The invention relates to the field of antenna systems used in satellite communications where orthogonal polarizations are employed to increase system capacity.
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 TE01 or TE10 for square waveguides and orthogonally disposed TE11 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.
An object of the invention is the capability to receive and/or transmit dual orthogonally polarized signals with selection between linear and circular states.
Another object of the invention is to achieve a high degree of polarization purity over a wide bandwidth to avoid co-channel interference of one signal to another.
Yet another object of the invention is to achieve a low lose design to increase system efficiency in antenna systems.
A further object of the present invention is to provide the means of transmitting and/or receiving two orthogonally polarized antenna signals with a high degree of polarization purity and with low loss and the capability to select either linearly or circularly polarized polarization states.
Yet a further object of the present invention is to provide the capability for a dual polarized, selectable polarization state waveguide capable of operation for multiple frequency bands.
The present invention is directed towards a waveguide switch having a plurality of switch positions for communicating a signal between at least one input port and a respective plurality of output ports through a respective plurality of dissimilar waveguide sections. In the preferred form, the waveguide switch has two output ports respectively connected to the input port through a straight waveguide section and a bent waveguide section. The waveguide switch is preferably used to receive and/or transmit dual polarized signals through an antenna feed input port between a linear output port using the bent waveguide section coupled to a linear polarization state sensitive probe and a circular output port coupled to a circular polarized probe using the straight waveguide section providing the capability to select either linearly or circularly polarized polarization state signal transmitted through the antenna feed port. This present invention provides a high level of polarization purity needed to separate two independent signals by polarization. The present invention is directed to selectable waveguides having selectable waveguide sections to perform the polarization state selection, and the loss incurred by these sections is much less than the losses in hybrid combining circuitry used in the conventional polarization state transformations. The waveguide sections can be sized, cascaded and coupled to frequency sensitive tapers and couplers for both polarization state selection and frequency selection of signals in applications where multiple frequency or multiple polarization state operation is required, for example, in simultaneous C band and Ku band operation.
The preferred selectable waveguide has two positions for respectively selecting one of two waveguide sections within the selectable waveguide. The selectable waveguide is capable of propagating the two independent orthogonal polarized channels. A waveguide is connected to an antenna feed capable of propagating two independent orthogonally polarized communication channels. A selector switch, knob, or other mechanical means on the waveguide is used to select one of the two waveguide sections to thereby select one of the two independent orthogonally polarized communication channels. Output ports of the selectable waveguide are used for separating the respective polarization states of the channels using respective polarization sensitive probes. The waveguide switch is thus used to route the transmitting or receive channel signals into either the circular polarized output port realized by an orthomode transducer capable of high polarization purity over wide bandwidths or to the linear polarized output port realized by an orthogonal linear polarized probe in the waveguide capable of high polarization purity over wide bandwidths.
Preferably, the selector switch is used to transfer either linear or circular polarization signal components to respective ports. Like the conventional waveguide switch, the selection is preferably accomplished by mechanical rotation. Unlike conventional switches, however, the improved selectable waveguide has dissimilar waveguide sections that can respectively operate in two dominant modes. One switch setting consists of a straight waveguide section so that higher order modes and mode coupling does not occur. The second switch setting changes the direction of propagation by ninety degrees using a waveguide miter bend to avoid higher order mode generation. The axis of rotation is offset to permit the rotation of the switch and the port alignment. The improved selectable waveguide switch of the present invention is effectively a single-pole double-throw waveguide switch using three ports.
These selectable waveguide switches can be frequency sized and cascaded for multiple frequency applications. Such cascading can be readily performed when the switch has the straight waveguide section. When the switch is placed in the position of bent section containing a miter bend, the conducting miter is replaced by a frequency selective surface to allow passage of the higher frequency signals to subsequent selector waveguide switches. Frequency sensitive couplers and tapers can be coupled to the switches to various operational configurations for selecting the signal of desired frequency and polarization. In addition to the ability to maintain polarization purity, the waveguide sections of the selector switch have little loss in comparison to hybrid network losses in the conventional approach. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.