a b
Fig. 7. a Transmission coefficient phase of the metamaterial-based PRS shown in inset with different gap spacing g. b Measured gain patterns of the cavity antenna in the E-plane
φ = 90° at 10.5 GHz for g = ±100 µm, g = ±50 µm and g = 0 µm.
5. ELECTRONICALLY RECONFIGURABLE CAVITY ANTENNA
This section deals with the modeling and characterization of an active reconfigurable subwavelength metamaterial-based cavity antenna. The aim of this active reconfigurable cavity is to be able to control dynamically the antenna beam in the
near future. Electronically controllable textured surfaces have been applied to leaky wave antennas [19] and radomes [20]. The tunable PRS used in this section is composed of a composite phase varying metamaterial, by the insertion of
active electronic components, and is proposed for the design of the reconfigurable Fabry-Perot cavity antenna. The study and design of this active phase-varying metamaterial tunable PRS is described and its application for a frequency
switching cavity antenna is presented.
5.1 Electronically phase-varying metamaterial
This PRS is the same as the one as illustrated in Fig. 6b. Instead of applying a linear variation of the gap spacing g in order to create a locally variable phase, we now use active components to make the phase of the PRS shift in frequency.
Varicap diodes are thus incorporated into the capacitive grid between two adjacent metallic strips Fig. 8a and depending on the applied bias voltage, the phase of the PRS varies with frequency. A prototype is designed and
fabricated where all the gaps have the same capacitance according to the bias voltage applied. The variable capacitive grid of the tunable phase PRS used for this work consists of a lattice of metallic strips with varicap diodes connected
each 6 mm s = 6 mm between two adjacent strips. The width of the strips and the spacing between two strips of the capacitive grid is respectively w = 1 mm and g = 2 mm Fig. 8b. Concerning the inductive grid, the width of the strips
and the spacing between two strips are respectively w
1
= 2 mm and g
1
= 4 mm Fig. 8c. Note that the inductive grid is not made tunable.
Measurements are done in an anechoic chamber so as to observe the frequency shift of the reflection phase or also the phase jump at a desired frequency when the bias voltage is tuned. Fig. 9a shows the measured reflection phase
coefficient of the PRS for a bias voltage of 0, 2 and 5 V. When the bias voltage is tuned from 0 to 5 V, the transmission phase response shifts towards higher frequencies. Particularly the resonance frequency of the PRS, corresponding to the
null of the phase, shifts from 7.5 to 7.9 GHz. This is explained by the fact that when the bias voltage increases, the capacitance of the PRS decreases causing a shift towards higher frequencies.
a
b c
Fig. 8. a Structure of the cavity antenna. b Photo of the prototype showing the side of capacitive grid of the PRS with the varicap diodes. c Inductive grid of the PRS.
a b
Fig. 9. a Electronically phase-varying PRS. a Reflection phase coefficient S
11
deg for 0 V, 2 V and 5 V. b Measured return loss S
11
dB of the cavity for 0 to 10 V applied bias voltage.
5.2 Reconfigurable cavity antenna
