An apparatus and method for determining radiated performance of multiple antennas in a wireless system on the basis of capacity degradation comprising calculating a plurality of normalized IID channel matrices based on a plurality of complex gain patterns; calculating a plurality of colored channel matrices based on the plurality of normalized IID channel matrices; calculating a first channel capacity corresponding to the plurality of normalized IID channel matrices; calculating a second channel capacity corresponding to the plurality of colored channel matrices; and calculating a capacity degradation based on the first channel capacity and the second channel capacity.