Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRThe curvature of an array antenna’s aperture plane fundamentally affects parameters such as the mutual coupling of the radiating elements and, thus, the active scan impedance of each radiator. For practical applications, it is important to numerically predict these effects efficiently.
Coupling in Broadband Phased Arrays
The numerical study of radiation properties of electrically large array antennas is highly CPU-intense. For time reasons, idealized radiators are frequently modeled, whose properties are independent of their environment. The far field of such arrays can be specified easily, but in practice this approach is often too inaccurate. Real radiators couple, among other things, via wave propagation on the carrying structure, free-space radiation, or imperfection of feeding networks. This leads to noticeable deviations between an ideal behavior and measurements: from moderate performance degradation up to total loss of a radiating capability at certain scan angles (blind spots).
For future systems, it is not sufficient to only consider narrow-banded coupling or coupling within a single group. Due to the increasing number of broadband arrays on platforms, broadband coupling effects between multiple groups or between multiple modes must be analyzed.
Development of Tailored Modeling Tools
For electrically large planar structures with regularly arranged radiators, the coupling can be examined with good accuracy, at relatively low computational costs using so-called Floquet modes, with the exception of edge elements. However, if an electrically large array is curved, there are currently no suitable commercial modeling tools available. At Fraunhofer FHR, specialized numerical methods were tested with the goal of predicting efficiently the coupling between radiators while taking the curvature into consideration.
Metrological Investigations
To verify the developed methods, a broadband, dual-polarized antenna demonstrator was designed. It is designed in a modular format for the X-band. Individual linear array modules are used as basic elements. These consist of so-called »stacked patch antennas« (Figure 1). By means of »3D printed« parts (Figure 2), a set of linear array modules can be assembled under laboratory conditions for apertures with different radii of curvature (Figure 3). To be able to specifically observe the creation of interferences such as blind spots and grating lobes (significant radiation emittance into unwanted solid angles), the radiator pattern can be varied using printed blind modules. First measurements on isolated radiators show a high level of consistency with the simulation.
Outlook
Coupling has been measured for typical curvature profiles. The developed numerical methods will be validated for accuracy and used to reduce coupling. Known approaches include decoupling networks integrated into feed networks or purpose-built covers that, in the case of blind spots, reduce reactive storage of field energy before the aperture (Wide Angle Impedance Matching Structures, WAIM). Such WAIMs are to be designed at Fraunhofer FHR within the scope of the EDA project METALESA II for typically occurring curvature profiles and to be verified experimentally using the existing demonstrator.