A planar equiangular spiral antenna can provide the calibration signal for the ATA antenna. The spiral antenna is located on the primary of the ATA antenna near its edge. It is fed by a broadband noise source whose spectrum yields a 1 K signal at every wavelength. The spiral radiates a circularly polarized signal which feeds the two linear polarizations of the ATA with identical calibration signals. To operate at the longest wavelength of the system, the planar spiral must be 60 cm in diameter, one wavelength. With this diameter, it is one tenth the diameter of the primary and 1% of its area. At the edge of the primary, the feed illumination is approximately 10 db tapered, and the spiral will have little influence on the main antenna system. This scheme satisfies the requirements for calibrating gain instabilities in the analog signal path, including those for polarimetric observations.