A practical microwave correlation radiometer was built and tested. Feedback was used to ensure stability and make it unnecessary to match receiver channels. The rms temperature deviation was derived and found to be square root of 2 times that of a total power radiometer. Two separate systems to provide an ac multiplier output were built and evaluated. In one, the receiver channels were tuned to a slightly different frequency while in the other, phase reversal in one channel at an ac rate provided the modulation. The latter method proved easier to instrument. (Author).