Patent Publication Number: US-6657516-B1

Title: Wideband TE11 mode coaxial turnstile junction

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a junction for directing both satellite uplink and downlink signals, and, more particularly, to a coaxial turnstile junction for combining and directing satellite uplink and downlink signals where the junction has a taper in the wave launching section to provide impedance matching for waveguide irises. 
     2. Discussion of the Related Art 
     Various communications systems, such as certain cellular telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then retransmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bandwidths. For example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz. 
     The satellite is equipped with an antenna system including a configuration of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. Typically, the antenna system includes one or more arrays of feed horns, where each feed horn array includes an antenna reflector for collecting and directing the signals. In order to reduce weight and conserve satellite real estate, some satellite communications systems use the same antenna system and array of feed horns to receive the uplink signals and transmit the downlink signals. Combining satellite uplink signal reception and downlink signal transmission functions for a particular coverage area using a reflector antenna system requires specialized feed systems capable of supporting dual frequencies and providing dual polarization, and thus requires specialized feed system components. Also, the downlink signal, transmitted at high power (60-100 W) at the downlink bandwidth (18.3 GHz-20.2 GHz), requires low losses due to the cost/efficiency of generating the power and heat generated when losses are present. 
     These specialized feed system components include signal junctions, such as coaxial turnstile junctions, known to those skilled in the art, used in combination with each feed horn to provide signal combining and isolation to separate the uplink and downlink signals. The current turnstile junctions are limited in their ability to provide suitable impedance matching between the downlink waveguide and the junction over the complete downlink frequency bandwidth. Thus, there is a need to provide a junction that has better impedance matching between the junction and the downlink waveguides. It is therefore an object of the present invention to provide an improved coaxial turnstile junction for this purpose. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a coaxial turnstile junction is disclosed for combining and directing both satellite uplink and downlink signals, that includes a tapered section to provide an improved impedance matching for the downlink signal between the junction and the downlink waveguides. The junction includes a first end that is in signal communication with an antenna feed horn. The first end of the junction includes a cylindrical outer wall and a cylindrical inner wall that are coaxial and define an outer chamber and an inner chamber. The outer wall extends into the tapered section at a second end opposite the first end, where the tapered section contacts the inner wall and closes the outer chamber at that end. A plurality of symmetrically disposed waveguides are positioned around the outer wall and are in signal communication with the outer chamber through openings in the tapered section. Irises are provided at the connection between the downlink waveguides and the outer chamber for impedance matching purposes. 
     Satellite downlink signals propagate through the downlink waveguides to the feed horn through the outer chamber. Satellite uplink signals received by the feed horn are directed through the inner chamber and exit the second end to be sent to receiver circuitry. The dimensions of the irises and the flare angle of the tapered section are selected and optimized so that the downlink signal from the downlink waveguides is impedance matched to the outer chamber at the downlink frequencies. 
    
    
     Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a coaxial turnstile junction, according to an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of the junction shown in FIG. 1 in a longitudinal direction; and 
     FIG. 3 is a cross-sectional view of the junction shown in FIG. 1 in a transverse direction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a coaxial turnstile junction for combining and directing satellite uplink and downlink signals in a satellite communications system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     FIGS. 1-3 show various views of a coaxial turnstile junction  10  that is part of a satellite antenna system, according to an embodiment of the present invention. As will be described below, the junction  10  is a waveguide device that directs the satellite uplink signals from an antenna feed horn  12  (only shown in FIG. 2) to receiver circuitry, and directs the satellite downlink signals from transmission circuitry to the feed horn  12 . In one embodiment, the downlink signal is in the frequency range of 18.3 GHz-20.2 GHz, and the uplink signal is in the frequency range of 28-30 GHz. The dimensions of the junction  10  would be optimized for the particular frequency bands of interest. The antenna system on the satellite would employ several feed horns and associated junctions in a particular array, and may also employ a plurality of such arrays. Additionally, each array of feed horns may include a reflector system for collecting and directing the uplink and downlink signals. The feed horn  12  can have any dimensional shape suitable for the purposes described herein. 
     The junction  10  includes a waveguide structure  14  having an outer wall  16  and an inner wall  18  that define an outer waveguide chamber  22  and an inner waveguide chamber  24 . The walls  16  and  18  can be made of any suitable conductive metal for the purposes described herein, such as aluminum or copper. The chambers  22  and  24  are in signal communication with the feed horn  12  at one end  26  of the structure  14 . The inner wall  18  is cylindrical along the entire length of the structure  14 . The outer wall  16  includes a cylindrical section  28  and a tapered conical section  30 , where the cylindrical section  28  and the inner wall  16  are coaxial. The tapered section  30  extends from a rim  32  in the wall  16 , and contacts the inner wall  18  so as to define a flare angle θ therebetween. Thus, the end of the chamber  22  opposite the feed horn  12  is closed. The outer wall  16  and the inner wall  18  may take on other geometrical shapes, such as rectangular, as long as the section  30  is tapered. 
     In this embodiment, four downlink waveguides  38 - 44  are symmetrically disposed around the tapered section  30 . The waveguides  38 - 44  are in signal communication with the outer chamber  22  through impedance matching irises  46 - 52 , respectively. It is important that the waveguides  38 - 44  be symmetrically disposed about the structure  14  for signal matching purposes. However, in alternate embodiments, a different number of waveguides can be provided, such as two waveguides, around the structure  14 . In this embodiment, the waveguides  38 - 44  and the irises  46 - 52  are rectangular shaped, however, in alternate embodiments, the shape of these components may take on different configurations. 
     A satellite uplink signal received by the feed horn  12  is directed into the waveguide structure  14 . The uplink signal that propagates through the inner chamber  24  is directed to a microwave network and to receiver circuitry (not shown) through the end of the structure  14  opposite the feed horn  12 . The receiver circuitry may include a polarizer and an ortho-mode transducer, as would be well understood to those skilled in the art. In this embodiment, the internal chamber  24  is free space. In alternate embodiments, it may be necessary to change the dielectric constant of the internal chamber  24  for signal propagation purposes by providing a suitable dielectric therein. The uplink signal that enters the outer chamber  22  and propagates down the waveguides  38 - 44  is at the uplink frequency, and thus is filtered by the transmission circuitry. 
     The downlink signal to be directed by the feed horn  12  enters the waveguides  38 - 44  from suitable transmission circuitry (not shown), that may include phase matching networks and the like, as would also be well understood to those skilled in the art. Any impedance mismatch between the waveguides  38 - 44  and the waveguide structure  14  results in signal loss, thus providing loss of transmission energy. According to the invention, the tapered section  30  provides signal impedance matching and coupling for the signal propagating from the waveguides  38 - 44  into the outer chamber  22 . The impedance of the signal at different locations along the length of the tapered section  30  varies depending on the dimensions of the waveguide  14  at that location, thus providing the ability to use this section as an impedance matching tool. 
     The impedance matching and coupling provided by the tapered section  30  is designed in combination with the irises  46 - 52  to provide the desired impedance matching at the particular downlink frequency band. For example, the width and length of the irises  46 - 52  and the location of the irises  46 - 52  along the tapered section  30  are optimized for the particular frequency. Likewise, the flare angle θ and the length of the tapered section  30  is also optimized in combination with the size and position of the irises  46 - 52 . The waveguide structure  14  is designed to transmit the lowest fundamental (TE 11 ) mode. In one embodiment, for a downlink signal of about 30 GHz, θ is selected to be about 10°. One skilled in the art would know how to optimize these parameters for a particular frequency band. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.