Reflectarray antenna design
In recent years, there has been significant interest in reflectarray antennas and how they can be used to design antenna systems for the next generation of communication satellites.
What is a reflectarray antenna?
A reflectarray is an alternative to a reflector antenna to provide a low-cost high-gain antenna for many different purposes including space applications. A reflectarray consists of a feed element and a, normally, planar surface with the special property, that the phase of the reflected field can be controlled and varied over the surface. In this way, a focused reflected field can be obtained from a planar surface.
A planar pencil-beam reflectarray in an anechoic measurement chamber.
The history of reflectarrays goes back to the 1960s, where they were made of an array of short-circuited rectangular waveguides. By varying the length of the waveguides, the reflected phase could be controlled and thus creating a focused beam from an almost flat structure. Such a waveguide array is obviously a rather heavy structure and complicated to manufacture.
It was a great improvement when the printed circuit reflectarray was invented in 1980s. Here the waveguide array is replaced by a dielectric circuit board with a ground plane underneath and printed elements on the top. The elements can, be e.g., simple rectangular patches of conducting material. The incident field is diffracted at the edges of the patches and propagates further in the dielectric material beneath the patches from which it is reflected back to free space. By adjusting the size of the patches, the phase of the reflected field can be controlled. Such a configuration is much thinner, more lightweight and less expensive than the previous waveguide arrays.
Since then, much effort has been devoted to improve the simple patch elements with respect to phase range, bandwidth, polarization and other performance parameters. A huge variety of array-element designs for many different purposes can now be found in the literature. This includes contoured beams, LP (Linear Polarisation) to CP (Circular Polarisation) conversion, and also advanced designs for multibeam applications in which RHCP (Right Hand Circular Polarisation) and LHCP (Left Hand Circular Polarisation) are reflected in different directions.
A dual-band dual-polarised doubly curved reflectarray for multiple spot beam applications.
Research paper: Reflectarrays in future satellite antenna systems: Application and design
What is the benefit of reflectarray antennas?
The major benefit of reflectarray antennas is that it is possible to create a focused radiation from a passive, planar, thin and lightweight structure. Also, a reflectarray is easier and less costly to manufacture than a traditional curved metallic reflector, making them particularly interesting for nano-sat and CubeSat applications.
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A 8U CubeSat with a reflectarray antenna in an anechoic measurement chamber.
Furthermore, the enormous design freedom in the patch geometry has led to many innovative designs for special use, contoured beams, multiple spot beams, etc., as mentioned above.
Available tools for reflectarray antenna design
A reflector antenna can be designed by relatively simple software tools without any optimization algorithms. This is not the case for reflectarrys. Any decent design must use a periodic solver to compute the electromagnetic response from the array elements followed by the use of an advanced optimisation algorithm to adjust the geometrical parameters of the elements. Since there are often very many elements, the number of optimisation variables will typically be in the order of several thousands.
These complications have been a serious obstacle to the widespread use of reflectarryas, but now commercial software packages are available. The QUPES software is a dedicated tool for analysis and design of quasi-periodic surfaces such as reflectarrays, frequency selective surfaces, transmitarrays, and other similar surfaces. The software has been designed to be very efficient to allow optimization of a reflectarray within a computation time comparable to that of a traditional optimized reflector antenna.
When used in combination with other products in TICRA Tools, QUPES allows the user to design advanced high-performance reflectarray systems consisting of periodic-/quasi-periodic surfaces.
Visit product page for QUPES.