Communications: radio wave antennas – Antennas – With spaced or external radio wave refractor
Reexamination Certificate
2000-08-15
2002-05-28
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With spaced or external radio wave refractor
C343S754000, C343S757000, C343S766000, C343S91100R
Reexamination Certificate
active
06396448
ABSTRACT:
TECHNICAL FIELD
The present invention is generally directed to a mechanically-scanned directional antenna. More particularly described, the present invention is a scanning beam antenna using a rotatable combination of a lens and a reflective surface, such as a mirror, to scan a wide field-of-view.
BACKGROUND OF THE INVENTION
Omnidirectional antennas cover a 360 degree field-of-view with a single beam. A directional antenna with a narrow azimuth beamwidth can be used to increase gain or provide directional information. For example, a 10 degree, half-power beamwidth antenna will have approximately 15 dB more gain than an “omni” antenna with the same elevation beamwidth.
FIGS. 1A and 1B
emphasize this gain difference and illustrate the use of multiple narrow beams to maintain antenna coverage over a 180 degree field-of-view. As shown in
FIGS. 1A and 1B
, an antenna characterized by a narrow azimuth beam
14
of 10 degrees typically exhibits an increased gain when compared to a typical pattern
12
for an omnidirectional antenna. To obtain this desirable gain increase of a directional antenna over a wide field-of-view, multiple narrow azimuth beams
16
can be mechanically or electronically scanned to cover a 180 degree field-of-view.
Scanned antennas are typically implemented in one of two forms: electronic scan or mechanical scan. Electronically-gained antennas usually require beam forming networks which contain electronic RF switches or phase control devices. Mechanically-scanned antennas typically utilize a motor to rotate or position a directional antenna in different directions over the required field-of-view. Mechanically-scanned antennas are usually less expensive to construct than electronically-scanned antennas but have slower scanning ability and lower reliability due to the use of moving parts.
FIG. 2
illustrates an antenna with a typical reflecting mirror for scanning a narrow beam. Mirrored reflectors have been used to scan narrow beams over a small field-of-view. RF energy is normally collimated off the mirror, which can create spill-over loss if the mirror is tilted by a large angle. As shown in
FIG. 2
, a flared horn can emit an electromagnetic signal that is reflected by a reflecting surface, such as a mirror
22
, to direct electromagnetic energy away from the antenna transmission axis. By rotating tie mirror
22
proximate to the output slot of the flared horn
20
, the electromagnetic energy can be scanned over a relatively small field-of-view. Angle theta defines the angle between the mirror
22
and the reflection axis for the beam B reflected by the reflective surface of the mirror. To reflect electromagnetic energy from the reflective surface of the mirror
22
at an angle theta, the length L of the mirror
22
is defined by the ratio of the horn span H and the angle theta. The length of the mirror
22
is defined by Equation 1, as follows:
H
cos
(
theta
)
Equation
1
For example, if the angle theta is 45 degrees, then the length L is defined by 1.41×H. If the angle theta is 60 degrees, then the length L is defined by 2.0×H. If the angle theta is 75 degrees, then the length L is defined by 3.9×H.
Mechanical tracking antennas typically use motor-driven rotation with position knowledge or feed-back and can be commanded to point and dwell from one beam position to the next beam position. If the user application requires repetitive rapid scans of the same field-of-view, most mechanically-scanned antennas lose efficiency by decelerating at the end of the scan to allow a subsequent acceleration in the reverse direction. In addition to time inefficiency in reversing the angular momentum, the reverse rotation adds cost and complexity to the positioning system and causes wear and stress on the bearing and joints of the positioning system.
As shown in
FIG. 3
, a simpler and lower-cost tracking approach is provided by a continuous rotating, mechanically-scanned system. A horn antenna
32
can be rotated about a transmission axis by an RF rotary joint
34
driven by a motor
36
. The rotation of the horn antenna
32
results in the sang of a narrow beam along a predetermined field-of-view. For example, to scan a 180 field-of-view with a 10 degree bean in the azimuth plane, the horn antenna
32
can be rotated in accordance with Option
1
by reversing the direction of the scan based upon deacceleration and acceleration operations completed by the RF rotary joint
34
and the motor
36
. In the alternative, a horn
32
can be rotated to produce a continuous scan of the narrow beam in accordance with Option
2
, thereby resulting in “dead” time when the desired antenna coverage is 180 degrees. Although Option
2
has the advantage of re-scanning in the same direction, a recovery period results from the motion of the antenna when the desired antenna coverage is less than 360 degrees. The latency in revisiting a specific pointing direction is undesirable in a collision warning radar or missile detecting radar due to the closure movement of the target while a continuous rotating antenna is in the recovery period of its rotation. Recovery time can be reduced by faster antenna rotation but this increases cost, reduces reliability, and reduces the “dwell” time on the target due to the high angular rotation rate.
In view of the foregoing, there is a need in the art for an improved antenna that can efficiently scan an antenna beam over a wide field-of-view. Moreover, there is a need in the art to provide a mechanically-scanned antenna that can scan a narrow beam over a wide field-of-view in a reliable manner without the need for a complex positioning system. There is a further need in the art for a mechanically-scanned directional antenna that exhibits a near instantaneous reset or “fly-back” capability for applications requiring the re-scanning of a specific pointing direction. The present invention addresses these and other needs in the art by providing an antenna comprising at least one feed with a rotating dielectric lens having a reflective surface, such as a mirror, to scan a narrow beam over a relatively wide field-of-view.
SUMMARY OF THE INVENTION
The present invention addresses the needs of the prior art by achieving the desired characteristics of a low-cost, mechanically-scanned antenna with the high reliability and near instantaneous reset or “fly-back” capability of an electronically-scanned antenna. The present invention provides a low cost, reliable, mechanically-scanned directional antenna that can scan a wide field-of-view by rotating a reflecting lens/mirror assembly placed adjacent to a signal source. By keeping the signal source, such as a line source, stationary and scanning a lens/mirror assembly, the need for RF rotary joints or flexible transmission line and amplifier slip rings is eliminated for the antenna design. The lens can be implemented as one-half of a constant-K dielectric cylinder with a reflective surface or mirror, such as metal foil tape, applied to the flat portion of the lens. For example, a parallel-plate horn can scan 180 degrees of the azimuth plane by rotating a lens/mirror assembly positioned proximate to the horn output slot and within the transmission axis for the antenna beam. Installing a second half cylinder lens on the back side of this mirror can support the generation of two or more directional beams, thereby achieving a simultaneous scan of 360 degrees with the use of a pair of opposing horns. Switching the output of a single transceiver between two or more horns allows a “fly-back” re-scan capability.
In general, the present invention provides an antenna comprising a feed for delivering electromagnetic energy and a rotatable combination of a dielectric lens and a reflective surface. The combination of the dielectric lens and reflective surface, also described as a reflecting lens/mirror, is placed proximate to and in front of the energy feed. This supports the reflection of electromagnetic energy as the reflecting lens/mirror rotates over a predetermined range to scan the resulti
Black, Jr. Donald Nelson
Freeman Robert Alan
Marsik James Charles
Wann John Elliott
Zimmerman Kurt Alan
Chen Shih-Chao
EMS Technologies Inc.
King & Spalding
Wong Don
LandOfFree
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