Phase adjustment of large antennas

A technique is described for adjustment of phase paths within large antenna arrays or paraboloidal surfaces which are now in use, or are planned, for radio astronomy. After large paraboloids have been constructed, they suffer distortions which are very difficult to investigate and for which photogrammetry, millimeterwave radar and optical survey have been suggested. A new suggestion, based on experiment at Stanford with phase measurement of long paths, is to place modulated gas discharge tubes, acting as scatterers, at various points on the paraboloidal surface and to monitor the phase path from a signal generator through the feed at the focus to each discharge tube in turn, and back. By means of a second probe, say a dipole situated at the vertex of the paraboloid, it is possible to triangulate on deflections. The feasibility of this scheme has been established in connection with the large Stanford cross antenna which has an aperture of 1339 wavelengths at 9.1 cm. The phase of the modulated reflected wave produced by the discharge tube is determined by adding it to a reference continuous wave of large amplitude and applying the resultant to a receiver sensitive to the modulating frequency. A null is obtained when the two waves are in quadrature. The coherent detection system allows measurement of the phase of the modulated reflection even when its amplitude is below -130 dbm. Using a 10-mw -band signal generator, no difficulty was found in detecting the reflection from a small discharge tube placed 100 feet away from a 3 by 4 inch horn, which is sufficient range for applying the method to large paraboloids.