Microstrip reflectarray antennas promise to provide the best features of printed phased-array and parabolic reflector antennas. Such antennas comprise a feed horn, which illuminates a flat array of reflection elements. To produce a planar phase front in a desired direction, each element's reflection phase has to be adjusted within a 360-deg. range. For beam-scanning applications, the electronically tunable microstrip reflectarray technology provides advantages over an active phased array realized with transmit/receive modules. It eliminates the need for a beamforming network while requiring less active components. At the Ecole Polytechnique in Montreal, Canada, Mathieu Riel and Jean-Jacques Laurin designed a C-band electronically beam-scanning reflectarray that operates without a beamforming network.
This reflectarray comprises a microstrip patch that is printed on a flexible membrane substrate. It is then aperture-coupled to a transmission line loaded with two varactor diodes. When the aperture-coupled antenna is perfectly matched, the microstrip lines are lossless and radiation leakage from the slot can be neglected. The phase variation of the reflection co-efficient, Γtot, will be the same as the phase variation of the reflection co-efficient, Γe.
The designed element allows continuous tuning of the reflected signal's phase over a 360-deg. range with a maximum loss of 2.4 dB at 5.4 GHz. According to the measured results on a 30-element reflectarray breadboard, the main beam can be steered to large angles by adjusting the bias voltages on each element. See "Design of an Electronically Beam Scanning Reflectarray Using Aperture-Coupled Elements," IEEE Transactions on Antennas and Propagation, May 2007, p. 1260.