6.2RECONFIGURATION MECHANISMS

Methods for Achieving Radiation Pattern Reconfigurability  27

FIGURE 6.2:  Radiation pattern for U.S. coverage using reflector antenna equipped with the mechanically reconfigurable subreflector (Washington et al. [52], ©IEEE 2002).

Changes in an effective reflector have also been demonstrated using reconfigurable highimpedance surfaces [53]. Here, a lattice of small resonant elements produces high-surface impedance near their resonant frequency, creating, in effect, an artificial ground plane. Changing the capacitances between resonant elements across the lattice through mechanical means creates a phase gradient that can produce a steered beam when the lattice is used as a reflector. An electrically tuned version of this structure using varactors that can deliver two-dimensional beam steering is discussed in [54].

A closely related approach has also been used to develop a reconfigurable leaky-wave antenna using mechanical tuning [55]. In this case, a horizontally polarized antenna is used to couple energy into leaky transverse electric waves on a tunable impedance surface. A diagram of the surface, including the moveable top capacitance surface, is shown in Figure 6.4 [55]. The radiated beam from the surface can be steered in elevation over a range of 45° by changing the apparent capacitance of the surface through mechanically shifting the top capacitive plane. The antenna and the surface are shown in Figure 6.5 [55]. An electronically tuned version of this antenna that uses varactors can produce reconfigurable backward as well as forward leaky-wave beams [56, 57].

The electromechanically scannable trough waveguide antenna, first developed in 1960, is one early example of a pattern-reconfigurable antenna [58]. Based on an asymmetrical trough waveguide intended to feed a parabolic reflector [59], two different mechanisms were proposed to alter the phase velocity in the guide that resulted in scanned radiation patterns at a single frequency. The first mechanism involved the rotation of artificial dielectric structures along a longitudinal axis

28  Reconfigurable Antennas

FIGURE 6.3:  Radiation pattern for Brazil coverage using reflector antenna equipped with the mechanically reconfigurable subreflector (Washington et al. [52], ©IEEE 2002).

within the trough waveguide, and the second used a mechanical variation of the height of periodic structures located on the top of the center fin of the trough waveguide [58]. Although elegant in concept, the authors acknowledged at the time that both approaches presented major mechanical issues that they resolved using “cam-and-gear” solutions that were complex and cumbersome.

Huff and Bernhard [60] began with the concept of [58] and proposed the application of more modern perturbations and streamlined actuation mechanisms to achieve beam steering at a single frequency while preserving the wide bandwidth characteristics of the original antenna. The trough waveguide’s fundamental structure is shown in Figure 6.6 [60]. In the more modern embodiment, beam steering is achieved using micromachined cantilever structures fabricated within the trough waveguide, shown in Figure 6.7 [60]. These cantilevers, which can be actuated electrostatically or magnetostatically, create perturbations that change the apparent phase velocity in the waveguide and produce scanned radiation patterns. Simulations with actuation of individual cantilevers or groups of cantilevers demonstrate beam scanning at single frequencies through broadside for this traveling wave antenna [60].

30  Reconfigurable Antennas

FIGURE 6.6:  Trough waveguide dimensions (left), electric field lines of the dominant transverse electric mode with equivalent structure for design and field distribution (middle), electric field lines of the transverse electromagnetic mode (right) (Huff and Bernhard [60], ©Huff and Bernhard 2005).

FIGURE 6.7:  Trough waveguide with cantilever-type perturbations, showing staggered positions of actuated and rest states of the cantilevers (Huff and Bernhard [60], ©Huff and Bernhard 2005).

Another approach using micromachined components has also been demonstrated that uses mechanical changes in a hornlike antenna to produce reconfigurable radiation patterns with no appreciable changes in frequency response [61]. The basic antenna, shown in Figure 6.8 [61] is composed of a planar “V” structure with a coplanar transmission line feed. Micromachined rotational hinges fixed to the substrate material hold the ends of each of the “V” arms in place. MEMS actuators move the arms laterally to alter the radiated main beam direction and/or beam shape [61]. Figures 6.9 and 6.10 [61] show some of the possible beam variation achieved with this structure [61].

Chang et al. enable mechanical perturbation of propagation constants in a dielectric waveguide antenna operating at millimeter-wave frequencies to achieve pattern reconfigurability [62].