GB618615A - Telecommunication systems employing differently polarised electromagnetic waves - Google Patents

Abstract

618,615. Multiplex radio signalling; radio navigation; polarizing devices. STANDARD TELEPHONES & CABLES, Ltd. Aug. 29, 1946, No. 25967. Convention date, Jan. 29, 1942. [Classes 40 (v) and 40 (vii)] In a multiplex radio system the channels are differentiated by the states of polarization of their radiation, artificial " crystals " exhibiting rectilinear or circular pleochroism being used to separate the channels at the receiving stations. The crystals are formed by arranging small conducting masses or " molecules " in a crystalline network in a low-loss dielectric such as air, the substance known under the Registered Trade Mark " Trolitol," textile fibres, &c. The molecules may be of solid or hollow metal and of symmetrical or asymmetrical form. If they are asymmetrical, they may have identical or regularly alternated orientations along rows parallel to an axis of the crystal network. Thus, they may comprise spheres or globules, sticks or plane elements of various contours and the distances between successive molecules are small relative to the wavelength of the radiation employed, to avoid diffraction effects. The dimensions of a molecule are usually sufficiently different from those of a resonator tuned to the wavelength of the radiation to avoid selective absorption. The artificial crystal absorbs almost completely one of two rectilinear or circular vibrations while the other is relatively little absorbed. A rectilinear crystal may be made by superposing strips of dielectric material on which the molecules are fixed by weaving, printing, photography, cathodic precipitation, condensation, &c. Circular pleochroism may be provided by placing on rods of Trolitol or cords of textile fibres, molecules having successively different settings and regularly alternated, the rods or cords being placed parallel to one another in a dielectric container representing the outside surface of the crystal. Alternatively, strips of artificial crystal having rectilinear pleochroism may be superposed so that along a line perpendicular to the plane parallel strips successive molecules have different and regularly alternated settings. Three directional systems are described, Figs. 2, 3 (a) and (b) (not shown), in which a loop aerial is embedded in an artificial crystal or a dipole and parabolic reflector or a row of thermocouples and a parabolic reflector are associated with a crystal, so that they are directive and also responsive to radiation polarized in one plane only. Two channels may be afforded by two mutually perpendicular half-wave transmitting aerials using a common frequency and two loop aerials associated with suitably oriented rectilinear artificial crystals, Fig. 4 (not shown). A marker beacon may be produced by vertical and horizontal half-wave aerials AV, AH respectively, Fig. 5, producing radiation patterns V1, V2 and H respectively, Fig. 6. The aeroplane A, using the beacon, is provided with loop aerial-crystal combinations Cvv, Cvh on the vertical surface of the fuselage responsive respectively to the fields V1, V2 and H respectively which are differentiated by their planes of polarization and preferably also by coded tone frequency modulation, to enable the pattern to be aurally indicated and to permit station identification. Alternatively, a cathode-ray tube indicator O, Fig. 7, may be used, the signals from the aerial systems Cvv and Cvh being used to deflect the beam vertically and horizontally respectively. The display on the screen F as the aircraft approaches the beacon consists of two lines 1, 2 and 2, 3 traced in that order and the opposite trace as the aircraft recedes again. An alternative radiation pattern V1, V2, H1, H2, Fig. 8, may be produced by using a horizontal aerial one wavelength long. For transmission in a rectangular wave-guide G, Fig. 9, two channels are radiated from mutually perpendicular half-wave aerials D1, D2, and the similarly arranged receiving aerials R1, R2 at the other end of the guide co-operate with rectilinear artificial crystals C1, C2 which separate the differently polarized channel signals. In another embodiment, Fig. 12, the plane polarized waves from aerials D1, D2 are transformed respectively into right- and lefthand circularly polarized waves by the quarterwave plate L and separated at the other end of the circular wave-guide G by circular polarization artificial crystals C1, C2 of opposite sense. They are then transformed into linearly polarized waves by the quarter-wave plate L<1> for reception by mutually perpendicular aerials R1, R2. A similar system, Fig. 13, uses Fresnel parallelepipeds P1, P2 to circularly polarize the plane polarized waves from aerials D1, D2 and further Fresnel parallelepipeds P<1>1, P<1>2 to reconvert the circularly polarized waves to plane waves.