{{short description|Beam focusing, typically horn-fed planar array of unit cells}} [[File:reflectarray.png|300px|thumb|Planar reflectarray fed by a [[horn antenna]]. Structure showing unit cells.<ref name="deng" /> This antenna has fixed cell delays, configured by different cutout sizes in each cell, as can be seen in the top-right inset. Larger cutouts produce the dark rings visible in the overview.]]
A '''reflectarray antenna''' (or just '''reflectarray''') consists of an array of unit cells, illuminated by a feeding [[Antenna (radio)|antenna]] (source of [[electromagnetic radiation|electromagnetic waves]]).{{r|deng|pozar}} The feeding antenna is usually a [[Horn antenna|horn]]. The unit cells are usually backed by a [[ground plane]], and the incident wave reflects off them towards the direction of the [[Beamforming|beam]], but each cell adds a different [[phase delay]] to the reflected signal. A [[Phase (waves)|phase]] distribution of [[Zone plate|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.<ref name="nayeri" /> It is common to offset the feeding antenna to prevent blockage of the beam.<ref name="huang" /> In this case, the phase distribution on the reflectarray surface needs to be altered. A reflectarray [[Focus (optics)|focuses]] a beam in a similar way to a [[Parabolic antenna|parabolic reflector]] (dish), but with a much thinner form factor.
== Phase distribution ==
According to,<ref name="carrier_hum" /> 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: <math> \frac{2\pi}{\lambda_0} \left(r_{mn} - R_{mn}.r\right) - \Delta \phi_{mn} = 2 \pi N </math>
where <math>\lambda_0</math> is free space wavelength,
<math>r_{mn}</math> is the position vector of the <math>{mn}</math>th element/unit cell relative to <math>(0,0,F)</math>,
<math>F</math> is the focal length,
<math>R_{mn}</math> is the position vector of <math>{mn}</math>th element relative to the origin <math>(0,0,0)</math> i.e. centre of the reflectarray,
<math>r</math> is the direction vector of the desired pencil beam,
<math>N = 1,2,3,...</math>,
and <math>\Delta \phi_{mn}</math> is the phase shift introduced by <math>{mn}</math>th unit cell of reflect array to its reflected field relative to the incident field.
For a feed [[Horn antenna|horn]] located at <math>(0,0,F)</math>, the formula for the optimal [[Phase (waves)|phase]] distribution on a conventional reflectarray for a beam in the boresight direction is given by:
<math> \Delta \phi(x_m, y_m) = \frac{2\pi}{\lambda_0}\sqrt{x^2 + y^2 + F^2} </math>
where <math>\Delta \phi(x_m, y_m)</math> is the [[Phase (waves)|phase]] shift for a unit cell located at coordinates <math>(x_m, y_m)</math>.
[[File:reflectarray rad pattern.png|300px|thumb|Simulated and measured [[radiation pattern]]s of a reflectarray antenna operating at 12.5 GHz.<ref name="deng" />]]
== Unit cell considerations ==
It is important to analyse the reflection magnitude <math>|S_{11}|</math> and the reflection [[Phase (waves)|phase]] <math>\angle S_{11}</math> across the frequency [[Bandwidth (signal processing)|bandwidth]] of operation. When designing a reflectarray, we aim to maximise the reflection magnitude <math>|S_{11}|</math> to be close to 1 (0 dB).<ref name="ghulam_ahmad2" /> The reflection [[Phase (waves)|phase]] <math>\angle S_{11}</math> at each unit cell determines the overall beam shape and direction. Ideally, the total phase shift range would be 360°.<ref name="deng" /> The [[aperture efficiency]], and hence [[Antenna gain|gain]], of the reflectarray will be reduced if the [[Angle of incidence (optics)|angle of incidence]] to the unit cells is not considered, or if spillover occurs or illumination of the reflectarray is not optimal (see also [[Transmitarray antenna|transmitarrays]]).<ref name="pozar" /> Similarly, phase errors due to [[Quantization (signal processing)|quantization]] into a discrete number of phase states for digital control can also reduce the [[Antenna gain|gain]].<ref name="ghulam_ahmad" />
A fixed reflectarray has a single beam direction per feed. Changing the shape of the unit cells alters their reflection phase. The unit cells cannot be reconfigured. This has applications in point-to-point communications, or for a [[satellite]] covering a specific geographic region (with a fixed beam contour).<ref name="legay" /> A reconfigurable reflectarray has unit cells whose [[Phase (waves)|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 diode]]s,<ref name="huanhuan_yang" /><ref name="zawawi" /> [[liquid crystal]],<ref name="bildik" /><ref name="perez-palomino_thesis" /><ref name="perez-palomino_2012" /><ref name="perez-palomino_2015" /> and novel materials. Each of these methods introduces loss which reduces the efficiency of the unit cells. [[Intermodulation|Linearity]] (such as distortion due to the diodes) also needs to be considered to minimise [[Adjacent-channel interference|out-of-band radiation]] which could interfere with users on adjacent frequencies.<ref name="sharahil" />
== Other types of reflectarrays ==
In [[satellite]] communications, it is necessary to produce multiple beams per feed, sometimes operating at different frequencies and [[Polarization (waves)|polarizations]].<ref name="martinez-de-rioja_EuCAP" /> An example of this is the four-color [[Cellular network#Frequency reuse|frequency reuse]] scheme.<ref name="martinez-de-rioja_EuCAP" /> [[Circular polarization]] is commonly used to reduce the effect of atmospheric depolarization on the communication system performance.<ref name="difonzo" /> A dual-band reflectarray has two different passband frequencies, for example for [[Telecommunications link#Uplink|uplink]] and [[Telecommunications link#Downlink|downlink]].<ref name="naseri" /> A bifocal reflectarray has two principle [[Focus (optics)|foci]], so can focus [[wavefront]]s to or from two feeding antennas simultaneously.<ref name="martinez-de-rioja" /> A dual reflectarray consists of two stages of reflection, in which the beam is first focused by one reflectarray, then by another. The [[Phase (waves)|phase]] distribution on each reflectarray must be carefully calculated to ensure that the phase [[Partial derivative|derivatives]] are consistent with the [[Angle of incidence (optics)|angle of incidence]] of the rays <ref name="martinez-de-rioja" /> The ratio of the sizes and positions of these reflectarrays can be used to achieve [[Quasioptics|quasi-optical]] [[magnification]] (narrowing of the beam).<ref name="scianella" />
== See also == *[[Phased array]] *[[Metamaterial]] *[[Transmitarray Antenna]] *[[Extremely high frequency|Millimeter Wave]] *[[Beamforming]]
== References ==
{{Reflist|refs= <ref name="pozar">[[David M. Pozar|D. M. Pozar]], S. D. Targonski, and H. D. Syrigos, "Design of millimeter wave microstrip reflectarrays," IEEE Transactions on Antennas and Propagation, vol. 45, no. 2, pp. 287–296, 1997.</ref> <ref name="deng">R. Deng, F. Yang et al., "A Low-Cost Metal-Only Reflectarray Using Modified Slot-Type Phoenix Element with 360° Phase Coverage," IEEE Transactions on Antennas and Propagation, vol. 64, no. 4, pp. 1556–1560, 2016.</ref> <ref name="nayeri">P. Nayeri, F. Yang, and A. Z. Elsherbeni, "Beam-scanning reflectarray antennas: A technical overview and state of the art," IEEE Antennas and Propagation Magazine, Aug. 2015, pp. 32-47.</ref> <ref name="huang">Y. Huang and K. Boyle, "Antennas: From Theory to Practice", 1st ed. Chichester, UK: John Wiley & Sons Ltd, 2008.</ref> <ref name="carrier_hum">S. V. Hum and J. Perruisseau-Carrier, "Reconfigurable reflectarrays and array lenses for dynamic antenna beam control: A review," IEEE Transactions on Antennas and Propagation, vol. 62, no. 1, pp. 183–198, 2014.</ref> <ref name="huanhuan_yang">H. Yang, F. Yang, et al, "A 1600-element dual-frequency electronically reconfigurable reflectarray at X/Ku-band," IEEE Transactions on Antennas and Propagation, vol. 65, no. 6, pp. 3024–3032, 2017.</ref> <ref name="martinez-de-rioja">E. M. Martínez de Rioja del Nido, "New Advances on Multi-Frequency and Multi-Beam Reflectarrays within Application to Satellite Antennas in Ka-Band," Ph.D. dissertation, Universidad Politécnica de Madrid, Spain, 2018.</ref> <ref name="martinez-de-rioja_EuCAP">D. Martínez-de-Rioja, E. M. Martinez-de-Rioja, and J. A. Encinar, "Preliminary Simulations of a 1.8-m Reflectarray Antenna in a Geostationary Satellite to Generate Multi-Spot Coverage," in Proc. 13th European Conference on Antennas and Propagation (EuCAP 2019), Krakow, Poland, 2019.</ref> <ref name="naseri">P. Naseri and S. V. Hum, "A Dual-Band Dual-Circularly Polarized Reflectarray for K/Ka-Band Space Applications," IEEE Transactions on Antennas and Propagation, 2020 (early access).</ref> <ref name="difonzo">D. F. DiFonzo, "The Electrical Engineering Handbook", 2nd ed. Boca Raton, FL, USA: CRC Press LLC, 2000, vol. 74, ch. Satellite and Aerospace.</ref> <ref name="sharahil">M. S. Shaharil, "Nonlinear characterisation of reconfigurable antennas," Ph.D. dissertation, University of Birmingham, UK, 2016.</ref> <ref name="scianella">C. Sciannella, G. Toso, "An Imaging Reflector System with Reduced Scanning Aberrations," IEEE Transactions on Antennas and Propagation, vol. 63, no. 1, pp. 1342–1350, 2015.</ref> <ref name="legay">H. Legay et al., "A 1.3 m facetted reflectarray in Ku band," in Proc. 2012 15th International Symposium on Antenna Technology and Applied Electromagnetics (ANTEM), 2012.</ref> <ref name="zawawi">M. N. Bin Zawawi, "New Antenna for Millimetre Wave Radar," Ph.D. dissertation, Université Nice Sophia Antipolis, France, 2015.</ref> <ref name="ghulam_ahmad">G. Ahmad, T. W. C. Brown, C. I. Underwood, and T. H. Loh, "How coarse is too coarse in electrically large reflectarray smart antennas? A phase quantization case study at millimeter wave bands," in 2017 International Workshop on Electromagnetics: Applications and Student Innovation Competition, May 2017, pp. 135–137.</ref> <ref name="ghulam_ahmad2">G. Ahmad, T. W. C. Brown, C. I. Underwood, and T. H. Loh, "An investigation of millimeter wave reflectarrays for small satellite platforms," Acta Astronautica, vol. 151, June 2018, pp. 475–486.</ref> <ref name="bildik">S. Bildik, S. Dieter, C. Fritzsch, W. Menzel, and R. Jakoby, "Reconfigurable folded reflectarray antenna based upon liquid crystal technology," IEEE Transactions on Antennas and Propagation, vol. 63, no. 1, pp. 122–132, 2015.</ref> <ref name="perez-palomino_thesis">G. Perez-Palomino, "Contribution to the Analysis and Design of Reflectarray Antennas for Reconfigurable Beam Applications above 100 GHz using Liquid Crystal Technology," Ph.D. dissertation, Universidad Politécnica de Madrid, Spain, 2015.</ref> <ref name="perez-palomino_2012">G. Perez-Palomino, J. Encinar, M. Barba, and E. Carrasco, "Design and evaluation of multi-resonant unit cells based on liquid crystals for reconfigurable reflectarrays," IET Microwaves, Antennas & Propagation, vol. 6, no. 3, pp. 348–354, 2012.</ref> <ref name="perez-palomino_2015">G. Perez-Palomino, M. Barba, J. A. Encinar, R. Cahill, R. Dickie, P. Baine, and M. Bain, "Design and Demonstration of an Electronically Scanned Reflectarray Antenna at 100 GHz Using Multiresonant Cells Based on Liquid Crystals," IEEE Transactions on Antennas and Propagation, vol. 63, no. 8, pp. 3722–3727, 2015.</ref> }}
{{Antenna Types}}
[[Category:Antennas (radio)]] [[Category:Radio frequency antenna types]]