CA2041572C - Dual band frequency reuse antenna - Google Patents

Dual band frequency reuse antenna

Info

Publication number
CA2041572C
CA2041572C CA002041572A CA2041572A CA2041572C CA 2041572 C CA2041572 C CA 2041572C CA 002041572 A CA002041572 A CA 002041572A CA 2041572 A CA2041572 A CA 2041572A CA 2041572 C CA2041572 C CA 2041572C
Authority
CA
Canada
Prior art keywords
meanderline
layers
traces formed
dimensions
layer
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Fee Related
Application number
CA002041572A
Other languages
French (fr)
Other versions
CA2041572A1 (en
Inventor
Terry M. Smith
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Maxar Space LLC
Original Assignee
Space Systems Loral LLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Space Systems Loral LLC filed Critical Space Systems Loral LLC
Publication of CA2041572A1 publication Critical patent/CA2041572A1/en
Application granted granted Critical
Publication of CA2041572C publication Critical patent/CA2041572C/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

Links

Classifications

    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q21/00Antenna arrays or systems
    • H01Q21/24Combinations of antenna units polarised in different directions for transmitting or receiving circularly and elliptically polarised waves or waves linearly polarised in any direction
    • H01Q21/245Combinations of antenna units polarised in different directions for transmitting or receiving circularly and elliptically polarised waves or waves linearly polarised in any direction provided with means for varying the polarisation 
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q15/00Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
    • H01Q15/24Polarising devices; Polarisation filters 
    • H01Q15/242Polarisation converters
    • H01Q15/244Polarisation converters converting a linear polarised wave into a circular polarised wave

Landscapes

  • Waveguide Aerials (AREA)
  • Waveguide Switches, Polarizers, And Phase Shifters (AREA)
  • Aerials With Secondary Devices (AREA)
  • Control Of Motors That Do Not Use Commutators (AREA)

Abstract

A dual frequency band antenna (10) having frequency reuse capability. The antenna waveguide (12) includes a four port waveguide network which transmits and receives orthogonal, linearly polarized signals of each of two frequencies. A
pyramidal horn (14) is engaged to the mouth of the waveguide, and a meanderline polarizes (16) is engaged to the aperture (17) of the horn (14) to convert the signals from linear polarizations to circular polarizations.

Description

Specification ~ ~ l~ ~ ~~ '~~
"Dual Band Frequency Reuse Antenna"

BACKGROUND OF THE: INVENTION
6 Field of the Invention This invention relates to antennas having frequency reuse 8 capabilities, and more particularly to antennas having a four 9 port network or quadruplexer located in the antenna waveguide, a feed horn attached to the waveguide, and a polarizes disposed 11 at the aperture of the antenna for converting linearly polarized 12 signals to circularly polarized signals.

14 Descrinti~r ~f the Prior art It has become well known in the field of satellite 16 communications to utilize a single antenna to transmit and 17 receive signals in two frequency bands with two orthogonal, 18 linearly polarized signal components within each band.
19 Waveguides that incorporate such features are known as four-port networks and/or quadruplexers. U.S. Patent 4,630,059 issued to 21 Morz on December 16, 1986 teaches a four-port network suitable 22 for satellite communication. Two orthogonal ports of the Morz 23 waveguide are utilized to introduce orthogonal linearly 24 polarized signals in the four GHz band which are converted to circularly polarized signals in the throat of the waveguide for 1 transmission through the grooved conical horn. Two other 2 orthogonally disposed ports are arranged to receive linearly 3 polarized signals in the six GHz band.
4 Another prior art four port waveguide network antenna has been designed by COMSAT Laboratories. This device includes two 6 coaxial waveguides, the outer waveguide being used for the 7 transmission and reception of the four GHz band and the inner 8 coaxial waveguide being utilized for the six GHz band. A
9 tunable configuration of screws and baffles within the waveguides are utilized to convert the linearly polarized 11 signals into circularly polarized signals. The device utilizes 12 a grooved conical horn to transmit and receive signals.
13 Additional prior art antennas that are of interest include 14 those described in U.S. Patent-4,797,681 to Kaplan et al. on January 10, 1989; U.S. Patent 4,707,702 issued to Withers on 16 November 17, 1987; U.S. Patent 4,573,054 issued to Bouko et al.
17 on February 25, 1986: U.S. Patent 4,358,770 issued to Satoh et 18 al. on November 9, 1982: U.S. Patent 4,219,820 issued to Crail 19 on August 26, 1980 and U.S. Patent 3,898,667 issued to Raab on August 5, 1975.
21 The efficiency of a satellite antenna which transmits and 22 receives different information .utilizing orthogonal 23 polarizations of the same frequency band depends to a 24 significant measure upon the elimination of cross-polarization between the orthogonal polarized signals. It is known that a n ; . "
.~~;> ~;~ø~

1 circularly polarized signal can be reduced to a linearly 2 polarized signal utilizing a meanderline polarizes. Such 3 meanderline polarizers produce minimal cross-polarization and 4 therefore promote efficiency. U.S. Patent 3,754,271 issued to Epis on August 21, 1973 describes a meanderline polarizes having 6 a plurality of stacked substantially identical arrays of 7 laterally spaced square-wave shaped meanderlines. The device 8 is positioned at the aperture of a pyramidal horn for conversion 9 of circularly polarized waves into linearly polarized waves.

12 The present invention is a dual frequency band antenna (10) 13 having frequency reuse capability. The antenna waveguide (12) 14 includes a four port waveguide network which transmits arid receives orthogonal, linearly polarized signals of each of two 16 frequencies. A pyramidal horn (14) is engaged to the mouth of 17 the waveguide, and a meanderline polarizes (16) is engaged to 18 the aperture (17) of the horn (14)~ to convert the signals from 19 linear polarizations to circular polarizations. The meanderline polarizes (16) includes five separated layers of meanderlines, 21 wherein the first and fifth layers (50 and 58 respectively) 22 include identical meanderlines, the second and fourth (52 and 23 56 respectively) layers include identical meanderlines that 24 differ from those of the first and fifth layers, and the third layer (54) includes meanderlines that differ from the others in ~/ F % a j_ »~L. i.

1 the first, second, fourth and fifth layers. I t i s a n 2 advantage of the present invention that it provides a dual band 3 frequency reuse antenna having minimal cross-polarization.
4 It is another advantage of the present invention that it provides a dual band frequency reuse antenna which includes a 6 linear-to-circular polarization device that is disposed in the 7 aperture of the feed horn to reduce cross-polarization effects 8 that axe created within the waveguide and the horn of the 9 antenna.
to It is a further advantage of the present invention that it 11 provides a dual band frequency reuse antenna which utilizes an 12 improved meanderline polarizes to provide reduced cross-13 polarization.
14 It is yet another advantage of the. present invention that it provides a dual band frequency reuse antenna including a four 16 port waveguide network incorporated into a square waveguide, a 17 pyramidal horn and a meanderline polarizes to achieve increased 18 signal gain and reduced cross-polarization.
19 It is yet a further advantage of the present invention that it utilizes a polarizes fabrication technique that provides 21 dimensional stability over a broad thermal range, whereby the 22 antenna is usable in an earth orbital environment.
23 The foregoing and other features and advantages of the 24 present invention will be apparent from the following detailed description of the preferred embodiment which makes reference ~~ F~ :y ~ n'' 1 to the several figures of the drawing.

4 Fig. 1 is a perspective view of the present invention;
5 Fig. 2 is a side elevational view of the antenna of the 6 present invention and a reflector;
7 Fig. 3 is a perspective view of the waveguide of the 8 present invention;
9 Fig. 4 is a side elevational view of the waveguide of the present invention;
11 Fig. 5 is an end elevational view of the waveguide of the 12 present invention;
13 Fig. 6 is a perspective view of the meanderline polarizes 14 of the present invention having~cutaway.portions; and Fig. 7 is a top plan view of portions of the meanderline 16 traces of the meanderline polarizes.

i9 As depicted in Fig. 1, the antenna 10 includes three main components, a waveguide 12, a horn 14 and a meanderline 21 ~ polarizes 16 that is attached to the aperture 17 of the horn 14.
22 As depicted in Fig. 2, the antenna 10 is preferably designed to 23 be used with a parabolic reflector 18, such that the antenna 10 24 is fixedly mounted to a structure (not shown) and the antenna beam is scanned by movement of the reflector l8 relative to the ~ W

1 fixedly mounted antenna 10.
2 As depicted in Figs. 3, 4 and 5, the waveguide 12 includes 3 a four port waveguide network. Two of the ports 20 and 22 are 4 designed for the transmission of orthogonal, linearly polarized signals of a first frequency, which in the preferred embodiment 6 is a 4.035 to 4.200 GHz transmission band frequency. The other 7 two ports 24 and 26 are designed for the reception of 8 orthogonal, linearly polarized signals of a different frequency, 9 which in the preferred embodiment is a 6.260 to 6.425 GHz l0 receiving band frequency. The four independent, linearly 11 polarized signals (1 from each port) are coupled into the common 12 square waveguide 12, which in turn excites the pyramidal feed 13 horn 14 . At the aperture 17 of the horn 14 , the meanderline 14 polarizes 16 then converts the~linearly polarized signals to circular polarizations, such that two oppositely, circularly 16 polarized fields are radiated from the antenna 10 at the 17 transmission band frequency. The meanderline polarizes also 18 converts two oppositely, circularly polarized signals to two 19 orthogonal, linearly polarized signals at the receiving band frequency.
21 Each port 20, 22, 24 and 26 of the four port waveguide 22 network includes an attachment flange 32, 34 and 36 30, 23 respectively, disposed about outer end to which signal its 24 transmitting or receiving devices(nat shown)are coupled.
In the preferred embodiment depictedin Figs. 3, 4 and 5, the 7 ~ s~'4 ;.y ~. ';;.~
1 orthogonal ports 24 and 26 feed directly into the side and 2 throat respectively of the waveguide 12, whereas orthogonal 3 ports 20 and 22 are provided with additional waveguide 4 structures 40 and 42 respectively which lead to the body of the waveguide 12.
6 As is known to those skilled in the art, the dimensions of 7 the various waveguide openings and structures are of 8 significance in obtaining acceptable antenna performance. For 9 ease of comprehension and enablement purposes, various significant dimensions, in inches, are provided in Figs. 3, 4, 11 and 5. The waveguide structures 40 and 42 comprise a series of 12 rectangular corrugations formed perpendicularly to the central 13 axis of the waveguide structures 40 and 42. In the preferred 14 embodiment, support straps 46 are engaged across the outer surface of the corrugations to provide structural rigidity to 16 the waveguide structures 40 and 42. The corrugated waveguide 17 structures 40 and 42 are dimensionally configured to act as a 18 short circuit to the six GHz signals while allowing the four GHz 19 signals to pass therethrough. Thus, the linearly polarized six GHz receiving signal does not propagate into waveguide 21 structures 40 and 42, but rather continues through the body of 22 the waveguide 12 to the ports 24 and 26. Additionally, a 23 central section 48 of the waveguide 12 located behind ports 20 24 and 22 is dimensionally sized to prevent the propagation of the four GHz transmission signals backwards through the waveguide ,l !' ~ ~~ r 1 rv ~ ,n _~. °.~ ' J

1 12 to the six GHz ports 24 and 26.
2 In the preferred embodiment, the feed horn 14 is a 3 pyramidal horn having a flare angle of approximately 10 degrees 4 and a square aperture having a side measurement of approximately 6 inches; its aperture 17 is located approximately 3.5 inches 6 towards the reflector 18 from the focal point 50 of the 7 reflector 18.
8 As is seen in Fig. 1, in the preferred embodiment, the 9 meanderline polarizes is oriented relative to the square aperture 17 of the feed horn 14, such that the meanderlines run 11 diagonally across the aperture 17 of the feed horn 14. The 12 improved meanderline polarizes 16 serves to transform the 13 linearly polarized signals into circularly polarized signals at 14 the aperture 17 of the antenna horn 14.~ Thus, the signals that propagate within the horn 14 and waveguide 12 are entirely 16 orthogonal, linearly polarized signals, and no circularly 17 polarized signals propagate within the horn 14 or waveguide 12.
18 This configuration results in the transmission and reception 19 within the waveguide of orthogonal, linearly polarized signals with significantly reduced cross-polarization, whereby improved 21 signal gain and reduced noise is achieved.
22 In the preferred embodiment, as depicted in Fig. 6, the 23 meanderline polarizes 16 is a sandwich structure including five 24 thin layers 50, 52, 54, 56 and 58, each having a plurality of meanderline traces 60, 62, 64, 66 and 68, respectively, formed A n' 4~ ~~
r". .r ... .... '~.1 ~4 YJ

1 thereon. Four foam-like spacers 70, 72, 74 and 76 serve to 2 separate the five meanderline layers. The use of meanderline 3 polarizers that are generally configured as described 4 hereinabove is well known in the art, as particularly taught in U.S. Patent 3,754,271 issued to J. Epis on August 21, 19?3. A
6 significant difference between the polarizes 16 of the present 7 invention and the prior art polarizers resides in the 8 utilization of meanderline traces of differing dimensions in the 9 various layers 50, 52, 54, 56 and 58. Specifically, the meanderline traces in layers 50 and 58 are identical, the 11 meanderline traces in layers 52 and 56 are identical, although 12 differing in dimensions from the meanderline traces in layers 13 50 and 58. The meanderline traces in layer 54 are different in 14 dimension from those of any other layer.
Proper selection of the meanderline trace dimensions 16 provides the required dual band conversion to pure circular 17 polarization. In the preferred embodiment, the polarizes is a 18 9.0~~ square by 2.0~~ thick sandwich construction. The sandwich 19 consists of the four spacers 60, 62, 64 and 66 composed of Stanthyne 817 Foam, and the five layers 50, 52, 54, 56 and 58 21 are composed of etched 1/2 oz. copper clad 3 mill Kapton bonded 22 together with Hysol 9309 adhesive. Bonding is done so as not 23 to cover the traces. The polarizes is bonded to a fiberglass 24 frame 19 which is bolted to the aperture 17 of the horn 14. The traces are preferably formed on the Kapton layers utilizing ~~ n < ,~ ~ ., 1 printed circuit board techniques to provide close tolerances and 2 reliability to the device.
3 As is depicted in Fig. 7, the dimensions of the meanderline 4 traces in each layer can be expressed by four parameters that 5 are designated as: A, the periodicity of a meanderline trace;
6 H, the height of the, meanderline trace; W, the width of the 7 meanderline trace; and B, the distance between adjacent 8 meanderline traces. The following table provides the dimensions 9 for each of the layers of the meanderline polarizer 16.

l0 11 __ ' _ 12 Layers 50 & 58 Layers Layer 54 52 & 56 14 A 0.046 0.174 0.134 H 0.180 0.336 0 16 W 0.011 0.043 .

17 B 0.782 0.782 .

18 ' .

i9 It is advantageous that the present invention provides a reuse frequency capability.
That_is, that the same frequency 21 can be used for transmitting two signals, one of which is 22 circularly polarized in a first sense and the other of whi h i c s 23 circularly polarized in an opposite sense.
Additionally, the 24 utilization of four ports in the waveguide network permits the simultaneous utilization of two reuse frequency signals, 26 approximately GHz and approximately GHz The use f .
o a 27 meanderline polarizer at the aperture of the feed harn 28 provides improved performance as compared to prior art devices 29 which attempt to convert signals from circular polarization to 11 ~-~ ~. 1 .i .., :M f ;,, ,'~ _'.;
1 linear polarization within the waveguide. The improved 2 meanderline polarizer reduces cross-polarization and thus 3 contributes to the improved performance of the invention.
4 While the invention has been particularly shown and described with reference to certain preferred embodiments, it 6 will be understood by those skilled in the art that various 7 alterations and modifications in form and detail may be made 8 therein. Accordingly, it is intended that the following claims 9 cover all such alterations and modifications as may fall within the true spirit and scope of the invention.
11 What I claim is:

Claims (15)

1. A dual band frequency reuse antenna comprising:
a four port waveguide network, two of said ports being configured for transmitting orthogonal, linearly polarized signals in a first frequency band, and two of said ports being configured for receiving orthogonal, linearly polarized signals in a second frequency band;
a feed horn being engaged to said waveguide network and adapted to enhance the transmission and reception of signals from and to said waveguide network respectively;
a signal polarizing means being engaged to the aperture of said feed horn and adapted to convert between linearly polarized signals and circularly polarized signals.
2. A dual band frequency reuse antenna as described in claim 1 wherein said signal polarizing means comprises a meanderline polarizer.
3. A dual band frequency reuse antenna as described in claim 2 wherein said meanderline polarizer comprises a plurality of layers, each said layer including a plurality of meanderline traces being formed thereon.
4. A dual band frequency reuse antenna as described in claim 3 wherein said meanderline traces formed on at least two of said layers are formed with differing dimensions.
5. A dual band frequency reuse antenna as described in claim 4 wherein said meanderline traces formed on a first layer differ in dimensions from said meanderline traces formed on a second layer, and said meanderline traces formed on a third layer differ in dimensions from said meanderline traces formed on both said first layer and said second layer.
6. A dual band frequency reuse antenna as described in claim 3 wherein said meanderline polarizer includes five layers, each of said layers having a plurality of meanderline traces formed thereon;
said meanderline traces formed on said first and fifth layers being substantially identical in dimensions;
said meanderline traces formed on said second and fourth layers being substantially identical in dimensions, said meanderline traces formed on said second and fourth layers differing in dimensions from said meanderline traces formed on said first and fifth layers; and said meanderline traces formed on said third layer differing in dimensions from said meanderline traces formed on said first, second, fourth, and fifth layers.
7. A meanderline polarizer for converting orthogonal, linearly polarized signals to circularly polarized signals, comprising:
a plurality of thin layers, each said layer having a plurality of meanderline traces formed thereon;
a plurality of spacers being disposed such that one said spacer is disposed between each said layer;

said meanderline traces formed on at least one of said layers differing in dimensions from said meanderline traces formed on another of said layers.
8. A meanderline polarizer as described in claim 7, wherein said meanderline traces formed on a first layer differ in dimensions from said meanderline traces formed on a second layer, and the meanderline traces formed on a third layer differ in dimensions from said meanderline traces formed on both said first layer and said second layer.
9. A meanderline polarizer as described in claim 8, wherein said meanderline polarizer includes five layers, each of said layers have a plurality of meanderline traces formed thereon;
said meanderline traces formed on said first and fifth layers being substantially identical in dimensions;
said meanderline traces formed on said second and fourth layers being substantially identical in dimensions, and said meanderline traces formed on said second and fourth layers differing in dimensions from said meanderline traces formed on said first and fifth layers; and said meanderline traces formed on said third layer differing in dimensions from said meanderline traces formed on said first, second, fourth, and fifth layers.
10. A method for transmitting and receiving signals in a dual band frequency reuse antenna comprising:
transmitting and receiving orthogonal, linearly polarized signals utilizing a four port waveguide network;

enhancing said transmission and reception of said orthogonal, linearly polarized signals utilizing a feed horn;
converting between said orthogonal, linearly polarized signals and circularly polarized signals at the aperture of said feed horn, whereby the structural configuration of said feed horn and said four port waveguide network do not contact said circularly polarized signals, such that cross-polarization is minimized.
11. A method for transmitting and receiving signals in a dual band frequency reuse antenna as described in claim 10, wherein said converting is accomplished utilizing a meanderline polarizer.
12. A method for transmitting and receiving signals in a dual band frequency reuse antenna as described in claim 11, wherein said meanderline polarizer includes a plurality of layers, each said layer including a plurality of meanderline traces being formed thereon.
13. A method for transmitting and receiving signals in a dual band frequency reuse antenna as described in claim 12, wherein said meanderline traces formed on at least two of said layers are formed with differing dimensions.
14. A method for transmitting and receiving signals in a dual band frequency reuse antenna as described in claim 13, wherein said meanderline traces formed on a first layer differ in dimensions from said meanderline traces formed on a second layer, and said meanderline traces formed on a third layer differ in dimensions from said meanderline traces formed on both said first layer and said second layer.
15. A method for transmitting and receiving signals in a dual band frequency reuse antenna as described in claim 12, wherein said meanderline polarizer includes five layers, each of said layers having a plurality of meanderline traces formed thereon;
said meanderline traces formed on said first and fifth layers being substantially identical in dimensions;
said meanderline traces formed on said second and fourth layers being substantially identical in dimensions, and said meanderline traces formed on said second and fourth layers differing in dimensions from said meanderline traces formed on said first and fifth layers; and said meanderline traces formed on said third layer differing in dimensions from said meanderline traces formed on said first, second, fourth, and fifth layers.
CA002041572A 1990-07-26 1991-05-01 Dual band frequency reuse antenna Expired - Fee Related CA2041572C (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US55903490A 1990-07-26 1990-07-26
US07/559,034 1990-07-26

Publications (2)

Publication Number Publication Date
CA2041572A1 CA2041572A1 (en) 1992-01-27
CA2041572C true CA2041572C (en) 1999-11-09

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Family Applications (2)

Application Number Title Priority Date Filing Date
CA002041572A Expired - Fee Related CA2041572C (en) 1990-07-26 1991-05-01 Dual band frequency reuse antenna
CA2046975A Pending CA2046975A1 (en) 1990-07-26 1991-07-12 Dual band frequency reuse antenna

Family Applications After (1)

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CA2046975A Pending CA2046975A1 (en) 1990-07-26 1991-07-12 Dual band frequency reuse antenna

Country Status (4)

Country Link
EP (1) EP0468620B1 (en)
JP (1) JP2651962B2 (en)
CA (2) CA2041572C (en)
DE (1) DE69115783T2 (en)

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH08139502A (en) * 1994-11-14 1996-05-31 Nec Corp Circular polarized wave generator
KR20090095518A (en) * 2008-03-05 2009-09-09 (주)인텔리안테크놀로지스 Device and method of transceiving multi band signals using horn antenna and reflector antenna
CN107093802B (en) * 2017-03-20 2019-07-23 东南大学 The equally distributed high-gain lens antenna of bore face phase and amplitude
US10665931B2 (en) 2017-08-01 2020-05-26 Lockheed Martin Corporation Waveguide aperture design for geo satellites
CN114709622B (en) * 2022-03-31 2024-04-23 重庆邮电大学 Polarization unit based on super-surface structure, polarization converter and preparation method

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3754271A (en) * 1972-07-03 1973-08-21 Gte Sylvania Inc Broadband antenna polarizer
DE3023562C2 (en) * 1980-06-24 1982-10-28 Siemens AG, 1000 Berlin und 8000 München Device for polarization conversion of electromagnetic waves
FR2488055A1 (en) * 1980-07-31 1982-02-05 Thomson Csf ANTENNA TRANSDUCER FOR EMISSION-RECEPTION ANTENNA AND PRIMARY ANTENNA SOURCE EQUIPPED WITH SUCH TRANSDUCER
FR2529392B1 (en) * 1982-06-25 1985-06-28 Thomson Csf MULTIPLEXING DEVICE FOR GROUPING TWO FREQUENCY BANDS AND MULTIPLEXER COMPRISING SUCH A DEVICE
JPS60176302A (en) * 1984-02-22 1985-09-10 Mitsubishi Electric Corp Polarizer
JPH0611085B2 (en) * 1987-02-23 1994-02-09 三菱電機株式会社 Circularly polarized array antenna
JPH01126803A (en) * 1987-11-12 1989-05-18 Mitsubishi Electric Corp Horn antenna system

Also Published As

Publication number Publication date
EP0468620B1 (en) 1995-12-27
CA2041572A1 (en) 1992-01-27
DE69115783D1 (en) 1996-02-08
JPH05136624A (en) 1993-06-01
DE69115783T2 (en) 1996-07-25
EP0468620A3 (en) 1992-05-20
CA2046975A1 (en) 1992-01-27
JP2651962B2 (en) 1997-09-10
EP0468620A2 (en) 1992-01-29

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