Passive millimetre-wave imaging with a planar diffraction antenna

A dispersive antenna, formed by locating a dielectric sheet waveguide in the near-field zone of a millimetric scale diffraction grating, is described for the application of passive millimeter-wave imaging in the W-band. Two dimensional radiometric contrast images are formed by frequency scanning in the radial direction of the image and mechanically scanning in the azimuthal direction. Mechanical scanning is by means of rotation of the antenna at a uniform angular speed, while frequency scanning is achieved by careful selection of antenna parameters to provide a near linear dependence of beam angular position with frequency of ∼ 1 degree per GHz. The imaging system operates over the frequency range 84–100 GHz and utilizes a single millimeter-wave LNA (Low Noise Amplifier) and mixer to produce a microwave frequency intermediate frequency; further down conversion stages result in 64 baseband channels. In this way, the 16 GHz operational bandwidth is divided into 64 equal width frequency channels, giving the system a field of view of ∼ 16 degrees.