U.S. patent application number 12/155051 was filed with the patent office on 2009-03-26 for real-time, cross-correlating millimetre-wave imaging system.
Invention is credited to John William Archer, Gooffron James, Oya Sevimli.
Application Number | 20090079619 12/155051 |
Document ID | / |
Family ID | 27809298 |
Filed Date | 2009-03-26 |
United States Patent
Application |
20090079619 |
Kind Code |
A1 |
Archer; John William ; et
al. |
March 26, 2009 |
Real-time, cross-correlating millimetre-wave imaging system
Abstract
A method and apparatus are disclosed for forming an image from
millimetre waves. A field of view scanned using two geometrically
orthogonal, intersecting copolarized fan beams (110, 120) to
receive millimetre wave radiation. The received millimetre wave
radiation from said fan beams are then cross-correlated (250, 650).
Also, a method and antenna (400, 610) for receiving millimetre wave
radiation are disclosed. The antenna includes first and second fan
beam antennas (410, 420) for receiving millimetre wave radiation
and a filter (430, 440) for rotating polarization of incident
millimetre wave radiation through 90 degrees received by the second
fan beam antenna (410). The respective first and second beams (110,
120) intersect and are co-polarized and geometrically orthogonal to
each other. Still further, a millimetre wave imaging system (600)
and method are also disclosed, which utilise an antenna (610) for
receiving millimetre wave radiation, process the received
millimetre wave radiation from the antenna (610), and build up the
image (682) using a filtered, cross-correlated signal.
Inventors: |
Archer; John William;
(Pennant Hills, AU) ; Sevimli; Oya; (North Epping,
AU) ; James; Gooffron; (Epping, AU) |
Correspondence
Address: |
KENYON & KENYON LLP
1500 K STREET N.W., SUITE 700
WASHINGTON
DC
20005
US
|
Family ID: |
27809298 |
Appl. No.: |
12/155051 |
Filed: |
May 29, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10518758 |
Jul 22, 2005 |
7385552 |
|
|
PCT/AU03/00897 |
Jul 11, 2003 |
|
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12155051 |
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Current U.S.
Class: |
342/179 |
Current CPC
Class: |
H01Q 19/195 20130101;
H01Q 3/08 20130101; G01S 13/89 20130101; H01Q 15/248 20130101; H01Q
19/138 20130101; H01Q 3/18 20130101; G01S 13/887 20130101 |
Class at
Publication: |
342/179 |
International
Class: |
G01S 13/00 20060101
G01S013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2002 |
AU |
2002950196 |
Claims
1. A method of forming an image from millimetre waves, said method
including the steps of: scanning a field of view using two
geometrically orthogonal, intersecting co-polarized fan beams to
receive millimetre wave radiation; and cross-correlating components
of said received millimetre wave radiation from said fan beams.
2. The method according to claim 1, wherein polarizations of the
electric fields of said two fan beams are arranged to be
substantially parallel in alignment.
3. The method according to claim 2, further including the step of
polarization rotation filtering of said millimetre wave radiation
received in one of said fan beams.
4. The method according to claim 1, wherein said scanning step is
performed in azimuth and elevation defining a scan range, and an
intersection region of said two fan beams is able to cover any
point in said scan range.
5. The method according to claim 4, wherein said scan range
determines said field of view and a beam width of each fan-beam in
a narrow direction determines an angular resolution of said
image.
6. The method according to claim 1, further including the step of
measuring said cross-correlated output at each point in said field
of view to produce a map of brightness.
7. The method according to claim 6, further including the step of
controlling said two geometrically orthogonal, intersecting fan
beams to generate said cross-correlated output at each fan beam
intersection point in said field of view.
8. The method according to claim 1, wherein said scanning step is
implemented using a dual fan-beam antenna.
9. The method according to claim 8, wherein said dual fan-beam
antenna has two modified pill-box antennas and a polarization
rotator to change the direction of incident polarization for one of
said modified pill-box antennas.
10. An apparatus for forming an image from millimetre waves, said
apparatus including: an antenna for scanning a field of view using
two geometrically orthogonal, intersecting co-polarized fan beams
to receive millimetre wave radiation; and a receiver for
cross-correlating components of said received millimetre wave
radiation from said fan beams.
11. The apparatus according to claim 10, wherein polarizations of
the electric fields of said two fan beams is arranged to be
substantially parallel in alignment.
12. The apparatus according to claim 11, further including a
polarization rotation filter for said millimetre wave radiation
received in one of said fan beams.
13. The apparatus according to claim 10, wherein scanning by said
antenna is performed in azimuth and elevation defining a scan
range, and an intersection region of said two fan beams is able to
cover any point in said scan range.
14. The apparatus according to claim 13, wherein said scan range
determines said field of view and a beam width of each fan-beam in
a narrow direction determines an angular resolution of said
image.
15. The apparatus according to claim 10, further including a
processor for measuring said cross-correlated output at each point
in said field of view to produce a map of brightness.
16. The apparatus according to claim 15, further including a
controller for controlling said two geometrically orthogonal,
intersecting fan beams to generate said cross-correlated output at
each fan beam intersection point in said field of view.
17. The apparatus according to claim 10, wherein said antenna is a
dual fan-beam antenna.
18. The apparatus according to claim 17, wherein said dual fan-beam
antenna has two modified pill-box antennas and a polarization
rotator to change the direction of incident polarization for one of
said modified pill-box antennas.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to millimetre
imaging systems and in particular to a realtime millimetre imaging
system for detecting millimetre wave radiation and generating a
corresponding image.
BACKGROUND
[0002] Millimetre-wave imaging systems produce a picture of a scene
by detecting thermally generated radiation in the 30-300 GHz range,
which is emitted or reflected by objects in the field of view of
the instrument. Such systems offer advantages over equivalent
instruments detecting infrared and visible light, because the
millimetre-wave radiation can penetrate low visibility and
obscuring conditions (e.g., caused by clothing, walls, clouds, fog,
haze, rain, dust, smoke, sandstorms) without the high level of
attenuation that occurs at the other noted wavelengths. This is
particularly the case in specific "windows" for atmospheric
transmission of radio waves that occur between 90 and 110 GHz and
between 210 and 250 GHz.
[0003] Millimetre-wave imaging systems may be used in a range of
important applications such as: aids to aircraft landing; collision
warning in air, land and sea transport; detection and tracking of
ground based vehicular traffic; covert surveillance for intruders,
contraband and weapons. In such applications, the availability of
real-time, "movie-camera" like imaging is highly desirable.
However, for such systems to find wide acceptance in the commercial
market-place, the sensing instrumentation must be light in weight,
small in size, and affordable in cost.
[0004] A range of millimetre-wave imaging systems have been
reported, but fail to meet the size, weight, and cost requirements
for wide commercial acceptance of the technology, while at the same
time offering real-time moving images. Such systems use two
distinct technologies: mechanical scanning of the beam of a single
antenna, and two-dimensional arrays.
[0005] Mechanical scanning of the beam of a single antenna
connected to a single receiving system is performed in a raster
pattern over a scene to detect the emitted radiation and produce a
map or image of the brightness. The angular resolution of the
resultant image is determined by the width of the antenna beam,
whereas the scan angle determines the field of view. Rapid
real-time imaging is difficult or inadequate, because physically
large and cumbersome antenna elements (required to achieve high
angular resolution) must be moved quickly at high rates.
[0006] Two-dimensional arrays of electrically-small antennas and
integrated receivers sample the magnitude of the received
millimetre-wave signal at the focal plane of an antenna system.
This information is then used to produce a snap-shot of the
brightness in the field of view of the instrument. In any given
plane, the angular resolution of the resultant image is determined
by the number of elements across the array and the outer dimensions
of the array. In contrast, the field of view is determined by the
beam-width of the individual antenna-array elements. Rapid
real-time imaging can be achieved with these systems. However, this
occurs at the expense of large numbers (1000's) of millimetre-wave
receiving sub-systems and complex electronic phase shifting and
amplitude weighting networks. Because of the large number of
receivers required, heterodyne systems are avoided (in view of the
local oscillator distribution problems) in favour of direct
detection systems, with the attendant problems of gain stability
and poorer sensitivity. Coherent local oscillator distribution to
such a large number of millimetre-wave heterodyne receivers
presents significant difficulties.
[0007] Thus, a need clearly exists for an improved real-time
millimetre-wave imaging system capable of producing real-time,
movie-like imaging, in which the system is more compact, less
complex, and less expensive to produce.
SUMMARY
[0008] In accordance with a first aspect of the invention, an image
is formed from millimetre waves. To do so, a field of view is
scanned using two geometrically orthogonal, intersecting
co-polarized fan beams to receive millimetre-wave radiation. The
components of received millimetre-wave radiation from the two fan
beams are cross-correlated. The polarizations of the electric
fields of the two fan beams are arranged to be substantially
parallel in alignment. This may be achieved by polarization
rotation filtering of the millimetre-wave radiation received in one
of the fan beams. The two fan beams may be scanned in azimuth and
elevation defining a scan range. The intersection region of the two
fan beams is able to cover any point in the scan range. The scan
range determines the field of view and a beam width of each fan
beam in the narrow direction determines an angular resolution of
the image. The cross-correlated output is measured at each point in
the field of view to produce a map of the brightness. The position
of the two geometrically orthogonal, intersecting fan beams may be
controlled to generate the cross-correlated output at each fan beam
intersection point in the field of view. Preferably, the scanning
is implemented using a dual fan-beam antenna. The dual fan-beam
antenna may have two modified pill-box antennas and a polarization
rotator to change the direction of the incident polarization for
one of the modified pill-box antennas. An image may be formed from
millimetre waves of a different polarization by having a
polarization rotator to change the direction of the incident
polarization for a different modified pill-box antenna, only one
polarization rotator being used at any time.
[0009] In accordance with a second aspect of the invention,
millimetre-wave radiation is received. A field of view is scanned
using a fan beam to receive millimetre-wave radiation. Polarization
of incident millimetre-wave radiation is rotated through 90
degrees, and the field of view is scanned using another fan beam to
receive the polarization-rotated millimetre-wave radiation. The fan
beams intersect and are geometrically orthogonal to each other, yet
the radiation is co-polarized. The fan beams are provided by
respective fan-beam antennas. Each such antenna may include a
modified pill-box antenna. Preferably, the modified pill-box
antenna includes: a metal housing with an elongated aperture in at
least one side of the housing, a curved primary reflector surface
located within the housing and opposite the aperture, a feed horn
within the housing, and one or more sub-reflectors for coupling the
feed horn to the primary reflector surface. At least one of the
sub-reflectors is designed to rotate, providing one-dimensional
beam scanning in the narrow direction of the fan beam. The
polarization rotation for a fan beam may be implemented using a
polarization rotating transreflector. Preferably, the
transreflector includes: a planar metallic reflector, and a grid of
closely spaced wires. The wires are preferably spaced
n.times..lamda./4 from the planar metallic reflector, where n is an
odd integer and .lamda. is a wavelength of the millimetre-wave
radiation. The polarization rotating transreflector may be
positioned at a 45 degree angle relative to the aperture of the
second fan-beam antenna and at a substantially 45 degree angle
relative to the direction of incident millimetre-wave radiation.
The polarization rotation for a fan beam may be switched by
exchanging a polarization rotating transreflector and a planar
metallic reflector, both aligned in the same way. An exchange may
be effected by turning a polarization rotating transreflector by
180 degrees to use its back surface as a planar metallic reflector.
An exchange may be effected by making the wires of a polarization
rotating transreflector out of a material that has a switchable
conductivity.
[0010] In accordance with a third aspect of the invention,
millimetre wave radiation is received for generating an image. To
do so, millimetre wave radiation is received in accordance with
first and second fan beams. The first and second fan beams are
geometrically orthogonal to each other and intersecting. The
millimetre wave radiation received in accordance with the second
fan beam is co-polarized with the millimetre wave radiation
received in accordance with the first fan beam. Components of the
millimetre wave radiation received in accordance with the first and
second beams are downconverted to generate respective intermediate
frequency (IF) signals. The IF signals are cross-correlated. The
resulting cross-correlated signal is filtered to provide a value
proportional to brightness at each point in the scene. The received
millimetre wave radiation may be amplified in accordance with the
first and second beams prior to the step of downconverting.
[0011] In accordance with a fourth aspect of the invention,
millimetre-wave imaging is disclosed. To do so, millimetre-wave
radiation is received. The receiving includes: receiving
millimetre-wave radiation by scanning a field of view using a fan
beam, rotating the polarization of incident millimetre-wave
radiation through 90 degrees, and receiving the
polarization-rotated millimetre-wave radiation by scanning a field
of view using another fan beam. The fan beams intersect and are
geometrically orthogonal to each other. The received
millimetre-wave radiation is processed. The processing step
includes: receiving components of millimetre-wave radiation from
the antenna received in accordance with the fan beams,
downconverting respective components of the received millimetre
wave radiation received to generate respective intermediate
frequency (IF) signals, cross-correlating the IF signals; and
filtering the resulting cross-correlated signal. The filtered,
cross-correlated signal is proportional to the brightness at each
point in the field of view as the antenna beams are scanned. In
this way, an image of the scene may be built up. The scanning of
each fan beam may be independently controlled as required so that
the image can be generated from the filtered, cross-correlated
output signal which provides a value proportional to the brightness
of the scene at each point in said field of view.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A small number of embodiments are described hereinafter with
reference to the drawings, in which:
[0013] FIG. 1 is a radiation pattern of two crossed fan beam
antennas in accordance with the embodiments of the invention;
[0014] FIG. 2 is a simplified block diagram of a real-time
millimetre-wave imaging system in accordance with an embodiment of
the invention;
[0015] FIG. 3 is a perspective view of an example of a pill-box
antenna for implementing a scanned-beam imaging system in
accordance with another embodiment of the invention;
[0016] FIG. 4 is a perspective view of a combination of two
pill-box antennas and a metallic reflector for producing a
dual-scanning beam antenna with co-polarized far-field response in
accordance with a further embodiment of the invention;
[0017] FIG. 5 is a perspective view of a combination of two
pill-box antennas and two polarization rotating transreflectors
that may be exchanged for planar metallic reflectors, for producing
a dual-scanning beam antenna with co-polarized far-field response
of either of two polarizations, in accordance with a further
embodiment of the invention; and
[0018] FIG. 6 is a block diagram illustrating a real-time
cross-correlating millimetre-wave imaging system in accordance with
a further embodiment of the invention, incorporating the dual
fan-beam antenna of FIG. 4 or FIG. 5 in a modified millimetre-wave
imaging system of FIG. 2.
DETAILED DESCRIPTION
[0019] A method and an apparatus for forming an image from
millimetre waves, a method and an antenna for receiving millimetre
wave radiation, a method and an apparatus for receiving millimetre
wave radiation for generating an image, and a method and a system
for millimetre wave imaging are disclosed. In the following
description, numerous specific details are set forth. In the other
instances, details well known to those skilled in the art may not
be set out so as not to obscure the invention. It will be apparent
to those skilled in the art in the view of this disclosure that
modifications, substitutions and/or changes may be made without
departing from the scope and spirit of the invention.
[0020] The embodiments of the invention involve improved imaging
methods, antennas, and systems that enable the realization of a
simple, low-cost instrument, capable of real-time imaging of moving
targets. In broad terms, the embodiments produce a map or image of
the millimetre-wave brightness in the field of view of the
instrument by cross-correlating the signal received from two
orthogonal, intersecting fan-beams.
Fan-Beam Antennas Generally
[0021] An antenna with a fan-beam radiation pattern detects
radiation from a region in the field of view that is of narrow
angular extent in one direction only, while possessing a broad
pattern in the orthogonal plane. Typically, a fan-beam can be
generated by an antenna, or array of antennas, which is essentially
one-dimensional (e.g., a long narrow slot, a linear array of slots,
or a linear array of patch antennas). The width of the beam in the
narrow direction is inversely proportional to the electrical length
of the aperture or array. In contrast, the beam-width in the broad
direction is inversely proportional to the width of the aperture or
an individual element of the array. The angular position of the
fan-beam in the narrow direction may be scanned across the field of
view by producing a varying linear gradient in the phase of the
electrical excitation across the aperture or across the elements of
the array.
[0022] In accordance with embodiments of the invention, two such
fan beams are arranged so that the beams intersect at right angles
in the field of view of the instrument.
[0023] FIG. 1 is a plot illustrating the radiation pattern 100 of
two crossed fan beam antennas. The pattern 100 includes an E-plane,
fan-beam antenna pattern 110 and an H-plane, fan-beam antenna
pattern 120, and a pencil beam pattern 130. The polarization of the
electric field in each beam is arranged to be parallel in
alignment. As the fan-beams 110, 120 are scanned in azimuth and
elevation, the intersection region 130 can be made to cover any
point in the scan range. Thus, the scan range determines the field
of view of the instrument and the beam-width of the fan-beam in the
narrow direction determines the angular resolution of the image.
The millimetre-wave brightness at any point in the image is
proportional to the cross-correlation between the signals received
by the two antenna systems.
Imaging Receiver System
[0024] A significant component of the imaging system is the
receiver, which takes the output from the antennas, amplifies the
signals, and then down-converts the amplified signals to a
convenient intermediate frequency at which the cross-correlation
can tale place. There are a number of possible implementations for
such receiving systems, depending upon the design of the fan-beam
antenna.
[0025] An imaging receiver system 200 in accordance with an
embodiment of the invention shown in FIG. 2 uses only two
receivers, one connected to an antenna 202 scanning in the vertical
direction and the other to an antenna 204 scanning in the
horizontal plane, to sample the whole image. The antenna 202 is an
E-plane antenna, and the antenna 204 is an H-plane antenna. The
E-plane antenna is coupled to one or more radio frequency (RF) low
noise amplifiers (LNAs) 212a, 212b. The output of the one or more
low noise amplifiers 212b is coupled to a respective block down
converter 232. Similarly, the H-plane antenna 204 is coupled to one
or more LNAs 214a, 214b. The output of the LNA 214b is coupled to a
further block down converter 234. A local oscillator 220 provides
an input to both block down converters 232, 234.
[0026] The respective block down converters 232, 234 produce
respective intermediate frequency (IF) signals that are both
provided to a correlator 240. The output of the correlator 240 is
provided to a low pass filter 250, which produces the output signal
260. A map of the millimetre-wave brightness at each point in the
field of view is produced by scanning the antenna beams over the
field and at each field point measuring the cross correlation
between the receiver outputs using a broadband analogue multiplier
240.
[0027] A polarization rotating filter (not shown) may be placed in
front of one of the antenna apertures so that both fan beams
operate in the same polarization.
Antenna for Imaging System
[0028] In accordance with an embodiment of the invention, a simple,
inexpensive implementation uses a multiple reflector "pill-box"
style antenna 300 shown in FIG. 3. In this simplified example, a
shaped primary reflector 334 is coupled to a single feed-horn 330,
332 via a rotating sub-reflector 320, which provides beam scanning
as the sub-reflector 320 spins. More than one sub-reflector may be
practiced, with at least one sub-reflector rotating to provide beam
scanning. With careful mechanical and electrical design, in which
the rotating sub-reflector 320 rotates about its center of mass,
high speed scanning can be achieved. Preferably, the sub-reflector
320 is disc-like in form. A significant advantage of this system is
that only a single heterodyne receiver per beam is needed. This is
advantageous from the point of view of system simplicity and cost
and also because a simple local oscillator distribution system is
possible without the need for complex array phasing.
[0029] In a conventional "pill-box" antenna, a parabolic cylinder
is used as the reflector. The "pill-box" is formed by two parallel
planes which cut through the parabolic cylinder perpendicular to
the cylinder elements. Typically, the focal line of the cylinder is
positioned in the center of the aperture formed by the open ends of
the parallel plates. When a feed horn is placed at the focal line,
the feed horn blocks a significant portion of the aperture,
resulting in large sidelobes in the far-field pattern of the
antenna as well as standing waves within the "pill-box" itself.
[0030] Much improved performance can be obtained when an offset
feeding arrangement is used, so that only one side of the
"pill-box" is illuminated. The arc of the parabola does not include
its vertex, and the feed horn points to illuminate this arc. Even
though the illumination is asymmetric, good sidelobe performance is
obtained. Alternatively, the "pill-box" antenna may be symmetrical
about the axis of the parabola, but arranged as a folded lens to
avoid blockage. Such an antenna, however, is more difficult to
manufacture than an unfolded design.
[0031] The millimetre-wave fan-beam antenna 300 shown in FIG. 3
includes a metal housing 310 with a radiating aperture 312 formed
in one side of the metal housing. The length of the radiating
aperture 312 is approximately 200 wavelengths (.lamda.) and the
width of the aperture 312 is approximately one wave length
(1.lamda.). These measurements are preferred and other dimensions
may be practiced without departing from the scope and spirit of the
invention. The direction of the electric field at the aperture is
indicated by an arrow 314. Located within the metal housing 310 is
the primary reflector surface 334 coupled to the tapered wave guide
feed-horn 330 with a wave guide input/output 332 oppositely
positioned relative to the radiating aperture 312 within the
housing 310. At the bottom of the tapered wave guide feed-horn 330
within the metal housing 310 is the rotating sub-reflector 320 for
one dimensional beam scanning.
[0032] The antenna 300 uses one or more sub-reflectors 320 to
couple the feed horn 330, 332 in an offset "pill-box" structure.
The primary reflector 334 is shaped away from the traditional
parabola to provide enhanced off-axis scanning angle with good
sidelobe performance over the widest possible range of scan. The
primary reflector 334 is coupled to the single feed-horn 330 via
one or more sub-reflectors 320, which are also designed to have a
profile that enhances the scan performance of the complete antenna
assembly 300. One of these secondary mirrors 320 is arranged so
that this sub-reflector 320 rotates, providing main beam scanning
as the sub-reflector 320 spins. With careful mechanical and
electrical design, in which the rotating sub-reflector 320 rotates
about its center of mass, high speed scanning can be achieved.
[0033] For the imaging system, a pair of independently-scanned,
orthogonally-oriented fan beams are required, with the sense of
electric polarization aligned in each beam. Two "pill-box" antennas
410, 420 of the type shown in FIG. 3 are used, configured 400 as
shown in FIG. 4. The antenna 410 has an aperture 414 oriented
lengthwise in a horizontal sense, while the other antenna 420 has
an aperture 424 lengthwise in a vertical sense, as depicted in FIG.
4. The direction 412, 422 of the electric field in the respective
apertures 414, 424 are shown. Thus, the aperture 424 couples
directly to the observed scene, while the other aperture 414 is
arranged at a right angle so that the aperture 414 is coupled via a
passive reflecting screen 430, 440 and is oriented so that the
narrow dimension of the far-field pattern of the aperture 414 is at
right angles to the pattern of the other antenna 420.
[0034] The passive reflecting screen 430, 440 is generally
configured at an angle of 45.degree. relative the surface of the
fan-beam antenna 410 having the aperture 414. The passive
reflecting screen preferably has a planar metallic reflector 430
spaced apart by a multiple of a quarter wavelength (n .lamda./4)
from a closely spaced, fine wire grid 440. The grid 440 is located
between the reflector 430 and the antenna 410. The wires of the
grid 440 are aligned at 45.degree. to the direction of incident
field polarization. This arrangement 400 results in orthogonal
polarization in the far-field, if a standard plane reflector 430 is
used.
[0035] Another way to achieve a co-polarized far-field response may
be to modify the feed for the "pill-box" antenna 410, 420, so that
the E-field vector is rotated through 90 degrees and aligned
parallel to the long direction of the aperture. For this
configuration, small variations in the surface quality and spacing
of the metallic walls may cause significant degradation in antenna
performance. However, for this arrangement, the polarization
rotating filter 430, 440 is no longer required to be included. The
preferred way to achieve co-polarization is by the use of a
"transreflector" 430, 440. The transreflector 430, 440 consists of
the wire grid 440, with wires aligned at 45 degrees to the incident
electric field vector, backed by the planar metallic mirror 430
spaced away by an odd-multiple of a quarter wavelength at the
operating frequency. The wire spacing and wire diameter must both
be small compared to the operating wavelength. Over a limited
bandwidth determined by the spacing between the grid 440 and the
reflector 430 (the higher the number of quarter wavelengths, the
narrower the bandwidth), this arrangement results in a rotation of
the polarization of the incident wave through 90 degrees, without
significantly altering the far-field radiation pattern of the
antenna system.
[0036] Two "pill-box" antennas 510, 520 of the type shown in FIG. 3
configured 500 in an alternative manner are shown in FIG. 5.
Generally, FIG. 5 shows how two pill-box antennas can be placed
with their flat sides parallel and the apertures oriented 90
degrees apart. In front of both apertures is a polarization
rotating transreflector that can be exchanged with a planar
metallic reflector, such that only one aperture receives
polarization-rotated radiation at any time. This leads to a more
compact structure than FIG. 4 that is capable of forming an image
of either of two polarizations. The axes of the rotating
sub-reflectors are parallel, so a simple gearing mechanism can be
used to give the relative rotation rates needed for the
intersection of the fan beams to perform a raster scan.
[0037] The antenna 510 has an aperture 530 oriented lengthwise in a
horizontal sense, while the other antenna 520 has an aperture 540
oriented lengthwise in a vertical sense, as depicted in FIG. 5. In
front of both apertures is a polarization rotating transreflector
that can be exchanged with a planar metallic reflector, such that
only one aperture receives polarization-rotated radiation at any
time. In FIG. 5 the horizontal aperture 530 is coupled to the
observed scene via a transreflector 550, while the vertical
aperture 540 is coupled to the observed scene via a planar metallic
reflector 560. The transreflector 550 may be exchanged with a
planar metallic reflector, and the planar metallic reflector 560
may be exchanged with a transreflector, as indicated by the dotted
lines on the reflector 560. An exchange may be effected by turning
a polarization rotating transreflector by 180 degrees to use its
back surface as a planar metallic reflector. An exchange may be
effected by making the wires of a polarization rotating
transreflector out of a material that has a switchable
conductivity. The advantages of this configuration 500 over the
configuration 400 in FIG. 4 are that the configuration 500 occupies
a smaller overall volume and is capable of forming an image from
either of two polarizations. The axes of the rotating
sub-reflectors 320 are parallel in this configuration 500, so a
simple gearing mechanism (not shown) can be used to achieve
relative rotation rates that cause the intersection 130 of the fan
beams 110, 120 to perform a raster scan of the field of view.
[0038] FIG. 6 is a block diagram illustrating an implementation of
a real-time, cross-correlating, millimetre-wave imaging system 600
in accordance with a further embodiment of the invention. For
purposes of illustration only, the system is shown in FIG. 6 with a
tree 602 as the object of imaging in the field of view. A dual,
fan-beam antenna 610 is used to scan the object 602 and respective
horizontal and vertical scans 604, 606 generated by the antenna 610
are shown. The dual fan-beam antenna 610 is of the type 400 shown
in FIG. 4. Alternatively, the dual fan-beam antenna 610 may be of
the type 500 shown in FIG. 5. The dual fan-beam antenna 610
provides respective E-plane and H-plane outputs to an imaging
receiver system, similar to that shown in FIG. 2.
[0039] The E-plane output is provided to a low noise amplifier 612
and the H-plane output is provided to a different low noise
amplifier 614. In turn, the low noise amplifiers 612, 614, acting
as RF amplifiers, are coupled to respective mixers 620, 622.
Further, a local oscillator 630 is coupled to both of mixers 620
and 622. The respective outputs of mixers 620 and 622 are provided
as inputs to IF amplifiers 640, 642. The output of the IF
amplifiers 640, 642 are provided to a cross-correlator 652.
[0040] The output of the cross-correlator 652 is provided to a base
band filter 660. The base band filter 660 provides the output
signal for the system. The output of the base band filter 660 is
provided to an analogue to digital (A/D or ADC) converter 670. The
ADC 670 produces digital data from the output signal that is
provided as input to a computer 680. The computer 680 using
hardware and/or software can produce a computer image 682 using the
digital data from the ADC 670. In turn, using the digital data, the
computer 680 can provide scan control signals 690 (indicated by
dashed lines) to the dual fan-beam antenna 610. As shown in FIG. 6,
the scan control signals 690 are preferably provided to each of the
pill-box antennas.
[0041] The embodiments of the invention have various advantages
including one or more of:
[0042] Use of a "pill-box" antenna to implement a scanned-beam
imaging system;
[0043] A "pill-box" antenna in which the beam is scanned in one
dimension using a rotating sub-reflector;
[0044] Use of a wire-grid transreflector to achieve a
dual-scanning-beam system with co-polarized far-field response;
[0045] Use of two wire-grid transreflectors, exchangeable for
planar metallic reflectors, to achieve switchable polarisation of
the far-field response.
[0046] Use of a mechanically scanned beam so that only a single
heterodyne receiver per beam is needed.
[0047] Use of two intersecting fan beams so that each antenna is
required to scan only in one direction.
[0048] Thus, a method and an apparatus for forming an image from
millimetre waves, a method and an antenna for receiving millimetre
wave radiation, a method and an apparatus for receiving millimetre
wave radiation for generating an image, and a method and system for
millimetre wave imaging have been disclosed. In the light of this
disclosure, it will be apparent to those skilled in the art that
modifications, substitutions and/or changes may be made without
departing from the scope and spirit of the invention.
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