The present disclosure relates to antennas, and more particularly to a high power, low profile broadband antenna for aerospace applications.
Cavity-backed slot antennas are well suited for applications on aerospace vehicles and other vehicles. Such antennas can be recessed in the vehicle structure and create virtually no drag or effect to air flow over the vehicles surface. However, the maximum power handling capability of any antenna, at a given density altitude is determined by the maximum electric field strength that exists within the antenna. The maximum electric field strength of a cavity backed slot antenna is determined by the dimensions of the feed network geometry. Existing cavity-backed antenna elements typically use an air strip line feed to excite the slot radiator. The strip line has a very non-uniform current over the cross-section of the center conductor and the electric field between the center conductor and the outer conductor peaks significantly at the edges of the strip line center conductor (the field strength at the edge of the center conductor is typically 2-3 times higher than the field strength at the center of the center conductor. The electric current that flows on the strip line center conductor is crowded to the edges of the center conductor and the result is significant ohmic loss and heating under high power conditions. The transition from the coaxial feed to the strip line also typically results in enhanced electric field strengths in the transition region owing to the geometrical limitations of using the strip line.
In accordance with one embodiment, an antenna may include an enclosure formed by a front wall and a back wall opposite to the front wall, and a front face and a back face opposite to the front face. Both the front face and the back face extend between the front wall and the back wall to form a cavity within the enclosure. The enclosure further includes a slot formed in the front face to form a cavity backed slot. A radio frequency (RF) connector is mounted in the front wall. A shaped feed line is mounted within the cavity and is electrically connected to the RF connector to transmit and receive RF energy. The shaped feed line extends across the slot to couple the RF energy between the slot and the shaped feed line. The shaped feed line has a predetermined structure to substantially reduce electric field strength to improve power handing of the antenna.
In accordance with one embodiment, an antenna may include an enclosure including a front wall and a back wall opposite to the front wall, and a front face and a back face opposite to the front face. Both the front face and the back face may extend between the front wall and the back wall to form a cavity within the enclosure. The enclosure may further include a slot formed in the front face to form a cavity backed slot. A radio frequency (RF) connector may be mounted in the front wall. A shaped feed line may be mounted within the cavity and electrically coupled to the RF connector to transmit and receive RF energy. The shaped feed line may extend across the slot to couple the RF energy between the slot and the shaped feed line. The shaped feed line may include a rod shaped center conductor.
In accordance with one embodiment, an antenna may include an enclosure including a front wall and a back wall opposite to the front wall, and a front face and a back face opposite to the front face, wherein both the front face and the back face extend between the front wall and the back wall to form a cavity within the enclosure. The enclosure may further include a slot formed in the front face to form a cavity backed slot. A shaped feed line is mounted within the cavity and extends across the slot to couple RF energy between the slot and the shaped feed line. The shaped feed line may include a rod shaped center conductor disposed. A radio frequency (RF) connector may be mounted in the front wall and electrically coupled to the shaped feed line to transmit and receive the RF energy. The RF connector may include a transition section including a predefined shape to transition from a coaxial feed point to the shaped feed line.
Other aspects and features of the embodiments, as defined solely by the claims, will become apparent to those ordinarily skilled in the art upon review of the following non-limited detailed description in conjunction with the accompanying figures.
The following detailed description of embodiments refers to the accompanying drawings. Other embodiments having different structures and operations do not depart from the scope of the present disclosure.
The following detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments. Other embodiments having different structures and operations do not depart from the scope of the present disclosure.
The enclosure 102 further includes a slot 114 formed in the front face 108 to define a cavity backed slot 116. The cavity backed slot 116 may be substantially rectangular. The dimensions of the slot 114 (width “W” and length “L”) will be a function of the desired operating characteristics of the antenna 100, such as the frequency range or bandwidth of the antenna, operating power, and other operating parameters.
The enclosure 102 may be open on the sides between the front wall 104 and back wall 106 and between the front face 108 and back face 110. The enclosure 102 may define a low profile waveguide cavity with a radiating slot 114 for radiating RF energy for communications or other purposes. The enclosure 102 may be substantially rectangular in shape although other shapes, such as circular or multi-sided, may also be used depending upon the application or for other reasons. The enclosure 102 may also be non-planar depending upon the application. The size of the enclosure 102 may also be dependent on the application and operating parameters or characteristics of the antenna 100. For example, for an antenna operating between about 250 and about 350 megahertz (MHz), radiating about 20 kilowatts (kW) of peak power at about 40,000 feet altitude and at a temperature of about −20 deg Centigrade (C), the front and back walls 104 and 106 may be about 1.5 inches in height and the front face 108 and back face 110 may each have a width of about 10 inches and a length of about 20 inches. The slot 114 may have a length “L” of about 20 inches and a width “W” of about 2 inches. The walls 104, 106 of the enclosure 102 are formed from a metallic material. The front face 108 is made from a metallic material for radiating electromagnetic energy. The back face 110 is also made from a metallic material to provide the desired electromagnetic field pattern or distribution within the cavity 112.
The antenna 100 may also include a radio frequency (RF) connector 120 mounted in the front wall 104. The RF connector 120 may be adapted to connect the antenna 100 to a transceiver (not shown in
Referring also to
In accordance with an embodiment, a tee section 124 and a shunt tuning stub 126 may be electrically connected to the shaped feed line 122. One end of the tee section 124 is connected to the shaped feed line 122. An opposite end of the tee section 124 is electrically connected to one end of the shunt tuning stub 126. An opposite end of the shunt tuning stub 126 is electrically connected to the front wall 104 of the enclosure 102 to short circuit the shunt tuning stub 126 to the front wall 104 and the enclosure 102. The tee section 124 and the shunt tuning stub 126 include a selected length and diameter to tune the antenna to a desired impedance bandwidth. The tee section 124 and shunt tuning stub 126 may each have the same structure and cross-section as the feed line 122 or may each have a different structure and cross-section depending on the application and desired operating characteristics of the antenna 100. The tee section 124 and the shunt tuning stub 126 may be connected to the shaped feed line 122 at a location before the feed line 122 extends across the slot 114.
After the shaped feed line 122 extends across the slot 114 to couple RF energy that is radiated by the slot 114, the shaped feed line 122 may transition into a series tuning stub 128. The series tuning stub 128 may be formed in an elongated loop 129. The series tuning stub 128 is electrically connected to the back wall 106 of the enclosure 102 to short circuit the series tuning stub 128 to the back wall 106 of the enclosure 102. The series tuning stub 128 includes a selected length and diameter to tune the antenna 100 to a desired bandwidth.
If both the shunt tuning stub 126 and series tuning stub 128 are present, the stub diameters (characteristic impedances) and lengths may be selected or selectively tuned in combination to optimize the impedance bandwidth to the antenna 100 as described herein.
Referring back to
The antenna 100 may additionally include a support arrangement 132 mounted within the enclosure 102 to support the shaped feed line 122 within the cavity 112. The support arrangement 132 may be formed from a dielectric material and may include a form to substantially minimize any alteration of an electromagnetic field pattern or distribution within the cavity 112. In one embodiment, the support arrangement 132 may include filling the cavity 112 with a low loss, low density foam or other support material with similar properties that will not adversely affect the electromagnetic field pattern within the cavity 112. An example of a low loss, low density foam that may be used to fill the cavity 112 for the support arrangement 132 may be Eccostock available from Emerson & Cuming Microwave Products, Inc. of Randolph, Mass. Eccostock is a trademark of Emerson & Cuming Microwave Products, Inc. in the United States, other countries or both.
Another example for the support arrangement 132 that may be used to support the shaped feed line 122 and any tuning stubs 124, 126 and 128 is illustrated in
Each of the plurality of dielectric supports 134 may be formed from a block of dielectric material, such as a hard, durable engineering plastic. A hole may be formed through each block to provide a tight fit for the feed line 122. Each block may then be cut in half with some non-planar interlocking shape to permit easy assembly of the antenna 100. An important feature is that the form or size of the supports be electrically small to substantially minimize any alteration of the electromagnetic field pattern or distribution within the antenna 100. The supports may also be shaped to maintain the power handling performance of the antenna. The supports 134 may be attached to the interior of the enclosure 102 by an adhesive, such as epoxy or the like, by a non-conductive fastener or other means.
The RF connector 300 also includes a transition section 314. The transition section 314 may include a predefined shape to transition from a coaxial feed point 316 of the coaxial center conductor 306 to the shaped center conductor 312 of the feed line. The predefined shape of the transition section 314 may be substantially dome shaped if the shaped center conductor 312 is a circular rod as described herein as one example of a shaped center conductor of a shaped feed line. The predefined shape of the transition section 314 may also be other shapes depending upon the structure and/or cross-section of the shaped center conductor of the feed line of the antenna.
For comparison purposes, referring also to
The selected shape of the center conductor 606 may be a rod. The rod may be substantially circular shaped with an appropriate diameter and length to provide a chosen characteristic impedance, for example 50 ohms. The circular rod shaped center conductor 606 with a diameter of about 1.65 inches will have a characteristic impedance of 50 ohms. A shaped center conductor with this structure will have a maximum field strength for a one volt excitation of about 0.737 v/cm or less. This is approximately half the field strength for the strip line 400 with the same height “h” of 3 inches. Because RF breakdown is determined by the maximum electric field strength, the shaped feed line 600 can be powered or excited to approximately twice the voltage (4 times the power) of the strip line 400. Because the shaped feed line 600 spreads the current over a larger portion of the cross section of the line compared to the strip line 400, the selected shape or cross-section of the center conductor 606 of the shaped feed line 600 compared to the strip line 400 substantially reduces ohmic losses to improve antenna efficiency and reduce internal temperature of the antenna. Accordingly, these characteristics of the shaped feed line 600 provide improved RF power handling capability particularly in aerospace applications and particularly at high altitudes, such as for example at flight levels above about thirty thousand feet.
While the shaped feed line 600 has been described as having a rod shaped center conductor 606 with a substantially circular cross-section, other rods with cross-sections other than circular may also be used that provide similar or better operating characteristics compared to those described above.
Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown and that the embodiments may have other applications in other environments. This application is intended to cover any adaptations or variations of the embodiments. The following claims are in no way intended to limit the scope of the disclosure to the specific embodiments described herein.
This invention was made with Government support under contract number F19628-01-D-0016 awarded by the United States Air Force. The Government has certain rights in this invention.
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Number | Date | Country | |
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20110074642 A1 | Mar 2011 | US |