Phase Correcting Fresnel Zoneplates

Abstract

Although the simple Fresnel zoneplate antenna with alternative reflective and transparent half-wave zones is easy to fabricate, its efficiency is too low for many practical applications. There are two factors that cause low efficiency to the simple Fresnel zoneplate. First, from a reception point of view, half of the energy intercepted by the aperture of a Fresnel zoneplate antenna is completely rejected. Second, the other half of the energy intercepted by the aperture is not efficiently focused into the feed, as the secondary radiation from a transparent half-wave zone does not add up completely in phase at the focal point. The first problem can be easily solved for reflection zoneplates, by placing a conducting plate at a quarter distance behind the simple zoneplate to form a phase reversal zoneplate reflector [4, 6]. The solution to the second problem is to apply phase correction techniques to the zoneplate surface [5]. Since a continuous phase compensation is very difficult to achieve, two sub-optimal phase correction techniques have been used for high efficiency zoneplate antennas, the subzone phase correction and the optimal discrete phase correction [5, 14, 19]. This chapter is concerned with the first technique and the second one, which is to employ an array of discrete phase shifters, will be dealt with later in chapter 6.