The use of dielectric materials to enhance the reflectivity of a surface at microwave frequencies

The padding of a metal surface by one or more layers of a dielectric material is examined, and it is shown that an improvement in reflectivity can be obtained provided that the loss factor (tan ?) of the dielectric is small enough. For most materials the gain is approximately proportional to ??/tan ?, where ? is the dielectric constant. Since the reflectivity of a metal diminishes as the frequency is increased, it is to be expected that dielectric padding will become more advantageous at the higher frequencies which can now be generated. For known materials, no improvement is to be expected at frequencies much below 3000 Mc/s, but at that frequency a gain of two was observed for a ceramic material consisting mainly of titanium dioxide. By laminating a dielectric it is possible to reduce the effective loss factor. A reduction in ? must occur, but an overall increase in ??/tan ? can be obtained. In the case of titania, laminating can give a further improvement by a factor of three, and hence at 3000 Mc/s it is theoretically possible to produce a surface which is six times as reflective as a metal. At higher frequencies even greater gains may be possible.