Communications: radio wave antennas – Antennas – Antenna components
Utility Patent
1999-01-15
2001-01-02
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Antenna components
C343S840000
Utility Patent
active
06169524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of reflector antennas, and more particularly, to a reflector antenna which includes frequency selective or polarization sensitive zones to provide a plurality of antenna patterns having different polarizations or frequencies from a single reflector.
2. Description of the Prior Art
Reflector antennas are frequently used on spacecraft to provide multiple uplink and downlink communication links between the spacecraft and the ground. The downlinks operate at one frequency, typically around 20 GHz, and the uplinks operate at a second higher frequency, typically around 30 or 44 GHz. It is typically desirable for a single spacecraft to have multiple uplink and downlink antennas where each antenna provides a separate antenna pattern covering a predetermined coverage zone on the earth. It is also typically desirable to provide both an uplink and downlink antenna pattern having the same beamwidth so that users can both receive and transmit to the same spacecraft. For example, a single spacecraft may have one uplink antenna which provides a 3°×6° antenna beam at 30 GHz for uplink communications from the continental United States (CONUS), and, one downlink antenna at a frequency of 20 GHz which provides a 30°×6° beam for downlink communications to CONUS. The method typically used to provide multiple uplink and downlink antenna patterns from a single spacecraft is to provide separate reflectors for each uplink and downlink antenna. This requires a large amount of space on a spacecraft, is expensive and extracts a weight penalty.
One method attempted to save weight is to couple one uplink and one downlink antenna together in a single reflector body. To do so, an illumination source is configured to illuminate the reflector body with two RF signals, one having a frequency of 20 GHz and the other having a frequency of 30 GHz. The reflector is typically fabricated of a composite or honeycombed material coated with a reflective material, typically aluminum, which is reflective to RF signals of all frequencies. The disadvantage with this system is that it is difficult to provide antenna patterns having predetermined beamwidths at different frequencies from the typical reflector. The beamwidth of an antenna beam is inversely proportional to the size of the reflector and the frequency of illumination. From the same sized reflector, the uplink antenna pattern at 30 GHz would have a smaller beamwidth than the downlink antenna pattern at 20 GHz thereby covering a smaller coverage zone than the downlink antenna pattern. To address this problem, conventional reflector antennas have used specially designed feed horns configured to under illuminate the reflector at 30 GHz, the higher frequency, thereby generating an antenna pattern at 30 GHz having a wider beamwidth. This is inefficient and often difficult to do since feed horns are extremely sensitive to tolerance and bandwidth limitations.
A need exists to have a single reflector which provides a plurality of antenna patterns each having a predetermined beamwidth allowing a single spacecraft to carry the weight and expense of only one reflector while having the ability to provide multiple uplink and downlink antenna patterns.
SUMMARY OF THE INVENTION
The aforementioned need in the prior art is satisfied by this invention, which provides a reflector antenna having frequency selective or polarization sensitive zones to provide a plurality of antenna patterns from a single reflector body. A reflector antenna, in accord with the invention, comprises a single concave reflector body having a plurality of zones with each zone configured as a frequency selective or polarization sensitive zone. The zones can be partially, completely or not overlapping. An illumination source is configured to illuminate the reflector body with a plurality of RF signals with each zone reflecting one or more of the RF signals. The reflector body generates a plurality of antenna patterns from the reflected RF signals with the shape & beamwidth of the antenna patterns being determined by the shape and dimensions of each zone. The shape and dimensions of each zone is thus preselected to provide an antenna pattern having a desired shape and beamwidth.
For the preferred embodiment of the invention, the reflector body has two concentric zones comprised of an inner zone and an outer zone encompassing the inner zone. The two zones are illuminated with the RF signals having frequencies of approximately 20 GHz and 30 GHz. The inner zone is comprised of a material which is reflective to RF signals of all frequencies, and, the outer zone is comprised of a material which reflects RF signals of a 20 GHz frequency and passes RF signals having a frequency of 30 GHz. The 30 GHz signal is reflected only by the inner zone and is not reflected by the second zone. Antenna patterns are generated at 20 and 30 GHz from the 20 and 30 GHz reflected signals respectively with the size and shape of only the inner zone determining the shape and beamwidth of the 30 GHz antenna pattern and the shape and beamwidth of both zones determining the shape and beamwidth of the 20 GHz antenna pattern. The dimensions of the inner and first zone are preselected to generate 20 and 30 GHz antenna patterns having approximately equal shapes and beamwidth.
REFERENCES:
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patent: 4757323 (1988-07-01), Duret et al.
patent: 4831384 (1989-05-01), Sefton, Jr.
patent: 4851858 (1989-07-01), Frisch
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patent: 5136294 (1992-08-01), Iwata
patent: 5365245 (1994-11-01), Ho
Lee, et al., “Compound Reflector Antennas,” published in the IEEE Proceedings on Antennas and Propagation, Apr. 1992, pp. 135-138.
Chandler Charles W.
Wu Te-Kao
Nguyen Hoang
Thousand Connie
TRW Inc.
Wong Don
Yatsko Michael S.
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