coupled layers to achieve a phase shift, but has poor sidelobe level (SLL) performance when steering due to higher-order Floquet modes. The second approach enables wider steering, at the expense of increased hardware cost and complexity.
Reconfiguration methods
In a reconfigurable transmitarray, the focusing direction is determined by electronically controlling the phase shift through each unit cell. This enables the beam to be steered towards the user. Electronic reconfiguration can be achieved by several possible methods.