GB2242265A - Antenna for thermal radiation - Google Patents

Abstract

An antenna comprises a first reflector (2) for reflecting thermal radiation from a scene to an thermal detector (4), the field of view of the detector being wider than the solid angle of rays received from the first reflector (2), and a second reflector (8) for shielding the detector (4) from rays outside the solid angle, the second reflector (2) reflecting radiation from the detector (4) back onto the detector (4) and/or radiation from cold sky onto the detector (4). By so shielding the detector (4) it can have a field of view appropriate to a first reflector of greater diameter than first reflector (2) and therefore a smaller reflector may be used whilst still achieving the desired resolution. The reflectors may be replaced by refracting systems and the antenna may also be used for short wavelength radio waves. <IMAGE>

Description

An Antenna This invention relates to an antenna for receiving thermal radiation.

Such antennas have a number of reflective surfaces for gathering and directing radiation onto a detector sensitive to thermal radiation such as a radiometer.

The reflectors may include a planar scanning mirror which is pivotable about at least one axis in order to achieve a desired scan of a scene, and a parabolic reflector system which focuses the radiation received from the scanning mirror onto a detector.

It is the size of the parabolic reflector or the scanning mirror which usually determines the aperture of the antenna and its resolving power. In some applications, for example where the antenna is to be used in an aircraft, the space available is not sufficient for a reflector having the desired resolution.

In the past, a certain type of antenna has been used where the detector "over-illuninated" a main reflector, that is to say the detector had a field of view such that it was able to receive radiation from beyond an outer periphery of the main reflector. The term over-illuminating is used because rays being directed to the detector can be thought of, using ray convention, as originating from the detector.

over-illuminating the main reflector achieved a uniform illumination which was desirable since the main reflector then produced part of a predicable diffraction pattern. For a square reflector this was a part of a sin x pattern, the full pattern of which could be recovered by analysis with a microcomputer of the detector output. The sin x pattern was preferred X because of the ease with which it could be factorised.

By regaining the full pattern the resolution could be increased.

Such an antenna, with its advantageous resolution, could not however be used in some applications particularly where the detector was a cooled detector such as a radiometer, for the detector could accept radiation from beyond the main antenna. This radiation, called overspill resulted in noise entering the system degrading the detected image.

Accordingly, the invention therefore provides an antenna comprising: a first reflector for reflecting thermal radiation from a scene to an thermal detector, the field of view of the detector being wider than the solid angle of rays received from the first reflector; and a second reflector for shielding the detector from rays outside the solid angle, the second reflector reflecting radiation from the detector back onto the detector and/or radiation from cold sky onto the detector.

Since the field of view is wider than the solid angle of rays received from the first reflector these rays appear to the detector to have been reflected from part of a larger reflector. The remaining part of the larger reflector appears to the detector to be, making the analogy with visible light, black that is to say of zero or near zero intensity. This is because the second reflector shields the detector from rays outside the solid angle and reflects cold radiation from the cold detector and/or cold sky onto the detector. Therefore little or no spurious radiation or noise enters the detector.

Preferably, the antenna comprises means for determining from an output of the detector a signal appropriate to the field of view.

Specific embodiments of the invention will now be described by way of example only with reference to the drawing in which: Figure 1 shows a first embodiment of an antenna in accordance with the invention; Figure 2 shows a detector's field of view; and Figures 3 to 6 show further embodiments of antennas in accordance with the invention.

With reference to Figure 1, thermal radiation from a scene to be detected is reflected by a scanning mirror 1 to a parabolic reflector 2 which typically has a diameter of 1.2m. Scanning of the scene is achieved by pivoting the scanning mirror 1 about an axis 3 in a manner well know.

The parabolic reflector 2 reflects and focuses the radiation (represented by solid rays on the diagram) onto a cooled detector 4 via a concave relay mirror 5 often called a front or secondary reflector and a hole in the parabolic reflector 2.

The detector 4 is a radiometer which has a field of view such that it is able to accept radiation over the solid angle shown by dotted rays. This field of view corresponds to a larger parabolic reflector than is usable. The larger reflector extends by dotted portions 6 and 7 beyond the periphery of the parabolic reflector 2 which is used. As can be seen from the figure, this corresponds to an aperture a' which is larger than aperture a through which radiation is actually admitted from the scene to be detected.

An annular spherical mirror 8, prevents radiation being accepted by the detector 4 from beyond the periphery of the relay mirror 5 and reflects radiation from the detector 4 back onto itself in a deliberate narcissus effect. The detector 4 therefore views in its field of view a central disk 9 of radiation from the scene bounded by an annulus 10 as shown in Figure 2.

The annulus 10 appears of very low intensity because it is comprised of radiation narcissus reflected from the cooled detector 4 by the annular mirror 8 and appears black. It will now be understood that the annular mirror 8 prevents noise, that is spurious radiation, from entering the detector 4.

The radiation detected over the diameter of the parabolic reflector 2 corresponds to only part of the radiation pattern appropriate to the larger reflector.

A second embodiment of the invention is shown in Figure 3 where the spherical annular mirror 8 of the first embodiment has been replaced by an annular planar mirror 11. This mirror 11 is orientated so to reflect, from a region of cold sky, radiation to the detector 4.

Again, the detector 4 views in its field of view a central disk 9 bounded by a black annulus described above with reference to Figure 2.

Another embodiment of the invention is shown in Figure 4 where the use of a Casegrainian system has been avoided in order that full use may be made of the parabolic reflector 2. In this embodiment a reflective iris 12 defines the aperture of the system and is reflective in order to (in conjunction with spherical mirror 8) reflect cold radiation from the detector back onto itself.

In some applications, it may be more appropriate to place the annular mirror about the periphery of the parabolic reflector as shown in Figure 5. However, this would increase the effective size of the main reflector and hence the overall bulk of the antenna.

Another alternative position for the annular mirror is shown in Figure 6 where an annular spherical mirror 8 is placed about the periphery of a relay mirror 5. It is also envisaged that this annular spherical mirror could be formed as a part of the relay mirror 5 reducing the number of separate reflecting elements but this would, of course, make the relay mirror 5 more difficult to fabricate.

Alternatively, the reflecting system may be replaced by a refracting system with an annular mirror as before to prevent radiation entering the detector from outside the solid angle of rays provided by the refracting system and reflecting to the detector cold radiation from itself or the sky in a similar manner to that described above.

Although the invention has been described with reference to thermal radiation it is envisaged that the detection of radiation of frequency from 1 GHz upwards could be improved by utilization of the invention. The invention may be particularly appropriate for the detection of radiation in the frequency range of 35 300 GHz.

Claims (8)

1. An antenna comprising: a first reflector for reflecting thermal radiation from a scene to an thermal detector, the field of view of the detector being wider than the solid angle of rays received from the first reflector, and a second reflector for shielding the detector from rays outside the solid angle, the second reflector reflecting radiation from the detector back onto the detector and/or radiation from cold sky onto the detector.

2. An antenna as claimed in claim 1 comprising means for determining from an output of the detector a signal appropriate to the field of view.

3. An antenna as claimed in claim 1 or 2 wherein the first reflector is a parabolic reflector.

4. An antenna as claimed in claims 1, 2 or 3 wherein the second reflector is an annulus through which the rays pass.

5. An antenna as claimed in claims 1, 2, 3 or 4 wherein the second reflector is a spherical reflector.

6. An antenna as claimed in claims 1, 2, 3 or 4 wherein the second reflector is a planar reflector.

7. An antenna comprising; a refracting system for refracting thermal radiation from a scene to an thermal detector, the field of view of the detector being wider than the solid angle of rays received from the refracting system, and a refractor for shielding the detector from rays outside the solid angle, the reflector reflecting radiation from the detector back onto the detector and/or radiation from cold sky onto the detector.

8. An antenna as hereinbefore described with reference to and as illustrated by the drawings.