GB2254737A - Antenna apparatus for infrared and millimetre-wave radiation. - Google Patents

Abstract

A simplified construction of an antenna apparatus for both infrared and millimetre-wave radiation is obtained by embedding the infrared detector (2) in a mm-wave lens (3). The lens (3) of, for example, polystyrene transmits the mm-wave radiation to and from the transceiver (1), and the lens performance is not seriously degraded by the embedded detector (2) or its connections (33) or even by a cooler pipe (37) of the infrared detector (2). As well as providing reliable mounting of the infrared detector (2), the arrangement permits the mm-wave transceiver (1) to be located on the optical axis of the lens (3) in a coaxial concentric arrangement for viewing a common scene at both the infrared and mm wavelengths through a common aperture. <IMAGE>

Description

DESCRIPTION ANTENNA APPARATUS FOR INFRARED AND MILLIMETRE-WAVE RADIATION This invention relates to antenna apparatus for both infrared and millimetre-wave radiation, comprising an infrared detecting means, a millimetre-wave transceiver means, and an optical system for receiving incoming infrared and millimetre-wave radiation and for transmitting outgoing millimetre-wave radiation. Such an antenna may be used in, for example, a radar system of a so-called "smart munition". The infrared reception can operate in a passive mode which does not reveal its presence during a military operation and which is not susceptible to radar jamming techniques.

Furthermore, the observation of a scene or object through a common aperture in both the infrared and millimetre-wave regions of the spectrum can aid in the identification and positive recognition of a specific object or potential target, rather than simply detection.

United States patent specification US-A-4 791 427 describes one example of such a multi-spectral antenna apparatus in which the optical system comprises a collimating lens for both the infrared and millimetre-wave radiation. The infrared detecting means is located behind the lens and on its optical axis. A further lens focussing system for further focussing the infrared image received from the main collimating lens is present between the collimating lens and the infrared detector. Rotatable prisms are included in the optical system, and millimetre-wave radiation collimated by the main lens is transmitted from and to two millimetre-wave feeds which are offset from the optical axis of the collimating lens.

The whole contents of US-A-4 791 427 are hereby incorporated herein as reference material.

In the antenna apparatus of US-A-4 791 427, the collimating lens may be made of a single material (for example a polystyrene material or zinc sulphide or zinc selenide) or of different materials in different areas chosen in terms of their suitability for the different regions of the electromagnetic spectrum being used. In the arrangement described in US-A-4 791 427, the further lens focussing system for the infrared detector takes only that part of the infrared which was focussed by a central area of the collimating lens. Figure 2 of US-A-4 791 427 illustrates a compound lens having a different optical material in this central area which is used for the infrared collimation.Thus, such a lens has a first (outer) area for transmitting the incoming and outgoing millimetre-wave radiation to and from the off-axis millimetre-wave feeds and a smaller second (central) area for transmitting the infrared. The further lens focussing system separate from the collimating lens is still required for transmitting the infrared from the centre area of the collimating main lens to the infrared detector spaced behind the collimating lens and the further focussing system.

It is an aim of the present invention to permit a simplication of such an antenna apparatus and also to permit the millimetre-wave transceiver means to be located on the optical axis of the lens if so desired. The invention is based on a recognition by the present inventors that the lens can be designed to provide support for the infrared detecting means while also providing satisfactory transmission of the millimetre-wave radiation to and from the transceiver.

According to the present invention, there is provided an antenna apparatus for both infrared and millimetre-wave radiation, comprising an infrared detecting means, a millimetre-wave transceiver means, and an optical system for receiving incoming infrared and millimetre-wave radiation and for transmitting outgoing millimetre-wave radiation, which optical system comprises a lens having a first area for transmitting the incoming and outgoing millimetre-wave radiation and a smaller second area for transmitting the infrared to be detected by the infrared detecting means. In accordance with the present invention, the antenna apparatus is characterised in that the smaller second area of the lens comprises a hole in the lens, and the infrared detecting means is embedded in the hole.

Such an embedded arrangement in accordance with the present invention provides good mechanical support of the infrared detector means and permits a reduction in the number of separately mountable parts of the antenna apparatus; this can increase the robustness and reliability of the apparatus as well as permitting a more compact arrangement of the whole antenna apparatus. Although there is some degradation of the lens performance due to the partial obscuration of its aperture by the hole, it is found that the degree of degradation is acceptable for many applications.

Similarly, it is found that electrical connections of the embedded infrared detecting means may comprise thin wires or a pattern of conductor tracks extending across a surface of the lens, with only minor degradation of the lens performance. Furthermore this embedded arrangement can be designed with both the infrared detecting means and the millimetre-wave transceiver sharing the same optical axis. Thus, the hole may occupy a central area of the lens around its optical axis, and the millimetre-wave transceiver means may be located on the optical axis behind the embedded infrared detecting means.

Although the infrared detector means may be of a type which operates at ambient temperature, higher performance can be achieved with types in which the detector elements (for example of cadmium mercury telluride or indium antimonide) are cooled to cryogenic temperatures. A narrow pipe of the cooling means may in the latter case extend across a surface of the millimetre-wave lens without seriously degrading the performance of the lens.

These and other features in accordance with the invention are illustrated, by way of example, in a specific embodiment of the invention now to be described with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a sectional view of an antenna apparatus in accordance with the present invention; Figure 2 is an isometric view of the mm-wave lens of the apparatus of Figure 1; Figure 3 is a graph showing the variation with the azimuth angle o in degrees of a measured far field pattern of the mm-wave radiation intensity in dB transmitted through the lens of Figure 2 from the transceiver in the Figure 1 apparatus; Figure 4 is a graph comparable with that of Figure 3 but for two measured far field patterns, one from the lens of Figure 2 but with its hole empty, and the other for a lens of the same design but without any hole.

It should be noted that Figures 1 and 2 are diagrammatic, and relative dimensions and proportions of parts of this embodiment have been shown exaggerated or reduced in size for the sake of clarity and convenience in the drawings. In general the same reference signs are used to denote corresponding or similar parts in different embodiments.

The antenna apparatus of Figure 1 comprises a millimetre-wave transceiver 1, an infrared detector 2, and an optical system 3,4 for receiving incoming infrared and millimetre-wave radiation 10 and for transmitting outgoing millimetre-wave radiation 11. This optical system comprises a main collimating lens 3 for the mm-wave radiation 10 and 11, and in accordance with the present invention the infrared detector 2 is embedded in a hole 6 in this mm-wave lens 3.

The mm-wave transceiver 1 may be of known type and is illustrated with a feed-horn in Figure 1. The transceiver 1 may be designed to operate at a frequency of, for example, 35GHz or higher. The infrared detector 2 may also be of known type comprising an array of detector elements 20 of, for example, pyroelectric material operating at ambient temperature or of, for example, cadmium mercury telluride which is cryogenically cooled for operation. The infrared detector 2 may be designed to operate at wavelengths in an atmospheric window, for example in the 3 to 5 Mm or 8 to 14 ym wavebands. The apparatus of Figure 1 may form part of a radar, for example, for projectile guidance using signals derived from both the mm-wave and infrared radiation inputs.

The applicants have designed a particular plano-convex aplanatic collimating lens 3 for transmitting 35GHz radiation 10 and 11 to and from a transceiver 1 located on the optical axis 5 of the lens 3 at a distance D of 60mm behind the lens 3. A block of cross-linked polystyrene having a dielectric constant Er of 2.53 was machined to form the lens 3 with a diameter 2R of 1OOmm. The curvature of the front face of the lens design is such that the curve intersects the optical axis 5 at a distance T of 32.5mm from the flat rear face of the lens 3. Due to the long distance D = 60mm and the wide feed beam of the transceiver 1, there is relatively strong edge illumination of the lens 3.In order to reduce this edge illumination, the applicants have placed a ring of mm-wave absorbing material around the periphery of the lens 3.

The same aperture of the antenna apparatus of Figure 1 is used for simultaneous operation at both the mm-wave and infrared wavelengths. The infrared detector 2 does not require such a large aperture as the mm-wave transducer 1 since the infrared is of much shorter wavelength. The apparatus is designed to accommodate both the infrared detector 2 and the mm-wave transceiver 1 on the optical axis 5. This is achieved by machining the hole 6 of radius r centrally around the optical axis of the lens 3. The performance results given in Figure 3 are for a lens 3 with a hole 6 of radius r = 15mm. This hole 6 is large enough to accommodate various known types of infrared detector 2, for example a Peltier-cooled cadmium mercury telluride element array 20 in a metal-alloy housing of 28mm diameter.The larger annular area of the lens 3 surrounding the hole 6 serves to transmit the incoming and outgoing mm-wave radiation 10 and 11 to and from the transceiver 1.

Figures 3 and 4 show the far field pattern of the 35GHz radiation from the transceiver 1 at D = 60mm for this particular plano-convex aplanatic lens 3 with R = 50mm and T = 32.5mm. Curve a in Figure 4 was measured with this lens design before machining the hole 6. Curve b in Figure 4 was measured in the same set-up with the same lens 3 after machining the hole 6 of r = 15mm but with the hole 6 left empty. Comparison with curve a shows that the mm-wave performance of the antenna is not badly affected by the presence of the hole 6. The (empty) hole 6 does result in a reduction in gain of about 2dB, but the main beam obtained from this holed lens 3 is significantly narrower than that from the un-holed lens 3.The curve in Figure 3 was measured in the same set-up but with a metal-alloy plug inserted in the hole 6 to simulate an embedded detector housing 2. The results are still similar to curve b in Figure 4; the resulting far field pattern shown in Figure 3 is of acceptable shape, and (compared with curve a in Figure 4) the main beam width is narrowed with a reduction in gain for the filled hole 6 as for the empty hole 6. This slight degradation in mm-wave performance is acceptable for many applications.

Although the results of Figures 3 and 4 are for a lens 3 of cross-linked polystyrene material, other mm-wave optical materials may be used. Any of the other mm-wave transmissive materials mentioned in US-A-4 791 427 may be used to form the lens 3, for example zinc selenide, zinc sulphide or a sintered alumina ceramic. The material may be transmissive of both the mm-wave and infrared radiation. However, the lens material surrounding the hole 6 does not transmit the infrared to the infrared detector 2, and so it need not be infrared-transmissive. Thus, the annular lens 3 may filter the infrared from the incoming radiation 10 so that only the mm-wave part 10' thereof is transmitted to the mm-wave transceiver 1.

The internal construction of the infrared detector 2 is not shown in Figure 1 because it may be of various known types. The detector housing includes an infrared-transmissive window 24 which may be of sapphire in the case of a detector for 3 to 5ijm wavelength radiation. The detector includes electrical connections 23 which extend in known manner from the detector elements 20 to the outside of the housing. These electrical connections 23 of the detector 2 may be connected to conductors 33 extending across the lens 3 from the hole 6 to the lens perimeter. By way of example, Figures 1 and 2 illustrate a metallization pattern of conductor tracks 33 on the substantially flat rear face of the lens 3. The detector connections 23 are shown connected to the conductors 33 by short flying leads 32.However, if desired, the conductors 33 may be formed by long leads from the detector 2 which rest on the rear face of the lens 3. The provision of narrow conductors 33 in either of these forms does not significantly degrade the performance of the lens 3. Furthermore, cooling means 37 may extend to the infrared detector 2, across the rear flat face of the lens 3 when the embedded detector 2 is of a type comprising a cooler for cooling the elements 20 during operation. This cooling means 37 may be a narrow heat-pipe for the heat-sinking of thermo-electrically cooled elements 20 or a narrow high-pressure gas pipe for feeding a Joule-Thomson cooler for cooling the elements 20 to, for example, liquid nitrogen temperature.

In order to focus the incoming infrared onto the detector elements 20 of the detector 2, embedded in the hole 6, the optical system of the antenna apparatus of Figure 1 also includes at least one infrared-transmissive lens 4 mounted in the front of the hole 6. The lens 4 may be of, for example, germanium. Both the infrared lens 4 and the infrared detector 2 may be mounted securely in the hole 6 of the pre-formed lens 3 by an adhesive layer 26, for example of resilient material. A rigid frame may be present between the lens 4 and the detector housing to maintain the proper location of the lens 4 in relation to the detector elements 20.

Alternatively, shoulders may be formed in the wall of the hole 6 to determine the required location of the lens 4 in relation to the housing of the detector 2. However, instead of mounting the detector 2 and lens 4 in the preformed hole 6 of a preformed lens 3, the polystyrene or other suitable lens material may be moulded around an assembly of the detector 2 and infrared lens 4; thus, the mm-wave lens may be formed subsequent to this assembly 2,4, and the conductor tracks 33 may then be deposited on the rear flat surface of the lens 3 and also onto the rear of the detector 2 to form direct electrical connections.

From an operational performance viewpoint it is often desirable to locate both the mm-wave transceiver 1 and infrared detector 2 on the same optical axis 5. The resulting co-axial concentric embedded arrangement of the two sensors 1 and 2 in accordance with the present invention facilitates the task of the designer in achieving common optical alignment of both sensors 1 and 2 in viewing a scene through a common aperture. However, one or both sensors 1 and 2 may be located off axis. Thus, for example, the hole 6 need not be located at the centre of the lens 3, but may be offset from the axis 5. Furthermore more than one hole 6 may be provided in the mm-wave lens 3 so that more than one sensor 2 may be embedded; these sensors 2 may then operate at different wavelengths from each other.A plano-convex lens 3 is shown in Figure 3 and its flat rear face is useful when the conductors 33 are formed as deposited metallization tracks.

However, other forms of lens design may be used for the lens 3, for example a doubly convex lens which may have aspheric surfaces.

Although a hole 6 extending right through the thickness of the lens 3 has been illustrated, a blind hole 6 terminating within the lens 3 may be used instead, in either the front or rear surface. Thus, for example, the lens 3 may be moulded around an infrared detector assembly 2,4 with lead-frame connections 33 and a cooling pipe 37 so that the total component assembly 2,4,33 and 37 is embedded in the lens 3 with the parts 33 and 37 protruding therefrom. This totally embedded structure in which the hole 6 is blind at the rear of the lens 3 provides very good mechanical support for the infrared detector assembly.

From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art. Such variations and modifications may involve equivalents and other features which are already known in the design, manufacture and use of infrared and mm-wave radar and antenna, their optical systems, and component parts thereof, and which may be used instead of or in addition to features already described herein.

Although claims have been formulated in this application to particular combinations of features1 it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (8)

CLAIM(S)

1. An antenna apparatus for both infrared and millimetre-wave radiation, comprising an infrared detecting means, a millimetre-wave transceiver means, and an optical system for receiving incoming infrared and millimetre-wave radiation and for transmitting outgoing millimetre-wave radiation, which optical system comprises a lens having a first area for transmitting the incoming and outgoing millimetre-wave radiation and a smaller second area for transmitting the infrared to be detected by the infrared detecting means, characterised in that the second area comprises a hole in the lens, and the infrared detecting means is embedded in the hole.

2. Apparatus as claimed in claim 1, further characterised in that electrical connections of the infrared detecting means comprise a pattern of conductors extending across a surface of the lens.

3. Apparatus as claimed in claim 2, further characterised in that the lens is of plano-convex shape having a substantially flat rear face, and the conductors are formed as a metallisation pattern on this substantially flat rear face.

4. Apparatus as claimed in any one of the preceding claims, further characterised in that the infrared-detecting means comprises a cooling means having a pipe extending across a surface of the lens.

5. Apparatus as claimed in claim 1, further characterised in that the infrared-detecting means comprises electrical connections and a cooler pipe which are embedded in and protrude from the lens.

6. Apparatus as claimed in any one of the preceding claims, further characterised in that the hole occupies a central area of the lens around its optical axis, and the millimetre-wave transceiver means is located on the optical axis behind the embedded infrared detecting means.

7. Apparatus as claimed in any one of the preceding claims, further characterised in that the optical system comprises at least one infrared-transmissive lens which is mounted in the front of the hole and which serves to focus the infrared onto the infrared detecting means embedded in the hole.

8. Antenna apparatus having any one or more of the novel features described herein and/or illustrated in any of the drawings.