AU627493B2 - A circularly symmetrical reflector - Google Patents

Description

b"7493 44 44 t 4 #4 C ,44~ 444.

COMMONWEALTHi OF AUSTRALIA PATENTS ACT 1952-1969 COMVPLETE SPECIFICATION FOR THE INVENTION ENTITLED "A CIRCLALY SYMMETRICAL REFLECTOR" .444 4 4 44 4~ 4 14 4444 4 4 4 44 t *0 4 44~ The following statement is a full descr'iption of this invention, including the best method of per'forming it known to us:- This invention relates to an antenna having a circularly symmetrical reflector.

Background of the Invention There are several types of such an antenna: for example, the antenna may be a single-reflector antenna.

As described in the work by Mr. Nhu BUI HAI, entitled "Antennes microondes" ("Microwave antennas") (published by Masson, 1978), an antenna of this type having its reflector illuminated by a primary source placed at its focus is commonly used in frequency bands above 400 MHz.

Such an antenna comprises a reflector, which is generally circularly S° symmetrical, and a primary source which is generally of the horn type when the operating wavelength is in the centimeter range, and of the dipole type S including a reflector when the operating wavelength is in the decimeter range.

For a circularly symmetrical paraboloidal reflector having a surface tolerance of about +X/16, where X is the working wavelength, and for a horn type of primary source, the efficiency of such an antenna lies in the range 0.45 to 0.55.

Q 0 0 One of the main factors having a considerable effect on antenna effi- 2°0 ciency lies in loss of gain due to the surface tolerance of the circularly symmetrical paraboloidal reflector. Thus, a surface tolerance of +X/16 S loses about 0.4 dB and increases the diffuse radiation level by about dB.

The present invention seeks to reduce these effects considerably.

Another such antenna is an antenna having Cassegrain optics.

Antennas having Cassegrain optics with circularly symmetrical reflectors are well known. They comprise a main reflector of the paraboloidal type, a subreflector which is either a hyperboloid or an ellipsoid, and a primary source.

They provide the following performance: In co-polarisation: level of first secondary lobe about -16 dB/maximum; efficiency about 0.55 to 0.65; and far lobe levels in the range -5 dB to -15 dB below the isotropic level; and In cross-polarisation: on axis level about -35 dB; and maximum level -22 to -30 dB/maximum.

Assuming that the primary source provides very good performance (e.g.

<TO> a corrugated type of horn with an exponential profile), then the performance of a Cassegrain antenna depends essentially on the mechanical qualities of the reflectors, i.e.: the accuracy of the profiles of the main reflector and of the subreflec- 04 4 t tor; the accuracy of the relative positioning between these two reflectors; and the shape, quantity, and positioning accuracy of the support arms for the 64 0 V subreflector.

The worse these criteria, the worse the radiating performance of the 'o0 antenna. Thus, for a profile tolerance e relative to the wavelength i.e. for a ratio E/ Xof about the performance of a Cassegrain antenna 0 o having circularly symmetrical reflectors is as specified above.

When analog radio beams only were in use, such performance corresponded to requirements. Now that digital radio beams are being used, cross-polarisation performance has become crucial. It is a function, in particular, of the quality of modulation: 4, 16, 64, or 256 quadrature amplitude modulation (QAM).

Thus, for a given form of modulation, there may be a corresponding value of cross-polarisation, e.g. as follows: 16 QAM -22 to -32 dB/maximum 64 QAM -28 to -38 dB/maximum

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256 QAM -35 to -45 dB/maximum Consequently, with 64 QAM digital radio beams there already exists a need to select component parts for the antenna such that the crosspolarisation is lower than in existing antennas. And for 256 QAM digital radio beams, the cross-polarisation performance of existing antennas is quite unsatisfactory.

In addition, in order to increase the illumination efficiency in a Cassegrain antenna having circularly symmetrical reflectors, attempts are made to obtain amplitude distribution in its aperture which is uniform and equiphase, while continuing to use a primary source which provides tapering illumination. To do this, two new reflector profiles are defined and referred to as being "conformal". The main reflector is a pseudo-paraboloid and the subreflector is a pseudo-hyperboloid. By "conforming" the profile of the subreflector, the illumination of the main reflector is made uniform, and by "conforming" the main reflector, the illumination in the aperture of the antenna is made equiphase. However, when a pseudo-hyperboloid subreflector is used, the source which must be placed at the focus situated between the main reflector and the subreflector provides a degree of main reflector and the subreflector provides a degree of masking for the wave emitted or received by the antenna.

An object of the invention is therefore to solve these various problems.

Summary of the Invention The present invention provides an antenna including at least one circularly symmetrical reflector, wherein the reflector is made of a material having a liquid phase and a solid phase, and is obtained by centrifuging the material while in its liquid phase and subsequently passing to its solid phase.

Such an antenna having a centrifuged reflector makes it possible to obtain the following improvements: about 0.3 dB of gain; about ten decibels in diffuse radiation levels; a drop of about 10 dB to 15 dB in cross-polarisation level; and this is achieved using the same primary source.

Brief Description of the Drawings Embodiments of the invention are described by way of example with reference to the accompanying drawings, in which: Figures 1 and 2 are diagrams of a first variant of an antenna of the invention shown respectively in longitudinal section and in front view; Figure 3 shows the first variant of the invention as shown in Figure 1 together with the addition of other components; Figures 4 and 5 are graphs representative of the performance of the first variant of an antenna of the invention; f.l Figure 6 is a fragmentary longitudinal section view through a second variant antenna of the invention; Figure 7 is a front view of the second variant antenna of the invention; o Figures 8 and 9 show a variant of the polarisation duplexer used in the second variant of an antenna in accordance with the invention, shown respectively in front view and in side view; Figure 10 shows a variant of the source in the second variant antenna of the invention; and Figure 11 illustrates the operation of the second variant antenna of the invention.

Detailed Description A first variant of the antenna of the invention is shown in Figures 1 and 2 and comprises a single reflector antenna having a primary source which is a unipolar source having an access flange 11, and a reflector 12 obtained by centrifuging a material while in the liquid state and allowing it to solidify. The source 10 is held in place by support rods 13 which cI

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may be triangular in section, with the edge of the triangle facing the concave paraboloidal face of the reflector 12.

The invention thus consists in replacing the conventional circularly symmetrical paraboloidal reflector made either of laminated glass or else of metal, by a paraboloidal reflector of extremely small surface tolerance as obtained by centrifuging a substance in the liquid state such as a molten plastic or a molten metal copper or aluminum).

When the reflector is obtained by centrifuging a plastic material polyester), it is subsequently coated with a layer of metal by depositing a layer of zinc having a thickness of a few tens of micrometers, using the Shoop process).

0 The radius of curvature and the focal length of such a reflector depend on the speed of centrifuging. The tolerance of a reflector obtained in this way is about 0.1 mm.

As shown in Figure 3, a single reflector antenna may have a ring mounted therearound fitted with absorbent material and closed with a flat window 16 so as to obtain better resistance to wind and also to obtain a S maximum level of radiation at more than 800 from the axis which is ten to o 1fifteen decibels lower. This variant improves the radio performance of an antenna of the invention.

As also shown in Figure 3, in order to further increase crosspolarisation performance, the support rods 13 of the primary source may be S coated with microwave-absorbing material 17. Depending on the angle under consideration, cross-polarised radiation levels may thus be lowered by a few decibels to about ten decibels.

Advantageously, when a conventional paraboloidal reflector of an antenna is replaced by a centrifuged paraboloidal reflector of the invention having the same diameter and the same focal length, the fixing and mounting system remains unchanged. The only change lies in the radio performance of the antenna, and this is considerably improved.

r In order to take advantage of the improved cross-polarisation performance, it is advantageous to use a primary source of the corrugated horn type.

In one embodiment, an antenna having a centrifuged paraboloidal reflector illuminated by a primary source placed at its focus may be considered, having the following characteristics: antenna diameter 3.60 meters ratio of focal length to diameter 0.43; surface tolerance of the centrifuged reflector +0.1 mm; and ,i0 frequency band 5.925 GHz to 6.425 GHz.

The resulting curve 20 as shown in Figure 4 represents the envelope of the co-polarisation radiation pattern, i.e. E where 9 is an angle S measured in degrees, and this should be compared with the same curve 21 as S obtained for an antenna using a conventional paraboloidal reflector, and in this figure NI represents the isotropic level.

A curve 22 is also obtained representative of the envelope of the O 4 4 S cross-polarisation radiation pattern with E' f(9) as shown in Figure which should be compared with the same curve 23 obtained using an antenna having a conventional paraboloidal reflector.

A second variant antenna of the invention has Cassegrain optics, as I.o shown in Figure 6, and comprises: Two confocal paraboloid reflectors 110 and 111 having the same ratio of focal length to diameter, i.e. f/d F/D. The main reflector 110 is obtained by the above defined centrifuging technique, either using a metal such as copper or aluminum or else by applying a metal deposit 126 to polyester, for example. The subreflector 111 may be obtained by machining a solid. The accuracy of the paraboloidal profiles is thus excellent: errors less than +0.1 mm peak-to-peak. This is to be compared with current reflectors which are manufactured either by molding laminated polyester material or else by metal spinning or else by stamping. Any of these techj -I niques generally gives rise to a peak-to-peak error of more than one millimeter for reflectors having a diameter of four meters. These parameters contribute to a large extent to the reduced cross-polarisation values.

A primary source 112 of the horn type having an exponential profile 118. The source is defined to have a phase center 0 which is as stationary as possible, thereby making it possible to obtain excellent crosspolarisation performance over a wide frequency band. A polarisation duplexer 113 is disposed at the free end of the corrugated horn 118.

This polarisation deplexer 113 operates with two orthogonal :i0C polarisations that are vertical and horizontal, and comprises a portion 114 in the form of a circular waveguide together with two accesses 115 and 116 S in the form of rectangular waveguides, with the second access 116 being in Salignment with the circular waveguide 114 and with a reflector plate 117 S being disposed between the level of the first access 115 and the second access 116.

This duplexer thus serves to combine these two horizontal and vertical orthogonal linear polarisations. If a bipolar wave arrives at the inlet to 0 S t the circular waveguide 114, the horizontally polarised wave strikes the re- S flector plate 117 which is parallel thereto. It is reflected and passes into the first access 115 whereas the vertically polarised wave passes normally and perpendicularly to the reflector plate 117 and reaches the second access 116. Since reciprocity applies, a wave arriving via the first access 115 is reflected on the reflector plate 117 and exits via the circular waveguide 114. the second access 116 is "balanced" since the wave reaching this access enters the circular waveguide 114 via its centre.

However, the first access 115 which is connected to the side of the circular waveguide 114 is "asymmetrical" and not balanceJ.

There is a lens 119 at the aperture of the corrugated horn 118. It serves to transform the spherical wave from the corrugated horn into a plane wave. It has parabolic and flat surfaces, with the focus of the lens 8 119 coinciding with the phase centre 0 of the corrugated horn 118. It is made of a dielectric material, e.g. polytetrafluoroethylene or "Teflon".

Most current high-efficiency Cassegrain antennas (efficiency about 0.70 to 0.75) have main and auxiliary reflectors 110 and 111 with "conformed" profiles, i.e. profiles which are deformed in such a manner that the illumination phase reflected from the main reflector 110 becomes very small in practice (a few degrees instead of several tens of degrees), and the anplitude reflected by the subreflector 111 becomes uniform. However, a n an antenna of the invention, the profile of the main reflector 110 is 10 constrained to be paraboloidal because of the centrifuging technique. A deformed or "conformed" profile cannot be obtained by such centrifuging.

However, since the subreflector 111 is machined in a solid, it can be con- S formed so as to have a different profile. The efficiency of this antenna is about 0.65 to 0.70.

In this variant, in order to obtain an antenna having inp roved effi- 0 S ciency, the two reflectors 110 and 111 as described above may be retained S 4 S while the lens 119 is, in addition, "conformed" in profile so as to alter its phase pattern, thereby enabling the main reflector 110 to be illuminated in a manner which is as ejuiphase as possible. The efficiency is then inproved a little, tending towards 0.67 to 0.72, i.e. for a centrifuged main reflector 110 and a conformed subreflector 111, the lens 119 can be con'ormed in such a manner for waves emitted or received by the main reflector 110 that conformation of the lens is practically eluivalent to conformation of the main reflector 110. Such a variant of the antenna of the invention can be made in two different ways, in particular: the first comprises: a centrifuged main reflector 110 having a profile which is necessarily paraboloidal; and a subreflector 111 which is machined in a solid and which has a conformed profile, however this solution corresponds to a "halfconformed" solution; the second comprises: a centrifuged main reflector 110 whose profile is necessarily paraboloidal: a subreflector 111 which is machined in a solid to have a profile which is conformed; and a lens 119 whose profile is phase conformed.

As shown in Figures 6 and 7, the subreflector support 111 is constituted by four rods 120 (or arms) holding and supporting the subreflector 111 accurately. These rods are advantageously placed in a crossconfiguration. The four arms 120 are fixed around the circumference of the S main reflector 110. In this way, the profile of the reflector retains per- S fect paraboloidal continuity and is therefore not altered where the four arms are fixed thereto as in prior art antennas. Similarly, the "cross" profile of the four arms, as opposed to an profile makes it possible to avoid having an influence on cross-polarisation since each field is concen- 0 trated at 450 to the horizontal and vertical axes. In addition, the section of each arm 120 is preferably triangular (an isoceles triangle) with a vertex pointing towards the paraboloidal face of the main reflector 110.

In this way, any reflection of the radiated field on the arms 120 is *0 minimised. This contributes to reducing cross-polarisation.

In a variant of the duplexer, as shown in Figures 8 and 9, the first access 115 is obtained by means of a "magic-T" whose two arms 122 and 123 meet two diametrically opposite accesses 124 and 125 (of waveguide dimensions) on the circumference of the circular waveguide 114. This device is balanced.

In order to reduce the space occupied by the primary source 112, the corrugated horn may be "folded" by means of a 450 plane as shown in Figure 10, with the horn taking up a vertical position.

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Il.rllL~-L ilPPI-*J 0e I so a I a ao a ae r e «e o a os o a ra ao a o a a o o A In operation, as shown diagrammatically in Figure 11, and assuming that transmission is taking place, a spherical wave E 1 is formed in the horn aperture 118.

It is transformed into a plane wave Y 2 after passing through the lens 119. The place wave E 2 is reflected on the paraboloidal subrcflector 111 and becomes a spherical wave E 3 which, on being reflected on the paraboloid main reflector 110, becomes a plane wave F 4 at the outlet from the antenna.

Naturally, the reciprocity principle applies for reception. A plane wave F 3 coming from infinity is reflected on the paraboloid main reflector 110. It becomes a spherical wave Y 3 after reflection and it strikes the paraboloidal subreflector I 11.

On leaving the subreflector it becomes a plane wave 1 2 which strikes the lens 119.

The lens transforms it into a spherical wave Z 1 which propagates along the corrugated horn 118 and leaves via the accesses of the polarisation duplexer 113.

In an example of operation of this second variant of the invention, the following values are taken into consideration: 15 frequency band 6.43 GHz to 7.11 Ghz; diameter of main reflector 10 D 4 m; diameter of subreflector I I d 0.60 m; ratio of focal length to diameter 0.45; the main reflector 110 is made by ccntrifuging, this reflector may be obtained, 20 for example, by centrifuging a plastic material and then depositing a layer of metal on the plastic, e.g. by depositing a layer of zinc having a thickness of a few tens of micrometers using the Schoop process (or spraying using a molten metal flame pistol); the subreflector 111 is made by machining a solid, e.g. made of a metal such as aluminum; reflector profile tolerance 0.1 mm; primary source 112: an exponential profile corrugated horn having an aperture with a diameter of 0.60 m and a length of 0.90 m; I I i i i lens 119 in the aperture of the horn diameter 0.60 m; four triangular section support arms 120 fixed to the circumference of the main reflector in a "cross" configuration; cross-polarisation value better than 42 dB; and efficiency better than 0.65.

Naturally, the present invention has been described and shown merely by way of preferred examples and its component parts could be replaced by equivalent parts without thereby going beyond the scope of the invention.

Thus, the primary source 112 may be square in shape, rectangular, or circular, and it may be fed from a square section, a rectangular section, or a circular section waveguide, respectively.

Thus, the subreflector 111 need not be confocal with the main reflector 110, but may be a hyperboloid or an ellipsoid. In either case the primary source is then a horn that does not include a lens. In this case, antenna efficiency is reduced but its characteristics remain very good by virtue of the centrifuged main reflector.

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Claims (8)

1. An antenna comprising a ccntrifugally cast main reflcctor having a circularly symmetrical substantially concave paraboloidal metallic face, with a diameter of at least approximately 3.6m; a subreflector having a circularly symmetrical convex shaped face machined from a solid metal to a surface tolerance of no more than ap- proximately 0.1 mm and facing the concave face of the main reflector, said sureflector having a diameter no greater than approximately 0.6m, said main reflector and said subreflector forming a Casscgrain optical system wherein the two reflectors have the same ratio of focal length to diameter of approximately 0.45; a subreflector support in the form of four arms in a cross-configuration, each of said arms being fixed at one end to the periphery of the main reflector and having a cross section in the form of an isosceles triangle whose apex points in the direction of the main re- flector; an exponential profile corrugated horn aligned with the subrcflector and the main reflector for receiving or transmitting microwaves, said corrugated horn defining 15 a phase centre and having an aperture end and a free end; a lens made of a dielectric material and disposed at the aFprture end of the corrugated horn with a focus of the lens coinciding with the phase centre of the corrugated horn; and a polarization duplexer disposed at the free end of the corrugated horn such that two fields are concentrated at 45' relative to the four arms of the subrcflector support. 20

2. An antenna for a frequency band of 6.43 GHz to 7.11 GHz having a cross polarization better than 42dB and efficiency better than 0.65, said antenna compris- ing a centrifugally cast main reflector having a circularly symmetrical substantially concave paraboloidal metallic face, with a diameter of approximately 4m: a subre- flector having a circularly symmetrical convex shaped face machined from a solid metal to a surface tolerance of no more than approximately 0.1mm and facing the concave face of the main reflector, said subrcflector having a diameter no greater than approximately 0.6m, said main reflector and said subreflector forming a Casscgrain optical system wherein the two reflectors have the same ratio of focal length to di- ameter of approximately 0.45; a subrcflcctor support in the form of four arms in a cross-configuration, the arms being fixcd at one end to the periphery of the main re- fector and each having a cross section in the form of an isosceles triangle whose apex points in the direction of the main reflector; an exponential profile corrugated horn aligned relative to the subreflctor and the main reflector for receiving or transmitting r microwaves, said corrugated horn defining a phase centre and having a circular ap- c c crture end having a diameter of approximately 0.6m and a rectangular free end scp- \,F6-t-G 14 arated by a length of approximately 0.9m; a lens made of a dielectric material also having a diameter of approximately 0.6m and disposed at the aperture end of the corrugated horn with a focus of the lens coinciding with the phase centre of the corrugated horn; and a polarization duplexer disposed at the free end of the corru- gated horn such that two fields are concentrated at 45' relative to the four arms of the subreflector support.

3. An antenna as claimed in claim 2, wherein said polarization duplexer further comprises: a circular waveguide portion; a first rectangular waveguide access, a sec- ond rectangular waveguide access in alignment with the circular waveguide portion; and a reflector plate disposed between the first access and the second access.

4. An antenna as claimed in claim 1, wherein said centrifugally case main re- flector comprises a centrifugally cast plastic and said metallic face comprises a metal coating deposited over said plastic.

5. An antenna as claimed in claim 4, wherein said plastic comprises a polyester Si 15 and said metal coating comprises zinc.

6. An antenna as claimed in claim 2, wherein said centrifugally cast main reflec- tor. comprises a centrifugally cast plastic and said metallic face comprises a metal coating deposited over said plastic.

7. An antenna as claimed in claim 6, wherein said plastic comprises a polyester 20 and said metal coating comprises zinc.

8. An antenna substantially as herein described with reference to Figures 1 to I I of the accompyning drawings. DATED THIS SECOND DAY OF JUNE 1992 ALCATEL N.V. S ,e //7 L