Improved resolution in microwave holographic images

Microwave holography is a useful experimental technique for imaging remote or inaccessible objects and for diagnostics of antennas, radomes, and scatterers; however, diffraction restricts image resolution. A method is described for improving resolution in microwave holography. The holograms are spherical or circular. Porter's scalar theory of curved holograms is extended to vector fields by using rectangular components to treat the effects of wave polarization. The mathematical formulation is a Helmholtz diffraction integral. We show that this integral can be written as a convolution for currents on line segments. The convolution is applied to the spatial frequency spectra of images. Theoretical examples, an infinitesimal dipole and a half-wave dipole, are reconstructed exactly, by a theory of analytic continuation. An experimental example is described; it is diffraction of a half-wavelength wide slit in a conducting screen. The analytic continuation of the holographically reconstructed near-field produced images with resolution approximately one quarter wavelength. Before continuation, resolution was 0.6 wavelength. In addition, the boundary condition of the vanishing tangential field over the metal screen is better satisfied by the image produced by continuation.