GB2155245A - Antenna systems - Google Patents

Abstract

An antenna system, which with a single feed horn has omnidirectional coverage in azimuth and low angle sectoral coverage in elevation, comprises a main reflector 4 - Figure 4, 15 - Figure 13, which is symmetrical about the Z-axis and is shaped to give the required low angle response with a TMO1 Field at horn 11 a vertically polarised signal at all azimuth angles is obtained. Figure 13 shows a dual reflector system with a parabolic subreflector 14 and wave-absorbent material 16. A circle of four feeds may replace the single feed 11 giving a four lobe response in azimuth (Figures 17a and 17b). This may permit direction finding or jam-resistant reception. <IMAGE>

Description

SPECIFICATION Antenna systems This invention relates to antenna systems and in particular, but not exclusively to millimetre wave antenna systems.

According to the present invention there is provided an antenna system including a main reflector which is symmetrical about a first axis and has a reflecting surface shaped such that when radiowave energy is directed thereat from feed means on the first axis it can be reflected azimuthally omnidirectionally therefrom in such a manner as to obtain a far-field pattern with a predetermined sectoral elevation.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 illustrates the coverage of an azimuthally omnidirectional antenna with sectoral elevation; Figure 2a and 2b illustrate a section and a view, respectively, of a conventional Cassegrain antenna system; Figure 3 illustrates one reflector shape (TT) for sectoral elevation; Figure 4 illustrates another reflector shape (TB) for sectoral elavation; Figure 5a illustrates a (TB) reflector with a linearly polarised feed horn and showing the far-field plane of polarisation rotating with aximuth (0); Figure 5b shows the TE1, linearly polarised feed horn pattern which is slightly asymmetric about the z-axis;; Figure 6a illustrates a (TB) reflector with a radially polarised feed horn (Two) and showing constant vertical polarisation with azimuth (0) in the far field; Figure 6b is the TM01 feed pattern which is symmetrical about the z-axis; Figure 7a illustrates a waveguide feed (TMo,) and Figures 7b and 7c illustrate the field pattern taken at two sections thereof; Figure 8a illustrates comparisons of computed and measured values for relative power versus Ol for TM01 feed, Ol being as defined in Figure 8b;; Figure 9 illustrates a main beam copolar power pattern for a conical feed horn with a feed pattern of the form cosq (at1), with a reflector of one cross-sectional shape; Figures 1Oa and lOb illustrate the far-field elevation pattern as given by physical optics (solid line) and geometrical optics (dashed line) for an antenna height h = 14X and 140X, respectively, for a single reflector; Figure 11 illustrates a cross-section through another reflector shape and the associated ray geometries for a TM01 feed pattern; Figure 12 illustrates the far-field pattern as given by physical optics for an antenna height h = 188X; Figure 13 illustrates a basic dual reflector construction of an azimuthally omnidirectional antenna with sectoral elevation;; Figure 14 illustrates a cross-section through the two reflectors of dual reflector construction and the associated ray geometries for a TMo1 circular waveguide feed; Figure 15 illustrates the far-field near-field criteria computed for the dual reflector construction of Figure 14, together with a ()blOckcalculation; Figure 16 illustrates schematically a practical dual-reflector construction, and Figure 17a and 17b show respectively a multi-element feed and the associated azimuthal patterns of the individual elements.

A requirement for surface to surface communication between vehicles, ships or low-flying aircraft is an antenna system having omnidirectional coverage in azimuth, and sectoral coverage in elevation distributed about the horizon. This will enable nearly constant transmission and reception overall azimuths with reasonable pitch and roll of a platform to which the antenna system is mounted. The overall coverage of such an antenna as mounted on a tank 1 is illustrated in Figure 1, which also indicates the pitch direction and the roll direction.

The gain of such an antenna over a true omnidirectional coverage is purely due to the restricted elevation coverage, from ( - sector)i2) to ((Ir + Osector)12), for the sectoral antenna as opposed to the full elevation coverage, from 0 to Ir, for the true isotropic omnidirectional antenna.Integration over the elevation element of solid angle sin 0 d 0 gives the gain (G) of the sectoral antenna with respect to the isotropic antenna as G = 10 lOgro [sin (Osectori2 ]-1 For a sector angle of sr/3 (+ 1r/6 with respect to the horizon) which might be typical of platform pitch and roll G = 10 log10 [1/2]-1 = 10 logo02 = 3dB.

At microwave frequencies, the cost of providing such an antenna outweighs the cost of providing the relatively modest increase in 1-way transmit power of 3dB necessary if a simple truly omnidirectional antenna were to be employed. In addition the physical size of the antenna may be inconvenient.

However, at mm-wave frequencies transmitter power is more costly and may, in any case, be limited by physical considerations. Since the size of the antenna scales down proportionately with wavelength, at mm-wave frequencies such an antenna becomes particularly attractive, for example, for communication system, orwhen incorporating azimuthal direction-finding for electronic surveillance receiver and radar purposes.

The basis concept of the antenna design of the present invention is derived from the feed and reflector of a Cassegrain antenna system, such as shown in Figures 2a and 2b. A feed 1' is disposed at the focus of a sub-reflector 2, there being a main reflector 3. The function of the sub-reflector 2 is to collect feed energy to provide azimuthally symmetric radiation to the main reflector 3 covering the elevation range 0wedge to it12.

Considering now the feed and sub-reflector alone; if the sub-reflector shape is progressively deepened, the elevation angles of its coverage patterns will correspondingly decrease whilst the azimuthal pattern will remain isotropic. Thus a desired distribution of elevation angle of the coverage pattern about the horizon can be achieved.

Two possible basic forms of the reflector shape both having identical geometrical optics radiation patterns are indicated in Figures 3 and 4. Figure 4 shows a TB (top-to-bottom) solution, so called because the top ray from the feed is associated with the bottom ray in the far-field, and vice-versa. Figure 3 shows a TT (top-to-top) solution.

The reflecting surface 4 (Figure 4) is comprised by a radially outer surface of an element which tapers generally in the direction of the z axis towards the feed 1' which is spaced apart from the apex of the tapering element.

The azimuthally omnidirectional coverage requirement means that the reflector is symmetric about the z-axis, which eases the design of the reflector shape and makes production a cost-effective proposition. The cross-sectional shape of the reflector in any plane containing the z-axis must satisfy the laws of reflection and power conversation and is determined by the desired far-field pattern and the feed pattern employed.

With synthesis of the reflector shape based on the principles of geometrical optics, the vertical height of the antenna (the dimension presented to incident radiation in the elevation domain) must be greater than about 1 OA if the actual pattern is to be a reasonable approximation to that given by geometrical optics.

An antenna system comprises both a feed and its particular associated reflector, and the type of feed to be employed is an integral part of the reflector design. The desirable property, mentioned above, of the azimuthal symmetry of the reflector is also dependent on the feed pattern being azimuthally omnidirectional.

A further important consideration is the polarisation properties of the feed. The use of a circular feed aperture with three types of fields is discussed below. If as illustrated in Figure 5a, a reflector 4 is fed by a linearly polarised feed, such as an open-ended circularwaveguide or circular horn 5 excited in the TW" mode, there results linearly polarised far-field radiation whose plane of polarisation rotates with changing azimuth. This is likely to be undesirable in a system for communications use. A typical plot of a computed elevation for such a feed is shown in Figure 5b for ka = 3.588, where a is the waveguide radius at the final feed aperture and k = 27r/A. The elevation patterns differ slightly in the H and E planes, that is the feed pattern is not truly azimuthally symmetric.In the case of a circularly polarised feed such as the E feed described above but in conjunction with an orthomode transducer, constant circular polarisation is produced in the far-field of the reflector at all azimuth angles. A particularly attractive feed for the reflector shape proposed is an open-ended circular waveguide or horn in the TM01 mode. All of the E-vectors are radial (radially polarised feed) and the patterns are truly azimuthally symmetric. Such an arrangement is shown in Figure 6a. Two beneficial consequences of this arrangement are that the far-field radiation from the reflector is vertically polarised over all azimuths and that there is a null on the feed boresight.The latter is convenient because it greatly reduces the illumination of the tip of the reflector 4 and hence reduces possible diffraction from that point which could affect the desired far-field patterns. A computed elevation pattern for a typical value of ka = 3.873 for a TMo1 open-ended circular waveguide feed is shown in Figure 6b. It indicated that this feed pattern is azimuthally symmetric.

Figures 7a, b and c illustrate an experimental model of a radially polarised TM01 feed (ka = 5.50) which has been tested at the X-band. The diameter of the cylindrical section 6 is 30mm, the diameter (2a) of the final feed aperture 7 is 59.5mm and the launching stub 8 is 8.5mm long. The lowest order modes are the TE11 mode (fundamental) and the TMol modes. In order to cut off modes higher than TE11 and TM01 the diameter of section 6 equals X/1 .133 so that it is 10% below the cut-off frequency for the next mode (TE21).To ensure that the TMol mode is excited in preference to the TEa1 mode, the radial symmetry of the TM01 mode is mirrored by that of the launcher used. The launching stub 8 is A/4 long so that backward travelling waves, originating in the vicinity of the transistion region 9 between the coaxial section 10 and the TMol region 11, are cancelled upon reflection from the rear end 12. The length of the cylindrical section 6 from the transition region 9 to the beginning of the flared section 13 (TMol region 11) is such as to permit pure TMo1 mode excitation to be achieved. The waveguide diameter is gradually increased over the flared section 13 in order to maintain mode purity and to provide the final feed aperture necessary to produce relatively narrow feed beamwidths.

Measurement of relative power (dB) against ()1 (Figure 8b) have been made for ka = 5.50 and the measured pattern is compared with theoretical computed values in Figure 8a. The goodness of fit and the depth of the boresight null are considered good indications of the purity of the excitation of the TM01 mode as opposed to TE11. This type of transmission is rather narrow band, but its simplicity is ideally suited for demonstration model purposes. More complex but broadband transmissions are available.

The profile of reflector 4 (Figure 6a) (single-reflector antenna design) for two types of azimuthally symmetric feed have been synthesised using geometrical optics. The synthesis was checked by a physical optics analysis of the far-field patterns of the resulting profiles with their associated feeds and observing the deviation of these patterns from the ideal sectoral requirement.

The mainbeam copolar power pattern of a conical feed horn in TE11 mode can be represented by P = cosq A01. A value of q = 1.5 was chosen and A = 1.724 was determined as the value required to produce -1 OdB illumination at the reflector edge where Ol = 45". The far-field was required to be of constant power over the elevation range 60 S O S 120 and zero elsewhere.

The result of this first synthesis is shown in Figure 9, which is a cross-sectional cut through the reflector with representative ray geometries. The reflector aspect ratio (h /r) is very low (0.585) making it bulky and difficult to manufacture. In addition, the linear polarisation of the feed leads to the drawback of a rotating plane of polarisation in the far-field of the reflector as mentioned above.

The far-field elevation patterns derived using a physical optics analysis of the reflector profile using reflectors of different heights are shown in Figures 1 Oa and lob. Figure 1 0a shows the far-field elevation pattern for h = 14A. The main feature of the sectoral pattern (O = 600 to 1200) and the spillover in the region around û= 45" are clearly identifiable. Figure 1 Ob shows the far-field elevation pattern for h = 140k. The sectoral region follows the geometrical optics ideal (dashed-line) much more closely, as would be expected, although the spillover level remains high.The interaction behind the reflector, where û < 45 , due to residual illumination of and diffraction from the reflector rim can now be seen more clearly.

Figure 11 shows a computer synthesised profile of a reflector and representative ray geometries for a TM01. The feed radius a is given by ka = 6.86 (k = 2irak). the feed angle (ü1) subtended at the reflector rim was chosen so that the feed gave -10dB illumination there, giving ü = 42" for ka = 6.86.

To achieve reasonably high reflector aspect ratios, it has been found that ka must be large. For instance, ka = 5.67, giving -10dB illumination at011 = 53", results in a reflector aspect ratio of only 0.514, whilst the corresponding values for kA = 14.708 are 18 and 0.988. Thus at first sight it would appear advantageous to make ka as large as possible. It should be noted, however, that although the spillover is always 1 OdB down on the peak feed level, the far-field signal strength is independent of the feed particular being always about 3dB up on the isotropic feed level. Thus as ka increases, the feed gets more directive.Hence the spillover level with respect to isotropic also increases, so that as ka increases the spillover level begins to approach the far-field signal level, causing adverse interaction with it.

A compromise value of ka = 6.86 has been found to be optimum. However, the peak spillover is a merel .5dB down on the far-field level so that this remains an unsatisfactory solution. In addition, the reflector aspect ratio is small (h/r = 0.663), thus this reflector will be difficult to produce and bulky to use.

A physical optics analysis of the far-field pattern for this system is shown in Figure 12. This is for ka = 5.67 and an antenna height of 188k was assumed to enable the spillover and rim diffraction components to be studied in some detail. It can be seen that the peak spillover atO11 = 45 is within a dB or so of the sectoral far-field signal level, and the peak rim diffraction, in turn, marginally exceeds it. This is comparable with the simple estimates made previously for ka = 6.86 on the basis of geometrical optics.

It was considered that better control of spillover levels might be achieved with a dual-reflector design.

Whilst this involves some increase in system complexity, feeds with small apertures could be employed to control the spillover levels, and simultaneously higher (main) reflector aspect ratios could be achieved. Such a dual reflector system is illustrated in Figure 13. A parabolic sub-reflector 14 is placed so that its focus coincides with the feed phase centre. The function of sub-reflector 14 is to collimate the broad beam from the small aperture feed into a pencil beam comprising a bundle of parallel rays in the near field of the sub-reflector. The edges of the sub-reflector are made to correspond to the 1 OdB levels of the feed field. This value corresponds to a reasonable compromise between minimising diffraction effects from the reflector edges whilst maintaining aperture efficiency.The collimating action of the sub-reflector permits a main-reflector 15 in its near-field having a higher, more reasonable aspect ratio. The main reflector 15 has a reflecting surface comprised by the radially outer surface of an element which tapers generally in the direction of the Z axis and includes an aperture at the smaller radius end thereof. The taper is directed towards the sub-reflector.

Around the rim of the sub-reflector 14 is provided a substantially cylindrical wall of RAM (radiowave absorbant material) 16 to obstruct and further reduce unwanted spillover radiating directly from the feed into the far-field pattern, without interfering with the desired illumination of the main reflector 15. The feed, indicated as TM01, is disposed within the reflector element so that microwave energy can be directed from the aperture in the reflector 15. TMo1 was again selected primarily on the basis of its constancy of radiation polarisation in azimuth. However, an additional advantage is the null on the boresight which leads to a very low back illumination of the feed by reflection from the centre region of the sub-reflector 14. This is indicated in Figure 13.

The geometrical optics synthesis for the dual reflector arrangement of Figure 13 is illustrated in Figure 14 for ka = 3.873. The main reflector is 10k high. The main reflector aspect ratio (h/r) = 0.927, thus providing a neat compact arrangement. The RAM wall reduced the peak spillover level to 1 OdB below the far-field sectoral level. In order to avoid excessive diffraction effects the RAM wall was assumed to extend to within 2k of the bottom far-field ray (ü = 120 ).

With the use of a RAM wall the spillover level can be further reduced by increasing ka. For example, with a RAM wall and ka = 5.67, the spillover level is 17dB below the far-field sectoral level. By contrast, without the RAM wall the spillover level rises with respect to the far-field level with increasing ka as in the single reflector case. Values range from -7dB for ka = 3.873 to OdB for ka = 8.6.

As ka increase the main reflector aspect ratio (h/r) increases only slightly. However, the distance between the main reflector and the sub-reflector starts to increase appreciably, so that the total system becomes less compact, and therefore less attractive for excessively large values of ka.

In addition, the criteria that the sub-reflector must be in the far-field of the feed, but the main reflector must be in the near-field of the sub-reflector, for the dual-reflector system to work, slightly favours smaller values of ka. Values of these far-field/near-field criteria for the example of ka = 3.873 are shown computed in Figure 15, the RAM wall being omitted therefrom. Also shown is the half-angle which the feed subtends at the sub-reflector (O1block). Now, a = Df/2 (Df is the diameter of the feed) and k = 2qr/A. Thus ka = Dfqr/X. For ka = 3.873, Df = 1.23k. 0,block = tan- ((Df/2)/Rs) where Rf is the distance from the feed to the sub-reflector.

To ensure that the feed is the far-field of the sub-reflector requires Rs > 2D2f /A = 3.04A. Since Rf is in fact 5.36k, this criterion is satisfied and 01block = tan-1(1 .23k/2)/5.36k) = 6.6 . 01block as calculated compares very well with the -10dB point of the TMol feed null (6.40" for ka=3.573) and demonstrates the value of this boresight null in reducing back illumination of the feed by the centre of the sub-reflector to relative insignificance.It might be thought that as ka increases, the half-angle subtended by the feed at the sub-reflector would increase in opposition to a decrease on the -1 OdB point on the null thereby the back illumination problem. In fact, because the main-reflector/sub-reflector distance increases also with increasing ka, both angles decrease and the trends do not conspire to work in opposition.

Thus, although the choice of ka is less clear-cut than in the single reflector case, for the dual-reflector the more compact solution resulting from a small ka, such as ka = 3.873, is generally preferable.

Figure 16 illustrates, schematically, a practical construction of the dual reflector system for a ground/ship platform. It is basically an upturned version of that already described. For mounting under a helicopter platform the original orientation would be preferable. The construction in Figure 16 comprises a cylindrical radome 17 which is closed at the uppermost end by a parabolic sub-reflector 18. Inside the cylindrical radome is mounted a primary (main) reflector 19. The feed aperture of a TMo1 feed 20 being disposed at the tip of the reflector 19. The cylindrical radome 17 is mounted at its lower end to the outer skin of a platform 21.

A waveguide transmit/receive connection 22 extends through the outer skin and the radome to the TM01 feed 20. The orientations proposed both facilitate connections to and from the feed through the platform outer skin without blockage due to waveguide runs etc. The structure which supports the reflector elements, that is the cylindrical radome, extends from the platform outer skin by an amount sufficient to prevent obscuration/multipath in the immediate vicinity of the platform. The radome cylinder is partially lined with radio absorbant material 23 to reduce spillover as described above. Both of the reflectors 18 and 19, and possibly the TMo1 feed, may be fabricated from metallised plastics material, enabling them to be both very accurately and cheaply mass-produced. The whole structure constitutes an inherent relatively low drag configuration.

The reflector arrangements so far described place the feed at the reflector/reflector combination azimuth hnd elevation focus so that, on transmit, power is radiated equally in all azimuths and in the desired sector in evation. However, if the feed position is displaced slightly upwards or downwards from the original focus, a single feed can be replaced by a multiplicity of feeds in a ring as indicated in Figure 17a for a simple reflector design. The major features of the sectoral elevation pattern will be maintained relatively undisturbed, whilst the azimuth patterns will be changed from omnidirectional to distinctly directional.

Considering the system in the transmit mode it can be seen (Figure 17b) that for a ring of feed elements 31-34 having a relatively large ring diameter, the radiation in certain azimuthal sectors is likely to become largely controlled by the excitation of particular elements in the ring. Therefore this antenna system on receive can be employed as a direction finding device by using simple inter-element response comparison or monopulse type processing on overlapping azimuthal beam patterns to determine the azimuthal angle of incident radiation. Full azimuthal direction finding of incident radiation can thus be achieved.

Alternatively, for a small diameter ring of feed elements where the resulting azimuthal far-field patterns associated with each individual element are likely to be identical, the small number of feed elements likely to be involved suggests the use of an adaptive signal processor based on the circular array concept. The reflector assembly here could then be regarded purely as a system for increasing the elevation directivity of the individual feed elements constituting the circular array. Such a system would provide simultaneous multiple signal direction finding and/or constitute a jam-resistant communication receiver system.

Claims (14)

1. An antenna system including a main reflector which is symmetrical about a first axis and has a reflecting surface shaped such that when radiowave energy is directed thereat from feed means on the first axis it can be reflected azimuthally omnidirectionally therefrom in such a manner as to obtain a far-field pattern with a predetermined sectoral elevation.

2. An antenna system as claimed in claim 1 and including feed means comprising a microwave feed.

3. An antenna system as claimed in claim 2, wherein the shape of the reflecting surface satisfied the laws of reflection and power conservation and is determined by the desired far-field pattern and the microwave feed pattern employed.

4. An antenna system as claimed in claim 3, wherein the reflecting surface is comprised by the radially outer surface of an element which tapers generally in the direction of the first axis towards the microwave feed which is spaced apart from the apex of the tapering element.

5. An antenna system as claimed in claim 4, wherein the microwave feed is comprised by a radially polarised TMo1 feed horn.

6. An antenna system as claimed in claim 3 and including a sub-reflector, wherein the reflecting surface is comprised by the radially outer surface of an element which tapers generally in the direction of the first axis, and includes an aperture at the smaller radius end thereof, the taper being directed towards the sub-reflector, the microwave feed being disposed within the element whereby microwave energy can be directed from the aperture towards the sub-reflector, which sub-reflector serves to reflect said microwave energy to the main reflector.

7. An antenna system as claimed in claim 6, wherein the microwace feed is comprised by a radially polarised TMo1 feed horn.

8. An antenna system as claimed in claim 6 or claim 7, wherein the sub-reflector is parabolic and disposed in th far-field of the microwave feed, the main reflector being disposed in the near field of the sub-reflector.

9. An antenna system as claimed in claim 8, wherein the sub-reflector is disposed with its focus coinciding with the microwave feed phase centre.

10. An antenna system as claimed in any one of claims 6 to 9, further including a substantially cylindrical wall of radiowave absorbant material around the rim of the sub-reflector whereby to minimise spillover.

11. An antenna system as claimed in claim 4 or claim 6, wherein the microwave feed is comprised by a plurality of feed elements disposed in a ring about the first axis whereby when microwave energy from the feed elements is directed towards the main reflector it is reflected therefrom in such a manner as to obtain an overall far-field pattern with the predetermined sectoral elevation and with respective directional azimuthal patterns corresponding to each feed element.

12. An antenna system as claimed in claim 11 and including means whereby when used on "receive" full azimuthal direction finding of incident radiation can be determined by comparison of the responses of the feed elements.

13. An antenna system substantially as herein described with reference to and as illustrated in Figure 3; Figure 4; Figure 5a; Figure 6a; Figures 9, 10a and 10b; Figures 11 and 12; Figure 13; Figure 14; Figure 15; Figure 16 or Figures 17a and 17b.

14. An antenna system substantially as herein described with reference to and as illustrated in Figure 5a; Figures 11 and 12; Figure 13; Figure 14; Figure 15 to Figure 16 and employing a TM01 feed horn substantially as herein described with reference to Figures 7a, 7b, 7c, 8a and 8b.