Doc Ewen: The Horn, HI, and Other Events in US Radio Astronomy

Slide 24: Microwave and Millimeter Wave Applications in the 1970s and 1980s

The figure shows the signal path from the Cassegrain antenna to the receiver location where each of the 8 bands are detected. The band numbers are shown on the optics table. Perforated ellipsoidal mirrors are located at points 8, 6, 4, 7, 5 and 3. The perforated mirrors are pass filters which transmit the desired band to a dielectric lens, where it is refocused. The beam is then reflected by a flat mirror through a hole in the table to a scalar feed-horn at the input to each receiver. These mirrors have a cutoff slope approximately 10 GHz wide, from maximum to minimum transmission. Signals below cutoff are reflected and refocused to the next consecutive mirror. Adjacent bands are detected in orthogonal linear polarizations; hence the purpose of the wire grid polarizer. Horizontal polarization is transmitted to odd numbered bands and vertical polarization is reflected to even numbered bands.

The wire grid polarizer separated the two orthogonal linear polarizations over a bandwidth of several waveguide bands. The electric field parallel to the wires excites current on the wires and is reflected. The wires are invisible to perpendicular field components in a properly designed grid. Credit: Photo and diagram courtesy of Doc Ewen.

Slide 25