Abstract:
An antenna feed system capable of simultaneously receiving and transmitting in multiple frequency bands is disclosed. In one embodiment, the feed system comprises a dielectric loading rod, an inner cylindrical waveguide, and one or more outer cylindrical waveguides. The dielectric loading rod lies along a central axis, as do the inner waveguide and outer waveguides. The axis of the inner waveguide and the rod may coincide. Each waveguide may be configured to receive and transmit a different frequency band simultaneously with the other waveguides. In addition, the axes of the outer waveguides also coincide with the central axis. The antenna feed system may further comprise one or more junctions disposed to propagate electromagnet radiation into and out of the inner waveguide and outer waveguides. A method and kit for simultaneously receiving and transmitting in multiple frequencies are also disclosed.

Description:
The United States Government may possibly retain an interest in this patent application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of wireless communications and, more particularly, to satellite antenna feed systems. 
     2. Description of the Related Art 
     Satellite communications systems convey information on carrier signals in a number of frequency bands approved for this purpose by regulatory organizations and standards bodies. Among the most widely-used bands are the C-band (3.625-4.200 GHz for downlink, 5.850-6.425 GHz for uplink), X-band (7.250-8.400 GHz), and Ku-band (10.950 to 12.750 GHz for downlink, 14.000-14.500 GHz for uplink). 
     Turning now to FIG. 1, a typical satellite communications system is shown. For uplink transmission from a ground station  110  to a satellite  109 , a data signal (which may be digital or analog) is first sent to a modulator circuit  112  in ground station  110 . There, the data signal modulates a carrier signal with a frequency in one of the permitted frequency bands. The modulated carrier signal is then sent to an input port on an antenna feed  102 . Antenna feed  102  is typically a waveguide assembly positioned such that its radiated output is efficiently coupled to a system of one or more reflector units  100 . Antenna feed  102  acts as a transducer that converts the modulated carrier signal into radiated electromagnetic waves  114  that illuminate the reflector units. The waves are then directed by reflector units  100  to satellite  109 . 
     Downlink transmission from satellite  109  to ground station  110  is commonly received by the same antenna system used for the uplink transmission. The above process is reversed for the antenna system to receive a signal from satellite  109 . A modulated carrier transmitted by satellite  109  is first directed by reflectors  100  into antenna feed  102 . Antenna feed  102  then acts as a transducer to route the received waves to transceiver  108 . A demodulator receives the modulated carrier signal from the receive ports and recovers the data stream transmitted by satellite  109 . 
     One potential limitation with prior art systems is that in order to transmit/receive in all three of the popular frequency bands C, X and Ku, three physically separate antenna feed structures are typically needed. For example, a C-band antenna feed with its own I/O port may be needed for transmitting/receiving in the C-band; an X-band antenna feed with its own I/O port may be needed for transmitting/receiving in the X-band; and a Ku-band antenna feed with its own I/O port may be needed for transmitting/receiving in the Ku-band. 
     Since three separate antenna feed structures are typically needed, it follows that the data transmission/reception from one parabolic reflector can usually occur only in one frequency band at a time. For example, before data transmission/reception can occur in the C-band, the C-band antenna feed may often be physically moved such that its I/O port is located at the focal point of the parabolic reflector. Then, to switch data transmission/reception to the X-band, the C-band antenna feed is physically moved out of the focal point of the reflector so that the X-band antenna feed can be physically moved to the focal point of the reflector. Consequently, the number of data streams which are transmitted/received simultaneously may be limited to the number of data streams which fit into one frequency band. Having to physically move the C-band, X-band and Ku-band antenna feed structures to and from the focal point of the reflector is a time-consuming and tedious operation. Furthermore, if the movement is not done accurately, misalignment problems between the reflector and the I/O port of the antenna feed structure may occur. 
     For example, when the I/O port of an antenna feed is misaligned with its reflector, the radiation pattern of the transmitted electromagnetic waves may become distorted. This distortion may in turn interfere with transmissions from other independent sources. Consequently, many ground stations limit their transmissions/receptions to just one of the three bands C, X, and Ku. 
     It is common to use antennas having paraboloidal reflectors (e.g., reflector  100  in FIG. 1) in applications such as space communications where radio frequency signals in the form of microwave frequency electromagnetic waves are transmitted between an earth station and a satellite or vice versa. Such antennas may be constructed in a prime focus configuration where microwave frequency energy is coupled to a transceiver by an antenna feed mounted near a focal point of the paraboloidal reflector. The antennas may also be constructed in other configurations such as Gregorian or Cassegrain. Doubly-shaped reflectors may be used as well. These configurations use a small hyberboloidal subreflector mounted near the focal point of the paraboloidal reflector, allowing the feed to be placed between the paraboloidal and hyperboloidal reflectors. Paraboloidal reflector antennas are also used in radar and other communications applications as well. 
     Regardless of the feed configuration or system application, it is the purpose of the feed to connect a transceiver to the paraboloidal reflector. Antennas intended for operation over multiple frequency bands may normally require a corresponding number of multiple feeds and subreflectors. U.S. Pat. No. 4,092,648 to Fletcher, et al., issued May 30, 1978, and assigned to the National Aeronautics and Space Administration of the United States Government, incorporated herein by reference in its entirety, shows a typical multiple-band antenna having a main reflector that diverts energy to a subreflector and then to a flange. The flange is arranged to pass radiation in a first frequency band to a first horn. Energy in a second frequency band is reflected by the flange to an auxiliary reflector. The auxiliary reflector is arranged to feed energy to a second horn. 
     If operation in more than two frequency bands is required, subreflector, auxiliary reflector, and multiple horn configurations may become more complicated. In some instances, it may be desirable to tilt and rotate the subreflectors about a symmetry axis in order to provide better tracking of the satellite or other signal source. This further complicates construction and operation of the antenna. It is typically desirable to keep the antenna assembly as small and simple as possible. 
     Various communication systems employ more than one frequency band for electromagnetic signals radiated from a transmitting station to receiving station. An important example of such a communication system is a satellite communication system wherein various bands of signals are transmitted between a satellite above the earth (synchronous orbit) and ground stations on the earth. As previously noted, three such bands of interest are the C band, X band, and Ku band, which together extend over two octaves of the communication frequency spectrum. Within each of the bands, there is frequency space allocated for reception of signals at the satellite and for transmission of signals from the satellite. The C band itself extends over approximately an octave, operates at both linear and circular polarizations, and includes a receive sub-band in the range of 3.625-4.200 GHz and a transmit sub-band in the range of 5.850-6.425 GHz. The X band includes a receive sub-band in the range of 7.250-7.750 GHz (gigahertz), and a transmit sub-band for transmission from the satellite in the range of 7.900-8.400 GHz. The Ku band operates at both linear and circular polarizations, and includes a receive sub-band from 10.950 to 12.750 GHz, and a transmit sub-band of 14.000-14.500 GHz. Collectively, these frequency bands extend over approximately two octaves of the communications spectrum. 
     Historically, it has been the practice to provide separate antennas for transmission or reception on each of the bands because there is insufficient bandwidth on any one of the antenna systems or terminals to transmit more than one of the bands. In some cases, where bands are close together and, collectively, do not occupy an excessive amount of spectral space, it has been possible to share a plurality of bands on one antenna. However, basically separate antennas have typically been employed for different portions of the spectrum. In particular, there is no adequate single-point antenna feed system which can cover plural octave bandwidths which include the C, X, and Ku bands. 
     A further problem arises in the case of satellite communication transportable earth stations in that there is a need to minimize the weight of the system. The use of numerous antennas for communication at various frequency bands may defeat this purpose. In addition, it is advantageous to employ a common phase center for all frequencies of radiation transmitted from the earth station and received at the earth station. This is typically lacking when several antenna feeds are mounted at different times upon an earth terminal. Furthermore, changing the feed system for each frequency band and refocusing the feed requires extra time and trained personnel. The same problems exist for an earth terminal at a fixed location that performs the difficult and tedious process of exchanging feeds and refocusing. 
     The foregoing problems are compounded by the previously described spectral utilization. The C band and the Ku band are commercial satellite bands which are spaced apart in the spectrum and, therefore, facilitate the filtering of signals in the two bands so as to permit transmission on one band without significant interference with signals on the other band. However, in some applications there is a need to employ the X band (which is a military band) in conjunction with the C band. However, due to the fact that the X band is contiguous to the C band, it is difficult to separate the two bands in a common antenna system. Presently available antenna and feed structures appear unable to accomplish this task adequately. 
     SUMMARY OF THE INVENTION 
     The problems outlined above may in part be solved by an antenna feed system configured to propagate electromagnetic radiation in a plurality of frequency bands simultaneously. In one embodiment, the antenna feed system comprises a dielectric loading rod, an inner waveguide, and one or more outer waveguides. The dielectric loading rod lies along a central axis, as do the inner waveguide and outer waveguides. The axis of the inner waveguide and the rod may coincide. In addition, the axes of the outer waveguides also coincide with the central axis. The antenna feed system may further comprise one or more junctions disposed to propagate electromagnet radiation into and out of the inner waveguide and outer waveguides. 
     In another embodiment, the antenna feed system may further comprise one or more external waveguides positioned to propagate electromagnetic radiation to the junction and away from the junction. The external waveguides may propagate linearly and/or elliptically polarized electromagnetic radiation into and out of the junctions. The antenna feed system may further comprise a sub-reflector. The sub-reflector may be positioned outside the waveguides along the central axis so that the sub-reflector may reflect electromagnetic radiation into and out of the waveguides. The sub-reflector may be positioned to further reflect the electromagnetic radiation from the waveguides to a main reflector (and vice versa). In one embodiment, the longitudinal axis of the waveguides may not coincide with the axis of the main reflector. The sub-reflector may also be positioned so that it does not lie on the axis of the main reflector. 
     In one embodiment, the inner waveguide and outer waveguides may each be configured to propagate electromagnetic radiation in a particular frequency band (e.g., the L band, the S band, the lower C band, the upper C band, the X band, the Ku band, or the Ka band). In another embodiment, the antenna feed system may comprise a plurality of waveguide means, each with an opening, and each configured to simultaneously transmit and/or receive a particular frequency band or sub-band. The antenna feed system may further comprise a means for reflecting electromagnetic radiation to and from the openings of the waveguide means. In this and other embodiments, the antenna system may further comprise a plurality of transceiving means coupled to the waveguide means. 
     A method for propagating electromagnetic radiation is also contemplated. In one embodiment, the method comprises propagating electromagnetic radiation along a feed. The feed may comprise a plurality of concentric cylindrical waveguides. A different frequency band may be simultaneously received and/or transmitted in each of the plurality of concentric cylindrical waveguides. The electromagnetic radiation may be propagated into and out of the feed a predetermined number of points along the length of the feed and at the end of the feed. The electromagnetic radiation propagated to and away from the points may be linearly and/or elliptically polarized electromagnetic radiation. In some embodiments, the method may further comprise reflecting electromagnetic radiation to and from a main reflector. 
     An antenna feed system for propagating communication signals in a plurality of frequency bands simultaneously is also contemplated. In one embodiment, the system may comprise at least three cylindrical conductors each having a different diameter and each being coaxially positioned. The antenna feed system may further comprise a fourth cylindrical conductor. The antenna feed system may be configured to transmit and/or receive two, three, or more frequency bands simultaneously. 
     In one embodiment, the cylindrical conductors may operate in a transverse electric (TE) mode. In another embodiment, the conductors may operate in the TE 1,1  mode. In one such embodiment, the second third and fourth conductors may form two outer waveguides that propagates electromagnetic waves in the C-band (e.g. the C-band received frequency range and the C-band transmitted frequency range). Similarly, the first and second conductors may form a third waveguide that propagates electromagnetic waves in the X-band frequency range. In one embodiment, the first conductor may form a fourth waveguide that propagates electromagnetic waves in the Ku-band frequency range. A fifth cylindrical conductor may be positioned coaxially with the first through fourth conductors. The fifth conductor may be configured to form a waveguide with the first conductor that propagates radiation in the Ka frequency band. 
     In one embodiment, the antenna feed system may further comprise a dielectric loading rod. The dielectric loading rod may be constructed of rexalite and may be positioned coaxially within the first conductor. The antenna feed system may further comprise one or more junctions coupled to the conductors. One or more rectangular waveguides may be coupled to the junctions. The junctions and the rectangular waveguide may serve as a transfer mechanism to transfer electromagnetic waves between the waveguides and a plurality of transceivers. In one embodiment, a rotatable flange may be coupled to one of the waveguides. Rotating the flange may change the polarization of the coupled waveguides. 
     In yet another embodiment, the antenna feed system may further comprise a plurality of polarization circuits. Each polarization circuit may be coupled to one of the waveguides. Each polarization circuit may comprise two waveguide combiners that are coupled to the rectangular waveguides. The waveguide combiners are each configured to receive two input signals from two rectangular waveguides coupled to different positions of a cylindrical waveguide. The combiners are configured to subtract the inputs to create combined signals. A highbred coupler may be included in the system to receive and combine the combined signals and generate an output signal therefrom. 
     In another embodiment, the antenna feed system may further comprise a choke coupled to an open end of the outer most cylindrical conductor. The choke may be configured to prevent spill over radiation. 
     A kit for creating an antenna feed system capable of transmitting and receiving a plurality of communication signals and a plurality of frequency bands simultaneously is also contemplated. In one embodiment, the kit may comprise a sub-reflector configured to receive wireless communication signals reflected from a parabolic dish; three or more cylindrical waveguides; and a plurality of junctions. The cylindrical waveguides may be positioned coaxially when each waveguide has a first end that is configured to face the sub-reflector, and each waveguide may be configured to independently and simultaneously convey a particular frequency band. Each junction may be coupled to a second end of the waveguides. The junctions may be configured to couple the waveguide to a plurality of communications transceivers via a plurality of external waveguides. 
     A method for transmitting and receiving a plurality of wireless communications frequency bands is also contemplated. In one embodiment, the method may comprise: positioning a parabolic dish antenna to receive the plurality of wireless frequency bands from a source; positioning a sub-reflector to reflect the plurality of wireless communications frequency bands from the parabolic dish antenna to a plurality of cylindrical waveguides having different diameters; receiving a first frequency band with the first cylindrical waveguide; and simultaneously receiving a second frequency band with the second cylindrical waveguide. In some embodiments, additional frequency bands (e.g. a third and/or a fourth frequency band) may be simultaneously transmitted and/or received. The method may further comprise changing the polarization of the frequency band received by one of the waveguides by rotating a flange coupled to the waveguide. The method may also comprise adjusting the frequency band received by one of the waveguides by inserting a dielectric loading rod. 
     A feed for multi-band satellite communications operation is also contemplated. In one embodiment, coaxial waveguides and a subreflector are utilized with dual-offset antennas. The feed design may consist of a formation of coaxial concentric waveguide cavities operating in the TE 1,1  mode. This mode, normally used in circular waveguide feed design, is also amenable for use in coaxial waveguides. The frequency of operation of this mode, in the coaxial waveguide, is dependent on the dimensions of the inner and outer radius of the waveguide. The cutoff wavelength is related to the ratio of the radii of the inner and outer cylinders. 
     In one embodiment, the largest, outermost cavities of the feed operate at the C-Band, the central cavities operate at the X-Band, and the innermost cavity operates at the Ku-Band. Because the commercial C-Band SATCOM frequency range covers a spectrum exceeding the operating limits of a single cavity, in one embodiment the feed incorporates two cavities to serve this frequency range, i.e., one for receive (3.4-4.2 GHz), and the other for transmit (5.850-6.650 GHz). The X-Band and Ku-Band frequency ranges, having smaller percentage bandwidths, are each served by a single cavity per band. In some embodiments, orthogonal rectangular waveguides may feed the antenna to permit polarization diversity, obtainable in all bands. 
     Advantageously, in some embodiments a multi-band antenna feed may provide the capability to operate a satellite antenna in all three frequency bands without the need for manual intervention. Other logistical advantages may also be present, depending upon the implementation, e.g., fewer parts may be used and training requirements may be reduced. In some embodiments, retrofitting existing earth station systems to increase their capabilities may also be possible. For example, the feed may potentially be modified (e.g., by adding, deleting, or changing the size of the coaxial cavities) to include any number of frequency bands up to 100 GHz. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which: 
     FIG. 1 shows one embodiment of a satellite antenna system. 
     FIG. 2 is a diagram of one embodiment of a satellite antenna system utilizing one embodiment of a multi-band antenna feed. 
     FIG. 3 is a diagram of one embodiment of an antenna feed connected to a set of transceivers by a set of waveguides. 
     FIG. 4 is a cross-section of one embodiment of a multi-band antenna feed. 
     FIG. 5 is a cross-section of one embodiment of a dielectric loading rod. 
     FIGS. 6 a, b  and  c  are diagrams of one embodiment of a flange. 
     FIGS. 7 a, b  and  c  are diagrams of one embodiment of an interface. 
     FIGS. 8 a, b  and  c  diagrams of one embodiment of a link. 
     FIGS. 9 a  and  b  diagrams of one embodiment of a flange. 
     FIGS. 10 a  and  b  diagrams of one embodiment of a junction. 
     FIGS. 11 a  and  b  are diagrams of one embodiment of a link. 
     FIGS. 12 a, b  and  c  are diagrams of one embodiment of a flange. 
     FIGS. 13 a  and  b  are diagrams of one embodiment of a junction. 
     FIGS. 14 a, b  and  c  are diagrams of one embodiment of a link. 
     FIGS. 15 a  and  b  are diagrams of one embodiment of a waveguide. 
     FIGS. 16 a  and  b  are diagrams of one embodiment of a choke. 
     FIG. 17 is an isometric view of one embodiment of an antenna feed. 
     FIG. 18 is a diagram of a circuit for receiving and transmitting circularly polarized signals. 
     FIGS. 19 a, b  and  c  are sets of plots of radiation patterns generated by one embodiment of an antenna feed. 
     FIG. 20 is a diagram of one embodiment of a dual offset geometry configuration. 
     FIGS. 21 a  and  b  are diagrams of one embodiment of a junction. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to FIG. 2, one embodiment of an antenna system  90  utilizing a multi-band antenna feed  102  is shown. In this embodiment, antenna system  90  includes a main reflector  100 , a subreflector  101 , and an antenna feed  102 . The main reflector  100  is preferably paraboloidal and is, with subreflector  101  and feed  102  in a Gregorian configuration in which subreflector  101  is near a focal point  103  of main reflector  100  and feed  102  is near a focal point  104  of subreflector  101 . A support member  105  supports subreflector  101  and feed  102  in positions away from main axis  106  of the main reflector  100 . Waveguides  107  connect the feed  102  to a plurality of transceivers  108 . While three transceivers  108  are shown, other combinations of transmitters and receivers are possible. 
     When antenna system  90  is being used to transmit signals to a remote station  109  such as an earth satellite, transceivers  108  send electromagnetic radiation down waveguides  107  to feed  102 . The electromagnetic radiation exits feed  102  and travels to subreflector  101 . Subreflector  101  reflects the electromagnetic radiation toward main reflector  100 . The main reflector reflects the electromagnetic radiation toward the remote station  109 . 
     When the antenna system is being used to receive signals transmitted by a remote station  109 , the remote station transmits electromagnetic radiation toward main reflector  100 , which reflects the electromagnetic radiation toward its focal point  103  and the nearby subreflector  101 . Subreflector  101  in turn reflects the electromagnetic radiation toward feed  102 . The electromagnetic radiation propagates through the feed to waveguides  107 . Waveguides  107  carry the electromagnetic radiation to the transceivers  108 . 
     Referring now to FIG. 3, the structure and the relative positions of the antenna feed  102  and the subreflector  101  are shown in more detail. In the following discussion, the end of feed  102  near subreflector  101  is referred to as the “front” end and the opposite end is referred to as the “rear” end. In one embodiment, antenna feed  102  has a junction  201  for the entry and exit of electromagnetic radiation in the lower part of the C frequency band, a junction  202  for the entry and exit of electromagnetic radiation in the upper part of the C band, a junction  203  for the entry and exit of electromagnetic radiation in the X band, and an interface (or flange)  204  for the entry and exit of electromagnetic radiation in the Ku band. As used herein, the term “bands” shall refer to both bands and distinct sub-bands. Waveguides  107  connect junctions  201 ,  202 , and  203  and interface  204  to transceivers  108 . 
     When the antenna system is being used to receive signals (as from remote station  109  in FIG.  1 ), electromagnetic radiation is reflected from subreflector  101  into the open front end of feed  102 . Electromagnetic radiation signals in the lower C band propagate down a part of the length of the feed  102  to junction  201 , where they exit the feed  102  and propagate away from the feed  102  through one set of waveguides  107  to one of the transceivers  108 . Electromagnetic radiation signals in the upper C band propagate down a part of the length of the feed  102  to junction  202 , where they exit the feed  102  and propagate away from the feed  102  through one set of waveguides  107  to one of the transceivers  108 . Electromagnetic radiation signals in the X band propagate down a part of the length of the feed  102  to junction  203 , where they exit the feed  102  and propagate away from the feed  102  through one set of waveguides  107  to one of the transceivers  108 . Electromagnetic radiation signals in the Ku band propagate down the length of the feed  102  to the rear end of the feed  102 , where they exit the feed  102  and propagate away from the feed  102  through one set of waveguides  107  to transceivers  108 . 
     When the antenna system is being used to transmit signals (as to remote station  109  in FIG.  1 ), electromagnetic radiation propagates from transceivers  108  through waveguides  107  to junctions  201 ,  202 , and  203 , and to interface  204  to the feed  102 . Electromagnetic radiation in the Ku band enters the feed  102  at the rear end of the feed  102 . Electromagnetic radiation in the X band enters the feed  102  at junction  203 . Electromagnetic radiation in the upper C band enters the feed  102  at junction  202 . Electromagnetic radiation in the lower C band enters the feed  102  at junction  201 . After entering the feed  102 , the electromagnetic radiation in all four bands propagates to the front end of the feed  102 . The electromagnetic radiation in all four bands exits the feed  102  at the its front end and propagates to the subreflector  101 , where it is reflected to the main antenna  100  of FIG.  1 . The electromagnetic radiation is then reflected to the remote station  109  of FIG.  1 . 
     Antenna feed  102  and subreflector  101  enable an antenna system to transmit and receive signals simultaneously. While one embodiment enables transmission and reception of signals in the lower C, upper C, X, and Ku bands simultaneously, other embodiments enable such simultaneous transmission and reception of signals in the L and S bands; in the Ka and Ku bands; and in the lower C, upper C, X, Ku, and Ka bands. 
     Referring now to FIG. 4, the internal structure of the antenna feed  102  is shown in more detail. In one embodiment, the components of the feed  102  are made of brass; in another embodiment, they are made of aluminum. In one embodiment, a coaxial cavity  301  for the propagation of electromagnetic radiation in the Ku band runs down the length of the feed  102 . In one embodiment, the Ku band cavity  301  is 27.300 in. long and has an inner diameter of 0.500 in., and the waveguide forming the outer surface of the Ku band cavity  301  may have an outer diameter of 0.580 in. The cavity  301  may contain a dielectric loading rod  302 , which adjusts the frequency response of the cavity  301  to allow the propagation of electromagnetic radiation in the Ku band. In one embodiment, the rod  302  is made of rexalite, a material with a dielectric constant of approximately 2.3. FIG. 5 shows the rod  302  in more detail. In one embodiment, the rod has a cylindrical part which is 0.500 in. in diameter and 26.000 in. long and a part 1.100 in. long that tapers to a point. In one embodiment, the cutoff frequency for the cavity  301  is lowered to 8.665 GHz by the use of the rod  302 . 
     The waveguide forming the outer surface of cavity  301  is soldered into a flange  303  into which radial slots are cut to allow the polarization of the signals in the cavity to be adjusted manually. FIGS. 6 a-c  show the flange in more detail. FIG. 6 a  is a view from the front, FIG. 6 b  is a view from the side, and FIG. 6 c  is a view from the rear. In one embodiment, the front component of the flange  303  is 0.797 in. in diameter, 0.100 in. thick and has a central hole of diameter 0.500 in.; the middle component is 1.900 in. in diameter, 0.100 in. thick, and has a central hole of diameter 0.580 in.; and the rearmost component is 3.000 in. in diameter, 0.100 in. thick, and has a central hole of diameter 0.500 in. In one embodiment, there are two 90° radial slots in the rearmost component that are 0.170 in. wide at a radius of 1.250 in. from the longitudinal axis of the flange. 
     The cavity  301  is also connected to an interface  304  that connects the cavity  301  to a waveguide  107  of FIG. 1 of standard size in the industry. FIGS. 7 a-c  show the interface  304  in more detail. FIG. 7 a  is a view from the front, FIG. 7 b  is a view from the side, and FIG. 7 c  is a view from the rear. In one embodiment, the outer diameter of the disk that forms the front part of the interface  304  is 3.200 in., the inner diameter of the ring at the rim of the disk on the front side is 3.000 in., and the inner diameter of the hole in the center of the front part is 0.500 in. In one embodiment, the thickness of the disk at the front is 0.100 in., and the thickness of the ring is 0.200 in. In one embodiment, the length of the interface  304  from the front edge of the ring at the front to the rear surface is 1.200 in. In one embodiment, the component of the interface  304  which forms the rear part of the interface is 2.000 in. in diameter and the hole through the center of the interface  304  is 0.673 in. in diameter at the rear surface. 
     Link  309  connects cavity  301  to cavity  305 . The waveguide forming the outer surface of cavity  301  is inserted through the center of link  309  to form this connection. FIGS. 8 a-c  show link  309  in more detail. FIG. 8 a  is a front view, FIG. 8 b  is a side view, and FIG. 8 c  is a rear view. In one embodiment, the front of the link  309  comprises a round cavity of inner diameter 0.797 in. and a front-to-rear depth of 0.500 in., the rear of the link  309  comprises a cavity of inner diameter 1.900 in., and the front-to-rear length of the link  309  is 0.700 in. In one embodiment, the outer diameter of the wall of the cavity is 2.000 in. 
     Junction  203  is an orthomode transducer through which electromagnetic radiation in the X band enters and exits a coaxial cavity  305 . The longitudinal axis of the cavity  305  coincides with the longitudinal axis of cavity  301 . The inner conductor of the cavity  305  is formed by the outer conductor of the cavity  301 . The cavity  305  runs from the junction  203  to the front end of the feed  102 . 
     In one embodiment, the cavity  305  is configured for the propagation of electromagnetic radiation in the X band. In one embodiment, the X band cavity  305  is 24.880 in. long and has an inner diameter of 0.797 in. In one embodiment, the waveguide forming the outer surface of the X band cavity  305  has an outer diameter of 0.895 in. 
     The waveguide forming the outer surface of cavity  305  is soldered into a flange  310 . FIGS. 9 a-b  show the flange in more detail. FIG. 9 a  is a view from the front. FIG. 9 b  is a view from the side. In one embodiment, the front component of the flange  310  is 1.245 in. in diameter, 0.100 in. thick, and has a central hole of diameter 0.950 in. at the front, narrowing at a 45° angle to a diameter of 0.900 in.; and the rear component is 1.900 in. in diameter, 0.100 in. thick, and has a central hole of diameter 0.797 in. 
     The junction  203  permits the propagation of electromagnetic radiation from the cavity  305  to waveguides  107  of FIG. 2 of rectangular cross-section, and from those waveguides  107  to the cavity  305 . FIGS. 10 a-b  show the junction  203  in more detail. FIG.  10   a  is a side view and FIG. 10 b  is a front view. In one embodiment for coupling an X band cavity  305  to the waveguides  107 , the distance from the front to the rear of the interior the junction cavities and of each waveguide  107  at the point it joins the junction  203  is 0.497 in., the inner width is 1.122 in., and has rounded comers of 0.125 in. radius. In one embodiment, the hole through the longitudinal axis of the junction  203  has a diameter of 0.797 in. In one embodiment, the junction  203  is 2.200 in. from front to rear and 2.250 in. wide and 2.250 in. high. 
     Link  311  connects waveguide forming the outer surface of cavity  305  to the waveguide forming the outer surface of cavity  306 . The waveguide forming the outer surface of cavity  305  is inserted through the center of link  311  to form this connection. FIGS. 11 a-b  show link  311  in more detail. FIG. 11 a  is a side view. FIG. 11 b  is a rear view. In one embodiment, the front of the link  311  comprises a round component of diameter 3.000 in. and a thickness of 0.200 in., the rear of the link  311  comprises a component of diameter 2.000 in. and a thickness of 0.200 in., and these two components are joined by a cylindrical component 4.350 in. in length. The front end of the link  311  is connected to junction  202  and the read end of the link  311  is connected to junction  203 . 
     Junction  202  is an orthomode transducer through which electromagnetic radiation in the upper C band enters and exits a coaxial cavity  306 . The longitudinal axis of the cavity  306  coincides with the longitudinal axis of cavity  305 . The inner conductor of the cavity  306  is formed by the outer conductor of the cavity  305 . The cavity  306  runs from the junction  202  to the front end of the feed  102 . 
     In one embodiment, the cavity  306  is configured for the propagation of electromagnetic radiation in the upper C band. In one embodiment, the upper C band cavity  306  is 17.630 in. long and has an inner diameter of 1.281 in. and the waveguide forming the outer surface of the X band cavity  305  has an outer diameter of 1.441 in. 
     The waveguide which forms the outer surface of cavity  306  is soldered into a flange  312 . FIGS. 12 a-c  show the flange in more detail. FIG. 12 a  is a view from the front. FIG. 12 b  is a view from the side. FIG. 12 c  is a view from the rear. In one embodiment, the front component of the flange  312  is 2.029 in. in diameter, 0.250 in. thick, and has a central hole of diameter 1.535 in. at the front, narrowing at a 45° angle to a diameter of 1.450 in.; and the rear component is 2.900 in. in diameter, 0.220 in. thick., and has a central hole of diameter 1.291 in. 
     The junction  202  permits the propagation of electromagnetic radiation from the cavity  306  to waveguides  107  of FIG. 2 of rectangular cross-section, and from those waveguides  107  to the cavity  306 . FIGS. 13 a-b  show the junction  202  in more detail. FIG. 13 a  is a side view and FIG. 13 b  is a front view. In one embodiment for coupling an upper C band cavity  306  to the waveguides  107 , the distance from the front to the rear of the interior the junction cavities and of each waveguide  107  at the point it joins the junction  202  is 0.622 in., the inner width is 1.372 in., and has rounded comers of 0.250 in. radius. In one embodiment, the hole through the longitudinal axis of the junction  202  has a diameter of 1.281 in. In one embodiment, the junction  202  is 2.900 in. from front to rear, 3.470 in. wide, and 3.470 in. high. 
     Link  313  connects waveguide forming the outer surface of cavity  306  to the waveguide forming the outer surface of cavity  307 . The waveguide forming the outer surface of cavity  306  is inserted through the center of link  313  to form this connection. FIGS. 14 a-c  show link  313  in more detail. FIG. 14 a  is a front view. FIG. 14 b  is a side view. FIG. 14 c  is a rear view. In one embodiment, the front of the link  313  comprises a round component of diameter 3.250 in. and a thickness of 0.200 in., the rear of the link  313  comprises a component of diameter 3.000 in. and a thickness of 0.200 in., and these two components are joined by a cylindrical component 7.075 in. in length. The front end of the link  313  is connected to junction  201  and the read end of the link  313  is connected to junction  202 . 
     Junction  201  is an orthomode transducer through which electromagnetic radiation in the lower C band enters and exits a coaxial cavity  307 . The longitudinal axis of the cavity  307  coincides with the longitudinal axis of cavity  306 . The inner conductor of the cavity  307  is formed by the outer conductor of the cavity  306 . The cavity  307  runs from the junction  201  to the front end of the feed  102 . The junction  201  permits the propagation of electromagnetic radiation from the cavity  307  to waveguides  107  of rectangular cross-section, and from those waveguides  107  to the cavity  307 . 
     The junction  201  permits the propagation of electromagnetic radiation from the cavity  307  to waveguides  107  of FIG. 2 of rectangular cross-section, and from those waveguides  107  to the cavity  307 . FIG. 21 show the junction  201  in more detail. FIG. 21 a  is a front view and FIG. 21 b  is a side view. In one embodiment for coupling a lower band cavity  307  to the waveguides  107 , the distance from the front to the rear of the junction cavities and of each waveguide  107  at the point it joins the junction is 1.145 in., the inner width is 2.290 in., and has rounded corners of 0.250 in. radius. In one embodiment, the hole through the longitudinal axis of the junction  201  has a diameter of 2.029 in. In one embodiment, the junction  202  is 2.950 in. from front to rear, 4.000 in. wide and 4.000 in. high. 
     Waveguide  314  forms the outer surface of cavity  307 . FIGS. 15 a-b  show the waveguide  314  in more detail. FIG. 15 a  is a rear view. FIG. 15 b  is a side view. In one embodiment, the circular component at the rear of the waveguide  314  has a diameter of 3.250 in. and a thickness of 0.200 in. In one embodiment, the cavity surface is formed by a cylindrical component with a length of 7.505 in., an inner diameter 2.029 in., and an outer diameter of 2.129 in. for all but the front 2.300 in. of its length, where it has an outer diameter of 2.129 in. 
     In one embodiment, choke  308  is attached to the front end of the waveguide forming the outer surface of cavity  307 . Choke  308  narrows the radiation pattern of the cavity  307  by preventing spillover around the edges at the front end of the cavity. FIGS. 16 a-b  show the choke  308  in more detail. FIG. 16 a  is a front view. FIG. 16 b  is a side view. In one embodiment, the front of the choke  308  comprises a round cavity of inner diameter 2.629 in and a front-to-rear depth of 0.750 in., the rear of the choke  308  comprises a cavity of inner diameter 2.129 in., and the front-to-rear length of the choke  308  is 1.250 in. In one embodiment, the outer diameter of the wall of the cavity is 2.729 in. 
     One embodiment uses a “double slug” approach to change the characteristic impedance of the cavity  307 . In the double slug approach, two annular rings of metal or dielectric are placed between the inner and outer conductors forming the cavity  307 . Different placements of the rings result in different characteristic impedances. 
     Turning now to FIG. 17 an isometric view of feed  102  is shown. As the FIG. illustrates, in one embodiment junctions  201 - 203  comprise four ports equally spaced (e.g., by 90 degrees). Each port is configured to be coupled to an external waveguide  107  (in this case, rectangular) that will convey the received waves to transceivers  108  (or vice versa). Each port conveys a different phase of the waves. In one embodiment, the rectangular waveguides  107  pass the waves through a receiving circuit before they arrive at the transceivers  108 . 
     Turning now to FIG. 18, one embodiment of a polarization circuit  400  for circularly polarized radiation signals is shown. Circuit  400  includes a waveguide combiner  410  with inputs  411  and  412 , a second waveguide combiner  420  with inputs  421  and  422 , and a hybrid coupler  430  coupled to waveguide combiners  410  and  420 . 
     When the antenna system is receiving signals, electromagnetic radiation from each of the orthomode junctions  201 ,  202 , and  203  propagates from the feed  102  through waveguides  107  to corresponding circuits  400 . Waveguides  107  convey electromagnetic radiation to the corresponding circuits  400  from coupling ports on the orthomode junctions  201 ,  202 , and  203 . 
     Each of the orthomode junctions preferably includes four coupling ports. The four coupling ports are arranged in a square geometry to couple to linearly polarized electromagnetic radiation in the corresponding cavities ( 307 ,  306 ,  305 ). The four ports comprise two pairs of opposing ports; one of the pairs couples to radiation linearly-polarized in a particular direction—a “vertical” direction, while the other pair of opposing ports couples to radiation linearly-polarized in an orthogonal direction—a “horizontal” direction. Four waveguides  107  convey electromagnetic radiation from an orthomode junction (one of  201 ,  202 , and  203 ) to a circuit  400 . 
     Each of the four waveguides is coupled to one of the four coupling ports and to one of four inputs  411 ,  412 ,  421 , and  422  on circuit  400 . The four wave guides, included in the waveguides  107 , are configured to convey radiation from one of the pairs of opposing ports to inputs  411  and  412 , and to convey radiation from the other of the pairs  10  of opposing ports to inputs  421  and  422 . To constructively add the four radiation signals received through the four waveguides, waveguide combiner  410  subtracts the signal from input  412  (which is shifted by 180° in phase) from the signal from input  411  to generate a combined signal  413 . Similarly, waveguide combiner  420  subtracts the signal from input  422  (which is shifted by 270° in phase) from the signal from input  421  (which is shifted by 90° in phase), to generate a combined signal  423 . Hybrid coupler  430  receives the two combined signals  413  and  423  further combines them to generate an LCP  431  output signal. Hybrid coupler  430  also combines the two combined signals in an orthogonal manner to generate a RCP output signal  432 . 
     If the radiation received by circuit  400  was left-hand circularly polarized (LCP) in the corresponding cavity of feed  102 , then circuit  400  operates to constructively add the signals received at the four inputs  411 ,  412 ,  421 , and  422  so that the output appears in LCP output signal  431 . Conversely, if the radiation received by circuit  400  was right-hand circularly polarized (RCP) in the corresponding cavity of feed  102 , then circuit  400  operates to constructively add the four received signals so that the output appears in RCP output signal  432 . 
     When the antenna system is transmitting signals, the procedure is reversed. A transmission signal applied at port  431  is converted to an LCP wave in the corresponding waveguide, and a transmission signal applied at port  432  is converted to an RCP wave in the corresponding waveguide. In more detail, circuit  400  operates to separate LCP and RCP signals into linearly polarized signals for propagation through the waveguides  107 , where they are added together at the corresponding junction (one of  201 ,  202 , and  203 ) so that LCP and RCP signals propagate through the associated cavity (one of  307 ,  306 , and  305 ) and out the front end of feed  102 . 
     LCP input signals  431  and RCP input signals  432  are separated by hybrid coupler  430  into signals  413  and  423 . Signal  413  represents the vertically polarized components of signal  431  and  432 , and signal  423  represents the horizontally polarized components of signals  431  and  432 . Combiner  410  separates the signal  413  into two signals,  411  and  412 , where signal  412  is shifted by 180° in phase from signal  411 . Combiner  420  separates the signal  423  into two signals  421  and  422 , where signal  421  is shifted by 90° in phase from signal  411 , and signal  422  is shifted by 270° in phase from signal  411 . Signals  411 ,  412 ,  421 , and  422  propagate along waveguides  107  to the coupling ports included in the orthomode junction  201 - 203 , where they are added together in the orthomode junction  201 - 203  to form the original LCP or RCP signals  431  and  432 . 
     In one embodiment, to receive or transmit a linear polarization, hybrid coupler  430  is manually switched out of the circuit. 
     Referring now to FIGS. 19 a-c,  selected radiation patterns created by one embodiment of the feed  102  are shown. FIG. 19 a  shows the primary radiated power pattern at 10.950 GHz. FIG. 19 b  shows the primary radiated power pattern at 7.250 GHz. FIG. 19 c  shows the primary radiated power pattern at 3.625 GHz. These wide patterns may influence the design of the subreflector  101 . 
     Referring now to back to FIG. 2, the design of the subreflector  101  will be discussed in more detail. In the embodiment shown, the subreflector  101  is designed for use in dual-offset Gregorian geometry in which both the antenna feed  102  and subreflector  101  are placed on support element  105  away from the axis  106  of the main reflector  100 . This arrangement prevents blockage of the aperture of the main reflector  100  by the feed  102  and the subreflector  101 . This arrangement may also allow the transceivers  108  to be placed below and behind the main reflector  100 , where they can be connected to the feed  102  with relatively short lengths of waveguide  107 . The main reflector  100  is an offset section of a paraboloid with circular aperture of radius 2.4 meters, and defined by the focal length f and height of the midpoint y c . With the origin located at the focus the paraboloidal surface is defined in x, y, z coordinates as: 
     
       
           x   2   +y   2 =4 f ( f−z )  (1) 
       
     
     The offset subreflector  101  is an ellipsoid defined by an eccentricity e and the interfocal distance 2c. In dual reflector antenna systems, one focus of the subreflector  101  ellipse is located confocal with the focus of the main reflector  100  parabola. The subreflector  101  axis is tilted by an angle β with respect to the z-axis of the paraboloid.                y   c     =         -   4                   fe                 sin                 β       1   +        2     -     2                 e                 cos                 β                 (   2   )                                
     θ H  is the cone angle from focus F 1  that defines the rays that illuminate the edge of the subreflector  101 , which reflects them on to the edge of the main reflector  100 . The angle θ H  is computed from                tan        (       θ   H     2     )       =       R        (     1   +        2     -     2                 e                 cos                 β       )         2        f        (     1   -        2       )                   (   3   )                                
     Since θ H  is defined by the primary pattern of the coaxial feed  102 , equations 2 and 3 
     were solved simultaneously to obtain angle β and eccentricity e. 
     The coaxial feed  102  is located so that its phase center is located at focus F 1  and is tilted by an angle α to the subreflector  101  axis so the feed  102  axis points toward the midpoint of the subreflector  101 . The angle α is found by                tan                 α     =         (     1   -        2       )        sin                 β           (     1   +        2       )        cos                 β     -     2      e                 (   4   )                                
     The subreflector  101  surface may be written in x, y, z coordinates about F 0  as 
     
       
           x   2   +y   2   +Z   2   =e   2 ( z  cos β− y  sin β+ d ) 2   (5) 
       
     
     where              d   =     c          1   -        2         2        e   2                   (   6   )                                
     Due to the wide-angle nature of the coaxial feed  102 , the tilt angles for the subreflector  101  and feed  102  may be relatively large. This may cause some problems when trying to calculate the secondary patterns of this geometry. FIG. 20 shows one embodiment of the dual offset geometry configuration. 
     Table 1 shows the values that are considered useful for one embodiment of feed  102 . 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Antenna System Parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Circular Aperture Size 
                 2.4 meters 
               
               
                 Parabolic Reflector Height 
                 110.4778 inches 
               
               
                 Focal Length (f) 
                 47.245 inches 
               
               
                 Parabola Offset from Vertex of Parent Parabola (y c ) 
                 57.245 inches 
               
               
                 Angle Beta (β) 
                 66.9026 degrees 
               
               
                 Angle Alpha (α) 
                 99.496 degrees 
               
               
                 Eccentricity (e) 
                 0.2826 
               
               
                 Interfocal Distance (2c) 
                 6 inches 
               
               
                 Subreflector Diameter 
                 15.4103 inches 
               
               
                 Feed Arm Length 
                 61.9574 inches 
               
               
                   
               
             
          
         
       
     
     The nature of the feed  102  design lends itself to being expandable to more frequency ranges both up and down in frequency. The Ka band is becoming more popular in many systems. The feed design may allow a straightforward expansion into Ka-Band should the requirement arise for a Ka, Ku, X, and C band antenna system. The addition of the Ka band may involve the addition of a circular waveguide to the center of the K band waveguide. This extension would then make the Ku band portion a coaxial waveguide section and may be handled similarly to the C and X band sections. At Ka band frequencies, however, the surface tolerance of the antenna may become more critical, and with the size of the reflector needed for C band the surface tolerance needed for Ka band may be more difficult to achieve. 
     This technique for the design of a multi-band feed could also be used to design a Ku band and Ka band dual-band feed which may be more practical than a quad-band feed in some implementations. By going to a high frequency dual band feed, the size of the reflector may possibly be reduced. The technique could be used to go down in frequency and produce an L and S band feed. Because of the modularity of the feed, any combination and number of different frequency bands up and down the spectrum might be used. 
     Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.