This paper introduces a proposed imaging system suitable for millimeter and submillimeter wavelengths, based on heterodyne mixing at the focal plane of an optical system. We have designed a prototype planar antenna array and quasi-optical system and present measurements at 9 GHz and at 140 GHz of beam patterns and sensitivity for a single two-element, full-wave, printed dipole array incorporating a GaAs beamlead diode. Scale model studies were carried out in the 8-10 GHz range, where the design was optimized. Subsequent measurements were made with a tunable carcinotron at 128-140 GHz on an appropriately scaled array.