A full-wave analysis of planewave scattering from an infinite, periodic array of microstrip patches on a ferrite slab under different bias conditions is presented. The magnetic bias field is applied in a direction parallel to the slab. The scattering coefficient which is directly related to the monostatic radar cross section is seen to move around in frequency as the bias field strength changes. This effect indicates that microstrip antennas could be made invisible or to hide in frequency when not in transmitting or receiving mode. The bias field could be applied when the antenna is not transmitting but one wishes to hide it.