GB2458663A

GB2458663A - Radome for a radar pressurised horn antenna

Info

Publication number: GB2458663A
Application number: GB0805484A
Authority: GB
Inventors: Anne Fuller; Sang Tu; Mike Tyrrell
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2008-03-26
Filing date: 2008-03-26
Publication date: 2009-09-30
Anticipated expiration: 2028-03-26
Also published as: GB0805484D0; GB2458663B

Abstract

A radome for a pressurised horn 4 of a radar antenna comprises a fluoropolymer film 6 whose periphery is supported by a closed cell rigid foam plastics frame 7, 8. The said film 6 may be made of fluronated ethylene propylene FEP and may have a thickness of 0.1 to 0.5 mm. The frame 7, 8 may be formed with two flat surfaces between which the periphery of the film 6 may be sandwiched and secured into place by an epoxy resin adhesive. The frame 7, 8 may be made of a closed cell foam plastic material such as polymethacrylimide PMI with good flame-inhibiting properties. The frame 7, 8 may have a polygonal opening which conforms to that of a polyhedral horn. The radome may hermetically seal the mouth of the horn of a radar pressurized horn antenna, typically operating with a transmission power level of 1.5 to 2 kW, which is suitable for aircraft applications.

Description

RADOME

This invention relates to a radome suitable for a pressurised horn of a radar antenna.

High power radar antennae for aircraft typically comprise a polyhedral, e.g. octagonal section, horn sealed hermetically at its mouth by a radome, the horn being pressurised and filled with an inert gas such as nitrogen. The radome necessarily absorbs some of [lie fflicrowave radiation transmittcd by the antenna, and its materia! has to withstand high temperatures as a result of the microwave absorption. It has been difficult to find a suitable radome which can withstand typical fluid pressures of a pressurised horn whilst resisting burning from the microwave absorption.

The invention provides a radome for a pressurised horn of a radar antenna, comprising a fluoropolymer film whose periphery is supported by a closed cell rigid foam plastics frame.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view, including a separate and enlarged view of a portion, of a high power, high-pressure horn radome installed in an airborne radar antenna, embodying the present invention; Figure 2 is an axial longitudinal cross-section through the dome of the radar antenna of Figure I, taken along the line 2-2 of Figure 3; Figure 3 is a front elevation of the dome of Figure 2; and Figure 4 is a perspective view of the dome of Figures 2 and 3.

A radar antenna suitable for airborne use and capable of high power transmission is shown in Figure 1. The radar antenna I comprises a dish 2 and a transmitting and receiving portion 3, which is also shown to an enlarged scale in Figure I. The transmitting arid receiving portion 3 comprises a metal wave-guide of rectangular section for microwave communication with a dome 4, which is shown in greater detail in Figures 2 to 4.

The dome 4 is hermetically sealed to the external atmosphere and, in use, is filled with an inert gas such as nitrogen which must be dry. The differential pressure can be of the order of one atmosphere, although we have discovered in tests that a horn embodying the invention can withstand over two atmospheres.

The mean transmission power of the radar is typically in the range of 1.5-2 kW.

The horn 4 comprises a wafl 5 which, as shown more clearly in Figure 4, is polyhedral, and octagonal in cross-section. This shape sets up desirable vertical and horizontal polarisations to give a well-defined power distribution across the horn, but it will be appreciated that this particular shape is not essential to the present invention.

The mouth of the horn 4 is planar, as shown in Figure 2, and its generally polyhedral outer surface is enlarged near its mouth so as to provide flat surfaces 9 parallel to the horn axis, for supporting a radome 12 as shown in Figure 2.

The radonie 12 comprises a membrane or film 6 of fluorinated ethylene propylene, FEP, which in this example is 0.254 mm thick but which could be in the range of 0.1 to 0.5 mm, preferably 0.2 to 0.3 mm. The membrane 6 has a peripheral shape, in this example octagonal, to conform to the mouth of the horn 4. FEP has been found to have satisfactory mechanical properties as well as the ability to bond well to a support frame 7, 8. It can be made stronger and thinner than the radome materials of existing structures, and as a result it can withstand high power microwave radiation without degrading or burning. This was surprising in view of its reputation as being rather brittle.

The membrane is gripped between flat opposed surfaces of two components 7, 8 of a frame. The axially inward component 7 of the frame has an internal surface 9 which fits closely over, and is bonded by adhesive to the external surface of the mouth of the horn 4 as shown in Figure 2. The axially outward component 8 of the frame has a frusto-conical surface 10 with the same half angle as the conical horn wall 5. The flat surfaces of the frame components 7, 8 which face each other cover the same area of the periphery of the membrane 6, but on opposite sides of the membrane, as shown in Figure 2. A groove 11 is cut into the exterior of the opposed surfaces of the frame components 7, 8, so that the groove follows the peripheral edge of the membrane 6.

An epoxy resin adhesive, preferably Araldite (registered trade mark) bonds the surfaces of the periphery of the membrane 6 to the opposed flat surfaces of the frame components 7, 8, and the adhesive also fills the groove 11 and is flush with the external flat surface othe frame 7, 8 as shown in Figure 2.

To minimise the risk of electric discharge within the horn, the internal surfaces are smooth, and the surface is uninterrupted as it tapers outwardly from the wall 5 of the horn to the wall 10 of the radome. The risk of discharge is also minimised by the presence of the inert gas.

The frame 7,8 is of a closed cell rigid foam plastics material, which in this case is based on polymethacrylirnide, PM1. PMI is described for example in US Patent 5698605 and in WO/2003/078514, and it is known to have the ability to bond to epoxy adhesives, as well as to have good flame-inhibiting properties. By sandwiching the membrane 6 between the two frame components 7, 8, the arrangement has good mechanical strength and the ability to withstand changes in pressure and also shock from acceleration, without tearing and without breaking the adhesive bond.

It will be appreciated that all internal faces of the horn and the radome are to be free from adhesive.

The external surfaces of the radome are free from paint, but the horn is painted with an epoxy resin enamel.

Claims

CLAIMS: I. A radome for a pressurised horn of a radar antenna, comprising a fluoropolymer film whose periphery is supported by a closed cell rigid foam plastics frame.
2. A radome according to Claim 1, in which the film has a thickness of 0.1 to 0.5 mm.
3. A radome according to Claim 3, in which the film has a thickness of 0.2 to 0.3 ni m.
4. A radome according to any preceding claim, in which the frame comprises two components with flat surfaces facing each other and gripping therebetween the periphery of the film.
5. A radorne according to any preceding claim, in which the fluoropolymer comprises fluorinated ethylene propylene.
6. A radome according to any preceding claim, in which the frame has a polygonal opening to conform to a polyhedral horn.
7. A radome according to any preceding claim, comprising a layer of adhesive bonding the periphery of the film to the frame.
8. A radome according to Claim 4, comprising a layer of adhesive bonding each surface of the periphery of the film to the flat surface of the respective frame component.
9. A radon-ic according to Claim 7 or 8, in which the adhesive is an epoxy resin.
10. A radome according to any preceding claim, in which the plastics frame is of a closed cell foam.
11. A radome according to Claim 10, in which the closed cell foam comprises polym ethacryli m ide.
12. A pressurisable horn for a radar antenna, comprising a radome according to any preceding claim hermetically sealing its mouth.
13. A radar comprising a pressurisable horn according to Claim 12.
14. An hermetically sealed, pressurised radar horn having a radome according to any ofClaims Ito II.
15. A radome substantially as described herein with reference to Figures 2 to 4 of the accompanying drawings.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS: 1. A radome for a pressurised horn of a radar antenna, comprising a fluoropolymer film whose periphery is supported by a closed cell rigid foam plastics frame.2. A radome according to Claim 1, in which the film has a thickness of 0.1 to 0.5 mm.3. A radome according to Claim 2, in which the film has a thickness of 0.2 to 0.3 mm.4. A radome according to any preceding claim, in which the frame comprises two components with flat surfaces facing each other and gripping therebetween the periphery of the film.5. A radome according to any preceding claim, in which the fluoropolymer comprises fluorinated ethylene propylene.6. A radome according to any preceding claim, in which the frame has a polygonal ::::. opening to conform to a polyhedral horn.7. A radome according to any preceding claim, comprising a layer of adhesive * S., bonding the periphery of the film to the frame. *.SS::*. 8. A radome according to Claim 4, comprising a layer of adhesive bonding each I...surface of the periphery of the film to the flat surface of the respective frame component.9. A radome according to Claim 7 or 8, in which the adhesive is an epoxy resin.10. A radome according to any preceding claim, in which the plastics frame is of a closed cell foam.11. A radome according to Claim 10, in which the closed cell foam comprises polymethacrylimide.12 A pressurisable horn for a radar antenna, comprising a radome according to any preceding claim hermetically sealing its mouth.13. A radar comprising a pressurisable horn according to Claim 12.14. An hermetically sealed, pressurised radar horn having a radome according to any of Claims Ito ii.15. A radome substantially as described herein with reference to Figures 2 to 4 of the accompanying drawings.