When illuminated with a plane wave a variable cross-coupling partial reflector reflects a specific amount of a cross-polarized field and a specific amount of a co-polarized field and transmits the remaining power with low attenuation. This is achieved with a pair of frequency selective surfaces (FSS) that are rotated with respect to the incident plane wave. The FSSs can be fixed with a given alignment for a particular application or a tuning mechanism can be provided to independently rotate the surfaces and adapt the reflected co- and cross-polarized fields to changing requirements. Of particular interest is the ability to provide a specific amount of cross-polarized reflected power while reflecting no co-polarized field over a certain range of wavelengths. This will be useful to increase power efficiency in, for example, wave power sources that utilize quasi-optical power by causing oscillations in reflection amplifier arrays.