Focussed reflections off spherical radomes for active phased arrays

Spherical radomes have successfully been used as environmental protection for large reflector and passive array antennas for many years. However, with the advent of active solid-state phased arrays, an additional phenomenon must be considered that previously was not important. This new phenomenon is the focusing properties of spherical surfaces. When the antennas transmits, a portion of the energy is reflected off the surface of the radome and focused to a small spot about half way between the surface and the center of the radome. The power density in this focal region is not large enough to be of concern for passive arrays of reflectors. However, if it happens to be focused on a small group of solid-state modules, it can exceed the power handling capability of the solid-state receive module. This paper presents an analysis of this phenomenon and provides radome design criteria to avoid potential problems. An analogy is made between analysis of this problem and the design of spherical reflectors, and two modeling methods are presented. A design criterion used to avoid the problem is presented. Two radome geometries are analyzed. Finally, measured data are used to validate the models