GB2454727A

GB2454727A - Planar antenna array with shunt radiating slots and shunt coupling slots

Info

Publication number: GB2454727A
Application number: GB0722571A
Authority: GB
Inventors: Sang Tu; Brendan Knowles
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2007-11-16
Filing date: 2007-11-16
Publication date: 2009-05-20
Also published as: GB0722571D0

Abstract

A planar antenna array 10 for a radar system comprises parallel linear arrays A1-A18, B1-B16 of coplanar shunt radiating slots S1-S6 and coplanar coupling shunt slots C1 -C18, D1-D16 in a waveguide spaced from the plane of the radiating slots S1-S6. The waveguide may be one or more parallel elongate waveguides 13, 14 which are coupled to a different set of coplanar linear arrays A1-A18,B1-B16 and are perpendicular to their radiating slot linear arrays and may be used for transmitting and/or for receiving radio frequency signals. The coupling slots C1-C18, D1-D16 may be alternately displaced either side of the centre line CL8, CL9 of a waveguide 13, 14 by an amount which may be varied to achieve a desired power distribution. Each of the linear arrays A1-A18, B1-B16 may also be alternately displaced such that the longitudinally equally spaced radiating slots S1 -S6 of a linear array may have pairs of slots equally spaced either side of their corresponding coupling slot C1-C18, D1-D16. Feed slots 1310-1330, 1410-1440 may be used to transmit signals into or out of a waveguide 13,14. A method of transmitting or receiving signals using such an antenna array is also claimed.

Description

1 2454727

PLANAR ARRAY RADAR ANTENNA

This invention relates to a planar array radar antenna and to a method of using it for transmitting or receiving radar signals.

Planar array radar antennas are well known, and one source of information is "Antenna Engineering Handbook", Second Edition, by Richard C Johnson and Henry Jasik. A planar array antenna used in airborne radar systems is described in "Introduction to Airborne Radar" Second Edition, by George W Stimson, published by Scitech Publishing Inc. Such antennas have a smooth planar radiating panel in which multiple arrays of elongate slots are cut. An opposing wall forming the rear face of the antenna is formed with coupling slots through which radiation to and from the interior of the antenna passes to and from receiver and/or transmitter waveguides attached to the rear of the antenna.

Conventionally, the slots of each array in the radiating plane at the front of the antenna are all parallel and are aligned with the longitudinal direction of the array: such slots are referred to as shunt slots because of their lengthwise orientation. Conventionally also the coupling slots, for communicating with the waveguide at the rear of the antenna, are series coupled, meaning that they are inclined at an acute angle to the longitudinal direction of the waveguide, which is transverse to the linear arrays of radiating slots.

The slant angles of the inclined series coupling slots are determined by the phase signal to be received or transmitted.

It is also conventional in planar array radar antennas to stagger the transverse positions of the radiating shunt slots in each linear array. Thus alternate slots are offset in opposite directions from a longitudinal centreline of each linear array.

Correspondingly, the inclined series coupling slots are also alternately staggered, so that their slant angle alternates in direction. This staggering has been found to improve antenna performance by reducing cross-coupling.

One important measure of performance of such an antenna is its frequency bandwidth expressed as a percentage of the centre frequency of the radar signal; bandwidth being defined as that band which gives better than 10dB return loss overall and better than 20dB side lobe loss. The conventional type of planar array radar antenna described above requires, at each end of the waveguide, a half wavelength short circuit component, due to the inclined series slots and the radiating shunt slots configuration.

This half wavelength short circuit is conventionally provided using a "folded short", which is an end portion of the waveguide folded over the edge of the antenna. With this optimisation provided by the folded short, such a conventional antenna can provide typically a maximum of 6% of the centre frequency in its bandwidth as defmed above.

The usual level of performance is around 3% of centre frequency.

A purpose of the present invention is to improve bandwidth both for transmission and reception.

Accordingly, the present invention provides a planar array radar antenna comprising: mutually parallel radiation slots for emitting or receiving radiation from the exterior, arranged as shunt slots in coplanar, parallel linear arrays; and coupling slots arranged as coplanar shunt slots in an elongate transmitter or receiver waveguide spaced from the plane of the radiation slots, for coupling radiation between selected radiation slots and the interior of the waveguide.

The provision of shunt slots as the coupling slots has been found by computer modelling to enhance significantly the bandwidth, and the performance of such an antenna can typically be in the range of 10- 15% of centre frequency. A 10% figure can be expected for the transmitter, and a 15% figure for each receiver, in the embodiment of the invention described below.

The use of shunt slots as the coupling slots also means that a quarter wavelength short circuit is required at each end, rather than a half wavelength short circuit, and this can, for some configurations, avoid the need for the folded short altogether.

The antenna could be provided with just one transmitter or more than one transmitter coupled to one or more corresponding waveguides at the rear of the antenna, and the same applies to the receiver or receivers. It is possible to have just one microwave feed, used alternately for reception and transmission by the provision of a circulator.

In a preferred embodiment, the antenna comprises at least one receiver waveguide and at least one transmitter waveguide parallel thereto and spaced therefrom, each waveguide being coupled by its respective coupling slots to a different set of the coplanar linear arrays.

Preferably, each waveguide comprises feed slots, spaced lengthwise of the waveguide, for coupling radiation between a respective receiver or transmitter, through the interior of the waveguide, with respective ones of the coupling slots.

To minimise cross-coupling, preferably the shunt coupling slots of each lengthwise half of the waveguide are transversely and alternately staggered.

With this alternate staggering, it is preferred that each shunt coupling slot of at least one set thereof is offset transversely from a longitudinally-extending centre line of the waveguide by a distance which decreases with the distance of the slot along the waveguide from a predetermined transverse baseline at the middle of the set of coupling slots.

It is further preferred that each waveguide comprises feed slots, spaced lengthwise of the waveguide, for coupling radiation between a respective receiver or transmitter, through the interior of the waveguide, with respective ones of the coupling slots; and for each feed slot there is a respective aligned baseline of a respective set of coupling slots.

Again for the minimising of cross-coupling, it is preferred that the radiation slots of each linear array are transversely and alternately staggered.

Preferably, the radiation slots are all transversely offset from a centre line of the linear array by an equal distance.

With any of these preferred arrangements for reducing cross-coupling, it is preferred that the radiation slots of alternate linear arrays are staggered longitudinally of the arrays such that alternate radiation slots, at the same corresponding position in each array, are offset by a different amount from a line transverse to the linear arrays.

It is then preferred that a normal projection of each coupling slot onto the plane of the radiation slots, which are perpendicular to it, is symmetrically disposed about the centre line of a respective linear array of radiation slots.

As with conventional antennas of this type, the coupling slots are preferably equally spaced lengthwise of the waveguide. The radiation slots are also preferably of equal size and shape, as are the coupling slots preferably of equal size and shape. The coupling slots are preferably equal in size and shape to the radiation slots, although this is not essential.

The invention also provides a method of receiving radar signals using such an antenna, comprising receiving radiation through the radiation slots and thence through the coupling slots and thence through the waveguide to a receiver.

Further, the invention also provides a method of transmitting radar signals from such an antenna, comprising transmitting radiation from a transmitter through the waveguide and thence through the coupling slots and thence through the radiation slots to the exterior of the antenna.

By way of explanation, the separation along the longitudinal direction of each linear array between adjacent slots is normally a half wavelength of the radar signal, but the distance between the end slots closest to an edge of the antenna and that edge, would normally be a quarter wavelength, in the preferred embodiment.

In order that the invention may be better understood, a preferred embodiment will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a front elevation of a planar array radar antenna embodying the invention; Figure 2 is a side elevation from the right hand side of the antenna of Figure 1; Figure 3 is a rear elevation of the antenna of Figure 1; Figure 4 is a front elevation, to an enlarged scale, of the antenna of Figure 1; Figure 5 is a cross-section taken along the line 5-5 in Figure 4, to an enlarged scale; and Figure 6 is a cross-section taken along the line 6-6 in Figure 4, to an enlarged scale.

The planar array radar antenna 10 of Figures 1-6 has a smooth, planar front face 11 spaced normally from a parallel rear planar metallic wall 12 and connected to it by a narrow, metallic side wall 15, Figure 2, so that the air space inside the antenna is enclosed by metal surfaces. Apertures 16 around the periphery of each planar surface 11,12 are provided for receiving connecting screws; three apertures 16' are provided across the body of the antenna for a similar purpose. Some or all of these connecting screws may also be used to secure the antenna to external apparatus.

A receive waveguide 13 is elongate with a generally rectangular cross-section, and is secured by screws at each end through corresponding apertures 16. The receive waveguide has three equally spaced enlargements which are square when viewed from the rear of the antenna as shown in Figure 3. These three enlargements 131, 132 and 133 each have a corresponding vertical slot 1310, 1320, 1330 respectively to allow microwave communication from the interior of the waveguide to a corresponding receiver (not shown).

An elongate transmit waveguide 14 is also attached to the rear of the antenna over the rear wall 12, and is arranged parallel to but below the receive waveguide 13, as shown in Figure 3. In this example, a single transmitter (not shown) communicates through respective vertical slots 1410, 1420, 1430 and 1440 with the interiors of respective rectangular enlargements 141, 142, 143 and 144 which are spaced equally along the length of the waveguide 14.

As shown in Figure 4, the mutually parallel radiation slots formed in the front planar face 11 are grouped into eighteen linear arrays A1-A18 for reception, and into sixteen linear arrays B 1-B 16 for transmission. Each linear array has six mutually parallel radiation slots SI -S6, of equal size and shape, alternately staggered transversely. Thus in array Al, for example, alternate slots Si, S3 and S5 are offset by an equal distance to the right of the centreline CLIO of the linear array, whilst the alternate slots S2, S4 and S6 are offset the same distance but to the left of the centreline CLIO. Further, the linear arrays themselves are staggered alternately along the width of the antenna, i.e. they are alternately offset in opposite directions relative to a centreline CL8 in respect of the receive arrays, and a centreline CL9 in respect of the transmit arrays. Thus for example linear arrays Al, A3, A5,... A17 are raised, whilst the alternate ones A2, A4,... A18 are lowered. This is important because it corresponds to the alternate staggering of the shunt coupling slots as described below.

The rear wall 12 of the antenna is formed with shunt coupling slots in front of each corresponding waveguide 13, 14, for the purpose of providing microwave communication between the interior of the corresponding waveguide and the interior of the antenna between planar walls 11 and 12. In this example, the coupling slots Cl-C18 and Dl-D16 are all of the same size and shape, which is also the same as the radiation slots S1-S6 of each linear array. The coupling slots are shunt slots, since they are longitudinal of the waveguide, and accordingly they are perpendicular to the radiation slots. Each waveguide is coupled by its respective coupling slots to a different set of the coplanar linear arrays. The coupling slots are spaced lengthwise of the waveguide by equal distances which are equal to the spacings between the linear arrays of radiation slots, with which they are aligned.

The coupling slots of each lengthwise half of the waveguide are transversely and alternately staggered. Accordingly, this is true of the coupling slots to the left hand side of the vertical centreline CL1, and it is also true of the half of the waveguide to the right hand side of centreline CL1. The slots Dl -Dl 6 are disposed symmetrically, but the slots C1-C18 are disposed asymmetrically, about the centreline CL1.

The coupling slots are arranged into sets, for communication with each corresponding feed slot 1310, 1320, 1330; and 1410, 1420, 1430 and 1440. Taking as an example the left hand receive portion 131, coupling slots Cl, C3, C5 and C7 are offset upwards relative to the centreline CL8, whereas the alternate coupling slots C2, C4 and C6 are offset downwards. A similar pattern applies to each set of coupling slots, relating to the other areas of the waveguides.

In order to balance the power across the antenna, the coupling slot of each set is offset transversely from the longitudinally-extending centreline of the waveguide by a distance which decreases with the distance of the slot along the waveguide from a predetermined transverse baseline at the middle of the set of coupling slots. To take an example of the left hand receive portion of the waveguide, coupling slot C3 is offset from the centreline CL8 by a distance equal to the opposite offset of coupling slot C4. For the pair of coupling slots C2 and C5 which are the next farthest from the transverse baseline CL2 at the middle of the coupling slots, the offset distance is equal but is less than the offset for coupling slots C3 and C4. The equal offset for the farthest pair of coupling slots, Cl and C6, is even less. In this particular example, the offsets from the centreline CL8 are about 2.5mm, 2mm and 1.5mm. To put this in the context of the antenna as a whole, each slot has a width of 1.80mm and length of 9.50mm, and the dimensions of the antenna body are 250mm by 180mm. The baseline for each respective set of coupling slots extends through the corresponding feed slot. Thus baseline CL2 extends through feed slot 1310; baseline CL1 extends through feed slot 1320; baseline CL3 extends through feed slot 1330; baseline CIA extends through feed slot 1410; baseline CL5 extends through feed slot 1420; baseline CL6 extends through feed slot 1430 and baseline CL7 extends through feed slot 1440.

For optimal distribution of power and equality of phase, the normal projection of each coupling slot onto the plane of the radiation slots, which are perpendicular to it, is symmetrically disposed about the centreline of a respective linear array of radiation slots. To take an example in Figure 4, the normal projection of coupling slot Ci onto the plane ills symmetrically disposed about the centreline CLIO of the linear array Al.

The radiation slots S1-S6 are equally spaced lengthwise of each linear array, and pairs of them, such as S3, S4, are equally spaced from the respective coupling slot Cl, etc. This also optimises the distribution of power and quality of phase.

In use, radar signals are received through the radiation slots and thence through the coupling slots and thence through the corresponding waveguide to a respective receiver or receivers (not shown). Correspondingly, radar signals are transmitted from a transmitter or transmitters through the corresponding waveguide and thence through the coupling slots and thence through the radiation slots to the exterior of the antenna.

Whilst a detailed embodiment of the invention has been described with reference to Figures 1-6, it will be appreciated that many modifications could be made. There could be any number of receivers and any number of transmitters, and the configuration of the shunt radiation slots and of the shunt coupling slots could be changed to suit particular performance requirements. Whilst the slots are described as being of equal size and shape, they could be varied, and in any event some manufacturing tolerance is necessary.

Claims

CLAIMS: 1. A planar array radar antenna comprising: mutually parallel radiation slots for emitting or receiving radiation from the exterior, arranged as shunt slots in coplanar, parallel linear arrays; and coupling slots arranged as coplanar shunt slots in an elongate transmitter or receiver waveguide spaced from the plane of the radiation slots, for coupling radiation between selected radiation slots and the interior of the waveguide.
2. An antenna according to claim 1, comprising at least one receiver waveguide and at least one transmitter waveguide parallel thereto and spaced therefrom, each waveguide being coupled by its respective coupling slots to a different set of the coplanar linear arrays.
3. An antenna according to claim 2, wherein each waveguide comprises feed slots, spaced lengthwise of the waveguide, for coupling radiation between a respective receiver or transmitter, through the interior of the waveguide, with respective ones of the coupling slots.
4. An antenna according to any preceding claim, in which the shunt coupling slots of each lengthwise half of the waveguide are transversely and alternately staggered.
5. An antenna according to claim 4, in which each shunt coupling slot of at least one set thereof is offset transversely from a longitudinally-extending centre line of the waveguide by a distance which decreases with the distance of the slot along the waveguide from a predetermined transverse baseline at the middle of the set of coupling slots.
6. An antenna according to claim 5, wherein each waveguide comprises feed slots, spaced lengthwise of the waveguide, for coupling radiation between a respective receiver or transmitter, through the interior of the waveguide, with respective ones of the coupling slots; and for each feed slot there is a respective aligned baseline of a respective set of coupling slots.
7. An antenna according to any preceding claim, wherein the radiation slots of each linear array are transversely and alternately staggered.
8. An antenna according to claim 7, wherein the radiation slots are all transversely offset from a centre line of the linear array by an equal distance.
9. An antenna according to any preceding claim, wherein the radiation slots of alternate linear arrays are staggered longitudinally of the arrays such that alternate radiation slots, at the same corresponding position in each array, are offset by a different amount from a line transverse to the linear arrays.
10. An antenna according to claim 7 or 8 and any of claims 4-6, wherein a normal projection of each coupling slot onto the plane of the radiation slots, which are perpendicular to it, is symmetrically disposed about the centre line of a respective linear array of radiation slots.
11. An antenna according to claim 10, wherein the radiation slots are equally spaced lengthwise of the linear array and pairs of them are equally spaced from the respective coupling slot.
12. An antenna according to any preceding claim, in which the coupling slots are equally spaced lengthwise of the waveguide.
13. An antenna according to any preceding claim, in which the radiation slots are of equal size and shape.
14. An antenna according to any preceding claim, in which the coupling slots are of equal size and shape.
15. An antenna according to claim 13, wherein the coupling slots are equal in size and shape to the radiation slots.
16. An antenna according to any preceding claim, wherein the waveguide or each waveguide is perpendicular to the radiation slot linear arrays.
17. A planar array radar antenna substantially as described herein with reference to the accompanying drawings.
18. A method of receiving radar signals using an antenna according to any preceding claim, comprising receiving radiation through the radiation slots and thence through the coupling slots and thence through the waveguide to a receiver.
19. A method of transmitting radar signals from an antenna according to any of claims I to 17, comprising transmitting radiation from a transmitter through the waveguide and thence through the coupling slots and thence through the radiation slots to the exterior of the antenna.