Fresnel zone antenna

Lore & Background

The simplest Fresnel zone plate antenna is the circular half-wave zone plate, invented in the nineteenth century. It divides a plane aperture into circular zones so that radiation from each zone arrives at the focal point in phase within ±π/2 range. By suppressing or phase-shifting radiation from alternate zones by π, an approximate focus is obtained. Despite its simplicity, the half-wave zone plate remained mainly an optical device for a long time due to its low efficiency (less than 20%) and high sidelobe levels. In 1948, Maddaus published work on stepped half-wave lens antennas achieving sidelobe levels around -20 dB. In 1961, Buskirk and Hendrix reported a circular phase reversal zone plate reflector with sidelobes as high as -10 dB. Black and Wiltse in 1987 achieved about -20 dB sidelobe level with a stepped quarter-wave zone plate. Huder and Menzel in 1988 obtained 25% efficiency and -15 dB sidelobe level with a phase reversal zone plate reflector. NASA researchers in 1989 measured 5% bandwidth and -15 dB sidelobe level on a similar antenna. Until the 1980s, the Fresnel zone plate antenna was regarded as a poor candidate for microwave applications, but the development of DBS services provided a commercial push for research.

Reader's Guide

The significance of the Fresnel zone antenna lies in its unique combination of low cost, flat profile, and high gain, which makes it attractive for applications where traditional reflector or lens antennas are impractical. Its flat nature reduces wind loading to as little as 1/8 of conventional reflectors, and it can be flush-mounted, printed on windows, or made conformal to vehicles. At millimetre-wave frequencies, it can be integrated with MMICs, becoming competitive with printed antenna arrays. The offset Fresnel zone plate allows flush mounting on walls or roofs and can be produced by painting a zonal pattern on window glass or blinds for DBS reception. Phase correcting techniques, such as sub-zone phase correction and multilayer dielectric slabs, improve efficiency, though complexity may offset advantages. The single-layer printed flat reflector and the reflectarray concept—using an array of phase-shifting elements illuminated by a feed—offer reconfigurability and practical configurations. Beamsteering can be achieved with amplitude-only control, avoiding bandwidth-limiting phase shifters. Three-dimensional Fresnel antennas extend the concept further. Overall, the Fresnel zone antenna provides a versatile, cost-effective alternative for applications from DBS to millimetre-wave systems.

Did You Know?

• A flat Fresnel zone antenna is termed a Fresnel zone plate antenna.
• The wind loading force of a Fresnel zone plate can be as little as 1/8 of that of conventional reflectors of similar size.
• An offset Fresnel zone plate can be flush mounted to a wall, printed on a window, or made conformal to a vehicle body.