Reflectarray antenna
A reflectarray antenna (or just reflectarray) consists of an array of unit cells, illuminated by a feeding antenna (source of electromagnetic waves).
The unit cells are usually backed by a ground plane, and the incident wave reflects off them towards the direction of the beam, but each cell adds a different phase delay to the reflected signal.
A phase distribution of concentric rings is applied to focus the wavefronts from the feeding antenna into a plane wave (to account for the varying path lengths from the feeding antenna to each unit cell).
A progressive phase shift can be applied to the unit cells to steer the beam direction.
[3] It is common to offset the feeding antenna to prevent blockage of the beam.
[4] In this case, the phase distribution on the reflectarray surface needs to be altered.
A reflectarray focuses a beam in a similar way to a parabolic reflector (dish), but with a much thinner form factor.
According to,[5] in a reflectarray a constant phase of the entire reflected field is achieved in a plane normal to the direction of the desired pencil beam as expressed by:
is the direction vector of the desired pencil beam,
, the formula for the optimal phase distribution on a conventional reflectarray for a beam in the boresight direction is given by:
is the phase shift for a unit cell located at coordinates
It is important to analyse the reflection magnitude
When designing a reflectarray, we aim to maximise the reflection magnitude
at each unit cell determines the overall beam shape and direction.
Ideally, the total phase shift range would be 360°.
[1] The aperture efficiency, and hence gain, of the reflectarray will be reduced if the angle of incidence to the unit cells is not considered, or if spillover occurs or illumination of the reflectarray is not optimal (see also transmitarrays).
[2] Similarly, phase errors due to quantization into a discrete number of phase states for digital control can also reduce the gain.
[7] A fixed reflectarray has a single beam direction per feed.
Changing the shape of the unit cells alters their reflection phase.
This has applications in point-to-point communications, or for a satellite covering a specific geographic region (with a fixed beam contour).
[8] A reconfigurable reflectarray has unit cells whose phase can be electronically controlled in real-time to steer the beam or change its shape.
Several methods have been used to implement reconfigurable reflectarray unit cells, including PIN diodes,[9][10] liquid crystal,[11][12][13][14] and novel materials.
Each of these methods introduces loss which reduces the efficiency of the unit cells.
Linearity (such as distortion due to the diodes) also needs to be considered to minimise out-of-band radiation which could interfere with users on adjacent frequencies.
[15] In satellite communications, it is necessary to produce multiple beams per feed, sometimes operating at different frequencies and polarizations.
[16] Circular polarization is commonly used to reduce the effect of atmospheric depolarization on the communication system performance.
[17] A dual-band reflectarray has two different passband frequencies, for example for uplink and downlink.
[18] A bifocal reflectarray has two principle foci, so can focus wavefronts to or from two feeding antennas simultaneously.
The phase distribution on each reflectarray must be carefully calculated to ensure that the phase derivatives are consistent with the angle of incidence of the rays [19] The ratio of the sizes and positions of these reflectarrays can be used to achieve quasi-optical magnification (narrowing of the beam).
