The interaction between the microwave antenna and the FSS is analyzed by using a modified plane wave spectrum approach. The analysis of the antenna is carried out by using CFDTD technique in order to obtain its aperture fields which are in turn used as an excitation for the FSS. Both the aperture electric and magnetic fields of the antenna are used in deriving the spectrum. The original truncated FSS problem has been transformed into an equivalent one that renders it far more manageable to treat than its original version. This is especially true for large radomes, comprising hundreds, if not thousands of elements. The proposed method is validated by comparison with measurement. The method is completely general and can handle various types of microwave antennas, FSS elements of different shapes, and FSS radomes with arbitrary boundary truncations.