Mirror antenna dual-band lightweight mirror design

The mirror antenna, sometimes called an inverse Cassegrain antenna, is used in modern radar systems where efficient wide angle scan is required in combination with light weight and small size. Although previous applications have been single band, an additional advantage of the mirror antenna is that it is practical to implement dual-band configurations which can greatly enhance the capability of radar or other radiating systems. The most difficult part of a dual-band mirror antenna design is the polarization rotating mirror (half-wave plate). The challenge is to achieve effective performance at two bands and maintain that performance over a wide range of aspect angle. Basic mirror antenna operation and design and fabrication techniques for a dual-band half-wave plate mirror are described. The solution for design parameters is derived for perfect polarization rotation at a given aspect angle for each of the two bands. The design approach includes a process for optimizing the design for the full desired range of aspect angles. Practical fabrication considerations are discussed along with inclusion of the effects of capacitive susceptance of a protective mirror coating in the design theory. Theoretical performance is compared with measurements of mirror samples.