ATTENUATION OF MICROWAVE SIGNALS

Abstract: It is sometimes necessary to reduce the power of micro­ wave transmissions, e.g. to avoid overloading a receiver or to reduce a transmitter's potential to cause interference. The in­ vention reduces the power by inserting radiation absorbent material (14) into the aperture of the antenna (10). In the preferred embodiment a membrane of absorbent plastics is stretched across the aperture of the dish reflector.

Description: This invention relates to the controlled attenuation of microwave signals and in particular it relates to the control of microwave radio links used for telecommunications. Microwave radio links may be used to carry signals from one place to another. It is sometimes desirable to be able to reduce the power of the transmitted microwaves either temporarily or permanently. This might be for any one or more of a number of reasons e.g. (a) to prevent overloading at the receiver of a short link; (b) to prevent interference into the receivers of other links in the locality; (c) to prevent interference into more distant receivers; (d) to check the operating margin of a link on installation or subsequently; (e) to check the interference susceptibility of a link on installation or subsequently. Conventional techniques for reducing the power include inserting an attenuator between the transmitter and the antenna, adjusting (either manually or electronically) the attenuation of an attenuator built into the link equipment, and varying the power output of the microwave source. The first method can take an inconvenient time to carry out and may involve breaking an airtight seal to the detriment of the equipment reliability; the second can add considerably to the cost of the equipment which is especially undesirable if only occasional changes of attenuation are required; and the third can be detrimental to the stability and cost of the microwave source. It is known to use screens of attenuating material to limit radiated/received signal strength. In FR-A-2304192, a radar test-aid has a screen of attenuating material fitted in an assembly, between two antennas. It is an object of this invention to facilitate the reduction of power, including both long and short term reduction, when this is necessary. According to the invention there is provided a microwave telecommunications link comprising: a transmitter feeding an antenna, and means for adjusting the transmitted power, characterised in that the adjusting means comprises an absorptive sheet mounted across the aperture of the antenna, which sheet comprises any one of the following: a) at least one layer of carbon filled polyurethane; b) at least one resistive metallic film with a sheet reistance between 10 and 1000 ohms per square supported on a substrate; or c) a planar array of pairs of dipoles supported on a substrate, the dipoles of each pair being spaced apart and being resistively coupled. Preferably the absorptive sheet is stretched across one end of a tubular support member, the other end of which is adapted for attachment to the periphery of the antenna. Preferably the signal attenuation means reduces the signal strength by at least 2dB, eg 5 to 30dB. The invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic view of a dish antenna incorporating a signal attenuation means in the form of an absorptive sheet; Figure 2 is a cross section of a first embodiment on line X-X of Figure 1; Figure 3 is a cross section of a second embodiment on line X-X of Figure 1; Figure 4 is a front view of a third embodiment; and Figure 5 is a detail of an element of Figure 4. As shown in Figure 1, an antenna 10 according to the invention comprises a conventional reflector 11 and a feed 12. An absorptive sheet 14 is stretched across the aperture of the reflector 11. The sheet 14 which is canted at an angle to reduce coherent reflections which sometimes occur, eg should the sheet 14 become wet with rain, is supported on extension tube 13 (so that it does not foul the feed 12) being removably fixed by one or more clips or a circumferential hoop 15. The sheet 14, various versions of which are described below, absorbs electromagnetic radiation passing through it and it degrades this energy to heat. This means that, in the transmission mode, the amount of energy radiated to the outside world is less than the amount of energy radiated by the feed 12. In the receive mode the amount of energy reaching the feed 12 is similarly reduced. (To avoid using a minus sign to denote attenuation we will use the convention that: dB = 10 log₁₀ Io/I where Io = power flux density on the incident side of sheet 14, and I = power flux density on the other side of sheet 14). The material and/or structure of the sheet 14 is chosen so that it attenuates signals by at least 1dB. Higher levels of attenuation, eg up to 30dB, may be achieved by using material with inherently higher absorptive properties or by using more of the same material, ie more layers and/or thicker layers. Preferably the sheet 14 is chosen to give an attenuation of between 5 and 25dB. Dielectric heating is regarded as the preferred mechanism for attenuation. Carbon filled polyurethanes are good compositions having suitable dielectric properties. (Films of such compositions are commercially available, eg under the trade name "ECCOSORB"). Figure 2 shows a cross section through a sheet consisting of two absorbing layers, 20 and 21 each of the layers being a film of carbon filled polyurethane. Each of the films is 3mm thick. Layer 22 is a weather resistant supporting film having little effect upon the electric properties of the antenna. Each of the films is a separate entity so that the number of layers 20, 21 and 22 can be adjusted and arranged to suit the circumstances. A single film (of polyurethane) produces an attenuation of 5dB and the two layers 20 and 21 as shown produce 10dB. Thus five films would be needed to give 25dB. (As alternatives, not shown, to separate films, laminates or thicker or more absorbent layers could be used.) The layer or layers is/are mounted as shown in Figure 1. An alternative to the dielectric mechanism, heat dissipation is provided by one or more continuous resistive layers of high sheet resistance, eg 200 ohms per square. Such layers are implemented as films of metal, eg nickel-chromium alloy deposited on an inert substrate. Figure 3 shows a cross section through a suitable composite in which a substrate layer 31 supports a metal layer 32; preferably layer 31 is weather resistance. In use the composite is mounted as shown in Figure 1 and electromagnetic radiation induces random currents in metal layer 32, and because this layer has a finite resistance, heat is produced. As a third alternative a pattern of dipole antenna elements each with a resistive load is located in the aperture of the antenna. The pattern is implemented by deposition of the required paths on an inert substrate. This third alternative is illustrated in Figure 4 and 5. Figure 4 shows a plan of the whole aperture. The absorptive membrane comprises an inert substrate 40 upon which is deposited a pattern of dipoles 41 individually illustrated in Figure 5). The pattern extends over the whole of the aperture of the antenna. In Figure 4 each dipole is shown as a single line wherein each line represents the loaded dipole. Figure 5 shows that each dipole comprises a pair of elements 50 and 51 with a conventional gap between them. The gap is filled with resistive material which provides an electrical connection between elements 50 and 51 (which are conveniently implemented as areas of copper deposited on the substrate 40). Area 52 is conveniently a thin film of nickel/chromium alloy to act as a resistive load, eg about 150 ohms, between the elements 50 and 51. The substrate 40 is mounted across the aperture of an antenna as shown in Figure 1. A plurality of layers may be so mounted if desired. In the use of the structure shown in Figures 4 and 5 the radiation induces alternating current to flow between elements 50 and 51 of each dipole antenna of the pattern. This current flows via resistive element 52 whereby radiant energy is attenuated by degradation into heat.

Claims: A microwave telecommunications link comprising: a transmitter feeding an antenna (10), and means (14) for adjusting the transmitted power, characterised in that the adjusting means comprises an absorptive sheet (14) mounted across the aperture of the antenna, which sheet comprises any one of the following: a) at least one layer (20, 21) of carbon filled polyurethane; b) at least one resistive metallic film (32) with a sheet reistance between 10 and 1000 ohms per square supported on a substrate (31); or c) a planar array of pairs of dipoles (41) supported on a substrate (40), the dipoles of each pair being spaced apart and being resistively coupled. A microwave telecommunications link as claimed in claim 1, wherein the absorptive sheet (14) is stretched across one end of a tubular support member (13) the other end of which is adapted for attachment to the periphery of the antenna (10). A microwave telecommunications link as claimed in claim 1 or 2, wherein the signal attenuation means (14) is adapted to attenuate the signal by between 5dB and 30dB. A microwave telecommunications link as claimed in claim 1, 2 or 3, wherein the sheet (14) is a resistive metallic film with a sheet resistance of the order of 200 ohms per square. A microwave telecommunications link as claimed in claim 1, 2 or 3, wherein the resistive loaded dipoles (41) are implemented as conductive areas supported on an inert plastics sheet (40).

Inventors: BUTCHER, MILES EDWARD, CARTWRIGHT, NORMAN EDWIN
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: H01Q17
IPC codes: H01Q17