Low Loss Steerable Reflectarray antenna for Space application Hervé LEGAY(1), Béatrice PINTE(1), Etienne GIRARD(2), Raphael GILLARD(2), Michel CHARRIER(3), Afshin ZIAEI(3) (1)

Alcatel Space Industries 26, Avenue J.F Champollion, BP 1187 31037 Toulouse Cedex 1, France Email: [email protected]; [email protected] (2)

IETR-INSA 20 avenue des buttes de coësmes, CS 14315 35043 Rennes Cedex France Email: [email protected]; [email protected] (3)

Thalès R&T Domaine de corbeville 91404 Orsay Cedex, France Email: [email protected], [email protected]

INTRODUCTION This communication presents some results issued from the French RNRT ARRESAT project, which concerns the feasibility study and the making of a phase-steerable reflectarray antenna for space applications. A reflectarray is composed of antenna elements including phase control through miniature switches. It is illuminated by a conventional antenna feed and each phase-shifting cell receives and transmits the wave from the feed with the required phase. It is consequently possible to steer and form a beam by controlling the phase of the phase-shifting cells. The novelty of a reflectarray is that it combines the advantages of an array (fixed antenna) and of a reflector (a single, high-efficiency source of power). The integration of phase-shifters inside the antenna elements also results in a high integration of the antenna. The steerable reflectarray is consequently of a far lower potential cost than an active antenna. The critical points identified are as follows: - beam forming performance from phase-shifting cells with a limited number of phase states; - feasibility of broadband reflecting phase-shifting cells; - design and production of MEMS switches; - feasibility of low-loss, reflecting phase-shifting cells through the insertion of MEMS switches;

BEAM FORMING WITH A STEERABLE REFLECTOR ARRAY Specifications A steerable reflectarray antenna is designed to meet in-flight reconfigurability requirements for space applications. These requirements may be of the following origins: Ø Adaptation to traffic, Modification of the mission; Ø Minimized interference; Ø Production of fixed spot beams on ground from an LEO satellite;

Application to a multimedia Ka-band system using a constellation of LEO satellites was defined as the scope of the study. The possibility of controlling the beam requires the design of steerable phase-shifting cells. The number, size and especially the potential number of phase states of these cells is optimized to be able to comply with performance for minimum complexity. Coverage specifications are given below. An antenna forms a spot beam on a fixed zone on ground and coverage changes as the satellite rotates in its low Earth orbit.

1

Footprint viewed from the satellite at t0 Figure 1: Coverage specification Antenna design The antenna proposed is presented in Figure 2.

Figure 2: Reflector array design

Same at t0+∆t

2

3

The shape of the array is optimized to make elliptical spot beams, whose ellipticity varies between 1 and 3, with elevation dispointing of between 0° and 50° with respect to the nadir. The array is composed of 130 phase-shifting cells. The cell diameter is 0.7 λ, where λ is the wavelength. Maximum gain is specified for spot beams on the edge of the Earth: it is mainly the shape of scanned spot beams which determines the geometry of the radiating aperture. The antenna in question will therefore be of an elliptical shape, favourable to making spot beams with an ellipticity of more than 1. The antenna points to a non-null elevation, to improve synthesized gain on the most dispointed spot beams. The shape of the spot beam is reconfigured for the centre spot beams (zooming effect) by one phase law only. Three specific spot beams, numbered 1, 2 and 3 in Figure 3 have been selected to optimize the antenna:

SPOT BEAM 1

SPOT BEAM 2 Figure 3: Synthesis of spot beams

Synthesis is optimum for spot beam 2 towards which the antenna is pointed. It is tricky for the center spot beam, for which the antenna is oversized. For the highly dispointed spot beam, an array lobe appears outside Earth coverage. Performance sensitivity to phase dispersions has been studied.

PHASE-SHIFTING CELLS design specifications Design specifications are as follows: • • •

circular polarization low loss broadband (17.8-19.3 GHz)

SPOT BEAM 3

phase-shifting principle Several techniques are used to phase shift the wave transmitted by a reflectarray. The first is to add a stub to array elements, where the length of the stub controls the phase shifting provided [1]. Another solution is to use array elements of different sizes [2]. These solutions were not selected because of their intrinsically low-band nature. The phase shifts very quickly to the resonance of the array elements. One promising solution, underlined by Montgomery in 1978, and adapted to circular polarization has been adopted. It consists in working with an array of identical elements which have undergone different rotations. The property used is as follows: The response of an element to a circularly polarized field depends on the orientation of this element [3]. If it is oriented by angle φ, the phase of the field reflected by the antenna element varies by 2φ. In this way we achieve broader band operation as the phase shift is not obtained through variation of the dipole resonance. The phase-shifting cell developed makes the most of this property. It is illustrated in diagram form in Figure 4. Upper etching is composed of dipole elements set out concentrically around a central pellet. This etching is located a quarter wavelength from a ground plane. Two opposed dipole elements are then connected using low-loss switches to the centre pellet to create a resonating dipole. Trous métallisés: passage des commandes Commandes BF

λ/4

Cellule mono-polar

Plan de masse Multicouche BF

Figure 4: Six phase state cell The active dipole reflects the component of the incident field parallel to it, while the orthogonal component is reflected by the ground plane. A distance of a quarter wavelength between the etching and the ground plane guarantees circular polarization. The phase shift obtained then depends on the orientation of the dipole built. To break the coupling between dipoles which disturbs cell operation, the cells are inserted in a metallic cavity. Consequently, the field reflected by a cell is independent of the orientation of neighbouring cells. The cell operates in guided mode. It is therefore considered to be independent of the angle of incidence of the illuminating plane wave.

Validation of operation through electromagnetic simulation The first simulations dealt with optimizing a passive phase-shifting cell. To simplify the simulation, the metal recess is of a square shape. The optimized etching is composed of dipole elements concentrically arranged around a centre pellet. Two opposed elements are directly connected to make a dipole. Orientation of this dipole is successively varied. z a a

H

y εr

h x

Figure 5: Description of the simulated passive mock-up Performances were simulated on an FDTD tool, and on the commercial HFSS simulator. Simulations confirm that dipole orientation generates electrical phase-shifting equal to double the angle of rotation of the dipole. The ellipticity simulated is presented in Figure 6. It is less than 0.4 dB throughout the 17.8-19.3 GHz band.

Axial Ratio (dB) 0.6 0.5 0.4 0.3 -60°and 60° 0.2 -30°and 30° 0.1 0° 0 17.8

18.3

18.8

19.3

Fréquence (GHz) Figure 6: Ellipticity over a frequency band for different dipole orientations (Simulations Results from FDTD Software)

DESIGN AND FABRICATION OF LOW-LOSS SWITCHES General Making RF MEMS switches is a technological challenge. The MEMS switch has the following advantages compared to a PIN diode switch: •

Very low consumption;

• • • •

Very low on-state loss; High linearity; High integration capacity A collective technology designed to reduce costs and production time

The critical points according to which design must be optimized are as follows: • • • •

Activation voltage; Switching time; Behaviour in microwave frequency; Reliability;

Making a switch The first step consisted in making the technological choices (materials, processes), and the electrical and mechanical design of an MEMS switch. The switch made is a “diaphragm” type. Several materials were selected for the substrate: Silicon and glass. Switches were made in Aluminium.

Ground membrane

MEMS switch principles à

command electrode

signal RF in

_______________________ Ground

First switches wafer æ

signal RF out

Figure 7: Manufacturing MEMS switches Performances measured on the switch made are summed up in the following table: ISOLATION INSERTION LOSSES START-UP VOLTAGE ON-STATE CAPACITY OFF-STATE CAPACITY Con/Coff RATIO RESPONSE TIME AIR GAP (between diaphragms and dielectric) DIAPHRAGM DIAPHRAGM METALLIZATION DIELECTRIC CONSTANT USED VSWR

32 dB (40 GHz) – 18 dB (20 GHz) 0.2 dB (40 GHz) – 0.1 dB (20 GHz) 32-38 Volts 30-40 fF 3-4 pF 100