GB2107936A - Antenna - Google Patents

Abstract

A strip transmission line antenna, especially for a Doppler radar navigation system on an aircraft, has three or four different radiation patterns respectively associated with the supply of energy to the antenna at different ports (1-4). In order to improve the radiation patterns and/or the aperture efficiency compared with an antenna in which all the antenna elements are connected to all the ports, the elements (5) are connected by feeders (6-9) in two separate arrays each occupying the same aperture, two of the ports (1, 2) being coupled to one array and the remaining port(s) (3, 4) to the other array; each array comprises one or more unidirectional feeders (7, 9 respectively) along which energy is supplied to the elements in only one sense irrespective of the port of that array at which energy is supplied. <IMAGE>

Description

SPECIFICATION Antenna The invention relates to an antenna formed in strip transmission line, the antenna comprising a plurality of antenna elements distributed over an antenna aperture which extends in each of two mutually perpendicular directions, at least three ports, and a plurality of feeders each for supplying energy from at least one of the ports to at least one element at each of a plurality of points spaced along the feeder, wherein the antenna has first, second and third radiation patterns which each have a single main lobe and which are respectively associated with the supply of energy to the antenna at a first, a second and a third of the ports, and wherein the angular orientations of the main lobes of the first and second radiation patterns differ substantially in a first of the two directions and wherein the angular orientation of the main lobe of the third radiation pattern differs substantially from the angular orientations of the main lobes of the first and second radiation patterns in the second of the two directions. The invention relates particularly but not exclusively to such an antenna for use in a Doppler radar navigation system on an aircraft.

In such a navigation system, signals are transmitted downwards from the aircraft in a beam inclined to the vertical and are received back at the aircraft after reflection from the earth's surface. Information on the aircraft's ground velocity can then be derived from the Doppler shift of the reflected signals. In order to determine both the forward and transverse components of velocity and to reduce errors due to pitching and roiling of the aircraft, signals are usually transmittedin a total of three or four beams with different orientations.The beams may be considered as being directed from a common point on the aircraft to the four corners (or three of the corners) of a rectangle in a horizontal plane beneath the aircraft, the common point on the aircraft being vertically aligned with the centre of the rectangle and, in navigation systems where the antenna is fixed relative to the aircraft, the longitudinal axis of the aircraft being in a vertical plane parallel to one pair of sides of the rectangle.

The beams may be produced one at a time in sequence. This has the advantages of allowing a single microwave source to be used, of maximising the radiated power for a given power of a single source by not dividing its power between a plurality of beams, and of alleviating possible interference that may occur with multiple sources and with multiple beams.

Two forms of waveguide antenna arrangement for such a navigation system are described in the paper "A Dual Beam Planar Antenna for Janus Type Doppler Navigation Systems" by H.

Saltzmann and G. Stavis, IRE Convention Record, Part 1, 1958, pp 240-247. With each arrangement, two diagonally-opposite beams are produced simultaneously. One arrangment comprises two separate antenna arrays which do not occupy the same aperture and only one of which is fed at a time. The other arrangement comprises two separate arrays which occupy substantially the same aperture and which are fed simultaneously, one of the arrays being fed via a polarity reversing switch whereby to select one or the other of the two pairs of diagonally-opposite beams.

For convenience, references in this specification to the operation of an antenna generally relate (as above) to the supply of power to the antenna i.e to use of the antenna for transmission, but might equally well relate mutatis mutandis to the derivation of power from the antenna, i.e. to use of the antenna for reception.

An antenna as set forth in the opening sentence is known from the papers entitled "A printed antenna/radome assembly (radant) for airborne doppler navigational radar" by T.W. Bazire, A.W.D.

Ludgate and R.H.J. Carry, Proceedings of the 4th European Microwave Conference, p.494 et seq., and "A printed antenna/radome (radant) for airborne doppler navigational radar" by T.W.

Bazire, R. Croydon and R.H.J. Cary, International Conference on Antennas for Aircraft and Spacecraft, 3-5 June, 1975, London, pp.

35-40. This arrangement is formed in triplate strip transmission line, and comprises a plurality of slot antenna elements connected to form a single rectangular array having four ports, one at each corner. More specifically, the array comprises a plurality of parallel rows of slots, the slots extending orthogonally to the rows. In each row, the slots are connected together by a respective common so-called "array track" extending in the direction of the row, and the array tracks are connected together at each end by a so-called "feed track" extending orthogonally to the rows.In this arrangement, the distribution of the coupling between the slots of a row and the respective array track (expressed as slot conductance) is necessarily symmetrical because the track may be fed from either end, depending on which port energy is supplied to, and because it is desirable that the radiation patterns respectively associated with the transfer of energy between the slot elements and each of the ports are mirror images one of another. (Even in the absence of the latter condition, an approximately symmetrical coupling distribution would generally be required to obtain radiation patterns of reasonable quality.) The coupling is therefore of a general form having a maximum at the mid-point of a row and decreasing towards each end of the row.In order that the proportion of power that is radiated by a slot near either end of a row, relative to the power radiated by a slot near the centre of the row, should not be too high when the respective array track is fed with power from that end, the coupling between such a slot and the array track must be small. However, this means that such a slot radiates very little power indeed when the array track is fed from the other end, bearing in mind that the amount of power travelling along the array track decreases with increasing distance from the end at which it is fed. Thus such an arrangement cannot make optimum use of the available antenna aperature, and the antenna aperture required to obtain a radiation pattern with a specified beam-width of the main lobe and a specified side-lobe ievel is necessarily larger than it might otherwise be.

It is an object of the invention to enable this disadvantage to be alleviated.

According to the invention, an antenna as set forth in the opening sentence of this specification is characterised in that the elements are connected by the feeders to form two antenna arrays each occupying substantially said aperture, in that the coupling of each of the first and second ports is substantially greater to the first array than to the second array and the coupling of the third port is substantially greater to the second array than to the first array, and in that at least two feeders of the first array and at least one feeder of the second array are each a unid'rectional feeder for supplying energy substantially in one a single sense along the feeder to at least one element at each of a plurality of points spaced along the feeder, the elements supplied respectively at those points being spaced from one another in the second direction.

In such an antenna comprising a fourth port, wherein the antenna has a fourth radiation pattern which has a single main lobe and which is associated with the supply of energy to the antenna at the fourth port, and wherein the angular orientation of the main lobe of the fourth radiation pattern differs substantially from the angular orientations of the main lobes of the first and second radiation patterns in the second direction and from the angular orientation of the main lobe of the third radiation pattern in the first direction, the coupling of the fourth port is suitably substantially greater to the second array than to the first array.

In an antenna embodying the invention wherein in operation the amount of energy supplied to said at least one element at each of said plurality of points spaced along a feeder, as a proportion of the energy supplied to the respective point from a port, varies along a feeder, the variation in a said unidirectional feeder is suitably substantially asymmetrical therealong, the proportion being substantially higher at a said point relatively remote from the or each port to which the feeder is connected than at a said point relatively close to the port(s). This is particularly suited to improving the associated radiation patterns and the aperture efficiency of the antenna.

Unidirectional feeders respectively of the two arrays may extend away from their respective port(s) in opposite senses. Each array may comprise a group of unidirectional feeders each for supplying energy to a respective set of the elements. These features can enable the elements to be interconnected without feeders crossing one another and can generally simplify the design of the antenna.

In order that the elements and feeders of the two arrays may be closely spaced, the antenna may have one or more of the following features: groups of feeders respectively of the two arrays may be interdigitated; elements may extend from adjacent feeders in opposite senses and be interdigitated; the elements of the two arrays may extend from their feeders in opposite respective single senses; the elements extending from pairwise adjacent feeders may be interdigitated.

Suitably, each unidirectional feeder supplying energy to a respective set of elements is terminated by a resistive load at its end remote from the respective port(s) to which the feeder is connected. This inhibits an undesired lobe in each radiation pattern at a symmetrical position relative to the desired main lobe.

An embodiment of the invention will now be described, by way of example with reference to the accompanying diagrammatic drawings, in wnich : Figure 1 shows a set of axes for a Doppler radar navigation system on an aircraft; Figure 2 shows contours of constant Doppler shift on the earth's surface, and Figure 3 is a plan view of a microstrip antenna embodying the invention.

Figure 1 illustrates a right-handed set or orthogonal axes X, Y, Z, for an aircraft which navigates with a Doppler radar system, the axes having an origin 0 on the underside of the aircraft.

The axis Yis vertical and the axes X and Z are horizontal, the axis Z extending forwards in the predominant direction of motion of the aircraft and the axis X being perpendicularly transverse to the latter direction.

Figure 2 is a plan view of contours of constant Doppler shift (so-called isodops) on the surface of the earth (assumed for simplicity to be a horizontal plane) for signals transmitted at any constant frequency from the origin 0 on the aircraft and reflected back to O by the earth's surface, for any constant velocity along the Z-axis. The point 0 and the X and Z axes as projected vertically onto the earth's surface are shown for reference. The approximate values in degrees of the angle between the Y-axis and a line joining any point on the earth's surface, as projected into the XY-plane and into the YZ-plane, can be deduced form the position of the point with respect to the marks spaced at 50 intervals along the projected X and Z-axes respectively, the numerical values being shown at 10 intervals. The contours are hyperbolae defined by the intersection of the plane of the earth's surface with cones each of which has the velocity vector (the Z-axis) as it axis and its apex at 0 and which have different respective apical half-angles 0 (the angle between the velocity vector and the direction of transmission of the signals). The contours have been drawn for values of 0 differing from 900 by integral multiples from 1 to 8 of 50, i.e. as far as (90 1 50)0. The contours of course respectively intersect the projected Z-axis at the above-mentioned marks therealong.

The choice of the angle to the vertical of the main lobe of each of the radiation patterns is governed by several factors. Two of the most significant are: (a) the strength of the reflected signal received back at the aircraft, which decreases as the angle of the beam to the vertical increases, because the backscattering coefficient decreases (especially for scattering by the surface of calm water) and the range for a given altitude of the aircraft increases; (b) the errors produced by pitching (in the measurement of velocity in the Z-direction) or rolling (in the measurement of velocity in the Xdirection), which increases as the angle of the beam to the vertical decreases. Consequently, the angles of the beam to the vertical as measured in the X and Z-directions are generally not more than about 300.

Since the single main lobe of a radiation pattern extends over a finite range of angles transverse to the isodops, signals which are transmitted in the single main lobe will be subject to a range of Doppier shifts, and there will also be weaker received signals with a wider range of Doppler shifts owing to transmission in the sidelobes of the radiation pattern. Generally speaking, the sharper the peak in the spectrum of Doppler shifts, the more accurately (in terms of percentage possible error can the aircraft's velocity be determined. The aircraft will usually have components of velocity in both the Z and X-directions. For the component in the X-direction, a set of isodops like those of Figure 2 but rotated through 900 about 0 with respect thereto can be drawn.Now it will be seen from Figure 2 that with angles typically used for the main lobes of Doppler navigator antennas, the isodops from the Z-direction component of velocity extend mainly in the X-direction. Hence a relatively large increase in the range of angles in the X-direction within which signals may be transmitted and received back at the aircraft after reflection from the earth's surface will produce only a relatively small broadening of the peak in the Doppler spectrum relating to the velocity inthe Z-direction and therefore will only slightly increase the percentage possible error in the determination of this velocity, but conversely will produce a relatively large increase in the percentage possible error in the determination of the velocity in the Xdirection.However, the component of velocity in the Z-direction will normally be much greater than in the X-direction, and the latter component may therefore have a larger percentage error for the same absolute error. Therefore the quality of the radiation pattern is less critical in the X-direction than in the Z-direction.

Figure 3 is a schematic plan view of a microstrip antenna embodying the invention. A suitable orientation for the antenna when mounted on the underside of an aircraft as part of a Doppler radar navigation system in the XZ plane is indicated by the X and Z axes at the top of the Figure (the antenna being viewed from below the aircraft). The antenna has four ports 1, 2, 3 and 4 respectively, and a large number of antenna elements 5 each in the form of an elongate half wave stub extending in the X-direction.Half of the total nu mnber of antenna elements are connected to ports 1 and 2 by a single feeder 6 extending in the X-direction and a group of feeders 7 extending in the Z-direction to form a first array; the other half of the elements are analogously connected to ports 3 and 4 by a single feeder 8 extending in the X-direction and a group of feeders 9 extending in the Z-direction to form a second array. Each of the feeders 7 and 9 is terminated by a respective resistive load 10 (indicated schematically).

The two arrays are interspersed so as to occupy substantially the same antenna aperture. In each array, the elements are uniformly spaced in each of the X and Zdirections, each group of feeders 7, 9 which extend in the Z-direction being connected to the respective single feeder 6, 8 which extends in the X-direction at points uniformly spaced along the latter feeder, and a respective set of antenna elements 8, which elements are spaced from one another in the Z-direction, being connected to each of the group of feeders at points uniformly spaced therealong. The two arrays are defined by respective planar conductive layers on the same major surface of a dielectric substrate 11 having a common conductive ground plane on the other major surface (not shown).The feeders and elements are all depicted diagrammatically by lines of the same uniform width, but the actual widths may vary across the array to provide desired impedances.

The supply of electromagnetic energy to the antenna at port 1 produces a radiation pattern with a single main lobe that is inclined to the positive Y-direction at an angle ce in the positive Xdirection and at an angle P in the positive Zdirection. The supply of energy at port 2 produces a radiation pattern with a single main lobe inclined to the positive Y-direction at substantially the same angle mainin the X-direction but in the negative sense with respect to that axis and at substantially the same angle,3 in the positiveZ-direction.Analogously, the supply of energy at ports 3 and 4 produces respective radiation patterns each with a single main lobe whose angular orientation with respect to the positive Y-direction is substantially a in respectively the negative and positive X-directions and is substantially ,B in the negative Z-direction in both patterns. Thus the antenna is backward-firing in both the X and Z-directions with respect to the supply of energy at each of the ports. The resistive loads 10 absorb energy not radiated by the elements.

Comparing the antenna and particularly its feeder arrangement with that in the abovementioned slotted triplate antenna, each of the feeders 6 and 8 extending in the X-direction corresponds to a "feed track" and each of the feeders 7 and 9 extending in the Z-direction corresponds to an "array track". Each of the feeders 6 and 8 of the two arrays is similar to a "feed track" in that the sense in which energy is supplied from a port along that feeder to the respective group of parallel feeders and thence to the elements of that array depends on which of the two ports of that array energy is supplied at.

Each of the feeders 6 and 8 may thus be termed bidirectional.

Considering either of the bidirectional feeders 6 or 8 and its associated group of feeders 7 or 9 respectively, the amount of energy supplied to each member of the group of feeders (and hence to each set of elements) at a respective junction with the bidirectional feeder, expressed as a proportion of the energy supplied to that junction along the bidirectional feeder from an associated port, may be the same at all the junctions, but suitably this proportion, which may be termed the power transfer ratio, varies along the bidirectional feeder so as to improve the radiation patterns of the array.However, if the radiation patterns obtained when energy is supplied to each of the ports of the array are both to be of reasonable quality and generally are to be mirror images of one another with respect to the YZ-plane, the variation in the power transfer ratio is subject to the constraint that it must be substantially symmetrical along the bidirectional feeder, the ratio being a minimum for the two outermost of the group of feeders and rising to a maximum midway therebetween.

The feeders 7 and 9 extending in the Zdirection differ from the corresponding "array tracks" of the known antenna in that for each of these feeders, energy is supplied to its respective set of elements in only a single sense along the feeder, irrespective of at which of the two ports of the respective array energy is supplied; these feeders may thus be termed unidirectional. The power transfer ratio for the junction of each element of the set and the feeder is therefore not subject to the constraint explained above with respect to the bidirectional feeders.It may be the same for all the elements of the set, but suitably it varies along the feeder in an asymmetrical manner: for example, taking into account that the amount of energy passing along the feeder decreases progressively with increasing distance along the feeder from the bidirectional feeder owing mainly to the dissipation by radiation by the elements, the ratio may vary so that the amount of energy supplied to each element of the set, as a proportion of the total energy supplied to all the elements of that set by the unidirectional feeder, varies in a symmetrical manner from a minimum at the first and last elements to a maximum midway therebetween, e.g. approximately as a (cos2) function.

The above-described variations in power transfer ratio may be obtained by varying the widths of the microstrip lines forming the feeders and the elements.

It may be noted that the groups of feeders 7, 9 of the two arrays extend respectively in opposite senses in one of the two directions (the Z-direction) in the plane of the antenna from respective single feeders 6, 8 extending respectively on opposite sides of the array in the other of the two directions (the X-direction). This assists in providing a compact arrangement in which feeders do not need to cross one another.

The phase differences between the energy supplied from a port to successive sets of elements at points spaced along a bidirectional feeder and between the energy supplied to successive elements of a set at points spaced along a unidirectional feeder, and hence (if the feeders are rectilinear) the spacing of the elements in the X and Z-directions will depend on the desired angles a and , the thickness and dielectric constant of the substrate, and the requirement that each radiation pattern should have only a single main lobe. If the elements are elongate (as in this embodiment), it may of course be particularly difficult to obtain sufficient spacing in the direction of elongation of the elements to enable the feeders and elements to be fitted in.

Each of the following features assists in enabling the two arrays to be closely interspersed: (a) the groups of feeders of the two arrays are interdigitated; (b) elements extend from adjacent feeders in opposite senses and are interdigitated; (c) the elements of the two arrays extend from their feeders in opposite respective single senses; and (d) the elements extending from pairwise adjacent feeders are interdigitated.

The last two features are particularly useful in enabling the elements of an array to be closely spaced in the direction in which the elements extend while still enabling the arrays to be interspersed.

A feeder need not extend directly between successive spaced points at which it supplies energy to elements (either directly or via further feeders). For example, each of the bidirectional feeders 6 and 8 may comprise a series of arcuate lengths of line each extending between a pair of adjacent unidirectional feeders. This increases the path length and hence the phase shift between adjacent unidirectional feeders without altering their spacing.

For systems which require only three different radiation patterns, one of the four ports may be omitted, and one of the bidirectional feeders 6, 8 will of course become unidirectional.

As a variation on the arrangement of Figure 3 in which the elements of an array are fed by a group of unidirectional feeders which in turn are supplied via a single bidirectional feeder from either of two ports, the elements of an array may be fed by a group of bidirectional feeders which in turn are supplied from either of the ports via a unidirectional feeder respective to each port. (It may be noted that this results in the first and second directions being interchanged with respect to the direction of elongation of the elements).

However, such an arrangment has the disadvantage that it will generally be necessary for feeders to cross one another.

As a further variation on the arrangement of feeders and elements shown in Figure 3, successive elements connected to the same feeder may extend therefrom alternately in opposite senses instead of all in the same sense; each feeder of one of the arrays may then suitably be mid-way between a respective pair of feeders of the other array (except of course for the outermost feeders of the antenna) instead of being further from a feeder on the same side of itself as its elements than from a feeder on the opposite side to its elements. Such an arrangement may be particularly suitable for an antenna in which the angles of the main lobes of the radiation patterns to the Y-direction in the direction of elongation of the elements are small.

Other variations on the arrangement of Figure 3 are of course possible within the scope of the invention. For example, the antenna elements may be of the form described in U.S. Patent 3 643 262; this form may be particularly suitable to enable feeders extending in the same general direction to be closely spaced relative to one another.

An antenna of the form shown in Figure 3 was constructed to test the feasibility of obtaining four radiation patterns each with a single main lobe from a four-port antenna comprising two distinct arrays occupying a common aperture. The antenna was designed for operation in the frequency range of 13.25-13.4 GHz and was made with usual printed-circuit technology on a sheet measuring 170 x 195 mm of "RT/duroid 5880", a copper-coated glass-reinforced PTFE material having a dielectric constant of 2.22 and a thickness of 0.045 inches. The resistive loads 10 were each a thin carbon-containing layer formed from a colloidal solution which is available under the trade name "Aquadag" and which was applied to the substrate on and adjacent the ends of each feeder and allowed to dry.Alternative forms for each of these loads are, for example, carbon-loaded paper which is available under the trade name "Teledeltos" and which may be secured to the substrate with adhesive so as to couple it to a feeder, or a lumped coaxial load which may be secured to the ground plane and to which the end of a feeder may be connected through the substrate.The angles a and ss were each in the region of 30 , but as the antenna had for simplicity been designed not to have any variation on the power transfer ratios along either the bidirectional or the unidirectional feeders, the radiation patterns were not of high quality especially in the X-direction; the presence in the radiation patterns in the Z-direction of a secondary lobe which was at a mirror-image orientation with respect to the YZ-plane of the main lobe and which had a fairly high level relative to the main lobe also indicated that the resistive loads used did not present an adequate match to the unidirectional feeders. Nevertheless, the VSWR at each port was better than 1.3. The coupling between two ports respectively of the same array was less than -15 dB, but the coupling between two ports respectively of the different arrays was slightly higher.

An antenna embodying the invention may find application other than in a Doppler radar navigation system, for example in a sequential lobing tracking radar.

Claims (14)

1. An antenna formed in strip transmission line, the antenna comprising a plurality of antenna elements distributed over an antenna aperture which extends in each of two mutually perpendicular directions, at least three ports, and a plurality of feeders each for supplying energy from at least one of the ports to at least one element at each of a plurality of points spaced along the feeder, wherein the antenna has first, second and third radiation patterns which each have a single main lobe and which are respectively associated with the supply of energy to the antenna at a first, a second and a third of the ports, and wherein the angular orientations of the main lobes of the first and second radiation patterns differ substantially in a first of the two directions and wherein the angular orientation of the main lobe of the third radiation pattern differs substantially from the angular orientations of the main lobes of the first and second radiation patterns in the second of the two directions, characterised in that the elements are connected by the feeders to form two antenna arrays each occupying substantially said aperture, in that the coupling of each of the first and second ports is substantially greater to the first array than to the second array and the coupling of the third port is substantially greater to the second array than to the first array, and in that at least two feeders of the first array and at least one feeder of the second array are each a unidirectional feeder for supplying energy substantially in only a single sense along the feeder to at least one element at each of a plurality of points spaced along the feeder, the elements supplied respectively at those points being spaced from one another in the second direction.

2. An antenna as claimed in Claim 1 comprising a fourth port, wherein the antenna has a fourth radiation pattern which has a single main lobe and which is associated with the supply of energy to the antenna at the fourth port, and wherein the angular orientation of the main lobe of the fourth radiation pattern differs substantially from the angular orientations of the main lobes of the first and second radiation patterns in the second direction and from the angular orientation of the main lobe of the third radiation pattern in the first direction, characterised in that the coupling of the fourth port is substantially greater to the second array than to the first array.

3. An antenna as claimed in Claim 1 or 2 wherein in operation the amount of energy supplied to said at least one element at each of said plurality of points spaced along a feeder, as a proportion of the energy supplied to the respective point from a port, varies along the feeder, characterised in that in a said unidirectional feeder the variation is substantially asymmetrical therealong, the proportion being substantially higher at a said point relatively remote from the or each port to which the feeder is connected than at a said point relatively close to the port(s).

4. An antenna as claimed in any preceding claim, characterised in that unidirectional feeders respectively of the two arrays extend away from their respective port(s) in opposite senses.

5. An antenna as claimed in any preceding claim, characterised in that each array comprises a group of unidirectional feeders each for supplying energy to a respective set of the elements.

6. An antenna as claimed in Claim 5, characterised in that groups of feeders respectively of the two arrays are interdigitated.

7. An antenna as claimed in any preceding claim, characterised in that elements extend from adjacent feeders in opposite senses and are interdigitated.

8. An antenna as claimed in any preceding claim, characterised in that the elements of the two arrays extend from their feeders in opposite respective single senses.

9. An antenna as claimed in Claim 7 or in Claim 8 as appendant to Claim 7, characterised in that the elements extending from pairwise adjacent feeders are interdigitated.

10. An antenna as claimed in Claim 5 or any claim appendant thereto, characterised in that each unidirectional feeder is terminated by a resistive load at its end remote from the respective port(s) to which the feeder is connected.

11. An antenna as claimed in any preceding claim, wherein the elements are elongate and wherein the antenna is backward-firing in the direction of elongation of the elements.

12. An antenna as claimed in Claim 11 wherein the main lobes of the first and second radiation patterns are at an angle of not substantially more than 600 to one another in the direction of elongation of the elements.

13. An antenna as claimed in any preceding claim, wherein the two arrays are formed in m icrostrip and comprise respective electrically conductive patterns on the same major surface of a dielectric substrate.

14. A Doppler radar navigation system on an aircraft, the system comprising an antenna as claimed in any preceding claim, wherein in operation the motion of the aircraft is predominantly in the second direction.

1 5. An antenna substantially as herein described with reference to the accompanying drawings.