Microwave metamaterial aperture imaging radar (MMAIR) is capable of generating high resolution images without using mechanical scanning or antenna arrays. In MMAIR, metamaterial elements are specifically embeded into a parallel plate waveguide, whose resonance frequencies vary among a wide bandwidth. Different radiation fields are gained by wide-band waveforms, and the scene information are measured by measurement matrix which consists of a set of radiation modes. According to the Compressed Sensing theory, MMAIR performance is restricted by the limited frequency measurement modes. Herein we propose a rotation-synthesis approach to synthesize radiation measurement modes. By rotating the metamaterial aperture panel around the panel axis, the approach exploits the radiation field's multi-beam spatial diversity, and with the azimuth rotation, the radiation field pattern varies relative to the scene under the radiation field. As a result, significant MMAIR imaging enhancement is achieved with the crucial increase of radiation measurement modes. The simulations verify the effectiveness and image improvement of the proposed method.