Investigations into phasing characteristics of printed single and double cross elements for use in a single layer microstrip reflectarray

In this paper, we focus our considerations on a single layer microstrip reflectarray because it is easy to manufacture. By selecting suitable phasing elements we aim to overcome the main problem faced in many single-layer microstrip reflectarray designs, which are reduced phase range and a large phase gradient. The reduced phase range and the sharp slope contribute to the phasing errors of the reflectarray that in turn degrade its directivity. We turn our attention to multi-resonant variable size planar antenna elements, for which we have shown in that they can offer an extended phase range when used in a unit cell of a single-layer reflectarray. Here we choose printed double cross elements to form a reflectarray unit cell with an extended phase range and a lower slope. In the presented investigations, we show that a unit cell containing a single resonant cross still faces similar phase range and slope problems as a unit cell with single dipoles or patches. By using a double cross element on the same substrate we demonstrate that the phase range can be doubled. Next, to reduce the slope and still keep the phase range more than 360deg, we use a thick composite material including a thin laminate and a thick layer of foam to support the double cross. Such substrate combination offers phase responses of reduced slopes over an acceptable phase range. We compare the obtained results with those for double stacked patches developed on similar thickness foam.