A new method developed for the design of microstrip reflectarrays is extremely efficient.
Traditionally microstrip or printed reflectarrays are designed using the “transmit mode” technique. In this method, the size of each printed element is chosen so as to provide the required value of the reflection phase such that a collimated beam results along a given direction. The reflection phase of each printed element is approximated using an infinite array model. The infinite array model is an excellent engineering approximation for a large microstrip array since the size or orientation of elements exhibits a slow spatial variation.
In this model, the reflection phase from a given printed element is approximated by that of an infinite array of elements of the same size and orientation when illuminated by a local plane wave. Thus the reflection phase is a function of the size (or orientation) of the element, the elevation and azimuth angles of incidence of a local plane wave, and polarization. Typically, one computes the reflection phase of the infinite array as a function of several parameters such as size/orientation, elevation and azimuth angles of incidence, and in some cases for vertical and horizontal polarization. The design requires the selection of the size/orientation of the printed element to realize the required phase by interpolating or curve fitting all the computed data. This is a substantially complicated problem, especially in applications requiring a computationally intensive commercial code to determine the reflection phase. In dual polarization applications requiring rectangular patches, one needs to determine the reflection phase as a function of five parameters (dimensions of the rectangular patch, elevation and azimuth angles of incidence, and polarization). This is an extremely complex problem.
The new method employs the reciprocity principle and reaction concept, two well-known concepts in electromagnetics to derive the receive mode analysis and design techniques. In the “receive mode design” technique, the reflection phase is computed for a plane wave incident on the reflectarray from the direction of the beam peak. In antenna applications with a single collimated beam, this method is extremely simple since all printed elements see the same angles of incidence. Thus the number of parameters is reduced by two when compared to the transmit mode design. The reflection phase computation as a function of five parameters in the rectangular patch array discussed previously is reduced to a computational problem with three parameters in the receive mode. Furthermore, if the beam peak is in the broadside direction, the receive mode design is polarization independent and the reflection phase computation is a function of two parameters only. For a square patch array, it is a function of the size, one parameter only, thus making it extremely simple.
The present method is substantially less intensive computationally. Since most practical antenna arrays require the design of a broadside beam or a single collimated beam, the receive mode design is expected to be substantially simpler than the traditional transmit mode design. In addition, when a designer needs to generate the reflection phase data using a computer intensive commercial software such as Ansoft HFSS, the reduction of computational effort in the receive mode will result in a substantial saving in design turnaround time. Similarly the receive mode analysis technique has potential to save computer time for large reflectarrays.
Microstrip reflectarrays have desirable features such as ease of design, manufacture, and deployment for application in many space-based radar and remote sensing systems. They are being investigated for many JPL systems such as SWOT (Surface Water Ocean Topography). The receive mode design and analysis technique is expected to find many future applications in NASA.