U.S. patent application number 12/864398 was filed with the patent office on 2011-04-28 for absorber assembly for an anechoic chamber.
Invention is credited to John F. Aubin, Mark Winebrand.
Application Number | 20110095932 12/864398 |
Document ID | / |
Family ID | 43222991 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110095932 |
Kind Code |
A1 |
Winebrand; Mark ; et
al. |
April 28, 2011 |
Absorber Assembly for an Anechoic Chamber
Abstract
An electromagnetic absorber assembly (10) capable of minimizing
reflectivity caused by reflected and diffracted waves within a test
chamber (1) is presented. The absorber assembly (1) includes a
plurality of first wedges (11) and a plurality of second wedges
(11) disposed in a symmetrical arrangement so as to form a
continuous and smoothly changing v-shaped pattern along one or more
walls (5-7) of an anechoic test chamber (1). Each wedge (11) has a
triangular-shaped first end (26) and second end (27) formed by a
pair of side walls (16, 18, or 19) and a base wall (17). One second
end (27) of each first wedge (11) contacts and covers one first end
(26) of each second wedge (11) along a contact plane (15). First
and second wedges (11) are disposed at a first angle (46) and a
second angle (47), respectively, about the contact plane (15) in a
symmetrical arrangement. The assemblies described could be
installed on a flat or shaped absorber base (44) or wall (20) to
divert reflected and refracted fields away from a quiet zone (3).
Interplay between the shaped absorber base (44) or wall (20) and
intersecting wedges (11) facilitates minimization of clutter and
secondary scattering.
Inventors: |
Winebrand; Mark;
(Montgomeryville, PA) ; Aubin; John F.; (New
Britain, PA) |
Family ID: |
43222991 |
Appl. No.: |
12/864398 |
Filed: |
October 29, 2009 |
PCT Filed: |
October 29, 2009 |
PCT NO: |
PCT/US09/62473 |
371 Date: |
July 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61181880 |
May 28, 2009 |
|
|
|
Current U.S.
Class: |
342/4 |
Current CPC
Class: |
H05K 9/0001
20130101 |
Class at
Publication: |
342/4 |
International
Class: |
H01Q 17/00 20060101
H01Q017/00 |
Claims
1. An absorber assembly for use within an anechoic chamber
comprising: (a) a plurality of first wedges which are substantially
parallel; and (b) a plurality of second wedges which are
substantially parallel; wherein said first wedges and said second
wedges are disposed so as to form an intersecting pattern which is
substantially v-shaped, each of said first wedges and said second
wedges having a substantially triangular-shaped cross section with
a pair of side walls and a base wall bound between a first end and
a second end, said second end of each said first wedges contacts
and covers one said first end of each said second wedges along a
contact plane, said first wedges and said second wedges disposed at
an angle about said contact plane, said absorber assembly
accounting for specular reflections and diffracted waves so as to
better control primary and secondary scattered fields within said
anechoic chamber.
2. The absorber assembly of claim 1, wherein said first wedges and
said second wedges are disposed in a convex and/or concave
arrangement about said contact plane along a wall.
3. The absorber assembly of claim 1, wherein at least one of said
side walls is linear shaped or non-linear shaped.
4. The absorber assembly of claim 1, wherein said first wedges and
said second wedges are disposed in an asymmetrical arrangement
about said contact plane.
5. The absorber assembly of claim 1, wherein said contact plane is
biased to one side of said absorber assembly.
6. The absorber assembly of claim 1, wherein said first wedges and
said second wedges are arcuate shaped along the length of said
wedges.
7. The absorber assembly of claim 1, wherein said angle varies
along the length of said absorber assembly.
8. The absorber assembly of claim 1, wherein said contact plane is
skewed.
9. The absorber assembly of claim 1, wherein said anechoic chamber
collects data for electromagnetic compatibility, far-field antenna
pattern, near-field measurements, or radar cross section.
10. The absorber assembly of claim 1, wherein said anechoic chamber
is a tapered or compact range design or illuminated by a direct
source antenna.
11. The absorber assembly of claim 1, wherein said plurality of
first wedges and said plurality of second wedges are applied to
partially cover at least one interior surface of said anechoic
chamber.
12. The absorber assembly of claim 1, wherein said anechoic chamber
includes at least one end wall which is shaped and includes a
plurality of pyramid-shaped or wedge-shaped absorbers.
13. The absorber assembly of claim 12, wherein said anechoic
chamber collects data for electromagnetic compatibility, far-field
antenna pattern, near-field measurements, or radar cross
section.
14. The absorber assembly of claim 12, wherein said anechoic
chamber is a tapered or compact range design or illuminated by a
direct source antenna.
15. The absorber assembly of claim 1, wherein at least one wall of
said anechoic chamber is shaped and said plurality of first wedges
and said plurality of second wedges at least partially cover said
wall.
16. The absorber assembly of claim 15, wherein said anechoic
chamber collects data for electromagnetic compatibility, far-field
antenna pattern, near-field measurements, or radar cross
section.
17. The absorber assembly of claim 15, wherein said anechoic
chamber is a tapered or compact range design or illuminated by a
direct source antenna.
18. A method for minimizing scattering within an anechoic chamber
comprising the steps of: (a) reflecting an electromagnetic wave
away from a quiet zone, a test item, and/or a source after a first
impingement with an absorber assembly; (b) controlling the
diffraction of said electromagnetic wave away from said quiet zone,
said test item, and/or said source after said first impingement;
and (c) controlling the diffraction of said electromagnetic wave
away from said quiet zone, said test item, and/or said source after
a second impingement by said electromagnetic wave in step (b);
wherein said absorber assembly includes: (i) a plurality of first
wedges which are substantially parallel; and (ii) a plurality of
second wedges which are substantially parallel; wherein said first
wedges and said second wedges are disposed so as to form a pattern
which is substantially v-shaped, each of said first wedges and said
second wedges having a substantially triangular-shaped cross
section with a pair of side walls and a base wall bound between a
first end and a second end, said second end of each said first
wedges contacts and covers one said first end of each said second
wedges along a contact plane, said side walls intersecting to form
a ridge line, said ridge line being substantially v-shaped along a
pair of contacting said first wedge and said second wedge, said
ridge line controls the diffraction of said electromagnetic wave
after said first impingement and said second impingement.
19. The absorber assembly of claim 18, wherein said anechoic
chamber collects data for electromagnetic compatibility, far-field
antenna pattern, near-field measurements, or radar cross
section.
20. The absorber assembly of claim 18, wherein said anechoic
chamber is a tapered or compact range design or illuminated by a
direct source antenna.
Description
1. TECHNICAL FIELD
[0001] The invention generally relates to a device which minimizes
reflectivity caused by reflected and diffracted waves within a test
chamber. Specifically, the invention is a wave absorber assembly
comprising a plurality of first wedge-shaped absorbers and a
plurality of second wedge-shaped absorbers which are disposed so as
to form a continuous and smoothly changing v-shaped pattern along
the interior surface of planar or non-planar walls of an anechoic
chamber. The arrangement of v-shaped absorbers improves control
over reflectivity within a chamber so as to better control primary
and secondary scattering effects which in turn improves quiet zone
reflectivity and reduces quiet zone clutter.
2. BACKGROUND ART
[0002] A typical anechoic chamber for electromagnetic compatibility
(EMC), far-field (FF) antenna, or radar cross section (RCS)
measurements includes a metallic enclosure with internal surfaces
covered by an absorbing material. An anechoic chamber also contains
a direct illumination source antenna, a compact range reflector
system, and/or a feed, the latter employed within a tapered
chamber. A chamber could also include positioning equipment to
rotate an antenna under test so as to acquire pattern data, to
rotate a device under test for electromagnetic compatibility
measurements, or to acquire incident collimated wave signals during
RCS measurements. The primary purpose of an anechoic chamber is to
create a test zone surrounding the antenna or device under test
wherein the electric field is as uniform as possible and
reflections are minimized.
[0003] The performance of a test zone within an anechoic chamber
depends on the geometry, size of the test zone as determined by the
dimensions of the antenna or device under test, source antenna
performance, separation between source and antenna/device under
test, and absorption properties and grades of the absorbing
materials lining the interior walls of the chamber.
[0004] For a conventional antenna/test item system, the dimensions
of an anechoic chamber and choice of source antenna depend on the
size of the test zone. Field uniformity within the test zone is
primarily determined by the electric field amplitude and phase
taper of the wave radiated into the test zone by the source
antenna. Field uniformity is improved by increasing the separation
distance between the source and antenna/device under test so as to
decrease the amplitude and phase taper. Typically, the separation
distance is greater than 2*D.sup.2/.lamda., where D is the maximum
aperture dimension of the antenna or device under test and .lamda.
the operating wavelength. For large aperture antennas operating at
higher frequencies, this distance is very large. However, the level
of reflections from walls, floor, and ceiling increases with
chamber length, thus offsetting the benefit of separation distance
in many applications. As a result, test zone performance is a
trade-off between the dimensions of the test zone and chamber
geometry.
[0005] For a compact range system illumination of a test zone, a
nearly uniform electric field can be achieved within the test zone.
However, reflections from the chamber enclosure, even when covered
by an absorbing material, negatively contribute to the uniformity
of illumination within the test zone.
[0006] When RCS measurements are performed, echo signals are
produced by scattering from the device under test. These signals
travel along direct and indirect paths to the source antenna. The
echo signals via indirect paths are often reflected from the walls
of the chamber, and as such could contaminate measurements when not
removed. Neither absorbing material nor time range gating is
capable of completely removing or eliminating all unwanted echo
signals. As such, some unwanted signals ultimately arrive back at
the source antenna at approximately the same time as the return
from the device under test, thus establishing the chamber
background clutter level which limits the lowest RCS signal
measurable.
[0007] For a tapered chamber system, reflections from the tapered
walls covered by absorbing materials negatively contribute to the
test zone, as described herein.
[0008] The requirements for test zone illumination and for the
overall performance of anechoic chambers are becoming ever more
stringent in response to advancements in antenna, RCS, and EMC
technologies. Numerous approaches are described in the related arts
to enhance the performance of anechoic chambers.
[0009] The shaping of surfaces in an anechoic chamber for EMC and
antenna measurements are described by Smith in U.S. Pat. No.
3,100,870, Buckley in U.S. Pat. No. 3,113,271 and U.S. Pat. No.
3,120,641, Hemming in U.S. Pat. No. 4,507,660, Sanchez in U.S. Pat.
No. 5,631,661, Shibuya in U.S. Pat. No. 4,906,998, Kogo in U.S.
Pat. No. 4,931,798, and Berg et al. in U.S. Pat. No. 6,008,753.
Shaping facilitates a reflecting screen or wall that deflects
reflected energy from the chamber wall, typically comprising an
absorber element and metallic backing, away from the test zone,
thus reducing reflectivity levels within the test zone, minimizing
or avoiding specular zones on the side walls, and avoiding the need
for absorbing materials along some areas of a chamber. For example,
Shibuya describes the use of shaping to deflect unwanted radiation
towards the open walls outside of a chamber and away from the test
zone. In another example, Kogo describes the use of shaping so that
reflections from walls are focused at some place within the chamber
outside of the test zone, usually behind the test zone. Insertion
of an absorption "ball" at this location further reduces echoes
within the chamber. In another example, Berg et al. describes the
use of shaping so that the echo signals reflected by the device
under test, and subsequently by the side walls, are focused at a
point located behind the source antenna, thus minimizing the
clutter level in RCS measurements.
[0010] Other inventions in the related arts have focused on the
design of absorber elements. Most absorber designs assume a flat
metallic backing along the absorber and ignore detailed
consideration of the scattering effect. For example, Hemming et al.
in U.S. Pat. No. 4,496,950 describes shaped absorber components to
create normal or close to normal incident angles, thus improving
absorption. In another example, Burnside et al. in U.S. Pat. No.
6,437,748 suggests the use of an R-card or "Chebyshev" pattern.
This latter approach assumes the absorber is highly efficient and
performance does not depend on a metallic backing plate.
[0011] The inventions described above are broadly based on the
principles of Geometrical Optics approximation. As applied to
anechoic chamber design, these principles assume that the walls of
a chamber, even when covered by a highly efficient absorbing
material, reflect a maximum signal level at about the same angle as
the arrival angle of the incident wave. As such, the related arts
have focused on minimizing the specular reflection of waves within
an anechoic chamber into a test zone, while ignoring the causes and
influence of diffracted waves. Furthermore, the related arts make
no effort to control the diffracted waves in a primary scattered
field resulting from the first impingement of a wave onto an
absorber and the diffracted waves in a secondary scattered field
resulting from the further impingement of the primary scattered
field onto an absorber, test item, and/or source within a chamber.
Accordingly, all presently known applications of absorbers within a
test chamber do not adequately control the reflection and
scattering of electromagnetic waves.
[0012] Therefore, what is required is an absorber assembly which
accounts not only for specular reflections, but also for the
primary and secondary scattered fields caused by diffraction within
an anechoic chamber so as to better control the reflected fields
therein.
3. DISCLOSURE OF THE INVENTION
[0013] An object of the present invention is to provide an absorber
assembly which accounts not only for wall specular reflections, but
also for the primary and secondary scattered fields caused by
diffraction within an anechoic chamber so as to better control the
reflected fields and direct them away from the quiet zone during
EMC and antenna measurements and from the quiet zone and source
during RCS measurements.
[0014] In accordance with embodiments of the invention, the
apparatus includes a plurality of first wedges and a plurality of
second wedges capable of absorbing electromagnetic energy. The
first and second wedges are arranged so as to form a continuous
repeating and/or smoothly changed pattern which is substantially
v-shaped. Each first and second wedge has a triangular or
like-shaped cross section with a pair of side walls and a base wall
bound between a first end and a second end. One second end of each
first wedge contacts and covers one first end of each second wedge
along a contact plane. The first and second wedges are disposed at
an angle about the contact plane in a symmetrical or asymmetrical
arrangement, so that the resultant angle is greater than 0.degree.
but less than 180.degree.. Wedges may be directly or indirectly
attached to a flat or shaped wall.
[0015] Several advantages are offered by the invention. The
invention minimizes the level of test zone reflectivity, so as to
facilitate a larger test zone. The invention extends test zone
performance to include lower frequencies than otherwise achievable
with similarly dimensioned anechoic chambers of conventional
design. The invention minimizes chamber size and volume for a given
minimum level of reflectivity and test zone size, thereby lowering
chamber implementation costs. The invention is based upon
Geometrical Optics and Geometrical Theory of Diffraction, so as to
account not only for specular reflections, but also for the overall
combined diffraction of waves from a wedge. The invention
facilitates a plurality of solutions within a chamber so as to
optimize reflectivity and control the path of diffracted waves
within all scattered fields.
4. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Additional aspects, features, and advantages of the
invention will be understood and will become more readily apparent
when the invention is considered in the light of the following
description made in conjunction with the accompanying drawings,
wherein:
[0017] FIG. 1 is an end view of an anechoic chamber from the
perspective of the end or hack wall illustrating arrangement of
absorber assemblies with wedges arranged in a continuous and
smoothly changing v-shaped pattern along the top, bottom, and side
walls in accordance with an embodiment of the invention;
[0018] FIG. 2a is a frontal view of an absorber assembly along view
2-2 in FIG. 1 illustrating arrangement of wedges in a symmetrical
v-shaped pattern in accordance with an embodiment of the
invention;
[0019] FIG. 2b is a frontal view of an absorber assembly along view
2-2 in FIG. 1 illustrating arrangement of wedges in an asymmetrical
v-shaped pattern in accordance with an embodiment of the
invention;
[0020] FIG. 2c is a frontal view of an absorber assembly along view
2-2 in FIG. 1 illustrating arrangement of wedges in a v-shaped
pattern offset above a centerline through source, test item, and
quiet zone in accordance with an embodiment of the invention;
[0021] FIG. 2d is a frontal view of an absorber assembly along view
2-2 in FIG. 1 illustrating arrangement of wedges in a v-shaped
pattern about a skewed contact plane in accordance with an
embodiment of the invention;
[0022] FIG. 3 is a sectional view of an absorber assembly along
view 3-3 in FIG. 2a illustrating the triangular-shaped interface
between intersecting wedges along a contact plane in accordance
with an embodiment of the invention;
[0023] FIG. 4 is a side view of an absorber assembly with wedges
illustrating attachment to a wall with a surface having an inward
angular profile in accordance with an embodiment of the
invention;
[0024] FIG. 5 is a side view of an absorber assembly with wedges
illustrating attachment to a wall with a surface having an outward
angular profile in accordance with an embodiment of the
invention;
[0025] FIG. 6 is a frontal view of a four-fold absorber assembly
illustrating both inward and outward angular features along a
single wall in accordance with an embodiment of the invention;
[0026] FIG. 7a is a cross-sectional view of a wedge illustrating
linear side walls in accordance with an embodiment of the
invention;
[0027] FIG. 7b is a cross-sectional view of a wedge illustrating a
composite arrangement of layers through the thickness of the wedge
in accordance with an embodiment of the invention;
[0028] FIG. 8 is a cross-sectional view of a wedge illustrating
convex-shaped side walls in accordance with an embodiment of the
invention;
[0029] FIG. 9 is a cross-sectional view of a wedge illustrating
concave-shaped side walls in accordance with an embodiment of the
invention;
[0030] FIG. 10 a frontal view of an absorber assembly illustrating
a paired arrangement of one set of arcuate-shaped wedges (other
wedges are not shown) wherein curvature is disposed along the plane
of the base wall in accordance with an embodiment of the
invention;
[0031] FIG. 11 a frontal view of an absorber assembly illustrating
a paired arrangement of one set of arcuate-shaped wedges (other
wedges are not shown) wherein curvature is disposed along the plane
of the base wall in accordance with an embodiment of the
invention;
[0032] FIG. 12 is a side view of an absorber assembly illustrating
a paired arrangement of arcuate-shaped wedges wherein curvature is
disposed along the plane perpendicular to the base wall in
accordance with an embodiment of the invention;
[0033] FIG. 13 is a frontal view of an absorber assembly
illustrating a paired arrangement of wedges wherein the angle
formed by intersecting wedges varies along the length of the
assembly in accordance with an embodiment of the invention;
[0034] FIG. 14a is a frontal view of an absorber assembly
illustrating the paired arrangement of wedges in the center of the
assembly and disposed between horizontal and vertical wedges in
accordance with an embodiment of the invention;
[0035] FIG. 14b is a frontal view of an absorber assembly
illustrating the paired arrangement of wedges in the center of the
assembly and disposed between horizontal and curvilinear wedges in
accordance with an embodiment of the invention;
[0036] FIG. 15 is a schematic diagram illustrating the reflection
of an incident wave from pyramidal-shaped elements;
[0037] FIG. 16 is a schematic diagram illustrating the reflection
and diffraction of an incident wave from an absorber assembly
comprised of wedges arranged in a v-shaped pattern in accordance
with an embodiment of the invention;
[0038] FIG. 17 is a schematic diagram illustrating the diffraction
of an incident wave from the ridge line of a wedge in accordance
with an embodiment of the invention;
[0039] FIG. 18 is a reflectivity versus frequency plot for an
absorber assembly in accordance with an embodiment of the
invention;
[0040] FIG. 19a is a contour plot of reflectivity through the
center of a test zone within a rectangular-shaped anechoic chamber
lined with pyramidal absorbers;
[0041] FIG. 19b is a contour plot of reflectivity through the
center of a test zone within a rectangular-shaped anechoic chamber
lined with intersecting wedges arranged in a v-shaped pattern along
shaped walls in accordance with an embodiment of the invention;
and
[0042] FIG. 20 is a perspective view of a pyramid-shaped end wall
illustrating the arrangement of a plurality of pyramid-shaped
absorber elements in accordance with an embodiment of the
invention.
5. MODES FOR CARRYING OUT THE INVENTION
[0043] This application is based upon and claims priority from U.S.
Provisional Application No. 61/181,880 filed May 28, 2009, entitled
Absorber Assembly for an Anechoic Chamber, which is hereby
incorporated in its entirety by reference thereto.
[0044] Reference will now be made in detail to several preferred
embodiments of the invention that are illustrated in the
accompanying drawings. Wherever possible, same or similar reference
numerals are used in the drawings and the description to refer to
the same or like pans. The drawings are not to precise scale. While
features of various embodiments are separately described throughout
this document, it is understood that two or more such features
could be combined into a single embodiment.
[0045] Referring now to FIG. 1, the interior of a test chamber 1 is
shown from the perspective of one end showing the location and
arrangement of absorber assemblies 10, a source 2, a quiet zone 3,
and a test item 4. The test chamber 1 could include a variety of
configurations understood in the art, examples including
conventional rectangular, compact range, and tapered chambers.
Absorber assemblies 10 are attached to the interior surface of the
side walls 5, top wall, 6, and bottom wall 7. Each absorber
assembly 10 includes a plurality of triangular or like-shaped
wedges 11. The absorber assemblies 10 could completely or partially
cover one or more interior surfaces within the test chamber 1.
[0046] Absorber assemblies 10 and wedges 11 are understood to be
composed of materials understood in the art, examples including but
not limited to foam or a combination of ferrite and foam,
fabricated via methods understood in the art. The source 2 is
understood to be an antenna or other device which emits
electromagnetic waves or the like. For example, the source 2 could
be a direct illumination antenna or horn antenna. The test item 4
is understood to include either a device or antenna under test.
[0047] Referring now to FIG. 2a, an absorber assembly 10 is shown
including a plurality of wedges 11 disposed in a symmetrical
arrangement about a contact plane 15 and at the same height as the
centerline 53 through a source 2 and a test item 4. The contact
plane 15 may be linear or non-linear. The wedges 11 are further
divided into a first group 8 and a second group 9. The first group
8 is arranged at a first angle 46 (.alpha..sub.1) so that the base
lines 13 and ridge line 14 of the each wedge 11 are oriented to the
left. The second group 9 is arranged at a second angle 47
(.alpha..sub.2) so that the base lines 13 and ridge line 14 of each
wedge 11 are oriented to the right. The sum of the first and second
angles 46, 47 is equal to the wedge angle 12 (.alpha.). In
preferred embodiments, the first angle 46 is equal to the second
angle 47 so that the first and second groups 8, 9 form a geometric
pattern which is substantially symmetric about the contact plane
15. When used in conjunction with wall shaping, the cones of
diffracted rays projected from the wedges 11 reduce intersects with
the quiet zone 3 and test item 4 during EMC and antenna
measurements and the rays reflected by the test item 4 and walls 5,
6, 7, reduce intersects with the source 2 during RCS
measurements.
[0048] The intersection of two wedges 11 forms a v-shaped
structure. The v-shaped structure is continuously repeated or
smoothly changes along the length of the absorber assembly 10 by
the intersection of the parallel wedges 11 comprising the first and
second groups 8, 9. This arrangement also provides base lines 13
and a ridge line 14 which are v-shaped and contiguous along the
length of each pair-wise arrangement of wedges 11.
[0049] Each wedge 11 comprising the first group 8 includes a first
end 26 and a second end 27. Each wedge 11 comprising the second
group 9 includes a first end 28 and a second end 29. The second end
27 of one wedge 11 in the first group 8 contacts and is attached to
a first end 28 of one wedge 11 in the second group 9.
[0050] When the first angle 46 (.alpha..sub.1) and second angle 47
(.alpha..sub.2) are equal, the triangular-shaped ends of the
contacting wedges 11 overlap in a complimentary arrangement, as
represented in FIG. 3. The cross section of each wedge 11 can
include a variety of shapes including those in FIGS. 7a, 7b, 8, and
9. The wedges 11 could be mechanically or adhesively fastened to an
absorber base 44, also represented in FIG. 3, and to one another
along the contact plane 15. The end-to-end arrangement of wedges 11
in a pair-wise fashion restricts the wedge angle 12 to a value
greater than 0.degree. and less than 180.degree., where the first
and second angles 46, 47 are disposed at an angle greater than 0
and less than 90.degree.. The first end 26 and second end 29 are
appropriately angled so as to align with the edges of the absorber
base 44. The second end 27 and first end 28 are appropriately
angled so as to facilitate complimentary contact along the
interface between two contacting wedges 11.
[0051] In some embodiments, the wedges 11 in the first group 8
could be oriented at an angle greater or less than that of the
second group 9, as generally represented in FIG. 2b. The
intersection of wedges 11 along the contact plane 15 could allow
along a profile which is generally v-shaped with arbitrary cross
section, however, including a pair of side walls 16 which are
asymmetrically shaped.
[0052] In other embodiments, the wedges 11 could intersect along a
contact plane 15 which is biased above or below the centerline 53,
as represented in FIG. 2c. The offset 52 between contact plane 15
and centerline 53 would be design dependent. The first angle 46 and
second angle 47 could be equal to provide for a generally
symmetrical arrangement of wedges 11 about the contact plane 15 or
not equal to provide an asymmetrical arrangement.
[0053] In yet other embodiments, the intersecting wedges 11 could
contact along a contact plane 15 which is skewed at a skew angle 45
with respect to the centerline 53, as represented in FIG. 2d. The
first angle 46 and second angle 47 could be equal to provide for a
generally symmetrical arrangement of wedges 11 about the contact
plane 15 or not equal to provide an asymmetrical arrangement.
[0054] In still other embodiments, it might be advantageous to
include either an inward or outward arrangement of wedges 11 along
an absorber assembly 10. Referring now to FIGS. 4 and 5, the first
and second groups 8, 9 are shown attached to a wall 20 having a
concave surface 42 and a convex surface 43, respectively. In FIG.
4, wedges 11 within the first group 8 are positioned along the
portion of the wall 20 oriented downward and wedges 11 forming the
second group 9 are positioned along the portion of the wall 20
oriented upwards about the contact plane 15, resulting in an
effective angle 48 (.beta.) less than 180.degree. measured from
projections perpendicular to the concave surface 42. In FIG. 5,
wedges 11 within the first group 8 are positioned along the portion
of the wall 20 oriented upwards and wedges 11 in the second group 9
are positioned along the portion of the wall 20 oriented downwards
about the contact plane 15, resulting in an effective angle 48
(.beta.) greater than 180.degree. measured from projections
perpendicular to the convex surface 43. The effective angle 48 is
chosen such that a normal vector at any point along the shaped
surface is at least tangent to or does not intersect the quiet zone
3.
[0055] The intersecting wedges 11 could also be disposed along any
wall which includes both concave surfaces 42 and convex surfaces 43
and two or more contact planes 15. In FIG. 6, concave surfaces 42
are disposed along the contact planes 15 and convex surfaces 43 are
disposed along a forty-five degree) (45.degree.) angle with respect
to the contact planes 15; however, other arrangements are possible.
The combination of surfaces along a single absorber assembly 10
could be used along the front, back, and/or other walls of a
chamber.
[0056] Referring now to FIGS. 7a-9, each wedge 11 includes a pair
of side walls 16, 18, or 19 and a base wall 17 disposed in a
generally triangular or polygonal arrangement. A base line 13 is
formed at the intersection of each side wall 16, 18 or 19 with the
base wall 17. A ridge line 14 is formed at the intersection of the
side walls 16, 18, or 19 opposite of the base wall 17. In preferred
embodiments, the side walls 16 and base wall 17 are planar shaped
elements with a linear profile, as represented in FIG. 7a. In some
embodiments, the wedge 11 could be comprised of a plurality of
layers 50 which traverse the thickness thereof, as represented in
FIG. 7b. In yet other embodiments, the side walls 18, 19 could
include a convex or concave profile, as represented in FIGS. 8 and
9, respectively. In preferred embodiments, the side walls 16, 18,
19 are symmetrically arranged about the centerline 49 of the wedge
11. However, the side walls 16, 18, 19 could be asymmetrically
disposed about the centerline 49 to facilitate contact along a
contact plane 15 when the first angle 46 and second angle 47 are
not equal. Furthermore, asymmetrical arrangements could include two
different wall profiles. The height and width of the wedges 11 are
design dependent.
[0057] Referring now to FIGS. 10 and 11, the otherwise linear
wedges 11 in FIG. 2 are now shown having a generally non-linear or
arcuate shape. In these embodiments, the side walls 16, 18, 19 are
curved so as to intersect the base wall 17 forming arcuate-shaped
base lines 13 and a ridge line 14 about the contact plane 15 in the
plane of the backing wall 25. For illustrative purposes, only one
pair of wedges 11 is shown, although it is understood that the
absorber assembly 10 would include a plurality of other arcuate
wedges 11 disposed in a generally parallel arrangement with the
wedges 11 shown in FIGS. 10 and 11.
[0058] Referring now to FIG. 12, the otherwise linear wedges 11 in
FIG. 2 are now shown having another generally non-linear or arcuate
shape about the contact plane 15. In this embodiment, the base wall
17 is curved so as to intersect the side walls 16, 18, 19 forming
an arcuate-shaped ridge line 14 and base lines 13 immediately
adjacent to the backing wall 25.
[0059] Referring now to FIGS. 13 and 14, the absorber assembly 10
is shown including a composite arrangement of wedges 11 so that the
angular orientation of wedges 11 varies along the length of the
absorber assembly 10. In FIG. 13, the first and second groups 8, 9
are shown orientated at three wedge angles 12, 21, and 22 along the
length of the absorber assembly 10 so that at least some wedges 11
contact in a pair-wise arrangement along the contact plane 15. In
FIG. 14a, the first and second groups 8, 9 of the innermost wedges
11 are positioned at an angular arrangement along the length of the
absorber assembly 10. The wedges 11 are disposed between a
plurality of horizontal wedges 23 and a plurality of vertical
wedges 24. The specific orientation of wedges 11, 23, 24 along the
interior surface of a chamber wall is based on the performance
specifications for the EMC, antenna, or RCS application. In some
embodiments, the absorber assembly 10 could include a plurality of
curved wedges 51, as represented in FIG. 14b.
[0060] Referring again to FIG. 13, the contact plane 15, along
which the pair-wise arrangement of wedges 11 intersect, is
preferred to be disposed in a horizontal arrangement along the
length of the absorber assembly 10 and at a height along the
centerline of the quiet zone 3, test item 4, and/or test chamber 1.
However, the contact plane 15 could be disposed at a skew angle 45
so that the wedges 11 disposed in a pair-wise arrangement are also
in a generally diagonal or skewed arrangement relative to the
centerline 53 of the quiet zone 3, test item 4, and/or test chamber
1 to minimize diffraction. It is likewise possible for the inward
and outward angular arrangements in FIGS. 4 and 5 to also be skewed
along the skew angle 45. Furthermore, the contact plane 15 could be
positioned at a height above or below the centerline of the quiet
zone 3, test item 4, and/or test chamber 1.
[0061] The side walls 5, top wall 6, and bottom wall 7 described
herein are used with flat and/or shaped walls along the front or
back ends of a test chamber 1. While a variety of designs are
possible, FIG. 20 shows an exemplary end wall 54 which is pyramidal
shaped having a plurality of pyramid-shaped absorbers 55
thereon.
[0062] Functional aspects of the prior art and instant invention
are now described with particular reference to FIGS. 15-17.
[0063] Referring now to FIG. 15, a plurality of pyramid-shaped
absorbers 31 are shown contacting and attached to an absorber base
44 adjacent to a backing plate 30. The absorber base 44 is an
integral part of the pyramid-shaped absorbers 31 and is composed of
an absorptive material. The backing plate 30 could be a metallic
plate or the wall of a test chamber 1.
[0064] The pyramid-shaped absorbers 31 generate multiple
reflections after an incident field 33 impinges the system at an
incidence angle 32 (.theta.) with respect to the horizontal.
Dominant reflections include a reflected wave 34 from the front
surface of the pyramid-shaped absorbers 31 and a reflected wave 35
from the surface of the backing plate 30.
[0065] As the incidence angle 32 approaches 90.degree. with respect
to the face of the pyramid-shaped absorbers 31, the reflected waves
34, 35 may cancel each other at a number of frequencies, thus
significantly improving the reflectivity of the system. The region
near the lowest frequency where reflections cancel is the lowest
operating frequency 38 of the system, as illustrated in the
reflectivity versus frequency plot in FIG. 18. The absorption
performance at intermediate frequencies is determined by the
absorption and scattering properties of the system, and is
characterized by multiple reflectivity nulls 39 at frequencies
where the two reflections cancel, as further illustrated in FIG.
18. Finally, the absorption performance at a higher frequency end
is determined by the absorption properties of the material and
scattering associated with reflections from the front surface of
the pyramid-shaped absorbers 31 only.
[0066] At off-normal incidence, the reflectivity of absorbing
materials decreases, in part due to the fact that the two dominant
reflections are not canceled. In addition, the scattering from the
front surface of the pyramid-shaped absorber 31 is not planar, but
rather three-dimensional being better described by Geometrical
Theory of Diffraction rather than Geometrical Optics, as
illustrated by the scatter region 36 in FIG. 15, which radiates in
many directions including paths intersecting the quiet zone 3
during EMC and antenna measurements and the source 2 after back
reflection from a device under test and the chamber walls in RCS
applications, regardless of the shape of the backing plate 30.
[0067] Referring now to FIGS. 16 and 17, a plurality of parallel or
smoothly changing wedges 11 are now arranged in the v-shaped
repeating pattern as described herein and attached to an absorber
base 44 adjacent to a backing plate 30. At normal incidence, the
system functions as described above. However, the absorber assembly
10 offers improved performance over the system described in FIG. 15
at angles less than normal by reducing the undesired effect of the
scatter region 36 so as to improve control over the diffracted
field. This control may be utilized to improve reflectivity
performance in the various applications described herein.
[0068] Referring again to FIGS. 16 and 17, the incident field 33
impinges the wedges 11 producing a cone-shaped diffractive wave 37
at an angle (.tau.) from a tangent vector 40 to the ridge line 14
of a wedge 11 equal to the incidence angle 32. If the orientation
of the wedges 11 is changed with respect to the incident field 33,
then the diffractive wave 37 does so also, thus facilitating
control of the diffractive wave 37 away from the quiet zone 3 in
EMC and antenna measurements and quiet zone 3 and a source during
RCS measurements. The formation of diffractive waves 37 and control
thereof is applicable to straight wedges 11 and others described
herein.
[0069] The absorber assembly 10 minimizes scattering within an
anechoic chamber via the control of reflections and diffractions.
During a first impingement of an incident field 33 with an absorber
assembly, the electromagnetic wave is partially reflected away from
the quiet zone, test item, and/or source by the absorber assembly
10. When used in conjunction with wall shaping, it is possible to
further reflect energy away from the quiet zone. The intersecting
and angular arrangement of wedges 11 also allows control of the
diffracted electromagnetic wave within the primary scattered field
away from the quiet zone, test item, and/or source. Any subsequent
or secondary impingement of the diffracted wave with an absorber
assembly 10 produces a secondary scattered field, of much lower
magnitude than the primary field, which further, via a plurality of
solutions, may likewise be directed in a controlled fashion away
from the quiet zone, test item, and/or source.
[0070] Referring now to FIGS. 19a and 19b, two exemplary plots
compare the predicted reflectivity performance of a test chamber 1
with a conventional arrangement of pyramid-shaped absorbers 31 and
intersecting wedges 11 described herein, respectively. Predicted
performance is for illustrative purposes only and not intended to
restrict or limit the scope of the claimed invention. Predictions
were performed using the software package CST Studio Suite sold by
CST of America, Inc. located in Framingham, Mass.
[0071] The pyramid-shaped absorbers 31 applied to the side walls 5,
top wall 6, and bottom wall 7 of the test chamber 1 yielded a large
number of reflections across and adjacent to the quiet zone 3 with
a calculated reflectivity of -28 dB, as illustrated in FIG. 19a. In
comparison, intersecting wedges 11 applied to a shaped wall yielded
fewer reflections across and adjacent to the quiet zone 3 with a
more uniform field across the test item 4 and a calculated
reflectivity of -36 dB, as illustrated in FIG. 19b.
[0072] The description above indicates that a great degree of
flexibility is offered in terms of the invention. Although various
embodiments have been described in considerable detail with
reference to certain preferred versions thereof, other versions are
possible. Therefore, the spirit and scope of the appended claims
should not be limited to the description of the preferred versions
contained herein.
6. INDUSTRIAL APPLICABILITY
[0073] As is evident from the explanation herein, the described
invention includes an arrangement of v-shaped absorbers, applicable
to walls with and without shaping, which improve control over
reflectivity within a chamber so as to better control primary and
secondary scattering effects, thus minimizing reflections within a
quiet zone and reducing quiet zone clutter.
[0074] Accordingly, the described invention is expected to be
applicable to a variety of test chambers which collect data for
electromagnetic compatibility, far-field antenna patterns,
near-field measurements, radar cross section, or other such similar
applications.
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