GB2141289A - Radars - Google Patents

Abstract

In order to avoid the problems arising when RF energy for transmission in a radar using an antenna array mounted on a rotating joint waveguide is provided by a plurality of stationary generators, combined, taken to the joint waveguide, and then split so as uniformly to feed the joint stationary waveguide portion, and when thereafter the energy abstracted from the joint rotating waveguide portion is then re-combined, taken to the antenna array, and then re-split to feed the individual antennas, the invention proposes to associate each of the joint input/output points with a different one of the individual generators/antennas, and then to connect each to the other directly, without any combiner and subsequent splitter. <IMAGE>

Description

SPECIFICATION Radars This invention relates to radars, and concerns in particular apparatus for feeding signals to a radar transmitting aerial.

Though some radars involve the use of a transmitting aerial comprising a single radiating antenna element backed by a beam-forming and -shaping reflector, many types of modern radars employ aerials constituted by an array of individual antennas each of which is fed with the RF (Radio Frequency) signal it is required to radiate, and by adjusting the relative phases of the signals so the energy radiated from the array may be shaped and directed in almost any way needed.

Certain varieties of radar require the antenna array to rotate (usually about a vertical axis), so the array is suitably mounted upon an appropriate supporting structure via a rotating joint. Though twill often be possible to mount the generator(s) (or perhaps merely their output stages) for the radiated signal immediately adjacent, and so rotating with, the antenna array, in some circumstances this may not be possible. In these latter cases, where the generator(s) must be mounted some distance from the array - and thus on the other side of the rotating joint -the manner in which the signals are fed to the array is necessarily complicated.Generally it is arranged that the rotating joint supporting the array be itself formed as a channel - specifically as a waveguide for RF energy, and a single, combined signal is fed through the joint, from the generator side to the antenna side, along this channel, and is then split into the appropriate parts for onwards transmission to each individual antenna.Indeed, with modern equipment, using in the signal generator solid state, semi-conductor devices with relatively low power outputs (rather than thermionic valves - tubes which can individually have outputs such that a single output stage amplifying tube can produce sufficient power to feed the whole antenna array, but tend to be less reliable, and require more complex power supplies), it is presently commonplace to employ a number of solid state signal-producing devices, add together all their low power outputs in a combiner, and feed the resulting high power signal along a suitable transmission line (usually itself a waveguide) to the channel constituted by the rotating joint, and then to take the combined high power signal from the joint along a further transmission line (again, usually a waveguide) to a splitter from which are taken the individual lower power signals needed to feed each separate antenna.

The situation may further be complicated if, as will often be the case, the position of choice for the rotating joint's channel portion - that volume occupying and immediately surrounding the joint axis - is already in use for some other purpose. It is then necessary to design the joint to include two channels; this may conveniently be done by enclosing the inner, axial, channel portion within a second, annular, channel portion; where the latter is itself a waveguide it thus is - and is used - rather like a section of coaxial cable.The present invention is concerned with the use of a double channel joint, and relates more particularlyto how the signal should be fed to and taken from the external, annular, channel portion, and how the joint should best be constructed so as to allow the signal to pass along that channel portion between its stationary part and its rotating part.

Joints incorporating co-axial channels each in the form of a waveguide have already been used in the art. They do, however, suffer from a number of problems; one of these is the difficulty of feeding RF powertothe external (annular) guide portion in such a way as to promote within the guide not only the desired mode but also a uniform distribution of the energy around the guide, while another is effectively the reverse of the first, namely the difficulty of withdrawing from the guide all the energy travelling along it. Both problems are connected with the actual size (the mean diameter) of the guide relative to the wavelength of the RF signal it is required to carry (in the chosen mode). In essence, the larger the guide relative to the wavelength the greater the tendency for the energy distribution within the guide to be non-uniform.The effects are most noticeable when the guide is fed from a single input point (and when the transmitted signal is likewise removed from a single output point), and it is conventional to mitigate them by feeding the energy into, or withdrawing it from, the guide at a plurality (two or more) of points spaced, preferably uniformly, around the circumference of the guide. There is thus the slightly absurd situation that RF power from a plurality of generators is first combined, taken to the joint waveguide, and then split so as uniformly to feed the joint outer waveguide portion, and that the power abstracted from the joint waveguide is then re-combined, taken to the antenna array, and then re-split to feed the individual antennas.The present invention proposes to simplify this, where power levels permit, by associating each of the joint input/output points with a different one of the individual generators/antennas, and then connecting each to the other directly, without any combiner and subsequent splitter.

In one aspect, therefore, this invention provides apparatus for feeding RF energy into, and/or withdrawing it from, a plurality of input/output points uniformly distributed around the stationary/rotating sections of a rotating joint annular RF energy channel portion, which apparatus includes transmission line means directly connecting each such point to a different one of a plurality of RF signal generators/radiating antennas.

The apparatus of the invention may be used to feed RF energy from a plurality of signal generators into the joint annular channel, or it may be used to take energy from the joint annular channel and deliver it to a plurality of radiating antennas, or it may be used to do both. Indeed, in a practical set-up it is likely to be used for the latter purpose - both to feed energy to, and to withdraw energy from, the channel - and such an arrangement is described in more detail hereinafter with reference to the accompanying drawings.

TheR F energy handled by the apparatus may in general be of any kind - of any wavelength and polarisation - as may be required. Thus, in a modern radar the energy can be in the millimetre, centimetre, or metre wavelength ranges. However, in one form, discussed in more detail hereinafter, the invention is of particular value for handling metric wavelength RF energy.

The energy is fed to and/or drawn from a plurality that is, at least two - of points uniformly distributed around the relevant section of the joint annular channel portion. The number of such-points re quired, and the manner of their distribution, is dependent upon the wavelength of the energy and the dimensions of the channel, and this concept is well-known in the Art, needing no detailed comment here.Nevertheless, to enhance the ease of under standing of the invention, it may here usefully be pointed out that for the-propagation-of symmetric electric and magnetic fields (whether in a coaxial line or- as discussed further hereinafter- in a capacita tively coupled planar ring) the number of feed points required for uniform excitation of the transmission line is a function of the line's mean circumferential dimension and the shortest wavelength of opera tin. in general, for a single feed point the mean circumference for uniform field excitation should be less than one free space wavelength (X) at the uppermost use frequency (the shortest use wavelength).Excitation at shorter wavelengths re sults in the propagation of asymmetric electric and magneticfields (in coaxial transmission line this is generally in the form of the TE11 mode), and for relative rotation between two halves of a channel this would cause unacceptable variations in trans mission parameters with rotation. It is usually the case that for a mean line circumference of the minimum number of feed points required for sym metric excitation is no 1. In addition, where more than one feed point is required it is very much preferred thatthe points beequi-spaced around the line circumference, and that all points be essentially excited by equal amplitude and equal phase signals.

In a typical embodiment of the invention, four identical amplifiers fed from a common drive would themselves feed four equi-spaced input probes in the stationary section of the annular channel; At a wavelength of 70cm, the probes could be on a 1 Ocm radius circle; this would make the probe separation less than one quarter of a wavelength, well within the limitation mentioned above, while leaving an adequate aperture in the middle of the metric joint for a microwave rotating joint and slip ring assem bly. In the rotating section of the annular channel four equi-spaced output probes would extract equal powers from the annular channel to feed four identical antennas.

Similarly, the nature of the input/output points needs no detailed discussion here, though again it may usefully be said that the points are in general fed from transmission lines operating in the TEM (Transverse electromagnetic) mode, and are usually in parallel connection with the main coaxial section, and that where the annular channel is itself a waveguide (as is discussed further hereinafter) the points will need to be shunted with quarter wavelength sections of short-circuited transmission line to excite the desired TEM mode in the coaxial section.

The joint annular channel portion will necessarily have a stationary section and a rotating section, and (while conceivably things could be the other way round) as might reasonably be expected the antenna array will be mounted upon the rotating side of the joint, its individual antennas being connected directliy to output points in the rotating section of the joint waveguide, while the signal generators are mounted upon the stationary structure itself carrying the stationary side of the joint, and are connected directly to input points in the stationary section of the jointwaveguide.

Thejointannularchannel portidn can be-an annular waveguide - that is, a short length of annular-section tube - behaving like a length of coaxial cable with a core conductor having a particularly large diameter. However, as intimated above in order properly to launch the wave energy thereinto the feed points for a waveguide channel need to be associated with chokes in the forum of quarter wavelength long short-circuited guide sections shunting each feed point. For wavelengths of about a metre, then, there must be provided a series of chokes each about 25cm long - and this may well prove unacceptably bulky in some circumstances.

Accordingly, there is most preferably employed an annular channel portion that is not a waveguide but is in effect a capacitor, the RF energy being capacitatively transferred from one annular plate fixedly mounted on the stationary structure supporting the joint to a second annular plate in close face-to-face relationship with the first and fixedly mounted on the rotating structure supported by the joint. In one preferred embodiment of such a capacitative annular channel portion, described in more detail hereinafter, each annular plate is a ring of width about 1cm (and overall diameter about 10cm), and the two are positioned face-to-face about 1 mm apart.

The inventive apparatus includes separate transmission line means directly connecting each input-or output point to a generator or antenna as appropriate, and this may be said to be the main feature of the invention. Each transmission line is direct in that it passes between the input/output point and the appropriate one of the generator and the antenna without any join to another line that would result either in any one generator feeding two or more input points or in any one output point feeding two or more antennas. Indeed, it is likely to be the case that each- line will be totally separate from all other lines, though it is envisaged that on occasions (perhaps to increase the power fed to any particular input point or antenna) lines may be joined so as to allow two or more generators to feed the same input point, or to allow two or more output points to feed the same antenna. An embodiment ofthis possibility is discussed hereinafter with reference to the accompanying drawings.

It is a particularly interesting, useful and slightly surprising advantage of the inventive arrangement that the combining in the joint channel itself of all the individual generated RF signals, and the withdraw ingtherefrom of the individual outputs to the antenna, has a significant smoothing effect upon the signals themselves (provided, of course, that the joint channel's dimensions are relatively small enough to prevent higher mode excitation), and this should be useful if the generators do not produce signals of precisely equal power.

Each transmission line means itself may be in any form used or suggested for use wifh RF signals of the chosen wavelength and mode, and no more need be said about this here. Nevertheless, for, say, signals of metric wavelengths an acceptable transmission line is a waveguide either of coaxial, semi-rigid form made using copper conductors and P.T.F.E. dielectric filling, or (as in the case of the preferred embodiment) in the form of a planar transmission line where a centre current-carrying conductor is placed midway between two earthed ground planes (these latter devices are produced by printed circuit techniques on dielectric boards as is well known in the Art).

In accordance with the invention each input/output point is directly connected to one of a plurality of RF signal generators/radiating antennas. The number of generators and/or antennas - and, indeed, the exact nature of these devices - may be any (though there should be at least two of each) used or proposed for use in the Art, and therefore needs no special comment here. Nevertheless, it may be said that the generators may be either vacuum tube or solid state devices, operating either as driven amplifiers fed from a common drive or as locked oscillators synchronised by a common locking signal. The waveforms may be pulsed or continuous wave or any variant of these alternatives appropriate to the specific application, and may be fed to the joint input points along striplines.

Likewise, the antenna elements may be dipoles, monopoles, slots, disc radiators, helices, spirals, or any other type of radiator appropriate to the system.

Although the transmission lines means directly connect the input/output points to the generator/ antenna, various active or passive components can be inserted in the line between one or other pair, as used or suggested for use in the Art. For example, if receiving facilities are to be incorporated, transmit/ receive devices may be included in the lines feeding the individual antenna elements. This enables monopulse difference receiving patterns as well as sum patterns to be generated. Again, if the antenna pattern is to be electronically scanned, phase shifters may be included in the lines feeding the individual antenna elements, and may be arranged either before or after the transmit/receive devices (if present), according to whether the receive beams are also to be electronically scanned or whether simultaneous multiple beams are to be generated.

One particular use of the invention is in dual-band radars in which the two aerials are back-to-back on a common rotating mount, the configuration leaving no room for signal generators/amplifiers to be mounted on the back of either aerial itself. In a metric/microwave radar like this, the microwave radar can use a high power rotating waveguide joint which occupies the axial volume of the joint (and slip ring) assembly itself, and the metric radar can use an annular channel positioned coaxial therewith.In this application the metric aerial might consist of a two-by-two array of individual half wave dipole antennas mounted against a reflecting screen, equally fed by four transmission lines from the rotating inner and upper annular section of the channel, while the relevant transmitter could comprise four or more generators (or the output stages thereof) equally spaced around the stationary lower and outer annular section of the channel.

The invention extends, of course, to a radar whenever using signal feeding/withdrawing apparatus as described and claimed herein.

Various aspects of the invention are now described, though only by way of illustration, with reference to the accompanying drawings, in which: Figure 1 shows in diagrammatic form the relative complexity of Prior Art combining/splitting networks used to feed power to or withdraw it from a rotating joint annular waveguide; Figures 2and 3 show in diagrammatic form the relative simplicity of two embodiments of the power feeding/withdrawing apparatus of the present invention; Figure 4 is a part section on the axis showing diagrammatically the construction of a double waveguide joint useable in the invention; Figures SAand B are part views sectioned respectively on the axis and transverse thereto of a waveguide/capacitor joint useable in the invention; and Figure 6 is a perspective, part see-through view of a metric/microwave radar using the apparatus of the invention.

In Figure 1 there is shown in diagrammatic form the various elements involved in PriorArt methods of feeding an RF signal to an antenna array. Four individual signal generators (1 1a,b,c,d) each produce a relatively low power RF signal, and these signals are fed to, and combined in, a combiner (12).

The combined signal is fed onwards to the stationary section (13a) of a rotating annular waveguide joint via three signal splitters (14 and 1 5a,b), each of which splits into two the signal it receives, the four signals from these splitters being fed into the annular guide 13a at four points (16a,b,c,d) equispaced around the joint.

The RF energy passes along the annular guide from its stationary section 13a to the corresponding rotating section (13b), and is extracted therefrom at four equispaced points (17a,b,c,d) and combined by three combiners (18a,b and 19), each of which "doubles up" the signal, for outwards transmission to the antenna array.

In this particular case the antenna array is comprised of four individual antennas (20a,b,c,d) each of which must be fed separately so that the combined RF signal is first split into four by the splitter 21.

Figures 2 and 3 show, like Figure 1, the various elements required to feed an antenna array, but in accordance with the invention. In Figure 2 four signal generators 1 1 a,b,c,d feed four individual antennas 20a,b,c,dvia an annular RF energy channel 23a,b built into the rotating joint, but instead of combiners and splitters the RF signal is fed directly fronn each generator 11 to one of the four input points 16a,b,c,d equispaced around the stationaryjoint input section 23a, and similarly from the rotating joint output section 23bathe signal is fed directly from each of the four output points 17a,b,c,d equi-spaced therearound to one of the four antennas 20.

The embodiment of Figure 3 differs from that of Figure 2 firstly in that though there are four signal generators (1 1a,b,c,d) two of them (1 b,c) have had their outputs combined (bycombiner 32), and the resulting three individual signals are fed to three input points (36a,b,c) spaced equally around the stationary joint input section 23a. It differs secondly in that though the signal is output at four points (17a,b,c,d) equispaced around the rotating joint output section 23b these are used tofeed only three antennas (30a,b,c), two of the signals being combined (in combiner 31) to feed one antenna (30b).

In Figure 4 is shown the right (as viewed) half of a section on the axis (A-A) of a double waveguide joint useable in the invention. The joint has two RF channels - Channel 1 (41), generally the higher frequency of the two channels, which can be realised in coaxial or waveguide form, but is here (42) shown in theformer, and Channel 2, which is of annular form and is "wrapped around" the outer conductor of Channel 1.

The joint has two main parts - a lower, stationary section (generally 43a, and shown cross-hatched) and an upper, rotating section (generally 43b). The latter fits on, and is supported by, the former via bearings (as 44a,b).

Channel 2 consists of a coaxial section that is excited by a feed network in the stationary part 43a realised generally in stripline form (as 45a). The number and disposition of these feed points is conventionai. The feed (input) points (as 46a) are supported by a shunt quarterwave section of radial transmission line (as 47a) terminated in a short circuit; this arrangement ensures a voltage maximum at the point of excitation of the coaxial mode.

The rotational part of the annular channel 42 is identical to the fixed half, and relative rotation between the two is afforded by electrically-choked mechanical breaks (at 48) one on either side. The electrical chokes consist essentially of a series section of low impedance line followed by a shunt (parallel) section of high impedance line terminated in a short circuit. Transformation of this short circuit backthrough these quarterwave sections of line produces a voltage minimum at the mechanical break in the conductor. Two such chokes appear in the Figure, oneforthe outer conductor (49a) and one for the inner conductor (49b). The take-off (output) points (as 46b) are, like the input points 46a, supported by a shunt quarterwave section 47b, and connect to an output network again generally in stripline form (as 456).

The joint arrangement illustrated in Figures 5A,B is the preferred variety for use in the invention. The joint has two main parts - a lower, outer, stationary part (generally 53a; this is shown on the left of the section of Figure 5B) and an upper, inner, rotating part (generally 53b; shown on the right in Figure 5B).

The latter fits in, and is supported by (by means not shown) the former, and in part overlaps the former (at 54) in a direction normal to the joint axis. Each of the two major (axially-normal) overlapping faces is annular (and coaxial with the joint's axis) and bears a conductive track, or strip, (55a,b) itself annular and so positioned that the two tracks are adjacent each other in a spaced face-to-face relationship. These two tracks are employed as the two plates of a capacitor, and in use RF energy is capacitatively transferred between them.The RF energy is fed to the stationary track 55a along one stripline network (56a,b,c...), and removed from the rotating track 55b along a second stripline network (57a,b,c...), and the joint is so constructed that its main supporting structure provides the ground planes while each track is mounted on an annular dielectric slap (58a,b) and its associated stripline network sandwiched between that slab and a second similar slab (59a,h), the internal and external radii of these slabs being arranged to provide the necessary overlap area 54.

The impedance of the input lines can be adjusted by the inclusion of quarter wavelength sections of lines of intermediate impedance to improve impedance matching of the annular channel. Moreover, because radiati-on can occur where the input line groundplanes are "broken" adjacent the output line groundplanes, shunt quarter-wave short-circuited lines are introduced (these are formed in the same plane as the input network, so that the stripline feed system operates in the radial cavityformed by this shunted section of transmission line).

Figure 6 shows in "outline" a trailer-borne radar system incorporating both millimetric and metric radars. The two aerials - a millimetric one (61) with a beam-forming and shaping reflector (not shown separately), and a metric one (62) consisting of a two-by-two half-wave dipole antenna (as 63) - are mounted back-to-back on a table (64) supported by a common rotating joint (not shown separately), and are protected from the weather by a radome (65).

Because of the severe shortage of space within the radome the generators for the metric antennas are carried not immediately adjacent each antenna but rather within the trailer body (66) - thus, on the other side of the rotating joint.

Claims (7)

1. Apparatus for feeding RF energy into, and/or withdrawing it from, a plurality of input/output points uniformly distributed around the stationary/ rotating sections of a rotating joint annular RF energy channel portion, which apparatus includes transmission line means directly connecting each such point to a different one of a pluraity of RF signal generators/radiating antennas.

2. Apparatus as claimed in claim 1 which is used both to feed RF energy from a plurality of signal generators into the joint annular channel, and to take energy from the joint annular channel and deliver it to a plurality of radiating antennas.

3. Apparatus as claimed in eitherofthe preced ing claims, wherein the RF energy is metric wavelength RF energy.

4. Apparatus as claimed in any of the preceding claims, wherein the energy is fed to and/or drawn from four points uniformly distributed around the relevant section of the joint annular channel portion.

5. Apparatus as claimed in any of the preceding claims, wherein the joint annular channel portion is a capacitor comprising one annular plate fixedly mounted on the stationary structure supporting the joint and a second annular plate in close face-to-face relationship with the first and fixedly mounted on the rotating structure supported by the joint, and the RF energy is capacitatively transferred from one to the other.

6. Apparatus as claimed in any of the preceding claims and substantially as described hereinbefore.

7. A radar whenever using signal feeding/withdrawing apparatus as claimed in any of the preceding claims.