WO1997004497A1

WO1997004497A1 - Antenna reflector

Info

Publication number: WO1997004497A1
Application number: PCT/CA1996/000462
Authority: WO
Inventors: Shyam Gupta; Ralph Pokuls
Original assignee: Spar Aerospace Limited
Priority date: 1995-07-14
Filing date: 1996-07-12
Publication date: 1997-02-06
Also published as: AU6296396A

Abstract

The reflector consists of a ground plane, enclosed in a ground plane frame, and a phasing plane, enclosed in a phasing plane frame. Spacers maintain the planes in connected, spaced relationship. The phasing plane includes a plurality of reactive elements sensitive to different frequencies in the bandwidth of the reflector. Thus, an E-M wave directed at the reflector is reflected to the angle of the received wave. In one embodiment, the antenna is flat and collapsible and simulates the response of a three dimensional reflector. In another embodiment, the reflector is parabolic in that an array of dipoles is suspended over a parabolic ground plane to assume a parabolic shape.

Description

ANTENNA REFLECTOR

Technical Field The invention relates to antenna reflectors which simulate the response of a three dimensional reflector. In one embodiment, the antenna reflector is flat and simulates a response of a normal parabolic reflector. The flat antenna reflector in accordance with this embodiment can also simulate other shaped reflectors. In another embodiment, the reflector is parabolic in that an array of dipoles is suspended over a parabolic ground plane to assume a parabolic shape.

The flat collapsible antenna reflector in accordance with the first embodiment of the invention is especially adaptable for use on space capsules.

Background Art

Collapsible antennas are known in art as illustrated in, for example, U.S. Patent 3,699,581, Hall et al, October 17, 1972, U.S. Patent 3,969,731, Jenkins et al, July 13, 1976 and U.S. Patent 5,132,699, Rupp et al, July 21, 1992.

The '581 Patent teaches a collapsible antenna arrangement for use in space. Foldable antennas are stowed in a cylindrical shroud during launch, and they are unfolded when the spacecraft body has been launched into space. As seen in Figure 6, antenna elements 42 are arranged on one side of the panels. As disclosed at column 3, lines 31 and 32, these elements are arranged in a manner of a phased array.

The '731 Patent teaches a mesh article useful as a reflector in space. The strands 2 of the mesh, as illustrated in Figure 1, are covered by a conductive material 4 along their entire length. The *699 Patent was selected as of interest in its teachings of a collapsible antenna comprising a plurality of panels each of which panel is inflatable. As seen in Figure 4, the panels comprise tubular elements 20 having disposed within them dipole elements 26.

Applications that require better bandwidth performance make use of a parabolic reflector. Given a predetermined feed location, the parabolic reflector will provide a circular illumination area. However, when used to illuminate a predetermined area from space, such as a country, a circular pattern is rarely adequate. Modifications are therefore made to the parabolic reflector and feed arrangement to duplicate to a certain degree, the shape of the area that needs to be illuminated.

One method is to mechanically distort the shape of the parabolic reflector such that the beam which is reflected from the surface of the reflector will have the required illumination pattern once it arrives on the surface requiring the signal.

Another method is to make use of several feeds disposed at different focal points such that the combined paths of the signals add and subtract to provide the required illumination pattern.

Summary of Invention

It is an object of the invention to provide a novel flat collapsible antenna reflector.

It is a further object of the invention to provide a flat collapsible antenna reflector which includes a ground plane and a spaced phasing plane.

It is a still further object of the invention to provide a flat collapsible antenna reflector wherein the ground plane and the phasing plane are made of flexible materials whereby both planes are foldable so that the entire antenna reflector is collapsible.

It is another object of the invention to provide such an antenna reflector whereby the dipoles are of the correct length and spacing so as to convert a received electromagnetic wave into a shaped beam.

It is another object of the invention to provide a flat collapsible antenna reflector whereby the dipoles are of the correct length and spacing such that the received electromagnetic beam is converted to a beam simulating the response of a three-dimensional reflector.

It is a further object of the invention to provide an antenna reflector in which an array of dipoles are disposed over a parabolic ground plane such that the dipole array assumes a parabolic shape.

It is another object of the present invention to provide an antenna reflector whereby the characteristics of the dipoles can be remotely adjusted so as to re configure the shape of the reflected beam.

In accordance with a particular embodiment of the invention there is provided a flat, collapsible, antenna reflector, for use in a predetermined bandwidth, and for reflecting an E-M wave, comprising: a ground plane enclosed in a ground plane frame; a phasing plane enclosed in a phasing plane frame; spacer means for maintaining said planes in connected, spaced relationship; said phasing plane including a plurality of reactive elements sensitive to different frequencies in said bandwidth; whereby, to cause said E-M wave to be reflected at a predetermined angle to the angle of reception thereof. In accordance with another embodiment of the invention there is provided a passive parabolic shaped antenna reflector, for use in a predetermined bandwidth, said antenna being configured to reflect a received E-M wave, said antenna comprising: a parabolic shaped ground plane; a parabolic shaped phasing plane; spacer means for maintaining said planes; said phasing plane including a plurality of reactive elements for causing a received E-M wave to be reflected from said reflector at a predetermined angle to the angle of reception thereof.

Brief Description of the Drawings The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which:

Fig. 1 illustrates generally the principles for forming a flat, collapsible, reflector antenna in accordance with one embodiment of the invention;

Fig. 2 illustrates a particular embodiment of a phasing plane in accordance with the invention;

Fig. 3 illustrates a second embodiment of the phasing plane in accordance with the invention; Fig. 4 illustrates a still further embodiment of a phasing plane in accordance with the invention;

Fig. 5 shows in greater detail a cylinder used in the Fig. 4 embodiment; Fig. 6 illustrates a further embodiment of a ground plane using the cylinders of Fig. 5;

Fig. 7 illustrates a still further embodiment of a ground plane;

Fig. 8 illustrates a still further embodiment of a phasing plane; Fig. 9a illustrates a top view of another embodiment of the invention;

Fig. 9b is a cross-section of the embodiment of Fig. 9a; Fig. 10a is a diagram illustrating the orientation relationship of cross-shaped dipole elements;

Fig. 10b is a top view of another type of dipole element for use with the antenna of the present invention; and

Fig. 10c is a schematic of a circuit for adjusting the electrical characteristics of a dipole element.

Brief Description For Carrying Out The Invention

Referring to Figure 1, it can be seen that the antenna reflector in accordance with a first embodiment of the invention comprises a ground plane, illustrated generally at 1, and a phasing plane illustrated at 3. The planes are connected by spacers 5 which maintain the two planes in connected, but spaced, relationship.

Turning to Figure 2, in one embodiment, the phasing plane 3 comprises a metallic sheet having cut out slots 9. The slots form single or crossed dipoles. In accordance with the invention, the slots are of unequal size and may be unequally spaced from each other.

With a phasing plane as illustrated in Figure 2, the ground plane can comprise a metallic sheet which would be of the same size as the metallic sheet 7 of Figure 2.

Turning now to Figure 3, in another embodiment, the phasing plane comprises a dielectric sheet 11. Metallic patterns 13 are printed onto the dielectric sheet to form the dipoles. Once again, the painted on metallic patterns are of different size and of different spacing there between. In the embodiment of Figure 3, the ground plane may also comprise a metallic sheet of the size as the dielectric sheet 11.

In a further embodiment, illustrated in Figure 4, the phasing plane comprise frame elements 15, 17, 19 and 21. The frame elements may be rigid members of, for example, a plastic material. Alternatively, they can be flexible members of, for example, a rope like material or the like. The embodiments of Figures 2 and 3 are also enclosed by frame elements.

The frame elements enclose a plurality of different sized and differently spaced cylinders 23. The cylinders are made of a metallic material. As can be seen, the cylinders are strung along vertical strands 25 and horizontal strands 27. As seen in Figure 5, each metallic cylinder 23 comprises a horizontal opening 29 extending along the axis of the cylinder, and a vertical opening 31 which extends transversely to the axis of the cylinder. As can be seen, the vertical strands 25 extend through the opening 31 and the horizontal strands 27 extend through the opening 29.

Figure 6 illustrates an embodiment of the ground plane using metallic cylinders. In Figure 6, the metallic cylinders 230 are all of equal size and there is equal spacing between the cylinders. Once again, the ground plane is enclosed by frame elements 15, 17, 19 and 21. The cylinders are strung by vertical strands 25 and horizontal strands 27.

A further embodiment of a ground plane is illustrated in Figure 1 . In Figure 7, the ground plane is also enclosed by frame elements 15, 17, 19 and 21. The ground plane is then made of vertical strands 250 and horizontal strands 270. The strands are made of a dielectric material, for example, Kevlar. Cylinders 231 are painted onto the Kevlar strands with a metallic paint. In the Figure 7 embodiment, each of these cylinders is of equal size and is equally spaced from every other cylinder. A phasing plane which uses the same approach as Figure 7 is illustrated in Figure 8. In Figure 8, once again, the frame elements 15, 17, 19 and 21 enclose the plane. The plane includes vertical strands 250 and horizontal strands 270. The strands are also of a dielectric material, for example, Kevlar. Cylinders 233 are painted onto the strands with a metallic material. In the Figure 8 embodiment, the cylinders are of an unequal size and are unequally spaced from each other.

On the phasing plane, the different approaches provide dipoles which, because of their unequal size and spacing, will have different reactions to an E-M wave of a given frequency. Thus, when an E-M wave of a given frequency is directed at the phasing plane, each dipole will cause it to reflect at a different phase angle, and the array of dipoles on the phasing plane of the reflector are adjusted to provide the proper phase relationships between the incident and reflected waves. The total reflected wave will constitute the sum of all of the reflected waves. The dipoles on the ground plane are made to be resonant at the center frequency of the bandwidth of the antenna reflector.

Both the ground planes and the phasing planes, especially as shown in Figures 4-8, can be folded up in the manner of a window blind with horizontal slats.

Referring now to Figure 9a, we have shown a top view of the parabolic dipole array according to another embodiment of the present invention. The array 90 consists of an array of dipoles or micro strip patches 91 forming phasing plane suspended over a parabolic shaped ground plane 92. The distance A and B between the dipole elements or patches is constant everywhere thus the dipole array assumes a parabolic shape as well. It will be known to those knowledgeable in this art that other antenna shapes can also be used. The ground plane 92 can be made of any electrical conductor such as aluminum or copper. The dipole elements or patches 91 form a phasing plane by being etched onto a Kapton sheet of about .001 inch thickness using standard etching techniques. The Kapton sheet is supported and separated from the parabolic ground plane 92 using a foam or Kevlar honeycomb structure 93.

The feed horn 94 is used to launch an electromagnetic wave at the antenna array. The feed horn is placed some distance in front of the antenna array. The exact location is determined by the length of the dipoles or patches. The dipole elements are of a predetermined length so as to convert the received electromagnetic wave into a shaped beam. Each element locally distorts the parabolic surface to the desired height or depth.

Any reactive element that has a well defined reflection phase may be used as an reactive element for the antenna array. Reactive elements such as rectangular micro strip patches, crossed dipoles, and spiral elements may be employed.

Elements that have two characteristic lengths corresponding to each polarization may be used. By individually adjusting the two characteristic lengths, each polarization will be reflected in a different manner. In essence, the local distortion will appear to be dissimilar for each polarization and thus two surfaces will be emulated even though only one physical surface is used. In addition, the focus point of each of the two surfaces may be selected independently. Hence, it is possible to have two separate horn positions, one for each polarization. The horns may be placed in the offset position if required.

Referring now to Figure 10a, we have shown a diagram illustrating the orientation of cross-shaped dipoles used as reactive elements with the present invention. As indicated above, dipoles of different lengths can be used to generate different reflection phases to generate shaped beam. With cross-shaped dipoles, each arm of the cross independently controls its corresponding polarization.

Incident RF energy causes a standing-wave to be set up between the dipole and the ground-plane. The dipole itself possesses an RF reactance which is a function of its length and thickness. This combination of standing-wave and dipole reactance causes the incident RF to be re-radiated with a phase shift 0, which can be controlled by variation of the dipole's length.

The value of this phase-shift is a function of the dipole length and thickness, distance from ground plane, dielectric constant and angle of the incidence RF energy.

The dipole lengths can vary between 0.25 and 0.60 wavelengths to achieve a full range of phase shifts depending on application. The ideal spacing from the ground plane is 1/4 to 1/8 wavelength. This parameter affects form factor, bandwidth and sensitivity to fabrication errors.

As an example, at C-Band frequencies, the wavelength is 3.0 inch. The ideal separation from the ground plane would therefore approximately be 0.5 inch. At Ku band frequencies, this separation would be of the order of 0.2 inch. Since the dipoles of different lengths produce a phase shift in the incident wave, by arranging the distribution and lengths of the dipole, the incident energy can be formed to provide a well defined focus. By placing a feed at the focal point, incident energy from a distant source can be collected.

For the embodiment of Fig. 10a, the dipoles 101 can disposed from each other a distance D of 0.5 wavelength. In order to obtain both linear polarization, cross dipoles are used. The dipoles 101 have a length L which vary between 0.25 to 0.6 wavelengths to achieve a full 360 degree range of phase shifts. The value of the phase shift is a function of the dipole length, thickness, its distance from the ground plane, the dielectric constant of the intervening layer, and the angle of the incident RF energy. The phase shift is further affected by mutual coupling among nearby dipoles. In order to determine the appropriate dipole lengths to achieve the required reflection phases, a commercially available FSS (Frequency Selective Surface) software is used. A provider of this type of software, is TICRA a company established in Denmark. TICRA's FLIP FLOP software can be used in the design of reflect arrays. FLIP FLOP is an FSS analysis package which analyzes an infinite array of crossed dipoles to provide the reflection and transmission coefficients of an electromagnetic wave which is incident on the dipole array.

The reflection and transmission coefficients are dependent on the lengths and widths of the crossed dipoles. A software package such as FLIP FLOP can be used to determine the reflection phases as a function of dipole lengths for a particular layout of dipoles. The result is a curve which is used for designing the antenna array.

Another software available from TICRA is POS. The POS (Physical Optics Synthesis) package determines the required aperture phase to achieve a given far-field pattern (i.e. "footprint"). The aperture phase is the field distribution that is required to be generated by the reflect array in order to achieve the desired pattern.

Knowing the reflection phase vs. dipole length curve calculated by FLIP FLOP and the required aperture phase calculated by POS, the required reflection phases on the surface of the reflect array, whether parabolic or flat can be calculated.

This is achieved by using geometrical optics to calculate the required reflection phase of each reflect array element (i.e. dipole) . The correct reflection phase is a function of the aperture phase (and hence the far-field pattern) and of the horn and it's position. Once the correct reflection phase has been determined, the output of the FLIP FLOP program is used to determine the correct dipole length.

This output is a listing of dipole lengths and positions which are used for manufacturing the antenna array. The antenna can also be designed for a dual band application by having two layers of dipoles, i.e. one for low frequency located at the back and one for high frequency located at the front.

Referring now to Fig. 10b, we have shown a spiral shaped dipole 110 which can be used as a reactive element with the antenna arrays of the present invention. The spiral shaped dipole is comprised of a series of spiraling dipole arms 111, 112 and 113. Spiral arm 111 spirals inwardly to the center and then outwardly towards end 115. The spiraling arms can be etched in copper on a dielectric surface 116.

The reflection phase of the dipole element can be adjusted by rotating the dipole element. The rotation of the spiraling arms will affect the polarization of the dipole element. The electrical length of each dipole element can be changed remotely by varying the electrical characteristic or reactance of the dipole arms. This can be done, whether using a cross-shaped or spiral shaped dipoles. As shown in Fig. 10c, the electrical length or reactance of a cross- shaped dipole 120 can be changed using a pair of diodes 121 and 122 connected at each end of a cross member. The diodes are controlled via a dipole controller 123, which is simply a circuit to provide the necessary biasing voltage across each diode. When a biasing voltage is placed across the diode, it changes the reactance of the diode thus resulting in a different dipole to provide a different RF incident energy.

The dipole controller 123 can be operated remotely, such as from an earth station for satellite applications or on board processors, in order to vary the reactance of each dipole element. This way, an antenna array located on a satellite, space station or other space vehicle can be electrically modified to provide a different beam shape, footprint, etc. This feature can be especially useful in situations where a satellite, for example, which is designed and launched to cover a certain geographic region of earth is required to cover a different geographic region. In the past, once the antenna array design was completed, it was not possible to change the antenna characteristics or footprint in order to efficiently illuminate a region having a different shape or size. By providing the ability to remotely modify the shape of a beam pattern or footprint, the operating efficiency of the satellite can be maintained.

Although several embodiments have been described, this was for the purpose of illustrating, but not limiting, the invention. Various modifications, which will come readily to the mind of one skilled in the art, are within the scope of the invention as defined in the appended claims.

Claims

1. A passive antenna reflector for use in a predetermined bandwidth, said antenna being configured to reflect a received E-M wave, said antenna comprising: a ground plane enclosed in a ground plane frame; a phasing plane enclosed in a phasing plane frame; spacer means for maintaining said planes in connected, spaced relationship; said phasing plane including a plurality of reactive elements, said reactive elements being sensitive to different frequencies in said bandwidth, said reactive elements causing a received E-M wave to be reflected from said antenna reflector at a predetermined angle to the angle of reception thereof.

2. A reflector as defined in claim 1, wherein said passive antenna reflector is flat and collapsible and said phasing plane including a plurality of reactive elements of different size and of different spacing there between, said reactive elements being sensitive to different frequencies in said bandwidth, said elements causing a received E-M wave to be reflected from said flat reflector in a radiation pattern which simulates the response of a three-dimensional reflector.

3. A reflector as defined in claim 2, wherein said reactive elements are cross-shaped dipoles.

4. A reflector as defined in claim 2, wherein said reactive elements are spiral-shaped dipoles.

5. A reflector as defined in claim 1 wherein said ground plane is parabolic in shape and said phasing plane is an array of dipoles suspended over the parabolic ground plane to form a parabolic reflector.

6. A reflector as defined in claim 5, wherein said reactive elements are cross-shaped dipoles.

7. A reflector as defined in claim 5, wherein said reactive elements are spiral-shaped dipoles.

8. An antenna reflector, for use in a predetermined bandwidth, said antenna being configured to reflect a received E-M wave, said antenna comprising: a ground plane; a phasing plane; spacer means for maintaining said ground and phasing planes in connected, spaced relationship; a plurality of reactive elements on said phase plane for causing a received E-M wave to be reflected from said reflector at a predetermined angle to the angle of reception thereof; and means for varying the reactance of said reactive elements, so as to control said predetermined angle and the shape of a reflected E-M wave.

9. An antenna reflector as defined in claim 8, wherein said means for varying the reactance of said reactive elements comprises diodes connected to the reactive elements at points located along each element, such that the electrical length of an element can be adjusted by biasing selected diodes.

10. An antenna reflector as defined in claim 9, wherein said antenna reflector is flat and collapsible, and said reactive elements being sensitive to different frequencies in said bandwidth according to how their electrical length is adjusted, said elements causing a received E-M wave to be reflected from said flat reflector in a radiation pattern which simulates the response of a three-dimensional reflector.

11. A reflector as defined in claim 10, wherein said reactive elements are cross-shaped dipoles.

12. A reflector as defined in claim 10, wherein said reactive elements are spiral-shaped dipoles.

13. A reflector as defined in claim 8, wherein said ground plane is parabolic in shape and said phasing plane is an array of reactive elements suspended over the parabolic ground plane to form a parabolic reflector.

14. A reflector as defined in claim 13, wherein said reactive elements are cross-shaped dipoles.

15. A reflector as defined in claim 13, wherein said reactive elements are spiral-shaped dipoles.

16. A parabolic shaped antenna reflector, for use in a predetermined bandwidth, said antenna being configured to reflect a received E-M wave, said antenna comprising: a parabolic shaped ground plane; an array of dipoles forming a phasing plane suspended over said parabolic ground plane; spacer means for maintaining said planes in connected, spaced relationship; said dipoles causing a received E-M wave to be reflected from said reflector at a predetermined angle to the angle of reception thereof.

17. An antenna as defined in claim 16, further comprising means for varying the reactance of said dipoles, so as to control said predetermined angle and the shape of a reflected E-M wave.

18. An antenna reflector as defined in claim 17, wherein said means for varying the reactance of said dipoles comprises diodes connected to the dipoles at points located along each dipole, such that the electrical length of an element can be adjusted by biasing selected diodes.

19. A reflector as defined in claim 18, wherein said dipoles are cross-shaped.

20. A reflector as defined in claim 18, wherein said dipoles are spiral-shaped.