Screen device

Abstract: The invention relates to a screen device. A frequency selective screen means (3) and a plate (4) displaceable relative to the screen device (3) are included in the screen device. In a first position of the plate (4) the screen device allows radiation to pass while in a second position radiation within the transmission range of the screen means is suppressed. The screen device is , for example, suitable to be include in a radome for a radar antenna and enables a reduction of the risk of being detected by, for example, an enemy radar.

Description: The present invention relates to a screen device comprising a frequency selective screen means selectively transmitting or preventing transmission of electromagnetic radiation in dependence on the radiation frequency incident and a plate movable relative to the screen means having electromagnetic radiation transmissible and non-transmissible sections, the cooperation between the screen means and the plate influencing the transmission characters of the screen device. The invention also relates to a radome comprising a screen device according to the invention. A screen device according to the above is previously known from EP-A-0 468 623. According to the known device one or more plates are disposed at a spacing from tile screen means. By moving the plate, the power or frequency characteristics are modulated. In this type of screen device with spaced apart elements there is always a considerable amount of transmission. Due to that fact the screen device is not suitable for application fields requiring a more effective suppression of the radiation within the transmission range of the screen device. For example it is not possible to obtain desired suppression in a radome for a radar antenna. Other examples of screen devices are previously known from US patent specifications nos. 4 684 954 and 4 797 683, respectively. According to US 4 684 954, the transmission range of the screen device is changed by changing the states of the comprised diodes from conducting to nonconducting state and vice versa. However, the change of the states of the diodes may only be used for suppressing within a very limited frequency range. According to US 4 797 683, the transmission range is changed for ever by removing an outer frequency selective layer by means of an explosive charge. The result of this is that the transmission range of the screen device may only be changed once. It is an object of the present invention to obtain a screen device not suffering from the above limitations and which offers a more effective suppression of the radiation within the transmission range of the screen means in a closed transmission state while simultaneously in an open transmission state transmitting the radiation within the transmission range of the screen means almost uneffected. The object of the invention is obtained by means of a screen device characterized in that the surface of the screen means facing the plate is constituted by a metal layer provided with slots or recesses, that the surface of the plate facing the screen means is provided with metal sections separated by non-metal sections and that said surfaces of the screen means and the plate are applied in mutual contact with each other in a first and second transmission state of the screen device, the first transmission state corresponding to a first position of the plate leaving the slots or recesses of the screen means open allowing radiation within the transmission range of the screen means to pass, the second transmission state corresponding to a second position of the plate hiding the slots or recesses essentially preventing the radiation within the transmission range of the screen means to pass. A relatively small displacement of the plate is needed between the two positions and due to that a rapid switching may be carried out. In the second position the metal layer provided with slots of the screen means together with the metal sections of the plate form a metallic area resulting in an effective screening of the electromagnetic radiation in question. Preferably, the plate is displaced between the first and the second position by means of a linear displacement in a plane parallel to the surface of the screen means facing the plate. In order to switch over between the two positions only a short displacement of the plate is needed and the displacement movement is easy to obtain by means of a mechanical movement transfer from a driving means. The invention will now be described in more detail by way of example with reference to the accompanying drawings in which: Figure 1 shows a screen device according to the invention comprised in a radome for a radar antenna, Figure 2 schematically shows a screen means and a plate comprised in the screen device according to the invention, Figure 3a shows a part of the surface of the screen means facing the plate, Figure 3b shows a section through the screen means according to 3b-3b in figure 3a, Figure 4a shows a part the surface of the plate facing the screen means, and Figure 4b shows a section through the plate according to 4b-4b in figure 4a. According to figure 1 a reflector antenna 1 is protected by a radome 2. In front of the opening of the antenna 1, the radome 2 comprises a screen means in the shape of a frequency selective plate 3, and a displaceable plate 4. The plates are inclined relative to the antenna opening in order not to reflect electromagnetic radiation incident in the lobe direction of the antenna and which is not allowed to pass trough the plates back in the direction from which the radiation is incident. Furthermore there is a rim comprised in the radome running around the periphery of the antenna. Together with the plates 3,4 the rim 5 form a closed space. In order to displace the displaceable plate 4 there is a driving means 6. The driving means is connected with the displaceable plate 4 by means of a transmission belt 7 guided around a number of guide elements 8-11. In order to keep the transmission belt stretched, spring means 12 may be included. The screen means 3 and the displaceable plate 4 included in the screen device are shown separately in figure 2. The screen means 3 is provided with a metal layer 13 in the surface layer facing the displaceable plate 4. As evident from figure 3 and the corresponding description below, there are slots in the surface layer. The displaceable plate is provided with metal sections 14 in the surface facing the screen means. As is apparent from figures 3a and 3b there are slots or recesses 15 in the surface layer 13 of the screen means 3. In these slots 15 the metal layer 13 is missing and the embodiment of the slots in essential determines the frequency selectivity of the screen means. Figures 4a and 4b show examples of how the metal sections 14 of the displaceable plate may be embodied. In the example shown, the metal sections are constituted by parallel elongated section separated by somewhat wider and elongated sections 16 free from metal. The invention is not in any way restricted to the embodiment described above, but within the scope of the invention as defined in the claims attached to the application there is room for a plurality of different embodiments. For example, surfaces adjacent to each other of the screen means and the displaceable plate may be given other structures. The displacement mechanism for the displaceable plate is another example of what may be given another embodiment.

Claims: A screen device comprising a frequency selective screen means (3) selectively transmitting or preventing transmission of electromagnetic radiation in dependence on the radiation frequency incident and a plate (4) movable relative to the screen means having electromagnetic radiation transmissible and non-transmissible sections ( 14 resp. 16). the cooperation between the screen means and the plate influencing the transmission characters of the screen device, characterized in that the surface of the screen means (3) facing the plate (4) is constituted by a metal layer (13) provided with slots or recesses (15), that the surface of the plate (4) facing the screen means is provided with metal sections (14) separated by non-metal sections (16) and that said surfaces of the screen means and the plate are applied in mutual contact with each other in a first and second transmission state of the screen device, the first transmission state corresponding to a first position of the plate leaving the slots or recesses (15) of the screen means open allowing radiation within the transmission range of the screen means to pass, the second transmission state corresponding to a second position of the plate hiding the slots or recesses (15) essentially preventing the radiation within the transmission range of the screen means to pass. A screen device as claimed in claim 1, characterized in that the plate (4) is displaceable between the first and the second positions by means of a linear displacement in a plane parallel to the surface of the screen means facing the plane. A radome (2) comprising a screen device as claimed in any of the preceding claims.

Inventors: LARSSON GOERAN, LARSSON, GOERAN, Larsson, Göran
Applicants: CELSIUSTECH ELECTRONICS AB
Application year: 1993
Main classification: H01Q15
IPC codes: H01Q15, H01Q1