Patent Publication Number: US-8537059-B2

Title: Cooling system for panel array antenna

Description:
FIELD OF THE INVENTION 
     The present invention relates to panel array antennas, and more particularly to a cooling system for an antenna such as a jet stream conformal panel array antenna. 
     BACKGROUND 
     Many types of aircraft, including combat airplanes, surveillance aircraft, and unmanned aerial vehicles, utilize panel array antennas. These antennas can be mounted on the outer skin of the aircraft, to radiate and/or receive radio frequency signals. Panel array antennas have a panel architecture, meaning that they are made up of several stacked panels or layers. These antennas may have a top layer that is exposed to the air flowing around the aircraft (the “jet stream”), a radiating layer (including the antenna elements that radiate and/or receive the radio frequency signals), an electronic circuit board layer including the electronics that generate the signal, and a bottom layer for mounting the antenna to the aircraft and connecting the antenna to the power and cooling systems on the aircraft. 
     Conformal panel array antennas are designed to conform to the exterior shape of the aircraft, so that they do not extend out from the aircraft substantially into the jet stream. Some panel array antennas extend out from the aircraft and into the jet stream flowing around the aircraft, but this design alters the flow of air around the aircraft, increases drag, and requires additional structural modifications and support. A conformal panel array antenna is mounted on or in the aircraft&#39;s outer skin, such that the antenna does not extend out into the jet stream. The overall radiation pattern of a conformal array results from the spatial superposition of all of the radiation patterns from the individual antenna elements making up the array. 
     Many aircraft would benefit from locating these conformal panel array antennas in various places around the aircraft&#39;s exterior skin, including the fuselage and wings, and including curved and flat surfaces on the aircraft. However, typical conformal panel array antennas require a cooling system in order to prevent the electronics within the various panel layers from overheating. In the prior art, a cooling plate is mounted on the rear side of the antenna, on the bottom surface of the antenna, opposite the jet stream. This cooling plate includes fluid circulation, fans, and/or heat sinks to draw heat away from the antenna. The cooling plate is powered by the aircraft&#39;s on-board power system, and it dissipates heat to the aircraft, such as to the aircraft&#39;s environmental control system, or to the aircraft&#39;s fuel. Thus, the cooling plate relies on the aircraft for power and cooling. 
     The need for a cooling element such as the cooling plate on the back surface of the antenna limits the use of conformal array panel antennas, because the cooling plate is typically flat, not curved, and requires operable connections to the aircraft for both power and heat disposal. Accordingly, a conformal panel array antenna with this cooling plate can be mounted on the aircraft skin only at locations where the cooling plate can be both structurally mounted to the aircraft and operably connected to the aircraft&#39;s power and cooling systems. Additionally, in drawing power and cooling from the aircraft, the cooling plate reduces the aircraft&#39;s available power, resulting in shorter flight duration for the aircraft and/or reduced power for other aircraft systems. The cooling plate also has other disadvantages, such as effectiveness (as it provides cooling only at the back surface of the antenna), weight, space, and cost. 
     A significant difficulty in designing more effective cooling systems for panel array antennas is the need to prevent leakage of the radio frequency signal that the antenna transmits. In order to prevent the signal from leaking, the antenna typically includes plates or layers that close out the antenna and prevent passage of radio frequency signals, so that the signal can be emitted in the desired direction, rather than radiating out in all directions. However, this closed structure also traps heat inside the antenna and makes cooling difficult. Another problem is the constrained space within the antenna. The electronic devices within the antenna are often packed closely together, limiting the available space for a cooling system. 
     Accordingly, there is still a need for an improved cooling system for a panel array antenna. 
     SUMMARY OF THE INVENTION 
     The present invention relates to panel array antennas, and more particularly to a cooling system for an antenna such as a jet stream conformal panel array antenna. In one embodiment, a panel array antenna for an aircraft includes a closed-loop fluid flow path that passes through the panel array assembly and dissipates heat to the jet stream outside the aircraft. A fluid such as pressurized air passes through this closed-loop path, flowing through strategically-placed openings in the layers of the panel array assembly and flowing over and around the hot electrical components in the panel assembly. The air is heated by these electrical components, and the heated air then flows through the flow path under the top sheet, dissipating the heat to the jet stream outside. The top sheet is the sheet of material that separates the internal components of the antenna from the jet stream and environment outside of the aircraft. This system uses the jet stream as a heat sink and integrates cooling into the antenna structure itself. In embodiments of the invention, the cooling plate mounted on the rear side of panel antennas in many prior art designs is not necessary, and as a result the closed-loop cooling system described herein reduces costs and enables the panel array antenna to be more efficiently and easily mounted at various locations on the aircraft. 
     In one embodiment, a panel array antenna includes a panel assembly having a top layer through which the antenna radiates or receives a signal, and a fluid flow path through the panel assembly. A first portion of the fluid flow path is disposed below the top layer such that a fluid passing through the first portion of the fluid flow path is in heat transfer proximity to the top layer. 
     In another embodiment, a panel array antenna includes a top layer; a radiating layer comprising one or more channels below the top layer; an intermediate layer comprising one or more screens below the radiating layer; an electronics layer comprising one or more openings and one or more electronic devices below the intermediate layer; a fluid flow path passing through the channels, the screens, and the openings; and one or more fans that circulate a fluid through the fluid flow path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an aircraft with two conformal panel array antennas, according to an embodiment of the invention. 
         FIG. 2  is a schematic representation of a cooling system for a panel array antenna according to an embodiment of the invention. 
         FIG. 3  is a perspective view of a panel array antenna according to an embodiment of the invention. 
         FIG. 4  is an exploded view of a panel array antenna according to the embodiment of  FIG. 3 . 
         FIG. 5  is an exploded view of a panel array antenna according to the embodiment of  FIG. 3 , showing a portion of a fluid flow path. 
         FIG. 6  is an exploded view of the panel array antenna of  FIG. 5 , showing another portion of the fluid flow path. 
         FIG. 7  is an exploded view of the panel array antenna of  FIG. 5 , showing yet another portion of the fluid flow path. 
         FIG. 8  is an exploded view of the panel array antenna of  FIG. 5 , showing still another portion of the fluid flow path. 
         FIG. 9  is a partial exploded view of a panel array antenna according to an embodiment of the invention. 
         FIG. 10  is a perspective view of a layer of a panel array antenna according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to panel array antennas, and more particularly to a cooling system for an antenna such as a jet stream conformal panel array antenna. In one embodiment, a panel array antenna for an aircraft includes a closed-loop fluid flow path that passes through the panel array assembly and dissipates heat to the jet stream outside the aircraft. A fluid such as pressurized air passes through this closed-loop path, flowing through strategically-placed openings in the layers of the panel array assembly and flowing over and around the hot electrical components in the panel assembly. The air is heated by these electrical components, and the heated air then flows through the flow path under the top sheet (which may be the skin of the aircraft), dissipating the heat to the jet stream outside. This system uses the jet stream as a heat sink and integrates cooling into the antenna structure itself. In embodiments of the invention, the cooling plate mounted on the rear side of panel antennas in many prior art designs is not necessary, and as a result the closed-loop cooling system described herein saves costs and enables the panel array antenna to be more efficiently and easily mounted at various locations on the aircraft. 
     Referring to  FIG. 1 , in one embodiment of the invention, an aircraft  10  includes two panel array antennas  12 ,  14 . The first panel array antenna  12  is mounted to the fuselage of the aircraft, and the second panel array antenna  14  is mounted to a wing. Both antennas  12 ,  14  conform to the exterior profile of the aircraft, so that in this embodiment they do not extend out into the jet stream  16  passing around the aircraft  10 . The conformal antennas can be mounted to the aircraft in several ways. In one embodiment, they are mounted to the exterior surface of the aircraft skin  18 , similar to a decal. In another embodiment, they are mounted into the aircraft&#39;s skin  18 , similar to windows cut into the aircraft. In this latter case, the antennas can be made flush with the outer skin  18  of the aircraft, so that they do not affect the jet stream  16  and do not create any additional drag or change the aircraft&#39;s radar signature. However, the invention is not limited to conformal antennas, and antennas according to an embodiment of the invention may extend out from the aircraft or other platform, rather than being mounted flush with the platform&#39;s exterior surface. 
     In embodiments of the invention, a panel array antenna with an improved cooling system is provided. A schematic view of such a cooling system is shown in  FIG. 2 . In the embodiment of  FIG. 2 , a panel array antenna  20  includes a panel assembly  22 ′, which includes various layers of the antenna, and a fluid flow path  24 ′ that passes around and through the various layers of the panel assembly  22 ′. Any suitable fluid may be circulated through the flow path  24 ′. In one embodiment, the fluid is air. 
     In the embodiment shown in  FIG. 2 , a panel assembly  22 ′ is made up of several layers, including a radome layer  28 , a radiating layer  32 , and an electronics layer  38 . When the term “radiating layer” or “radiating element” is used herein, it refers to the layer or element of the panel assembly that receives and/or transmits the radio frequency signal. This layer could receive only, transmit only, or both receive and transmit the signals. The radiating layer  32  includes the individual antenna elements that transmit radio frequency signals through the radome layer  28 . The outer surface  31  of the radome layer  28  is exposed to the jet stream  16 . 
     The electronics layer  38  is on the opposite side of the radiating layer  32  from the radome layer  28 . The electronics layer  38  includes electronic devices such as microchips, microprocessors, and/or memory devices that generate the radio frequency signals to be radiated out by the radiating layer  32 . These electronic devices generate heat during operation. The electronics layer  38  may generate the most heat of all of the various layers in the panel assembly  22 ′. Absent any cooling system, the electronics in this layer are at risk of overheating. Overheating of the panel assembly  22 ′ can lead to malfunction of the electronic devices, and/or delamination of the assembly  22 ′ from the aircraft or other structural failure of the assembly. 
     In one embodiment, the electronics layer  38  includes one or more fins  50  that are attached to the electronic devices. The fins extend out from the electronic devices and increase the surface area that is exposed for cooling purposes. Cool air is blown at these fins  50  to draw heat away from the electronic devices in the layer  38 . 
     The fluid flow path  24 ′ is shown in dotted lines in  FIG. 2 , in schematic form. The fluid in the flow path, such as air, flows through the radome layer  28  where it is cooled by the jet stream  16 . Heat  17  that the fluid has obtained from the panel assembly  22 ′ is dissipated to the jet stream  16  via conduction through the outer surface of the radome layer  28 , which is exposed to the jet stream  16  and therefore transfers heat to the jet stream by convection. This cools the air in the flow path  24 ′. The cooled air then flows through a set of fans or blowers  52  that circulate the fluid through the flow path  24 ′. The fluid passes from the fans  52  through a jet impingement layer  42 , which includes strategically placed openings such as nozzles  54  that direct the fluid toward the electronic devices in the electronics layer  38 . As indicated in  FIG. 2 , the fluid passes through the nozzle  54  and toward the fins  50  extending out from the electronics layer  38 . This fluid flows around the fins  50 , absorbing heat from the fins and cooling the electronics layer  38 . The fluid then flows back to the radome layer  28 , where the fluid dissipates the absorbed heat  17  to the jet stream  16 . 
     The radome layer  28  is provided above the radiating layer  32  to protect the radiating elements and other sensitive electronics in the assembly  22 ′ from the environmental elements such as rain, sunlight, dirt, etc. The radome layer  28  conceals the antenna below it, so that the existence and location of the antenna is not readily visible. The radome  28  also provides a smooth outer surface  31  over which the jet stream  16  flows. The radome layer  28  includes hollow space through which the radio frequency signals received or transmitted by the antenna can pass. In embodiments of the invention, this hollow space is also used as part of the flow path  24 ′. Fluid is circulated through this path  24 ′ to dissipate heat through the outer surface  31  to the jet stream  16 . 
     The flow path  24 ′ shown schematically in  FIG. 2  passes through the radome layer  28  itself. As the fluid flows from the radome layer  28  to the electronics layer  38 , and then from the electronics layer  38  back to the radome layer  28 , the flow path  24 ′ can take several alternative paths. In one embodiment, the flow path passes through passages such as ducting around the panel  22 ′, which transports the cool fluid  26   b  to the electronics layer  38 , and then through additional passages or ducting that transports the heated fluid  26   a  back to the radome layer  28 . In another embodiment, the flow path passes directly through the various layers of the panel assembly  22 ′, rather than through separate ducting. In such an embodiment, the flow path is integrated within the various layers of the panel assembly itself. The cool fluid  26   b  is diverted through the radiating layer  32  and through the electronics layer  38  to the jet impingement layer  42 , where it is sent through the nozzle  54  to circulate around the fins  50 . The heated air  26   a  then passes back through the electronics layer  38  and the radiating layer  32  to the radome layer  28 , where it dissipates the heat  17  to the jet stream  16 . 
     In one embodiment, the cooling system includes a pump  56  that is in communication with the flow path  24 ′, in order to maintain the fluid in the flow path at a sufficient pressure so that the fluid will circulate through the path  24 ′. In one embodiment the flow path  24 ′ is maintained at a pressure that is equal to atmospheric pressure at about 10,000 feet elevation. The pump  56  can also replenish the fluid in the flow path  24 ′ in the case of a leak. The pump  56  may be a local pump that draws air from the atmosphere, or it may draw from pressurized air inside the aircraft, using the aircraft&#39;s on-board pressurization system that keeps the aircraft cabin pressurized. 
     In one embodiment, the fluid flow path is a closed-loop path. That is, the fluid in the path is recycled and re-used. After the fluid passes through the panel assembly  22 ′, accumulates heat from the various layers and electronics in the assembly  22 ′, and dissipates this heat to the jet stream  16 , the fluid repeats this cycle. Of course, the fluid may be replenished periodically by a pump such as pump  56 , in the case of a leak, or for repairs or maintenance. However, in operation, the fluid in the flow path  24 ′ is recycled rather than replaced with each cycle through the flow path. This closed-loop design is efficient and compact. 
     Another embodiment of a panel array assembly  22  with a fluid flow path  24  is shown in  FIGS. 3 ,  4 , and  5 . The antenna  20  includes the panel assembly  22  made up of various layers. The outer-most layer is the top sheet  30 , which includes an outer surface  31  exposed to the jet stream. The area of the top sheet  30 , covering the radiating layer  32 , may also be referred to as the antenna aperture (the area through which the radio frequency signal is transmitted or received). The top sheet  30  may be made from a fiber reinforced resin, which allows both transfer of heat to the jet stream and passage of radio signals. In this embodiment, the fluid flow path  24  passes directly through the panel assembly  22 , rather than simply around it or along an end surface of it. The various layers of the panel assembly  22  include strategically-positioned holes, openings, and passages that allow the fluid to move through the panel assembly  22 , as described in more detail below. 
     Moving in order through the panel assembly  22 , the next layer is the radiating layer  32 . The radiating layer  32  includes the individual antenna elements or “stubs”  58  that transmit the radio frequency signal out from the antenna. The stubs  58  extend along the length of the radiating layer  32 , between opposite ends  32   a ,  32   b  (see  FIG. 4 ). The antenna elements  58  can be any radiating element such as continuous transverse stub (CTS) strips, cavity-back long slots, flared notches, flared dipole, or strips of conventional dipoles. These various options will be known to those skilled in the art. In one embodiment, the radiating layer  32  is adjacent the top sheet  30 , so that the radiating elements  58  are positioned to transmit signals directly through the top sheet  30  and away from the antenna  20 . 
     Between these stubs  58  are channels  60  that set the stubs  58  apart from each other. These channels  60  provide space around each stub within which the radio frequency signal from the stub travels. The particular sizing of the channels  60  and stubs  58  depends in part on the particular antenna, its desired performance, and the radiating frequency. The channels are closed at opposite ends by caps or seals  59 . The fluid in the flow path  24  passes through these channels  60  as described more fully below. In one embodiment, a filler piece such as a nonconductive strip  57  occupies a portion of the channel  60 . The fluid moving through the channel  60  passes over this strip  57 , so that the fluid passes close to the top sheet  30  to dissipate heat to the outside environment. In one embodiment, the strip  57  rests on caps  57   a  at opposite ends of the strip  57 . The caps  57   a  elevate the strip  57  to the desired location to move the fluid path  24  close to the top sheet  30 , and also prevent the fluid from passing under the strip  57 . Thus, the space below the strip  57  is occupied by static air that does not flow through the flow path  24 , while the space above the strip  57  forms part of the flow path  24 . Alternatively, instead of using the thin strips  57 , caps  57   a , and static air below the strips  57 , this space can all be occupied by one larger, thicker filler piece. However, this larger filler piece may increase the weight and cost of the panel array, in which case the thinner strip  57  with elevating caps  57   a  and static air below the strip  57  may be used to reduce weight. 
     The next layer is an intermediate layer  34 . This layer contains microwave circuitry and interconnects between layers  32  and  36 . At the same time, this layer closes out the radiating layer  32 , preventing leakage of the radio frequency signals from the stubs  58  back through the antenna in the wrong direction. That is, without capping or closing the radiating layer  32 , the signal transmitted by the stubs  58  could travel in all directions, including back through the antenna rather than out in the direction of the aperture, away from the antenna, as desired. The intermediate layer  34  may simply be a bottom layer of the radiating layer  32 , closing out the channels  60 . 
     In one embodiment, the intermediate layer  34  provides beam-steering functionality for the antenna. The layer  34  includes one or more varactor diodes, which are used in a phase shifter circuits to change the radiation profile of the antenna, to steer the radiated signal. The varactor diode changes the profile of the radio signal that passes through the stubs  58 , to steer the beam in a particular direction, as is well known to those skilled in the art. 
     The next layer is a fluid collection layer  36 , which diverts the fluid in the flow path  24  in a desired direction, as described in more detail below. The collection layer  36  may contain a series of protrusions such as pegs or discs  66  that extend out toward the electronics layer  38  (described next, with reference to  FIG. 7 ). These protrusions  66  can transmit radio frequency signals toward and/or away from the radiating layer  32 , and also carry structural load between the layers in the panel assembly  22 , to prevent the assembly from becoming bowed or sagging in the center, between opposite ends  22   a ,  22   b.    
     The next layer is the electronics layer  38 , which is a multi-layer mixed signal printed wiring board for distributing DC power, RF signals, and digital control signals to individual electronic devices  62  (see  FIG. 7 ). As mentioned before, the electronic devices in this layer generate the radio frequency signals that the antenna transmits. Below the electronics layer  38  is a fluid distribution layer  40 , a jet impingement layer  42 , and a fluid circulation layer  44 , all of which form part of the flow path  24  as described in further detail below. The surface of the fluid circulation layer  44  facing away from the top sheet  30  forms the bottom surface  64  of the panel assembly. 
     The fluid flow path  24  through these various layers will now be described. The movement of a fluid  26  is shown in arrows in  FIGS. 5-8 . Referring first to  FIG. 5 , the fluid  26  moves through the channels  60  along the radiating layer  32 , below the top sheet  30 . The fluid  26   a  at a first end  60   a  of the channels  60  carries heat from the panel assembly  22 . As mentioned above, the channels  60  provide space around each stub within which the radio frequency signal from the stub travels. In the present embodiment, that space is also used as a flow path for a moving fluid, rather than a static space. That is, the wave guide path is also used as a cooling path. As the fluid passes through these channels  60 , heat from the fluid radiates out into the jet stream through the top sheet  30 . The strips  57  position the fluid  26   a  close to the top sheet  30  as the fluid travels along the channels  60 . The portion of the fluid flow path passing through the channels  60  is disposed below the top layer  30  such that the fluid  26   a  passing through the fluid flow path is in heat transfer proximity to the top layer  30 . Thus, the fluid  26   b  at the opposite end  60   b  of the channels is cooler than the fluid  26   a.    
     The channels  60  are closed by the intermediate layer  34 . At each end  34   a ,  34   b  of the intermediate layer, one or more screens  68  are formed in the intermediate layer  34 . The screens  68  at the end  34   b  of the intermediate layer  34  allow the fluid  26  to flow out of the channels  60  and through the other layers in the panel assembly  22 . Thus, when the fluid  26   b  reaches the end  60   b  of the channels  60 , it is diverted downward through the screens  68  into the antenna structure. Each individual screen  68  is made up of several spaced-apart small holes  70  (see  FIG. 9 ). As shown in  FIG. 5 , the screens  68  allow the fluid  26  to flow through the small holes  70 , but do not allow radio signals to pass through the holes. The screens  68  are designed with these small holes  70  rather than one large opening, so that the screens can block the radio frequency signals emitted by the radiating layer  32 . Much like the screen provided on the door of a microwave oven, the screens  68  block the radio waves from the radiating layer  32  and prevent them from passing through the antenna toward the bottom surface  64 . Due to the wavelength of the radio signals, the waves cannot pass through these small holes  70 . As a result, the panel assembly  22  does not suffer from radio frequency leakage, despite the presence of the holes  70  in the intermediate layer  34 . The size of the holes  70  can be determined from the wavelength of the radio frequency signals transmitted and received by the antenna, as well as the acceptable level of radio frequency leakage. The wavelength and acceptable leakage depend on the desired performance of the antenna. 
     The fluid  26  passes from the screens  68  through openings  72  in the fluid collection layer  36 . These openings  72  are strategically placed to divert the fluid  26  toward the electronics layer  38 . In one embodiment, as shown in  FIG. 5 , the fluid  26   c  passes through the openings  72  and fans out to flow over the electronics layer  38 . The particular arrangement shown in  FIG. 5  is not the only option, and the openings  72  can be located and shaped to create any desired distribution of fluid toward the electronics layer  38 . In one embodiment, the fluid is diverted to flow toward the center of the electronics layer  38 . As mentioned above, the electronics layer  38  may the highest temperature layer in the panel assembly  22 , so the flow path  24  circulates over and around this electronics layer  38  in order to allow the fluid in the flow path to absorb heat from the electronics layer. 
     In one embodiment, the openings  72  in the collection layer  36  are not constrained by the radio frequency wavelength, as the screens  68  are. Thus, the openings  72  in the collection layer  36  can be sized as spaced to divert the fluid and spread it out in any desired direction to circulate over the electronics layer  38 . In other embodiments, the fluid can be fanned out in a different layer, such as below the electronics layer  38 , to circulate the fluid along a bottom surface of the electronics layer (see, for example,  FIG. 9 , where the fluid fans out over the bottom surface of the electronics layer on its way back up toward the top sheet). Thus, the particular arrangement shown in  FIG. 5 , and the way the fluid  26   c  spreads out from the collection layer  36 , is not the only way the layers and flow path can be arranged. In general, the flow path  24  can be modified based on the specific layers used in the panel assembly, and it is not limited to the particular arrangement shown in  FIGS. 5-8 . 
     As shown in  FIG. 5 , the electronics layer  38  includes small holes or openings  74  through which the fluid can pass. These openings  74  are strategically placed between the various electronic components on this layer  38 . As shown in  FIG. 7 , the electronics layer  38  includes various spaced-apart electronic devices  62  such as microchips. Thus, some portions of the layer  38  cannot accommodate a hole or opening without disturbing or displacing an electronic device  62 . The holes  74  are positioned away from the electronic devices  62  in areas where the electronics layer  38  can accommodate an opening. In one embodiment, these holes  74  are smaller than the openings  72  in the fluid collection layer  36 , as the holes  74  are constrained by the placement and spacing of various electronic components. The electronics layer  38  includes a sufficient number of openings  74  to allow the fluid to continue along the flow path  24  through the panel assembly  22 . 
     As shown in  FIG. 5 , the fluid  26   d  passes from the electronics layer  38  into the fluid distribution layer  40 . The distribution layer  40  includes one or more fluid flow channels  76  that divert the fluid  26   d  toward an opening such as slot  78  near the second end  40   b  of the layer  40 . The channels  76  are defined by rear and side walls  76   a ,  76   b , respectively, that contain the fluid  26  and direct it toward the slot  78 . In the embodiment shown, the channels  76  are formed in the distribution layer  40 , rather than on the electronics layer  38 , as the electronic devices on the electronics layer  38  constrain the space on that layer and reduce the space available for fluid channels to collect and redirect the fluid. However, in another embodiment, channels could be formed on the electronics layer, with the electronic devices rearranged to provide available space. 
     The fluid  26   d  passes through the slot  78  toward the jet impingement layer  42 . The flow path  24  then passes through the jet impingement layer  42 , through an opening such as slot  80 . In one embodiment, the fluid distribution layer  40  and the jet impingement layer  42  are made together as one piece, such as one machined piece of aluminum. This is true for other layers in the panel assembly  22  as well, which may also be combined together and made as one integral piece, or provided as separate layers. In general, the various layers in the panel  22  may be made from any suitable materials, including composites, plastic, metal-coated plastic, aluminum, magnesium, steel, and other materials. The choice of material depends on the particular design and application as is known to those skilled in the art. 
     The fluid  26   e  then reaches the fluid circulation layer  44 . In the embodiment shown in  FIG. 5 , this circulation layer  44  forms the bottom layer of the panel assembly, with the bottom surface  64  of the circulation layer  44  forming the bottom surface of the panel assembly  22 , facing away from the top sheet  30 . This layer  44  collects and re-circulates the fluid  26   e , sending it back up through the panel assembly  22 , back toward the radiating layer  32  to close the fluid path  24 . In an embodiment of the invention, the fluid circulation layer  44  includes fans, blowers, air movers, micro air movers, or other devices that give velocity to the fluid  26 , to keep it moving through the flow path  24 . The fans are shown schematically in  FIG. 2 . In  FIG. 5 , the fans may be contained within the circulation layer  44 , communicating with the flow path  24  to keep the fluid moving. In another embodiment, the fans may be contained elsewhere, and they may be designed to communicate with the flow path  24  to move the fluid through the flow path. 
     The circulation layer  44  includes a plenum  84  that receives the fluid  26   e  from the jet impingement layer  42 . In one embodiment, the fluid flows through the plenum  84  and through the fans or blowers in the circulation layer  44 . Referring now to  FIG. 6 , after the fluid  26   f  has passed through the fans, it flows back toward the jet impingement layer  42 . The plenum  84  and the fans in the circulation layer  44  are arranged to collect the fluid  26   e  from the jet impingement layer  42  and redirect that fluid  26   f  back toward the jet impingement layer  42 , effectively routing the fluid by about 180 degrees to send it back toward the top sheet  30 . 
     The first time the fluid passed through the jet impingement layer, as it was moving away from the top sheet  30 , it passed through the slot  80  at one end of the jet impingement layer  42 . After passing through the circulation layer  44 , the fluid  26   f  now passes through the nozzles  54  in the jet impingement layer  42 . These nozzles accelerate the fluid  26   f  toward the fluid distribution layer  40 . The accelerated air  26   g  exiting the nozzles flows through passages  86  in the distribution layer  40 . The nozzles  54  and passages  86  are strategically located to direct the fluid  26   g  toward the electronic devices  62  on the electronics layer  38  (shown in  FIG. 7 ). Depending on how the assembly  22  is stacked and how the devices  62  are distributed on the electronics layer, the plenum  84 , nozzles  54 , and passages  86  may be re-arranged or located as necessary to direct the fluid toward the devices  62 . 
     As shown in  FIG. 7 , the fluid  26   h  that flows through the passages  86  is directed toward the electronic devices  62 . The fluid  26   h  flows directly into and around these devices  62 . In one embodiment, the devices  62  include fins  50  (see  FIG. 2 ) that increase the surface area that contacts the fluid  26   h . The accelerated fluid  26   h  flows through and around these fins, absorbing heat from the electronic devices  62 . The fluid  26   h  passing through this portion of the fluid flow path is in heat transfer proximity to the electronics layer, so that it can absorb heat from the electronics layer. 
     After absorbing heat from the electronics layer  38 , the heated fluid  26   i  flows through the openings  74  in the electronics layer, as shown in  FIG. 8 . The collection layer  36  collects the heated fluid  26   i  and diverts it toward the first end  36   a  of the layer  36 , where the fluid can pass through the openings  72 . The collection layer  36  may include a separating structure such as a dividing wall  82  (see  FIG. 7 ) that separates the heated fluid  26   i  from the cool fluid  26   c . This wall  82  prevents the cooler fluid  26   c  from flowing back to the channels  60 , bypassing the rest of the flow path through the electronics layer  38  and circulation layer  44 . 
     From the collection layer  36 , the fluid passes through the screens  68  in the intermediate layer  34 , and back into the channels  60  in the radiating layer  32  (see  FIG. 8 ). The heated fluid  26   a  flows along the channels  60  and dissipates its absorbed heat through the top sheet  30  to the jet stream passing around the aircraft, as described above (see  FIG. 5 ). The cooled fluid  26   b  at the opposite end of the channels is diverted back through the panel assembly  22  to repeat the cycle. 
     Notably, in one embodiment, the collection layer  36  acts to fan out the fluid  26   c  in the flow path  24  as it flows away from the top sheet  30 , in order to circulate the fluid  26   c  over the hot electronics layer  38  (see  FIG. 5 ). The collection layer  36  also collects the heated fluid  26   i  (see  FIG. 8 ) and converges it back into a path through the screens  68  toward the top sheet  30 . As a result, the fluid passes all the way through the channels  60 , from one end  60   a  of the channels to the opposite end  60   b , to maximize the transfer of heat from the fluid through the top sheet  30  to the jet stream. Additionally, the channels  60  are closed out except for at the screens  68 , in order to prevent radio frequency leakage. Then, when the cooled fluid heads back through the panel assembly  22  away from the top sheet  30 , it is spread out to circulate fully through the various layers, to provide sufficient cooling. Thus, the fluid flow path  24  is not confined to the outer edges of the various layers in the panel assembly  22 . 
     A panel assembly  22 ″ according to another embodiment of the invention is shown in  FIG. 9 . In this embodiment, the collection layer  36  is not included, and the openings and fluid passages in the various layers have been rearranged. This embodiment gives just one example of how the layers in the assembly  22 ″ and the openings and passages through these layers can be arranged differently, according to the particular antenna and its desired performance. Specifically, in  FIG. 9 , the radiating layer  32  includes stubs  58  and channels  60  extending between the stubs  58 . The flow path  24 ″ passes through the channels  60  and then down through screens  68  in an intermediate layer  34 . Below the intermediate layer  34  is an electronics layer  38 , which includes various electronic devices that receive and/or transmit radio frequency signals. 
     In this embodiment, the electronic devices have been packaged on the electronics layer  38  in a compact way that allows the layer  38  to include large openings  88  at opposite ends  38   a ,  38   b  of the electronics layer  38 . Comparing to the embodiment of  FIG. 5 , the holes  88  in  FIG. 9  are larger than the small openings  74  that are spaced throughout the layer  38  in  FIG. 5 . The sizing and distribution of the openings in the electronics layer depends on the arrangement of electronic devices on this layer. Because the layer  38  in  FIG. 9  is arranged such that the larger openings  88  can be accommodated at the opposite ends of the layer  38 , the collection layer  36  is omitted. In  FIG. 5 , the collection layer  36  was used in part to fan out the fluid toward the electronics layer, in order to spread the fluid out so that it could pass through the smaller openings  74  that were distributed along the electronics layer in  FIG. 5 . By contrast, in  FIG. 9 , the fluid can continue to pass straight through the larger holes  88  without the need to distribute the fluid through smaller holes spread out across the electronics layer  38 . Similarly, when the fluid passes back up toward the radiating layer  32 , the fluid passes through the holes  88  at the first end  38   a  of the electronics layer  38 , and then directly up through the screens  68 . The collection layer  36  is not needed in this embodiment to collect the fluid from the smaller openings  74  (see  FIG. 5 ) and direct it toward the screens  68 . 
     Referring again to  FIG. 9 , after the fluid passes through the holes  88  in the electronics layer  38 , it passes through a slot  78  in a distribution layer  40 . Comparing again to  FIG. 5 , the distribution layer  40  in  FIG. 9  omits the channels  76  that direct the fluid in  FIG. 5  toward the slot  78 . As described above, the fluid in  FIG. 9  is not fanned out as it passes away from the radiating layer  32 , so the channels  76  that are shown in  FIG. 5  are not necessary to redirect the fluid back toward the slot  78 . Accordingly, the fluid passes through the slot  78  and then through a slot or opening  80  in a jet impingement layer  42 . From there, the fluid flows through a plenum  84  in a circulation layer  44 . The fluid passes through fans or blowers in the circulation layer  44  and then through nozzles  54  in the jet impingement layer  42 , which direct the fluid onto the electronic devices in the electronics layer  38 . The fluid absorbs heat from these electronic devices as the flow impinges on each device. The heated fluid then moves back up toward the radiating layer  32  through the openings  88  and screens  68 , and then back through the channels  60  to complete the fluid flow path  24 ″. 
     In one embodiment, the fluid cooling system described above improves the operating temperature of the antenna in two ways. First, the fluid dissipates heat to the jet stream, as described above, as the fluid passes through the channels  60 . Second, the fluid reduces the temperature gradient of the antenna. Typically the bottom surface  64  of the panel assembly has a much higher temperature than the top surface  31 , which is exposed to the cold jet stream  16 . However, when the heated fluid  26   i  reaches the first end  60   a  of the channel  60 , it is hotter than the jet stream, and thus the heated fluid  26   a  increases the temperature of the top sheet  30 . Also, the cooled fluid  26   b  travels down through the flow path toward the bottom surface  64 , reducing the temperature of the bottom surface. Thus, the two temperature extremes are brought closer together, with the fluid acting as a buffer between them. Reducing this temperature gradient can be beneficial, because a large temperature gradient can affect the structural integrity of the antenna and the mounting frame that attaches the antenna to the aircraft. Because different materials within the antenna have different coefficients of thermal expansion, they may expand at different rates, potentially leading to a structural failure of the antenna and/or its mounting structure. 
     In one embodiment, the flow path is closed-loop, such that the fluid  26  recycles through the path (see  FIGS. 5-8 ). As mentioned before, a pump can be provided, such as for example a pump mounted in or next to the circulation layer  44 , to replenish any fluid lost to leaks and to maintain the fluid in the flow path at a sufficient pressure to continue circulating through the path. 
     As shown in  FIGS. 5-9 , the flow path  24 ,  24 ″ moves generally along a first end  22   a  of the panel assembly as the fluid moves toward from the top sheet  30 , and the flow path moves generally along a second end  22   b , opposite the first end  22   a , as the fluid moves away from the top sheet  30 . The channels  60  extend between the two ends  22   a ,  22   b . Notably, the direction of the fluid through the channels  60  relative to the direction of the jet stream  16  is not important. The jet stream can flow in any direction over the top surface  31 . 
     Additionally, in one embodiment, the direction of fluid flow through the channels alternates.  FIG. 10  shows a radiating layer  132  with channels  160  between pairs of stubs  158 , with the channels  160  closed at each end by a cap  159 . A fluid  126  moves through the channels  160 . In a first channel, the fluid  126  moves from one end  132   a  of the layer  132  to the opposite end  132   b . In the next adjacent channel, the fluid  126  moves in the opposite direction, from end  132   b  toward end  132   a . Thus, the flow path has opposing flow directions in a single layer of the panel assembly, in order to mitigate adverse temperature gradients. In this embodiment the alternating flow paths are included in the radiating layer  132 , but in other embodiments they can be included in other layers, or in multiple layers. The flow path can be redirected as necessary throughout the panel assembly to route the fluid  126  in the opposing directions through the layer  132 . 
     In embodiments of the invention, a panel assembly with the closed loop fluid flow path can operate without a cooling plate attached to the bottom surface  64  of the panel assembly. The panel assembly dissipates its own heat to the jet stream  16 , without requiring any additional mechanism for heat dissipation. Thus, the panel assembly does not rely on the aircraft&#39;s own environmental control system or onboard cooling system to dissipate heat from the assembly. As a result, the panel assembly can be mounted in locations around the aircraft without the constraints of a cooling plate or connection to the aircraft cooling system. 
     When multiple panel assemblies are provided on an aircraft, each assembly may have its own internal cooling system as described above. The panel assembly  22 ,  22 ′,  22 ″ can be made in a variety of sizes. In one embodiment, the top surface  31  of the panel assembly is one square foot in area, or smaller. Each additional panel assembly added to the aircraft includes its own cooling system. 
     In one embodiment, the panel assembly  22 ,  22 ′,  22 ″ is powered by the aircraft&#39;s on-board power system. That is, the fans and (optionally) the pump are powered by the aircraft&#39;s on-board power. In another embodiment, they are powered by a battery. 
     As described above, the fluid flow path passes under the top sheet  30  to dissipate heat through the top sheet  30  to the jet stream outside the aircraft. Heat can be dissipated in this way if the jet stream is at a lower temperature than the heated fluid in the flow path. Typically, the antenna  20  is operated only while the aircraft is in flight, rather than when it is stationary on the ground. While the aircraft is in flight, the jet stream will typically be cooler than the heated fluid. However, in one embodiment, the cooling system is designed for sub-sonic flight, meaning that the speed of the aircraft is below Mach 1. Above that speed, it is possible for the jet stream passing around the aircraft to generate enough friction that it heats up to a higher temperature than the antenna, in which case the fluid in the flow path may not be able to dissipate heat to the jet stream. Accordingly, the antenna may be limited to use during sub-sonic flight conditions, or only brief periods of super-sonic flight. 
     In embodiments of the invention as described above, an improved panel assembly utilizes a unique closed-loop cooling system that is integrated into the panel assembly itself, passing through the antenna&#39;s functional architecture. The cooling system dissipates heat directly through the outer skin of the aircraft to the jet stream outside the aircraft. This panel assembly is more compact, efficient, and self-contained than prior art designs that require cooling plates or other external cooling systems attached to the antenna. As a result, the improved panel assembly can be mounted in many locations on the aircraft, such as on a curved surface like the aircraft wing, without the constraint of an external cooling system or connection. Additionally, the assembly requires less power from the aircraft as compared to the prior art, leading to longer flight durations and/or more power available for other systems. Initial modeling of the cooling system according to one embodiment of the invention showed the potential to provide 2-4 W/in 2  of heat rejection from the panel array antenna. 
     Although the present invention has been described and illustrated in respect to exemplary embodiments, it is to be understood that it is not to be so limited, since changes and modifications may be made therein which are within the full intended scope of this invention as hereinafter claimed. For example, the openings, holes. and flow passages in the various layers of the panel assembly can be arranged in different configurations, other than those specifically shown and described herein, to provide a fluid flow path through the panel assembly. The openings are not confined to the specific slots, holes, and passages shown. Additionally, while the panel array antenna has been described for use on an aircraft, it is not limited to that application, as it can also be used on other platforms such as ground vehicles, water vehicles, space vehicles, etc. Also, the antenna architecture is not limited to the specific layers and configuration described above. The various layers in the panel assembly can differ, with some layers being removed or additional layers being added, depending on the purpose and performance of the particular antenna.