Wave transmission lines and networks – Wave mode converters
Reexamination Certificate
1998-12-22
2004-11-09
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Wave mode converters
C033S123000, C033S123000
Reexamination Certificate
active
06816026
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of antenna systems used in satellite communications where orthogonal polarizations are employed to increase system capacity.
BACKGROUND OF THE INVENTION
The demands for satellite communication capacity have resulted in the implementation of several different techniques. One technique is to extend satellite capacity using orthogonal polarization states to send two independent signals to the same coverage region thereby doubling the information that can be delivered to that region. This technique is referred to as polarization reuse. The success of this technique depends in part on the ability to maintain the separation of the two signals to avoid mutual interference that degrades communication performance. The required signal separation in turn imposes requirements on the polarization purity of the signals.
Polarization reuse is very commonly used on commercial satellites operating at the C band (4-6 GHz) and Ku band (11-14 GHz) frequencies. The required separation between signals used in these systems depends on the power differences in the signal levels and the susceptibility of the reception to co-channel interference. A typical requirement for the polarization purity needed for signal separation is to limit the reception of the undesired signal to a level that is 27 dB lower, that is, {fraction (1/500)} of the power, than the desired signal component. The degree of polarization purity needed to satisfy this requirement is significantly more stringent than the polarization purity required to insure minimal signal loss caused by polarization mismatch.
Different satellite systems, however, are not consistent in the polarization states used. Some systems use orthogonal linear polarization states while other systems use orthogonal circular polarization states. Within a given satellite system, antenna systems for a single polarization state have been developed. However, if antenna systems are developed for use with several different satellite systems, the antenna system requires the capability to select the polarization state depending on the satellite system being used. Clearly, antenna systems capable of operating with different satellite systems afford advantages in flexibility and potential cost effectiveness. However, such antenna designs have to be fully compatible with the requirements for each satellite system. In view of the various polarization signaling methods, antenna systems designed for inter-program compatibility must be capable of processing dual polarization signals with either linear or circular polarization states and must meet system requirements for polarization purity.
The design requirements to achieve the requisite polarization purity must address the antenna, its feed system, and the ports for each polarization. These design requirements must be maintained over the entire bandwidth spanned by the satellite systems. The antenna, for example, must be designed with a high degree of symmetry so that cross polarized components are not generated that would degrade polarization purity. Similarly, the feed system must be designed to produce rotationally symmetric illumination of the antenna system and attention must be paid to the excitation of higher order modes that produce cross polarized components that degrade polarization purity. The terminals of the feed system must be constructed with precision to avoid polarization coupling, and any combining circuitry used to transform polarization states must satisfy stringent matching requirements to avoid the generation of cross polarized components that degrade polarization purity. The satisfaction of the overall system requirements for polarization purity is limited by the aggregate of the imperfections in the antenna, feed system, terminals and transforming circuitry.
One fundamental limitation in the development of designs that permit selection of the polarization state results from the inherent imperfections when hybrid combining circuitry is used to transform polarization states. The conventional approach to this problem is to combine one of the polarization states with hybrid circuitry to obtain the other polarization state. The limitation of this approach lies with the inherent imperfections of the hybrid. Quadrature hybrids needed to convert the linearly polarized state to the circular polarized state can maintain a ninety degree phase shift but the amplitude response is unequal over the bandwidth. This amplitude imbalance results in coupling between the two polarization states resulting in co-channel interference. When linearly polarized components are transformed to circularly polarized components, for example, the circular components are obtained from the addition of equal levels of each linearly polarized component with a ninety degree phase shift between the components. Such combining is typically implemented using a quadrature hybrid. Practical hybrids provide the appropriate ninety degree phase shift but exhibit the problem of an imbalance when combining the amplitudes that then varies over the required bandwidth. This amplitude combining imbalance is a limiting factor in achieving the polarization isolation needed to maintain signal separation. A similar limitation exists with one hundred and eighty degree hybrids used to combine circularly polarized components to obtain linearly polarized components. One problem with-one hundred and eighty degree hybrids is the resulting phase imbalance. A second problem is the insertion loss inherent when using combining circuitry results. Such insertion loss degrades system sensitivity. The insertion loss reduces transmitted power delivered to the antenna and also limits the power handling because the thermal energy resulting from the insertion lose must be dissipated. The insertion loss in receiving antennas not only reduces the received signal strength but also increases the total system temperature, a factor that is extremely important when modern low noise receivers are used.
A means of switching is also required to select between the polarization states. Three distinct switch technologies exist. Diode switch devices can switch very rapidly but are relatively lossy and limited in their power handling capability. Ferrite switching technology has somewhat less loss, slower switching time, and greater power handling capability and very low loss, but with disadvantageous slow switching times. The low loss and power handling capabilities are desired in this polarization reuse applications and rapid switching may not present a problem. Thus, waveguide switch technology is preferred in this polarization reuse application having low loss and high power handling capabilities, but with slow switching times. Conventional waveguide switch has a single dominant waveguide mode. A dominant waveguide mode may be TE
01
or TE
10
for square waveguides and orthogonally disposed TE
11
for circular waveguides. Tapers and frequency selective surfaces have long been used for frequency isolation. The most familiar waveguide switch uses rotating waveguide bends to route the signals between four ports. The conventional waveguide switch has two selectable position settings for aligning two curved waveguide section bends symmetrical about a rotating axis. The curved selectable waveguide section does not use reflecting surfaces, and is limited to rectangular cross section waveguide sections incapable of communicating orthogonally polarized signals. This dual position arrangement is analogous to a double-pole double throw switch. This configuration in commonly referred to as a baseball switch, because the waveguide bends resemble the stitching on a baseball. However, this switch technology is not capable of switching orthogonally polarized signals because the bends inherently result in coupling between the linear and circular polarized signals. These and other disadvantages are solved or reduced using this invention.
SUMMARY OF THE INVENTION
An object of the invention is the capability to receive and/or transmit dual orthogonall
Lee Benny
Reid Derrick Michael
The Aerospace Corporation
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