The dichroic microstrip antenna concept is reported which enables an antenna aperture to function at two widely separated frequenciesfHandfL. The innovation centres on the utilisation of printed frequency selective mesh conductors as microstrip patch antennas atfL, which in turn become transparent to near-field radiation atfH, from another microstrip array in close proximity. An approximate analysis shows that the mesh perimeter scattering has a dominant effect on the degree of invisibility achievable, and for simple configurations the NEC computer program models the behaviour which also includes a leaky wave action. Radiation pattern measurements on several microwave and millimetre dual band arrays using low permittivity (εr= 1.05) substrates illustrating the concept are presented and a further development concerning an array that also functions as an electromagnetic window. The measurements show the effect of mesh strip width, patch impedance levels, band separation and feeder radiation, but the small dissipative loss in the mesh is difficult to precisely ascertain. Despite the complexity of effects produced by the mesh its transparency is good and radiation pattern perturbations, reduced by the array factor, are typically 1 to 2 dB. However, the use of higher permittivity substrates may lead to some bandwidth restrictions and some increased mesh dissipation loss. Finally, numerous possible applications for the innovative dual band structures presented, are noted.