DIRECTIONAL RADAR ARRANGEMENT AND ANTENNA ARRAY

Abstract: A radar arrangement, for example a frequency modulated continuous wave (FMCW) radar arrangement, is provided with an antenna array which is operated off-boresight in order to provide beam scanning. The antenna array comprises a plurality of antenna elements (16.1 - 16.10) which are arranged to produce a wavefront which is inclined relative to the final scanning direction. The inclined wavefront is produced either by inclining the antenna elements (16.1 - 16.10 (Figure 6 - not shown)) or using prismatic elements (20.1 - 20.14 (Figure 4A)). A beam deflecting prism (22) produces a scanning wavefront which is propagated in the desired direction.

Description: DESCRIPTION DIRECTIONAL RADAR ARRANGEMENT Technical Field The present invention relates to a directional radar arrangement having particular, but not exclusive, application to a low cost frequency-scanned radar arrangement for use in road vehicles. The invention also relates to an antenna array suitable for use in such a radar arrangement. Background Art Radar systems are known which use antenna arrays to provide a scanning beam covering an arc of a circle by altering the relative phases of an oscillator signal applied to the antenna elements in the array. Such an antenna is known for example from Skolnik, Radar Handbook and is shown in Figure 1 of the accompanying drawings. One technique for providing the beam scanning involves feeding the elements in the array from a succession of equally spaced tap-off points on a serpentine or snake feed. The oscillator signal is coupled to the serpentine feed and the direction of the transmitted beam is determined by the frequency of the applied signal. At a centre frequency, the length of the feed between successive antenna elements is an integer number of wavelengths and the radiated signals are in phase with one another. This provides a straight ahead, or boresight, beam. As the frequency is increased, for example, the wavelength decreases and the distance in wavelengths between the tap-off points increases which provides an incremental phase difference between successive antenna elements. This provides a beam from the array which is off centre in one direction. Decreasing the frequency provides a beam which is off centre in the other direction. Alternatively the phase changes across the antenna array may be provided by phase shifters. A major difficulty with an antenna array is that slight mismatches between the feed lines (whether via phase shifters, delay lines or using a serpentine feed) and the antenna elements cause reflections of the oscillator signal back to the oscillator. On boresight, in other words normal to the length of the array, the antenna elements are fed in phase with one another and so the reflections add up in phase with one another leading to a significant drop in efficiency. The poor performance of such an antenna array close to boresight is illustrated by a sketch in Figure 2 of the accompanying drawings. Disclosure of Invention It is an object of the present invention to ameliorate this disadvantage. According to the present invention there is provided a directional radar arrangement comprising means for transmitting a radar signal, means for receiving a return signal and processing means for deriving range and direction information from the return signal, at least one of the means for transmitting and receiving a signal including a scannable antenna array, characterised in that the scannable antenna array comprises a plurality of antenna elements having a directional peak in their characteristic arranged off the normal to the array. It has been possible previously to operate an antenna array away from boresight and substantially avoid the problem of mismatch. However, this meant that the individual array elements were being operated over a portion of their characteristic which was rather poorer than that of the boresight direction and one of the grating lobes of the array fell near the peak of the element characteristics. This can cause significant problems for a directional radar such as false returns and poor sensitivity. The antenna array of the radar arrangement in accordance with the invention, however, alters the direction in which the individual elements best operate to overcome the problem. Another way of thinking of the antenna array is to say that the element characteristics and the array characteristics are decoupled from one another. The centre beam direction of this antenna array, or displaced boresight, will not be normal to the array and where space is a consideration, for example when mounting the antenna at the front of a car for collision avoidance purposes, it might prove to be too bulky. Accordingly it is possible to add a beam deflecting means, for example a prism, in front of the antenna array to deflect the whole beam back to substantially normal to the array. The scannable antenna array may be applied to the transmitted radar signal and a non-scanning antenna coupled to the receiving side of the radar. Alternatively, a non-scanning transmit antenna may be employed and the scanning array coupled to the receiver. However it is preferred to use the same antenna for both the transmit and receive sides of the radar as this provides better use of the transmitted power and does not cause extra desensitizing of the receiver. A circulator may conveniently be used to separate the transmitted and received signals within the radar arrangement. According to another aspect of the invention there is provided an antenna array comprising a plurality of antenna elements and a serpentine feed means for feeding the antenna elements, characterised in that the plurality of antenna elements have a directional peak in their characteristic arranged off the normal to the antenna array. If desired the antenna array may further comprise beam deflecting means arranged in the path of beams from the antenna elements for deflecting the beam of the antenna array towards a normal to the array. The beam deflecting means may comprise a prism shaped element arranged transverse to the wavefront from the antenna elements. In one embodiment surfaces of the prism shaped element through which the wavefront passes in operation are non-parallel to the antenna elements. In an alternative embodiment an anti-reflective coating is provided on a surface of prism shaped element where the wavefront is incident at substantially ninety degrees. Brief Description of Drawings The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a known antenna array having a serpentine feed, Figure 2 shows a sketch of the output of the antenna array against angular direction for a fixed-amplitude drive signal, Figure 3 shows a sketch of the output of a single antenna element against direction, Figure 4 shows a plan view of an embodiment of an antenna array made in accordance with the present invention, Figure 5 shows a block schematic diagram of a radar arrangement in accordance with the invention, and Figure 6 shows a schematic diagram of an alternative antenna arrangement. In the Figures, corresponding features have been identified using the same reference numerals. Modes for Carrying Out the Invention The antenna array 10 shown schematically in Figure 1 comprises an input/output connection 12 coupled to a serpentine or snake feed 14 and ten horn antenna elements 16.1 to 16.10 arranged in a straight line. The horn elements are coupled to successive tap-off points on the feed 14. Although ten antenna elements are shown for ease of description a larger number will usually be used, for example from 14 to 30. As is known, the larger the number, the narrower will be the beam width produced by the antenna array. Figure 2 shows a sketch of the antenna array performance as it is fed with a signal at various frequencies and a fixed amplitude. Since the variation in frequency results in beam scanning, the x-axis can be regarded either as frequency (f) or angle (ø). The point on the axis marked with a 0 represents the boresight of the antenna, corresponding to the wavelength of the feeding signal being an integer fraction of the feed length between adjacent antenna elements. It will be observed that the antenna array performance in this region is distinctly poor and that there is a peak of performance on each side of the boresight. If a scanning angle of less than half the maximum possible is sufficient, as it has been found to be for vehicle collision avoidance, one of these better performing parts of the characteristic can be used. A suitable portion of the characteristic for such use is shown between the vertical broken lines on the Figure. However a price is paid for operating the antenna array away from boresight in that the first grating lobe in the other direction from that in which the antenna is scanned will lie within the maximum part of the antenna element characteristic. The elements will typically have a performance characteristic against angle as shown by the curve N in Figure 3 where the vertical broken lines equate approximately with those of Figure 2. As can be seen, this is not the best performing part of the element range so the array performance will suffer from this in addition to the grating lobe problem. The present invention is based upon the realisation that if the peak in the element performance is adjusted towards the intended point of array operation a considerable reduction of the grating lobe problem and performance gains are possible. Figure 4 shows a plan view of an antenna array for use in a radar system in accordance with the invention. The serpentine feed has been omitted for clarity. Polystyrene prisms 20.1 to 20.14 (shown here moulded as one unit) are arranged in front of each of the antenna elements 16.1 to 16.14 to deflect the element characteristic away from the normal. For the best performance the peak of the element characteristic is made to coincide with the centre of the range of angles over which it is intended to use the array. Such a condition is shown by the curve S in Figure 3. Also shown in Figure 4 is a large prism 22 which is located in front of all of the array elements 16. The purpose of the prism 22 is to deflect the beam from the array, that is from the polystyrene prisms, so that the boresight of the skewed array is again normal to the array of antenna elements. This is of particular interest in a confined space, for example in the nose of a car when the radar is used for collision avoidance or intelligent cruise control purposes. Again, for use in the 77 GHz band, a polystyrene prism is particularly cost effective and this material is suitable for use at other millimetre waveband frequencies. Alternative materials and techniques will be known to those skilled in the art. The small prisms are ideally located as close as possible to the horns so that the beam from each element is supplied substantially to only one prism. The gradient of the angles of the small and large prisms is 10.4° and this has been found to give good performance for a 77GHz radar arrangement for vehicle collision avoidance. In the embodiment illustrated in Figure 4, the prism 22 is designed for constructional convenience as a right angled triangle and is arranged with the surface constituting the hypotenuse facing away from polystyrene prisms 20.1 to 20.14. With such an arrangement the scanning beams do not enter or leave either of the main surfaces at ninety degrees which would otherwise cause unwanted reflections. Optionally the prism 22 may be designed such that neither major surface is parallel to the wavefront at any angle through which the beam scans. In a non-illustrated variant of the embodiment shown in Figure 4 the prism 22 is rotated through 180° so that the surface constituting the hypotenuse faces towards the polystyrene prisms 20.1 to 20.14 but diverges therefrom going from left to right. The exit or re-entry main surface of the prism 22 is substantially parallel to the plane containing the openings of the antenna elements 16.1 to 16.14. However since reflections can occur at the exit or re-entry surface of the prism 22, an anti-reflection coating 24 is provided on the surface of the prism by applying a coating which is one quarter or any odd multiple of a quarter of a wavelength thick to the surface by any suitable known technique. The refractive index of the material constituting the coating has the geometrical mean of the dielectric and the air. As a general rule if unwanted reflections occur at the surface(s) of the prism 22 due to the angles of incidence of the scanning beams, an anti- reflective coating will attenuate or eliminate them. While the present invention is applicable to a pulse radar arrangement a preferred arrangement employs a frequency modulated continuous wave (FMCW) arrangement. Figure 5 shows a simplified block schematic diagram of a FMCW radar suitable for use with the antenna. Further information can be obtained from the Skolnik text referenced previously. A radar oscillator 40 is controlled by a sawtooth generator 42 so that the frequency of the oscillator output continuously rises and is reset to its starting value before rising again. The output of the oscillator 40 is connected to a first input terminal to a circulator 44 and travels through the circulator to a scannable antenna array 18 as described above by way of a second terminal which functions as an output and an input. A return signal from the antenna array 18 enters the second terminal of the circulator and leaves by a third terminal. The return signal is amplified by an amplifier 46. An output of the amplifier 46 is coupled to a first input to a mixer 50. A second input to the mixer is connected to a coupler 48 which taps off a portion of the oscillator output. The output of the mixer 50 is low pass filtered in a low pass filter 52 to derive the difference frequency signal which represents the range of the target(s). A processor 54 is connected to receive the output of the low pass filter 52 and to derive whatever target information is required by the application. In order that the angular location of the target(s) may be derived by the processor 54, the output of the ramp generator 42 is also supplied to the processor. Figure 6 shows a schematic view of an alternative construction of the antenna array in accordance with the invention. The individual prisms have been omitted and the antenna elements 16.1 to 16.10 are arranged in a line skewed with respect to each other to provide the decoupling of the element characteristic from the array characteristic. Again a single large prism 22 may be added to move the skewed boresight back normal, or substantially so, to the antenna array. The prism 22 may be arranged as shown in Figure 4B or be designed so that neither major surface is parallel to the wavefront at any angle through which the beam scans. From reading the present disclosure other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of radar arrangements 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 features, 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. Industrial Applicability Low cost frequency-scanned radar arrangement suitable for use in cruise control and/or collision avoidance in road vehicles.

Claims: CLAIMS 1. A directional radar arrangement comprising means for transmitting a radar signal, means for receiving a return signal and processing means for deriving range and direction information from the return signal, at least one of the means for transmitting and receiving a signal including a scannable antenna array, characterised in that the scannable antenna array comprises a plurality of antenna elements having a directional peak in their characteristic arranged off the normal to the array. 2. A radar arrangement as claimed in Claim 1, characterised in that the scannable antenna array is coupled to both the means for transmitting and the means for receiving radar signals. 3. A radar arrangement as claimed in Claim 1 , characterised in that the means for transmitting and receiving a radar signal comprises an frequency modulated continuous wave radar arrangement. 4. A radar arrangement as claimed in Claim 1 , characterised in that the antenna array comprises a serpentine-fed antenna array. 5. A radar arrangement as claimed in Claim 1 , characterised in that the antenna array further comprises beam deflecting means arranged in the path of beams from the antenna elements for deflecting the beam of the antenna array towards a normal to the array. 6. A radar arrangement as claimed in Claim 5, characterised in that the beam deflecting means comprises a prism shaped element arranged transverse to the wavefront from the antenna elements. 7. A radar arrangement as claimed in Claim 6, characterised in that surfaces of the prism shaped element through which the wavefront passes in operation are non-parallel to the antenna elements. 8. A radar arrangement as claimed in Claim 6, characterised in that an anti-reflective coating is provided on a surface of prism shaped element where the wavefront is incident at substantially ninety degrees at any angle over which the beam is scanned. 9. An antenna array comprising a plurality of antenna elements, and a serpentine feed means for feeding the antenna elements, characterised in that the plurality of antenna elements have a directional peak in their characteristic arranged off the normal to the antenna array. 10. An antenna array as claimed in Claim 9, characterised by beam deflecting means arranged in the path of beams from the antenna elements for deflecting the beam of the antenna array towards a normal to the array.

Inventors: STOVE ANDREW GERALD, STOVE, ANDREW, GERALD
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H01Q3
IPC codes: G01S7, H01Q3