Abstract
Introduction
Electrical Design
Features
Polarization Conversion
and Rotation
Polarization Isolation
Dual Frequency Applications
Conformal FLAPS
Low Cost Beam Scanning
or Switching Switching
Mechanical Design
Large Apertures with
Low Windloads
Large Portable Apertures
Summary
FLAPS
Technology is Patented by Malibu Research
FLAPS is a Registered Trademark of Malibu
Research
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A
FLAPS reflector is a thin (planar or
conformal) surface consisting of an
array of elements, each functioning
as a radiator and phase shifter. Unlike
a conventional planar array, however,
the elements on the FLAPS surface are
spatially fed using a feed assembly
as in a conventional reflector system.
This results in an antenna technology
that offers the advantages of both planar
arrays and reflector systems. Additionally,
FLAPS technology offers packaging and
deployment ease, and is suitable to
a variety of manufacturing processes
and procedures using low-cost materials.
Other features such as polarization
control, large apertures with low windloading,
and low-cost electronic beam switching
and scanning are also possible. Initially
developed for defense microwave and
millimeter-wave radar applications,
FLAPS antennas are now being developed
and fielded in many defense as well
as commercial radar and communications
systems.
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"Flat Parabolic Surface" (FLAPS)
at first may seem like an oxymoron but
in fact, it is possible to design a geometrically
flat surface to behave electromagnetically
as though it were a parabolic reflector. |
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The
FLAPS consists of an array of dipole scatterers.
The elemental dipole scatterer consists of
a dipole positioned approximately 1/8 wavelength
above a ground plane. Here, a crossed shorted
dipole configuration is shown with each dipole
controlling its corresponding polarization.
Incident RF energy causes a standing wave
to be set up between the dipole and the ground-plane.
The dipole itself possesses an RF reactance
which is a function of its length and thickness.
This combination of standing-wave and dipole
reactance causes the incident RF to be reradiated
with a phase shift f, which can be controlled
by a variation of the dipole's length. The
exact value of the this phase shift is a function
of the dipole length, thickness, its distance
from the ground-plane, the dielectric constant
of the intervening layer, and the angle of
the incident RF energy. When the element is
used in an array, as discussed later, it is
also affected by nearby dipoles.
Typically, the dipole lengths vary over the
range of 0.25 to 0.60 wavelengths to achieve
a full 360° range of phase shifts. The
ideal spacing between the ground-plane and
the dipole is 1/16 to 1/8 wavelength. The
spacing affects form-factor, bandwidth, and
sensitivity to fabrication tolerances.
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The FLAPS elemental scatterer
performs the function of a radiating
element and a phase shifter in a space
fed phased array. Since dipoles of different
lengths will produce a phase shift in
the incident wave, arranging the distribution
and the lengths of the dipoles will
serve to steer, focus or shape the reflected
wave. As the above figure illustrates,
an array of such elements is designed
to reradiate with a progressive series
of phase shifts so that an RF beam is
formed in a specific direction.
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In a simple application, a parabolic surface
can be directly replaced with a FLAPS.
It is possible to design a FLAPS as a
substitute for any conventional reflector
used in antenna design.
FLAPS
surfaces can be up to 95% efficient.
When designed as an offset reflector,
the feed may be offset up to 60°
from the flat surface. Bandwidths of
3% to 10% are achievable with a designed
center frequency in the range from 1
to 100 GHz.
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Typicaly, at the element level of a standard
FLAPS design, the orthogonal dipoles are designed
to reflect with the same phase shift relative
to each other. When designed in this manner,
the FLAPS surface will function as a standatd
metal reflector and have no influence on the
reflected polarization. Therefore, the antenna
polarization will be determined by the feed
design.
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The
polarization isolation between the orthogonal
dipoles is very high (greater than 50 dB).
This valuable feature allows independent
control of the separate eigenvectors of
the RF energy reflecting off the FLAPS.
Designing
the orthogonal dipoles to reradiate with
a 90° relative phase shift will result
in a surface that will convert 45° linear
incident RF into circular polarization.
In fact, a surface designed in this manner
will yield left and right hand circular
as well as horizontal and vertical linear
polarizations with a single linear polarized
feed depending upon the relative polarization
orientation of the feed. This eliminates
the requirement for a costly circular polarized
feed.
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In similar fashion, a FLAPS surface could
be designed to convert horizontal linear
to vertical linear. Using this feature
with a parallel-wire-grid subreflector,
a "twist-Cassegrain" antenna
is possible whereby the linear polarized
feed energy reflected back from the wire
grid subreflector will be columnated and
rotated 90° by the FLAPS surface and
passed through the subreflector, thus
eliminating the blockage effects normally
associated with the subreflector.
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FLAPS technology allows the designer to
independently control the RF reflecting
characteristics of the FLAPS for orthogonal
senses of polarization. This capability
eases the design of an antenna system
that requires dual linear polarization.
By designing the FLAPS surface to have
separate focal points for the orthogonal
linear polarizations, a dual polarized
feed is not required. This technique will
also result in high polarization isolation.
It is also interesting to note that this
feature allows two independant beam characteristics
to be achieved at orthogonal linear polarizations.
For example, the FLAPS surface can be
designed to achieve a pencil beam at vertical
polarization and a shaped beam at horizontal
polarization.
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In all the examples reviewed so far, the
dipoles are suspended in front of a solid
metal ground plane. By substituting the
solid ground plane of a FLAPS reflector
with an array of dipoles that are at resonance
at the frequency of design, the FLAPS
reflector has the additional feature of
being RF transparent at other frequencies
as illustrated above. Antennas designed
in this fashion can be placed in front
of a planar array for dual frequency applications.
This technique is well suited to modifying
existing airborne weather radar antenna
systems with a millimeter wave aperture
for landing radar and obstacle avoidance
applications. Antennas built in the fashion
also exhibit low racar cross section features.
Layered
FLAPS reflectors can also be designed
to operate at two or more frequencies.
In a MILSTAR application, for example,
a transparent FLAPS is designed to operate
at 44 GHz and placed directly in front
of another FLAPS designed to operate
at 20 GHz. In this example the antenna
feed is greatly simplified by designing
separate focal points for each frequency
and using separate feeds in lieu of
one costly dual frequency feed. Considerable
freedom is allowed in the design of
the feed locations.
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Nearly any geometrically shaped surface
can be "electromagnetically reshaped"
with FLAPS technology to yield the desired
reflection pattern characteristics. In
difficult siting environments the surface
of a building, fence, or parking lot,
for example, can function as the FLAPS
reflector of a fixed satellite earth station
antenna even though it is not normal to
the direction of the satellite of interest.
Furthermore the ability to design the
FLAPS so the feed may be at any defined
location with respect to the FLAPS greatly
simplifies the antenna design.
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Typically, the beam of a conventional parabolic
reflector is repositioned by moving the feed
and the reflector via a gimbal mechanism. By
using a unique non-parabolic shaped reflector
that is "phase corrected" with FLAPS
technology, however, it is possible to redirect
the beam as much as 90° by moving only the
FLAPS reflector and keeping the feed fixed.
This eliminates the need for a rotary joint
and greatly reduces the mass that must be moved.
The 2:1 scan effect also reduces the swept volume
of the reflector for a given required scan angle.
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Alternately, by placing several feeds at different
locations in front of a common "curved"
FLAPS surface, antenna beams can be directed
to different locations. These can be simultaneous
beams, or rapidly switched, whichever the application
requires. TILT FLAPS Technology is suitable
for a high gain digital link antennas that must
communicate with several other links, multibeam
antennas and millimeter-wave cameras.
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FLAPS surfaces can be fabricated in a variety
of ways. The only mechanical requirement is
to support the double-layer of dipoles with
the desired spacing between layers and between
dipoles, and with adequate mechanical integrity
to maintain the spacing and surface shape under
the anticipated operating loads. For most ground-based
and airborne defense applications to date, FLAPS
surfaces have been etched from double-layer
printed-circuit boards. These surfaces readily
produce the required surface shape (e.g. flatness)
and surface smoothness, even at 94 GHz. FLAPS
technology lends itself readily to low-cost
CAD/CAM fabrication. Very low-cost FLAPS surfaces
intended for direct-broadcast consumer TV reception
have been produced by silk-screening onto plastic
panels.
Very-low-mass
FLAPS surfaces are possible, since the dielectric
layer can be lightened considerably. Designs
based on lightened-foam-plastic dielectric
layers with the dipoles deposited on thin
Kapton films result in reflector masses as
low as 0.05 to 0.1 kg/m2, not including the
supporting structure.
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Most satellite earth stations must operate
in high winds. As the apertures get larger,
the windload forces require costly mounting
supports and tracking systems. At approximately
8 GHz and below, light weight FLAPS surfaces
can be fabricated that exhibit as little as
1/8 the windloading forces of conventional
solid or wire-mesh reflectors. This is achieved
by fabricating a frame and attaching KEVLAR
string in "tennis racket" fashion,
with a grid spacing of about 0.5 wavelength.
Dipoles are then attached on the string. The
groundplane surface is fabricated in the same
manner to complete a FLAPS surface as illustrated.
This fabrication technique significantly reduces
the mounting and positioning requirements,
which results in a much lower-cost antenna
system. Recently, a 20-foot C-band antenna
system has been developed. This antenna system
is designed for rapid deployment and is capable
of withstanding winds of 70 mph. This fabrication
technique is also applicable to large aperture
air traffic control radar applications. See
product photos for examples of products delivered
using this feature.
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Conventional reflector antenna systems usually
are difficult to disassemble and package for
portable and rapid deployment applications.
Even when sectioned, the curved reflector panels
do not store efficiently.
Planar FLAPS Surfaces Easily Fold, Stow and
Deploy
Thin, planar FLAPS reflectors, however, are
efficiently stowed in compact packages and deployed
via a simple hinging or assembly process. A
very simple hinge-folding scheme is illustrated.
One such antenna has been developed for a commercial
INMARSAT-B terminal. Another novel folding scheme
was used for a MILSTAR antenna .
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FLAPS reflectors can be configured in many
different ways. Packaging and deployment are
limited mainly by the imagination of the designer.
FLAPS surfaces do not have to be planar and
they do not have to be continuous. Small sections
are easily fabricated and later "tiled"
to complete the full aperture. When fabricated
in this manner, electrical continuity is not
a requirement. Provided the antenna designer
knows, in advance, the beam location and shape,
the feed location, and the reflector surface
geometry, FLAPS technology can be used to
electrically reshape the surface to perform
as a parabola. Conventional reflector antenna
calculations apply to determine surface tolerances,
gain, sidelobes, and other electrical antenna
parameters.
FLAPS
reflector antenna technology is a proven candidate
for low-cost commercial applications. It has
been fielded in a variety of defense radar
and communications applications and provides
many features preferred by commercial systems.
light-weight
low-windloading
simple deployment
packaging ease
polarization control
low recurring costs.
Product examples and photos of FLAPS antennas
are available for review elseware on this
web site. For additional information Contact
Malibu Research directly to obtain more
detailed information or to discuss a specific
requirement.
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