Abstract:
A multi-reflector antenna array capable of simultaneously transmitting and receiving communication signals at Ku-band frequencies is mounted on an exterior surface of an aircraft. The antenna array provides four cassegrain reflector antennas mechanically connected together in a group capable of being simultaneously mechanically scanned. A common support structure fixes the antennas with respect to each other. A drive mechanism and directional azimuth and elevation motors control the position of the array. The aerodynamic drag of the array is minimized using four antennas rather than a single large diameter antenna. Each antenna is positioned on a common horizontal centerline. Two centrally located antennas are positioned between two smaller diameter antennas. The antennas and positioning equipment are both mounted for rotation within a radome. A corporate power combiner/divider is provided to adjust both an amplitude and a phase of each antenna signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to RF communication antennas, and more specifically to aircraft Ku-band communication antenna systems required to simultaneously transmit and receive from a single aperture 
     BACKGROUND OF THE INVENTION 
     Aircraft mounted Ku-band communication antenna systems presently operate in receive only mode. There is a need for an aircraft mounted, Ku-band communication antenna system which can simultaneously transmit and receive from a single aperture. For this system, International Telecommunication Union (ITU) regulatory levels apply such that transmit Effective Isotropic Radiated Power (EIRP) antenna pattern levels cannot exceed ITU regulatory levels for Ku-band satellite interference. 
     A drawback of the currently used receive-only antennas is that their wide beam widths and high sidelobes cannot meet the beam width and sidelobe requirements for transmit operation under the ITU Ku-band satellite regulations. Use of conventional rectangular slotted waveguide and microstrip-patch array technology cannot be employed because of the high transmit to receive isolation, high efficiency and high cross polarization performance required over the combined transmit and receive operating frequency bandwidth, i.e., about 14.0 GHz to about 14.5 GHz and about 11.2 GHz to about 12.7 GHz respectively. 
     A large, circular reflector antenna, i.e., approximately 0.9 meters (m) (36 inches) diameter, could be used for the application. Several drawbacks exist, however, for an antenna of this size. The communication antenna(s) is required to be mounted on the external surface of the aircraft fuselage. The vertical height of a 0.9 m diameter antenna creates an aerodynamic vertical drag problem for the aircraft. A further drawback is that aircraft antennas are normally enclosed within a radome in order to protect the antennas and to control aerodynamic drag induced by the antenna(s). As the diameter of an antenna increases, the necessary height and length of the radome increases. The necessary sized radome for a 0.9 m (36 inch) diameter surface mounted reflector antenna produces unacceptable levels of aerodynamic drag. 
     In addition to the above drawbacks, the effective isotropically radiated power (EIRP) for a single, large antenna and single transmitter is less efficient than an array of smaller antennas and smaller transmitters. Exemplary vertical and horizontal solid state power amplifiers (SSPAs) for a single large antenna producing 20 watts have an efficiency of about 15 percent. The vertical and horizontal SSPAs of four smaller antennas producing an exemplary 5 watts each (for the same total of 20 watts output) have an efficiency of about 25 percent. It is therefore an efficiency drawback to use a single larger antenna if an appropriate number of smaller, more efficient antennas can be employed. 
     Reducing the antenna diameter, however, necessarily reduces the antenna aperture area. To maintain the total aperture area of a 0.9 m diameter reflector antenna by using a greater number of smaller diameter antennas requires balancing several factors. As noted above, using a plurality of smaller diameter reflector antennas decreases drag while increasing efficiency, but also increases system complexity (wiring, receiver differentiation, etc.). The use of a plurality of smaller reflector antennas requires a common support structure, increasing complexity with each antenna to account for the structure and mechanisms required to jointly mount and rotate the assembly. The antennas must be grouped to permit mechanical scanning with the least number of mechanical components, i.e., motors, wiring or gears, to control complexity and weight. A need therefore exists for a wide-band, low drag, mechanically scanned Ku-band communications antenna system which can simultaneously transmit and receive from a single aperture. 
     SUMMARY OF THE INVENTION 
     According to a preferred embodiment of the present invention, there is provided a multiple reflector antenna array. The antenna array includes a plurality of independent reflector antennas with each of the reflector antennas being fixed to a common antenna support structure. The collective group of antennas on the support structure is trainable to simultaneously receive and transmit RF signals. Cassegrain reflector antennas are preferably employed by the present invention. The support structure of the multiple cassegrain reflector antenna assembly is mechanically attached on an exterior surface of a fuselage of an aircraft. The assembly is enclosed within a radome to reduce aerodynamic drag on the aircraft. Multiple reflector antennas reduce the height of the required radome compared to the height of a radome enclosing a single large diameter reflector antenna. Each antenna is required to both simultaneously transmit and receive communication signals within the Ku frequency band. An exemplary transmit frequency is about 14.0 to about 14.5 gigahertz (GHz) and an exemplary receive frequency range is about 11.2 to about 12.7 GHz. 
     Since multiple reflector antennas are employed by the present invention, a corporate power combiner/divider is employed to process the transmit and receive signals from each of the reflector antennas. Individual service lines to provide both horizontal and vertical signal support to each of the smaller reflector antennas is provided. Through use of the corporate power combiner/divider, the antenna overall pattern performance can be controlled by adjusting each antenna&#39;s signal amplitude and phase within a corporate feed network provided. This adjustment is in addition to the amplitude and phase adjustment of the normal feedhorn/reflector system of these antennas. 
     A radome surrounds the multiple antenna arrangement and its aerodynamic vertical drag component is a function of its height. Radome height is determined by selecting antenna diameter. Radome length is a function of its height. Typically, the radome length is 10 times the radome height to minimize aerodynamic disturbances. Therefore, reducing radome height also reduces radome length and its length component of aerodynamic drag. 
     The present invention provides a wideband, low drag, mechanically scanned, Ku-band communications antenna system which can simultaneously transmit and receive from a single aperture. An antenna array system of the present invention meets the ITU regulatory levels for Ku-band GEO satellite interference. 
     In one preferred embodiment of the invention, a multiple element antenna array for both transmitting and receiving communication signals is provided. A plurality of reflector antennas forms an antenna array. The antenna array is arranged on a common horizontal axis. A support structure mounts the antenna array on the common horizontal axis. A drive mechanism permits multiplane movement of the support structure. At least one motor is provided to rotate the drive mechanism. 
     In another preferred embodiment of the invention, an antenna array is provided to both transmit and receive Ku-band communication signals for a moving platform. The antenna array comprises an array of three to four cassegrain reflector antennas. A support structure is provided for mounting each reflector antenna of the antenna array. A drive mechanism permits movement of the support structure to mechanically scan the array. A first motor controls vertical motion of the drive mechanism. A second motor controls horizontal motion of the drive mechanism. A radome encloses the antenna array. The radome has an internal volume sufficient to permit mechanical scanning of the array within the radome by the first and second motors. 
     In still another preferred embodiment of the present invention, an aircraft communication system is provided which comprises four cassegrain reflector antennas. A support structure mounts each of the four reflector antennas. A drive mechanism permits mechanical scanning of the support structure. A corporate power combiner/divider is electrically connected with each of the four cassegrain reflector antennas. The combiner/divider processes both a transmit and a receive signal for each of the four cassegrain reflector antennas. A radome encloses all four cassegrain reflector antennas. The radome reduces aerodynamic drag of the four cassegrain reflector antennas. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view of an aircraft employing a communication system and its radome of the present invention; 
     FIG. 1B is a plan view taken along Section  1 B— 1 B of FIG. 1A showing the radome; 
     FIG. 1C is a partial section view taken along Section  1 C— 1 C of FIG. 1B showing a portion of the reflector antenna array of the present invention within the radome; 
     FIG. 2A is a block diagram of a single circular reflector antenna; 
     FIG. 2B is a simplified drawing of a multiple circular reflector antenna array of the present invention; 
     FIG. 3 is a front elevational view of a four-antenna array of the present invention; 
     FIG. 4 is a plan view of a four-antenna array of the present invention; 
     FIG. 5 is a partial side cross sectional view of the four-antenna array of FIG. 4 taken along section line  5 — 5  in FIG. 4; and 
     FIG. 6 is a block diagram showing the antenna array of the present invention connected to a corporate power combiner/divider. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     Referring to FIGS. 1A through 1C, an exemplary aircraft  10  is shown on which an antenna system of the present invention is mounted. A radome  12  having height A and length B is shown on an upper surface of the aircraft fuselage  14 . Radome height A shown in FIG. 1C is determined primarily by the diameter of the individual antenna(s) employed in the antenna system. Radome length B shown in FIG. 1B is determined by the radome height A and increases in length in direct proportion to the height of the antenna equipment provided within radome  12 . The location of radome  12  shown in FIG. 1A is exemplary of a preferred location adjacent to a plane perpendicular to the aircraft longitudinal axis C at the wing leading edge D. However, the radome  12  can also be located in multiple locations along the crown of the fuselage  14  of crown of the aircraft  10 . 
     Referring to FIG. 2A, a single, circular reflector antenna  16  is shown. Single reflector antenna  16  is required to have a diameter E in order to both simultaneously transmit and receive Ku-band communication signals. The single reflector antenna  16  would have an exemplary diameter of about 0.9 m (36 inches). A 0.9 meter diameter antenna mounted within a suitably sized radome on the aircraft fuselage  14  would produce unacceptable drag levels. Referring to FIG. 2B, the preferred embodiments of the present invention therefore employ multiple preselected, smaller diameter, wide bandwidth, high gain, fan beam antennas mounted on the aircraft fuselage  14 . 
     One embodiment of the present invention provides four reflector antennas: a first reflector antenna  18 , a second reflector antenna  20 , a third reflector antenna  22  and a fourth reflector antenna  24  combined to form an antenna array  26 . Second reflector antenna  20  and third reflector antenna  22  each comprise a first diameter F. First reflector antenna  18  and fourth reflector antenna  24  each comprise a diameter G smaller than diameter F. An exemplary dimension for diameter F for the array centrally located reflector antennas, comprising second reflector antenna  20  and third reflector antenna  22 , is about 0.25 meters (10.0 inches). An exemplary dimension for diameter G for the antenna array  26  adjacently mounted reflector antennas, comprising first reflector antenna  18  and fourth reflector antenna  24 , is about 0.20 meters (8.0 inches). 
     Reducing antenna height by employing four smaller diameter antennas in antenna array  26  rather than the single reflector antenna  16  reduces the height A of radome  12  (shown in FIG.  1 ), which will reduce aerodynamic drag. FIGS. 2A and 2B compare single reflector antenna  16  having diameter E to the horizontally configured antenna array  26 . The array width H of the four antenna array  26  is about equal to the diameter E of single reflector antenna  16 , however, the aerodynamic drag of the four antenna array  26  is considerably lower because of reduced antenna diameters F and G which permits a shorter radome height A and length B. 
     Referring now to FIGS. 3 through 5, a more detailed illustration of the antenna array  26  of the present invention is shown. The reflector antennas  18 ,  20 ,  22  and  24  each have a sub-reflector  28 ,  30 ,  32 , and  34  respectively. Each reflector antenna  18 ,  20 ,  22  and  24  is mounted to an antenna support structure  36 . Antenna support structure  36  supports each reflector antenna  18 ,  20 ,  22  and  24  on a common horizontal centerline H. The antenna support structure  36  also provides a vertical centerline K for the antenna array  26  between second reflector antenna  20  and third reflector antenna  22  as shown. The vertical centerline K forms the azimuthal axis of rotation for the antenna array  26 . A space L on both ends of the antenna array  26  is filled with a radar absorbing material (RAM) to reduce or eliminate spurious radiation. 
     FIG. 4 shows a plan view of the antenna array  26  supported by the antenna support structure  36 . The antenna support structure  36  comprises a geared platen  38  which is rotated by an azimuth stepper motor  40  about an axis of rotation of vertical centerline K in the directions indicated as arrow M. A semi-spherical geared support member  42  is rotationally supported to the support structure  36  allowing antenna array  26  to be rotated by an elevation stepper motor  44  in engagement with the semi-spherical geared support member  42  about elevation rotation axis J. Reflector antennas  18 ,  20 ,  22  and  24  preferably comprise Cassegrain reflector antennas. Each sub-reflector  28 ,  30 ,  32 , and  34  is secured to its respective reflector antenna by a plurality of sub-reflector struts  46 . A support structure  36  rear face  48  is shown which covers at least the rearward facing surface areas of the combined antennas of antenna array  26 . In a preferred embodiment, rear face  48  comprises a graphite/epoxy covered foam to help align and support reflector antennas  18 ,  20 ,  22  and  24 . 
     FIG. 5 shows a simplified cross sectional side view of the arrangement of FIG. 4 taken along section  5 — 5  of FIG.  4 . The mechanism for supporting and rotating the four element antenna array  26  of the present invention is shown. Elevation stepper motor  44  provides the driving force for positioning the antenna array  26  in accordance with a desired elevation angle. A portion of semi-spherical support member  42  is geared and in mechanical communication with elevation stepper motor  44  to rotate the antenna array  26  about elevation rotation axis J in the directions indicated by arrow N. The support structure  36  employs the rear face  48  to cover and protect the antenna array  26 . As shown in FIG. 1C, the radome  12  has sufficient internal volume and height to permit scanning the antenna array  26  within the radome  12  in the directions indicated as arrow N in FIG.  5 . 
     FIG. 5 shows an exemplary second reflector antenna  20 , with its sub-reflector  30  secured to the second reflector antenna  20  by the sub-reflector struts  46 , in a first extreme rotation position with the sub-reflector centerline P horizontal. FIG. 5 further shows a phantom view of the second reflector antenna  20  in its opposite maximum rotated position having sub-reflector centerline P vertical. The semi-spherical support member  42 , attached to antenna array  26 , rotates with antenna array  26  between the extreme rotation positions. The angle of total rotation between the extreme rotation positions is about 90 degrees. The geared platen  38  is rotationally supported by a platen support  50 . The platen support  50  is connected to the aircraft fuselage  14  by other support structure (not shown) such that the platen support  50  is fixed in position and cannot rotate. 
     FIG. 6 shows an exemplary arrangement of signal lines into the antenna array  26 . A first vertical signal line  52  serving first reflector antenna  18  connects with a second vertical signal line  54  serving second reflector antenna  20 . A third vertical signal line  56  serving third reflector antenna  22  connects with a fourth vertical signal line  58  serving fourth reflector antenna  24 . First vertical signal line  52  and second vertical signal line  54  join as a combined vertical signal line  60 , and third vertical signal line  56  and the fourth vertical signal line  58  join as a combined vertical signal line  62 . Combined vertical signal lines  60  and  62  are connected as a vertical signal input/output line  64  for a corporate power combiner/divider  66 . 
     FIG. 6 also shows a first horizontal signal line  68  serving first reflector antenna  18  connecting with a second horizontal signal line  70  serving second reflector antenna  20 . A third horizontal signal line  72  serving third reflector antenna  22  connects with a fourth horizontal signal line  74  serving fourth reflector antenna  24 . First horizontal signal line  68  and second horizontal signal line  70  join as a combined horizontal signal line  76 . The third horizontal signal line  72  and the fourth horizontal signal line  74  join as a combined horizontal signal line  78 . Combined horizontal signal lines  76  and  78  are connected as a horizontal signal input/output line  80  for corporate power combiner/divider  66 . 
     Corporate power combiner/divider  66  processes the vertical and horizontal signals for each of the four reflector antennas. Within the corporate power combiner/divider  66 , a network (not shown) is employed which adjusts the amplitude and the phase of the signal from each of the antennas processed. This network is in addition to the processing which is conducted on the feedhorn/reflector system of the antenna array  26 . Antenna pattern performance is enhanced by adjusting the amplitude and phase of the individual antenna signals within the corporate power combiner/divider  66 . 
     Other structural support designs for the antenna array  26  are also possible without departing from the spirit and scope of the invention. These include, but are not limited to: (1) a single support plate having cutouts for each antenna, (2) supports comprising a round tube, a square tube, a flat strip or various geometric shapes, or (3) a single centrally located support member having one or more individual support arms for each antenna. A variety of materials for the array supports may be used including steels, aluminum and plastics. 
     Antenna array  26  can also be designed for less than 4 or more than 4 reflector antennas without departing from the spirit and scope of the invention. The four reflector antenna design disclosed herein is an exemplary design. Providing fewer than the exemplary 4 reflector antennas reduces structure at the cost of a larger height array having greater aerodynamic drag. Providing more than the exemplary 4 reflector antennas increases structural and electronics complexity but provides the benefit of a smaller height array having reduced aerodynamic drag. An optimum design point must be selected based on all the aircraft design parameters. 
     The plurality of sub-reflector struts supporting the sub-reflector for each antenna can also be replaced by a single dielectric tube (not shown) for each antenna. The dielectric tube must be dimensioned such that antenna array  26  can still be rotated within radome  12 . Exemplary vertical and horizontal solid state power amplifiers (SSPAs) for the single reflector antenna  16  producing 20 watts, have an efficiency of about 15 percent. The vertical and horizontal SSPAs of four smaller antennas in antenna array  26  producing an exemplary 5 watts each (for the same total of 20 watts output) have an efficiency of about 25 percent. It is therefore advantageous to use an appropriate number of smaller, more efficient antennas than a single larger antenna if smaller antennas can be employed. 
     The array of the present invention provides several advantages. By reducing the height of a wide-bandwidth reflector antenna by dividing the antenna aperture area into an array of smaller reflector antennas, the vertical height of the antenna array is reduced, which results in reduced aerodynamic drag on the aircraft. Antenna pattern performance is enhanced by the added control of the amplitude and phase of the individual antenna signals provided by the corporate feed network, in addition to the normally adjusted amplitude and phase of the feedhorn/reflector system. Also, the use of a multiple reflector array antenna system allows the use of smaller, more efficient, lower power solid state power amplifiers. The combined effect of using multiple antennas having multiple smaller power amplifiers provides more efficient power consumption than would be provided by power amplifier(s) of a single antenna. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.