Aiming for the sky

7 March 2013

Research from France presents a compact, single-source subwavelength metamaterial directive Fabry-Pérot cavity antenna with 56° of beam steering aimed at aerospace applications.

Streamlined design

There is growing interest in reducing the cavity size of Fabry-Pérot (F-P) cavity antennas in order to ease their integration into structures like airfames. Modern civil aircraft carry an increasing number of antennas for systems ranging from safety-critical flight systems to in-flight entertainment. Some of these also demand high-performance ‘smart’ antennas with features including frequency agility and beam steering.

One such application is the provision of in-flight Internet services which requires an active antenna capable of tracking satellites. Current designs for this kind of antenna are large due to the complexity required. The antenna presented in this issue by researchers from the Institut d’Electronique Fondamentale (IEF) raises the possibility of replacing such complex and bulky designs with flattened agile antennas able to track satellite signals or shift in operating frequency. The IEF team also believe these antennas may be suitable for conforming onto non-planar surfaces such as the cylindrical skin of an aircraft, further saving space and potentially even drag.

Running through the phases

The IEF researchers have presented a F-P cavity antenna with a beam emitted in an off-normal direction, with low profile and very high beam steering. The steering range is more than double that previously achieved using a single radiating source. The use of a single source avoids the need for complex feeding structures and the thickness of the F-P cavity itself is only 2 mm or around λ/10.

F-P cavity antennas are composed of a primary radiating source placed in a resonant cavity formed by two reflectors. Generally, one reflector is the source’s metallic ground plane and the other is a partially reflecting surface (PRS). The PRS is a metasurface with a reflecting coefficient around 90%, which assures multiple reflections in the cavity. Using specific phase values from metasurfaces allows the cavity thickness, and so antenna profile, to be reduced. For example, using a reflection phase of +180° will lead to a cavity thickness of λ/2, a reflection phase of 0° from an artificial magnetic conductor will allow λ/4 and phases of -90° and -145° will respectively allow cavity thicknesses of λ/8 and λ/16.

The PRS used in this work consists of a periodic lattice of LC resonant cells acting as an array of micro-antennas. The antenna function can be described by following the paths of the EM waves undergoing multiple reflections inside the cavity. Phase shifts are introduced by the path length, ground plane and the phase of reflection coefficient of the PRS. This means that the thickness of the cavity antenna can be considerably reduced with an LC resonant PRS since reflection phase varies from +180° to -180°.

The direction of the beam emitted is defined by the phase difference between the resonant cells of the PRS. By adjusting the transmission phase through the PRS, the IEF researchers are able to control the direction of the emitted beam through applying a local phase shift between each row of cells in the PRS. The idea is based upon classical antenna arrays where phase shift is applied between each element to achieve beam steering. Team member Dr Shah Nawaz Burokur explained that “since the PRS is a composite inductive-capacitive grid (LC resonance), we have in our case modified the inductive grid by changing the geometrical parameters of the LC cells for the phase variation.”

Multifunctional

The IEF team are developing different versions of these antennas for applications in transport (train, air and spacecraft) in collaboration with several academic and industrial partners. The antennas are designed for multi-function use as by using a PRS incorporating lumped elements such as varactor diodes, they are able to simultaneously control the emission frequency by globally changing the average transmission phase of the PRS, and also the direction of emission by locally varying the phase along the PRS. This was not possible with previous versions of cavity antennas.

The features of these antennas should give them a wide range of applications. “We can imagine replacing classical parabolic satellite television antennas with these antennas, which are very discrete. They can be easily integrated in the wall of buildings or houses” said Burokur.

The IEF group’s wider work on metamaterials and metasurfaces includes cloaking, invisibility and illusion devices using transformation optics and the application of transformation optics to control of the flow of light in infrared-frequency silicon devices.

Further reading

This article is based on the Letter:Inductive-varying grid for highly beam-steerable cavity antennas (new window).

Institut d’Electronique Fondamentale (IEF): http://www.ief.u-psud.fr/ (new window).

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, volume 50, issue 08

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