Patent GB-1082109-A (GB)
Title: Microwave identification of railroad cars
Inventor: BRADFORD WILLIAM R
IPC: B61L25/04
Filed: 1965-12-24
Abstract: 1,082,109. Aerials; radar; radio signalling. AMERICAN BRAKE SHOE CO. Dec. 24, 1965, No. 54786/65. Headings H4A, H4D and H4L. [Also in Division B7] Apparatus for identifying vehicles such as railway trucks as they pass by a fixed scanning station comprises a transmitter from which microwaves with a particular polarization are focused by a first zone plate lens upon a coded identification member carried by each truck. The microwaves are reflected back by the said member with changed polarization, and are focused by a second zone plate lens upon a receiver adjacent the transmitter. As described, two adjacent semi-circular zone plate lenses 51, 52, Fig. 2, each comprise a vertically disposed sheet of dielectric material 54 or 57 carrying a pattern of concentric semicircular bands of conductive material 53 or 58 alternated with clear spaces 55 or 59. A horizontal earthed conductive sheet 62 extends rearwardly from between the two lenses. The two sheets of dielectric material 54, 57 may be combined into a single sheet (Fig. 3, not shown). In an alternative form of construction, an increased thickness of dielectric is provided in place of each conductive band (Fig. 5, not shown). Formulµ are given for calculating the radii of the bands, and also for correcting for the thickness of the dielectric sheet when conductive bands are used or for calculating thickness of dielectric for the alternative construction. The lenses may be made with the two media of different transmission characteristics arranged in parallel bands instead of in concentric circles (Fig. 6, not shown). Microwave radiation, preferably of frequency 5-40 Gc/s., is transmitted from an aperture in the end of a waveguide 16A, Fig. 2, a septum 49 being provided to ensure that signals are polarized at 45 degrees to the horizontal. The lens 51 focuses the radiation into a narrow vertical band 56 upon a coded identification member 14A comprising a flat dielectric baseplate (65), Fig. 8 (not shown), having a number of equally spaced binary code positions (66) in each of which there may either be one or more vertically disposed half-wavelength narrow conductive elements (67) or one or more horizontally disposed similar elements (68). The elements (67) reflect vertically polarized radiation, and the elements (68) reflect horizontally polarized radiation, which is focused by the lens 52, Fig. 2, upon the apertures in the ends of two waveguides 18A, 18B. The waveguide 18A is provided with a vertical septum and accepts horizontally polarized radiation reflected by a code position containing suitably disposed conductive elements, which produces an output signal from a detector-amplifier 44. The waveguide 18B is provided with a horizontal septum, and accepts vertically polarized radiation, producing an output signal from a detector-amplifier 46. The outputs are connected to give opposite polarity voltages changes across a common load resistor 47, and these are fed to decoding circuits 48. In another embodiment, microwave radiation which is horizontally polarized is transmitted from an aperture in the end of a waveguide (16C), Fig. 9 (not shown), to the lens (51), and then passes through a horizontal polarization grid (101), consisting of a sheet of dielectric (111), Fig. 10 (not shown), having a series of metal bands (112) extending vertically across its face. The grid (101), Fig. 9, stops any vertically polarized components introduced by the lens (51). A coded identification member (148), Fig. 7 (not shown), comprises a conductive metal sheet (104) having at each code position either a resonant slot (105) or (106), or a blank. The slots (105) are arranged at +45 degrees to the vertical, and the slots (106) at - 45 degrees to the vertical, and both represent binary " ones." The blanks represent binary " zeros." Reflected radiation from the slots is polarized at + or - 45 degrees to the vertical and passes through a vertical polarization grid (102), Fig. 9, before being focused by the lens (52) upon an aperture in the end of a waveguide (18C) which is provided with a horizontal septum so that it accepts only vertically polarized radiation. An associated detector-amplifier (46) operates decoding circuits (48). The lenses and polarization grids may be combined, for example, a sheet of dielectric material may carry the rings for a lens on one side and a polarization grid on the other side. Alternatively, the rings of a lens may be divided vertically or horizontally to provide a polarizing effect (Fig. 11, not shown).