Composite unidimensional metamaterial with locally variable phase

3.2 PEC-PRS cavity antenna

In order to simplify the fabrication of the cavity antenna, another one using only one metamaterial-based surface reflector acting as the PRS and a PEC reflector similar to the cavity shown in Fig. 1a is designed. As we have seen from the reflection coefficients in Fig. 3a and 3b, losses are maximum at the resonance frequency of the metamaterial-based surfaces. Thus using only one reflector has also the advantage of presenting lower losses. A new PRS composed of simultaneously a capacitive and an inductive grid on the two faces of a dielectric substrate is designed Fig. 2b. Concerning the metallic patches of the capacitive grid, a period p 2 = 5 mm and a width w 2 = 4.8 mm are used. A line width l = 2.2 mm is considered for the mesh of the inductive grid. This PRS having a resonance frequency of about 8 GHz presents a reflection phase close to -150° for frequencies higher than 10 GHz Fig. 5 a . The use of such a reflector in conjunction with a PEC leads also to a subwavelength cavity since the sum φ PRS + φ r is very close to zero between 9 GHz and 11 GHz. A 1 mm λ30 thick cavity is designed with lateral dimensions of 10 × 10 cm 2 where the resonance is achieved at around 9.7 GHz. At this resonance frequency, the cavity is well matched. The antenna gain patterns in the E- and H-planes obtained from simulation and measurements are presented in Fig. 5 b and 5c. a b c Fig. 5. a Calculated and measured reflection phase for the PRS reflector. Radiation patterns of the cavity antenna formed by one metamaterial-based reflector PRS and a PEC reflector together with a patch antenna at 9.7 GHz: b in the E- plane φ = 90°, c in the H-plane φ = 0°. In this case, despite the use of only one metamaterial-based surface as the PRS and the use of smaller lateral dimensions than the two metamaterial-based cavity, the antenna directivity is found to be twice and equal to 160 22 dB.

4. PASSIVE STEERING BEAM CAVITY ANTENNA

In this section, we report the design of a steerable directive beam antenna using a resonant metamaterial-based subwavelength cavity Fig. 6a. Generally, phased-arrayed antennas are used by imposing electrical phase differences between the antenna elements so as to steer a beam into a desired direction [18]. Although this solution offers fast scan speed, it requires many phase shifters connected to each antenna element, which is very expensive for communication systems. Our approach here is based on a cavity made of a conventional PEC ground plane and a novel composite unidimensional metamaterial with a locally variable phase acting as a Partially Reflective Surface PEC-PRS cavity of Fig. 1a. The antenna used is a rectangular microstrip patch designed to operate near 10 GHz. So, first of all we will describe the study and design of this new phase-varying metamaterial and then its application for a cavity antenna.

4.1 Composite unidimensional metamaterial with locally variable phase

The PRS reflector used for the modelling of the steerable directive beam antenna consists of a periodic array of copper strips mechanically etched on each face of the 1.4 mm thick FR3-epoxy ε r = 3.9 and tan = 0.0197 substrate as shown in Fig. 6b. The periodicity a and the width w of the strips are respectively 5 mm and 1.2 mm, optimized to have a resonance near 10 GHz and to provide a sufficiently high reflectance. 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. By changing the gap spacing g between the metallic strips of the capacitive grid of the PRS in only the direction parallel to E x–direction is chosen here and keeping all the other geometric parameters unchanged, the capacitance of the metamaterial will also vary along this same direction. As a consequence, the phases of the computed reflection and transmission coefficients vary. This behaviour is illustrated by the simulation results of the transmission coefficient phase shown in Fig. 7a. Due to the different gap spacing g, each PRS has a different transmission phase characteristic. We can note that the variation of g accounts for the shift of the resonance frequency. An increase in the value of g causes a decrease in the value of the capacitance created between two cells, and finally a shift of the resonance towards higher frequencies. At a particular frequency the phase of the PRS increases with an increase in the gap spacing. This phase shift especially for the transmission coefficient is very important since it will help controlling the radiated beam direction of the antenna. a b Fig. 6. a Schematic view of the cavity composed of a PEC and a metamaterial-based phase-varying PRS. b Elementary cell of the composite metamaterial, showing the capacitive and inductive grids.

4.2 Steerable directive antenna