Radio Astronomy plays a dual role. Fundamentally a science, it often serves as a tool for engineers. Radio Astronomy has furnished accurate knowledge of dimensions and intensities of some radio stars. This information in turn helps determine those parameters of large-diameter antennas which cannot be accurately calculated by conventional methods. The large antenna may then be used to make absolute measurements of other radio sources. At AFCRL parameters of the 84- foot parabola were found by such methods. The antenna was then used during two lunar eclipses to record simultaneously lunar thermal emission at 1200 and 3100 megacycles which was found to be constant at

and

K respectively. The moon thus becomes another source which may be used to calibrate large antennas.