Bragg scattering of EM waves by microwave produced periodic plasma layers

Uses of artificial plasma layers as Bragg reflectors for applications in telecommunication links and over-the-horizon (OTH) radars are currently being investigated in a large chamber of Plexiglas. Two microwave beams at right angles to each other are directed through the S-band horns into the chamber containing air at a pressure comparable to that of the upper atmosphere (approximately 50 km). Inside the chamber, the microwave beams produce a series of equally spaced plasma layers parallel to the plane bisecting the two beams. A third beam is then incident on the plasma layers, and the reflected signal is collected for analysis. A sweep microwave generator (4-8 GHz) is used to generate a test wave which enters the chamber through a C-band horn. The test wave is swept in a frequency range much higher than the plasma cutoff frequency in order to separate the coherent Bragg reflection mechanism from the cutoff mechanism. The incident angle of the test wave with respect to the normal of the plasma layers is 45°. Hence, one of the S-band horns located at right angles to the adjacent side can be used as the receiver of a Bragg scattered test wave. The experimental results agree very well with the theory.