Electronically reconfigurable metamaterial for compact directive cavity antennas A. Ourir, S.N. Burokur and A. de Lustrac The modelling and characterisation of an electronically controllable metamaterial partially reflecting surface to a resonant cavity antenna near 8 GHz is considered. The metamaterial considered is composed of a composite phase varying metamaterial, by the insertion of active electro- nic components, and is proposed for the design of the reconfigurable Fabry-Perot cavity antenna. An adjustable resonance frequency between 7.9 and 8.2 GHz is obtained and a drastic enhancement in the directivity of the antenna is also observed for a cavity thickness as small as l=75. Introduction: The half wavelength restriction of a Fabry-Perot cavity antenna can be reduced to, respectively, a quarter wavelength [1] and a tenth (10th) wavelength [2] by making use of a metamaterial-based resonant cavity in order to design compact directive electromagnetic sources based on a single radiating antenna. The principle is based on the use of a partially reflective surface (PRS) with a frequency dependent reflection phase as a transmitting window. This PRS surface is made of a capacitive and an inductive grid and our group has lately further reduced the cavity thickness up to l=60 for applications to ultra-thin directive antennas [3]. In [4], we have also shown the possibility of obtaining a passive steerable directive antenna using a novel 1-D composite metamaterial with a locally variable reflection and transmission phase. In this present Letter, we present the modelling and characterisation of a resonant cavity for an electronically reconfigurable antenna near 8 GHz using an active phase-varying metamaterial. The cavity is composed of a PEC surface and a composite metamaterial acting as the PRS with a tunable reflection and transmission phase. The cavity resonance is presently shown to occur for a cavity thickness as small as 0.5 mm (l=75). This resonance frequency is adjustable between 7.9 and 8.2 GHz. Further- more, a drastic enhancement of the antenna directivity is obtained. Design and characterisation of electronically tunable phase metama- terial: The cavity considered here is composed of the patch antenna’s PEC ground plane and the PRS reflector placed at a distance h above the antenna (Fig. 1a). The PRS reflector consists of a periodic array of copper strips mechanically etched on each face of a 1.4 mm-thick FR3-epoxy (e r ¼ 3.9 and tand ¼ 0.0197) substrate as shown in Fig. 1b. The upper array where the strips are oriented parallel to the electric field E of the antenna plays the role of the inductive grid, whereas the lower array where the strips are oriented parallel to the magnetic field H acts as the capacitive grid. Instead of applying a linear variation of the gap spacing g in order to create a locally variable phase as in [4], we now use active components to make the phase of the PRS shift in frequency. Varactors are thus incorporated into the capacitive grid between two adjacent metallic strips (Fig. 1b) and depending on the applied bias voltage, the phase of the PRS varies with frequency. A prototype of the PRS is designed and fabricated where all the gaps present the same capacitance according to the bias voltage applied. To do so, 8  8 unit cells are used and the overall dimensions of the prototypes are 60  60  1.4 mm 3 . The variable capacitive grid of the tunable phase PRS used for this work consists of a lattice of metallic strips with varactors connected each 6 mm (s ¼ 6 mm) between two adjacent strips. The width of the strips and the spacing between two strips of the capacitive grid is, respectively, w ¼ 1 mm and g ¼ 2 mm (Fig. 1b). RF inductances are used in the microstrip circuit in order to prevent high frequency signals going to the DC system. Concerning the inductive grid, the width of the strips and the spacing between two strips are, respectively, w 1 ¼ 2 mm and g 1 ¼ 4 mm. Note that the inductive grid is not made tunable. By changing the bias voltage of the varactors of the PRS, the capacitance of the metamaterial will also vary. As a consequence, the phases of the reflection and transmission coefficients also vary as it has been observed in [4]. This behaviour is illustrated by the measurement results of the reflection coefficient phase shown in Fig. 2. The measurements are performed in an anechoic chamber using two horn antennas working in the [2–18 GHz] frequency band and a 8722ES network analyser as described in [3]. From Fig. 2, we can note that the variation of the bias voltage accounts for the shift of the resonance frequency of the PRS, i.e. the frequency where the phase crosses 0  . An increase in the bias voltage leads to a decrease in the value of the capacitance of the metamaterial, and finally a shift of the resonance towards higher frequencies. This phase shift is very important since it will help to tune the resonance frequency of the cavity antenna. Fig. 1 Schematic view of cavity composed of PEC and electronically phase-varying PRS, and photographs of both sides of composite meta- material, showing capacitive grid with varactors and inductive grid a Schematic view of cavity b Photographs of both sides of composite metamaterial Fig. 2 Reflection coefficient phase of PRS for different bias voltage V of varactors A shift towards high frequencies and a decrease of the slope of the phase variation around 8 GHz are noted when V increases Tunable resonant frequency cavity antenna: A prototype of the subwavelength cavity was fabricated. The PRS studied above is placed at a distance h ¼ 0.5 mm above the feeding source which is a rectangular patch antenna with dimensions 9  9 mm 2 . This patch antenna alone has a resonance frequency of 8.1 GHz. The resonance frequency of the proposed cavity, which depends on the phase of the reflection coefficient of the PRS and also the height, is found to be in the vicinity of 8 GHz for the different bias voltages applied. Fig. 3a shows the matching of the cavity antenna with different bias voltage of the varactors. Two important observations are made and are illustrated in this Figure. First, as for the reflection phase response, a shift towards high frequencies of the cavity resonance is noted when the bias voltage varies from 0 to 10 V. This is due to the decrease in the capacitance of the PRS. Moreover, it can be observed that the dip of the resonance increases when the bias voltage is tuned from 0 to 10 V, indicating an enhancement of the matching of the cavity. This enhancement is explained by the fact that the cavity antenna is better and better matched for the thickness h ¼ 0.5 mm fixed when the bias voltage varies from 0 to 10 V. If we change the thickness h of the cavity, we obtain a better ELECTRONICS LETTERS 21st June 2007 Vol. 43 No. 13