GB2192309A - Energy reflectors - Google Patents

Abstract

The invention provides a reflector particularly for use in a radio antenna system. The reflector is provided with energy absorbent material distributed across the effective aperture of reflection in such manner that the reflection of the energy outside of a main lobe of reflected energy tends to be reduced. The effect of the distribution of absorbent material across the aperture is made such that the reflected energy is a gaussian or normal distribution across the aperture.

Description

SPECIFICATION Improvements in or relating to energy reflectors This invention relatesto energy reflectors and in particularto energy reflectors forming part of antenna systems in which a feed antenna or radiator illuminates a reflector.

As is known, if a reflector of aperture D is illuminated uniformly by its feed without a taper the reflectorwill radiate with a radiation pattern as shown in Figure 1 of the accompanying drawings. This representsthewell known sin x E field distribution of power across the x angularvolume normal to the reflecting surface. As may be seen, in addition to the radiation in a main lobe there is alsothe reflections of reduced amplitude in so-calledsidelobes. In many cases the presence of sidelobes is undesirable and the feed is tapered or shaped to follow so asto follow some distribution which is not uniform but is greater at the centre of the reflectoraperture byas much as an order greaterthan the illumination atthe edges ofthe reflector aperture.

Such a distribution of illuminating power can result in the almost completesuppression ofsidelobes in the radiated energy pattern.

In some cases however, it is not practicable to achieve sidelobe suppression, or a desired degree of sidelobe suppression, by tapering or shaping the feed.

It is one object of the present invention to provide an improved antenna reflector or antenna system in which thisdifficultyis mitigated.

According tothis invention, a reflector for incident energy having an effective aperture of reflection is provided wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflectionofsaid energy outside of a main lobe of reflected energytendsto be reduced.

Expressed in otherterms, according to this invention a reflectorfor incident energy having an effective aperture of reflection is provided wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflectivity within said aperture is reduced so thatthe reflected energy has a substantially gaussian or normal distribution across said aperture.

Normally said energy will be radio energy and said absorbent material will be radar absorbent material (RAM) as know perse.

Preferably said reflector is a radio reflector for a radio antenna system.

Commonly, said effective aperture will be substantiallycircularinwhich case the reflecting surface of said reflector may be provided with absorbent material in the form of a plurality of lines with a radial nature butwith the length ofthe lines and closeness of the spacings between lines such asto give a near exponential densityofthe application ofthe material from the periphery ofthe aperture.

Again where the aperture is substantially circular, lines of absorbent material may be provided on the reflective surface of said reflector, which extend circumferentiallywith the spacing between lines reduced towards the periphery ofthe aperture of the reflector.

Preferably, the arrangement is such thatthe proportion of reflected power at the centre is near 100% and at the periphery is near zero.

Again where said aperture is substantially circular said absorbent material may be provided in the form of a radial and circumferentially mesh dimensioned to provide an inner exponential density ofthe application ofthe material from the periphery of the aperture of said reflector.

,Said absorbent material may also be applied in the form of discrete elements of circular or other shapes, such as diamond, square, triangular, orcrvstalform, with a density of application which is exponentially reduced towards the centre ofthe aperture of said reflector. In this last mentioned case the individual elements should be sufficiently small as to be well below the dimension of a wavelength of the shortest wavelength of energywhich isto be reflected by said reflector.

Said absorbent material may also be provided as a continuous layer of material having a thickness which reduced in exponential fashion towards the centre of the aperture of said reflector.

According to a feature of this invention, a radio antenna system includes a reflector as described above and a feed radiator provided to feed said reflector. Saidfeed radiator may, and usually will, be such asto provide near uniform illumination of said reflector. The feed ofthe feed radiator may, however, itself be shaped or tapered.

The invention will now be described in greater detail with reference to Figures 2 to 5 ofthe accompanying drawings.

Figures 2 and 3 illustrate various embodiments of the present invention, and Figures 4 and 5 are illustratory diagrams.

Referring to Figure 2, this schematically illustrates a feed horn 1 arranged uniformly to illuminate a radio reflector 2. The reflector 2 is shown as having a circular aperture which is divided in six different sections from Ato Fwhich forthe purposes of illustration are differentlytreated bytheapplication ofradarabsor- bent material, as known perse to the reflecting surfaces.

In segment A, the reflecting surface is sprayed with a plurality of lines with a radial nature but length ofthe lines and closeness of the spacing being such as to give a near exponential density of the application of the material from the periphery ofthe reflector so that the approximate amount of power is reflected in the desired distributed manner.

In segment B, lines of radar absorbent material are sprayed on to the reflected surfaces circumferentially and the spacing there is reduced towards the periphery so that the proportion of reflected power at the centre is near 100% andattheedgeiszero.

Whilst not illustrated, a combination ofthetwo techniques illustrated in segmentsAand Bcould be made so asto promebe a radial and circumferential mesh of radar absorbent material applied to the surfaces of the reflector which naturally lends itself to the application of a so-called absorbing paint.

Segment C illustrates the application of the paint in dots of circular or other shapes such as diamond, square, triangular or indeed in any other shapes such as those illustrated in Figures 3(a), (b), (c). These shapes are sufficiently small as to well below the dimension of a wavelength at the frequencies contem plated. The density of application of the pattern of shapes of radar absorbent material is exponentially reduced towards the centre ofthe reflecting surfaces as shown in section C of Figure 2.

Sectors, D, E and F illustrate the application of radar absorbent material which has a discrete thickness of its own as a sheet material and in sectorD it is shown applied in the form of discs distributed exponentially so asto achieve the desired reduction of reflecting capability at the periphery. Instead of discs, sheet in the shapes shown in Figure 3(a), (b) or (c) could be employed.

Sector E illustrates another method employing thorn shaped wedges cut from absorbent material thus providing a dual of the lines of paint-like absorber used in Sector A.

Sector F illustrates the radar absorbent material whose thickness is reduced towards the centre of the reflecting surface in order to achieve a similar result Whilst in relation to Figure 2 differentsegments of the aperture are described as being differently treated (which is possible) in practice it would be normal to treatthewhole oftheaperture in like manner. The aperture is shown segmented in Figure 2 merely to illustrate some of the techniques wh ich may be applied in carrying out the invention.

By controlling the reflectivity ofthe surface ofthe radiator 2 which is illuminated by the horn 1 a reflection may be achieved having a distribution across the aperture which is as illustrated in Figure 4.

This has the idealised shape ofthe normal distribution of the radiated energy pattern of a sidelobe suppression system, in which sidelobes are suppressed by shaping ortapering the feed as illustrated in Figure 5.

The reflection atthe edgeofthe aperture is reduced to substantiallyzeroand is at maximum atthe centreso thatthe re-radiated pattern from the reflector is appropriately shaped thus reducing the sidelobes from atypical performance such as that shown in Figure 1 to a performance such as is shown in Figure 5.

In other words, the reflectivity within the aperture is reduced sothatthe reflected power now has a substantially gaussian or normal distribution across the aperture.

Claims (16)

CLAIMS 1. A reflector for incident energy having an effective aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflection of said energy outside of a main lobe of reflected energy tends to be reduced. 2. A reflectorforincident energy having an effec- tive aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflectivity-within said aperture is reduced so thatthe reflected energy has a substantially gaussian or normal distribution across said aperture. 3. A reflector as claimed in claim 1 or2and wherein said energy is radio energy and said absorbent material is radar absorbent material (RAM) as known per se. 4. A reflector as claimed in any of the preceding claims and wherein said reflector is a radio reflector for a radio antenna system. 5. Areflectorasclaimed inanyofthepreceding: claims and wherein said effective aperture is substan- tially circular andthe reflecting surfaceofsaid reflector is provided with absorbent material in the form of a pluralityoflineswith a radial nature but with the length ofthe linesand closenessofthe spacings between lines such asto give a near xponential densityoftheapplication ofthematerialfromthe periphery of the aperture. 6. A reflector as claimed in any oftheabove claims 1 to4whereintheapertureissubstantiål'fycircularand lines of absorbent material are provided on the reflective surface of said reflector, which extend circumferential ly with the spacing between, lines reduced towards the periphery of the apertu'reofthe reflector. 7. A reflector as claimed in any ofthe above claims and wherein the arrangement is such that the proportion of reflected power at the centre is near 100% and atthe periphery is near zero. 8. A reflector as claimed in any ofthe claims 1 to 4 and wherein said aperture is substantially circular and said absorbent material is provided in the form of a radial and circumferentially mesh dimensioned to providean inner exponential density ofthe applica- tion ofthe material from the periphery ofthe aperture of said reflector. 9. A reflector as claimed in any of claims 1 to 4 and wherein said absorbent material is applied in the form of discrete elements ofcircular or other s hapes, such as diamond, square,triangular, orcrystalform, with a density of application which is exponentially reduced towards the centre ofthe aperture of said reflector. 10. Areflectoras claimed in any of the above claims 1 to 4 and wherein said absorbent material is provided as a continuous layer of material having a thickness which reduces in exponential fashion towards the centre of the aperture of said reflector. 11. A radio antenna system including a reflector as claimed in any of the preceding claims and a feed radiator provided to feed said reflector. 12. A system as claimed in claim 11 and wherein said feed radiator is such asto provide near uniform illumination of said reflector. 13. A system as claimed in claim 11 and wherein thefeed of the feed radiator is itself shaped ortapered. 14. A reflectorfor incidentenergy having an effective aperture of reflection substantiallyas herein described with reference to the accompanying drawings. 15. A radio antenna system substantially as herein described with reference to the accompany drawings. New claims or amendmentsto claims filed on 1 Superceded claimsAll New or amended claims:-All CLAIMS

1. A radio wave reflectorfor incident energy having an effective aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such manner that reflection of said energy outside of a main lobe of reflected energy tends to be reduced irrespective of any polarisation that said incident energy may have.

2. A reflector as claimed in claim 1 for incident energy having an effective aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflectivity within said aperture is reduced so that the reflected energy has a substantially gaussian or normal distribution across said aperture.

3. A reflector as claimed in claim 1 or 2 and wherein said said absorbent material is radar absorbent material (RAM) as known peruse.

4. A reflector as claimed in any ofthe preceding claims and wherein said reflector is a radio reflector fora radio antenna system.

5. A reflector as claimed in any the preceding claims and wherein said effective aperture is substan tiallycircularandthe reflecting surface of said reflector is provided with absorbent material in the form of a plurality of lines with a radial nature but with the length ofthe lines and closeness ofthe spacings between lines such as to give a near exponential densityofthe application ofthe material reducing fromthe periphery of the aperture.

6. A reflectorasclaimed in any ofthe above claims 1 to 4wherein the aperture is substantially circular and lines of absorbent material are provided on the reflective surface of said reflector, which extend circumferentially with the spacing between lines reduced towards the periphery ofthe aperture ofthe reflector.

7. A reflector as claimed in any ofthe above claims and wherein the arrangement is such that the proportion of reflected poweratthe centre is near 100% and at the periphery is nearzero.

8. A reflector as claimed in any of the claims 1 to 4 andwherein said aperture is substantially circular and said absorbent material is provided in the form of a radial and circumferentially mesh dimensioned to provide an inner exponential density of the application ofthe material from the periphery ofthe aperture of said reflector.

9. Areflectorasclaimed in anyofthe above claims 7 to 4and wherein said absorbent material is provided as a continuous layer of material having a thickness which reduces in exponential fashion towards the centre ofthe aperture of said reflector.

10. A reflector as claimed in any of claims 1 to 4 and wherein said absorbent material is applied in the form of discrete elements with a density of application which is exponentially reduced towards the centre of the aperture of said reflector.

11. Areflectoras claimed in claim 10 and wherein said elements are of circular, diamond, square, triangulardrcrystalform shape.

12. A radio antenna system including a reflector as claimed in any of the preceding claims and a feed radiator provided to feed said reflector.

13. Asystem as claimed in claim 12andwherein said feed radiator is such asto provide near uniform illumination of said reflector.

14. Asystemasclaimed inclaim 12andwherein thefeed ofthefeed radiator is itself shaped or tapered.

15. A reflectorfor incident energy having an effective aperture of reflection substantially as herein described with reference to the accompanying draw- ings.

16. A radio antenna system substantially as herein described with reference to the accompanying drawings.