The Luneberg lens or Luneberg reflector is a type of spherical lens or reflector that is designed to focus or redirect electromagnetic waves, typically microwaves or radio waves. It was originally invented by Rudolf Luneberg in 1944.
The design of the Luneberg reflector involves a gradient refractive index. Unlike traditional lenses that have a uniform refractive index, the refractive index of the Luneberg lens varies continuously from the center to the outer surface. The highest refractive index is at the center, and it gradually decreases toward the edges. This gradient refractive index allows the Luneberg lens to achieve unique focusing and beam steering properties.
Inside the Luneberg reflector, there are concentric layers of material with varying refractive indices. The specific materials used can vary depending on the application and the desired frequency range.
Regarding its operability with frequencies below the microwave range, it's essential to note that the design of the Luneberg reflector is primarily optimized for microwave and radio wave frequencies. The reason for this is that the principles governing wave propagation and electromagnetic behavior vary at different frequency ranges.
For the Luneberg reflector to function effectively, the wavelength of the electromagnetic waves should be comparable to the size of the lens. Microwave and radio wave frequencies generally have wavelengths in the centimeters to meters range, making them suitable for this type of lens. As you move to lower frequencies, such as those in the lower radio frequency (LF), very low frequency (VLF), or extremely low frequency (ELF) ranges, the wavelengths become significantly larger (ranging from kilometers to thousands of kilometers). At these lower frequencies, it becomes impractical to construct a Luneberg reflector of a feasible size to achieve useful focusing or reflecting effects.
In summary, the Luneberg reflector is primarily designed for microwave and radio wave frequencies due to the practical constraints of constructing an effective lens for lower frequencies with significantly larger wavelengths.