Compact phased array antenna system, and a method of operating same

An antenna system (10) includes circuitry (13) and an antenna unit (12). The antenna unit includes a multi-layer circuit board (21). The circuitry provides radio frequency signals, control signals and power to the circuit board. The circuit board has an array of antenna elements (23) on one side thereof, and has a plurality of modules (71, 72) soldered to and projecting outwardly from the opposite side thereof. The modules each have electronic circuitry thereon, which is electrically coupled to the circuit board. Each module includes a thermal transfer element (96), the heat generated by the electronic components on that module being thermally transferred by the thermal transfer element to a cooling section (51).

TECHNICAL FIELD OF THE INVENTION
 This invention relates in general to an antenna system and, more
 particularly, to a compact phased array antenna system suitable for use in
 a satellite, and a method of operating such an antenna system.
 BACKGROUND OF THE INVENTION
 Active phased array antenna systems are used in a wide variety of
 applications. As one example, a satellite may include an antenna system of
 this type in order to facilitate communication between the satellite and
 one or more ground stations on earth. In a phased array antenna system,
 especially for a satellite, it is desirable that the antenna system be
 relatively small in volume and relatively light in weight. On the other
 hand, antenna systems of this type typically use circuits such as
 monolithic microwave integrated circuit (MMICs). Circuits such as MMICs
 generate a substantial amount of heat during operation. As the frequency
 of antenna operation increases, there is an increase in the amount of heat
 which is emitted by these circuits, which in turn can affect temperature
 gradients across the array.
 In particular, in a phased array antenna system, the existence of
 temperature gradients across the array can produce phase errors, which
 affect the accuracy of the antenna system. The higher the frequency of
 antenna operation, the smaller the permissible temperature gradients
 across the array. For example, where the phased array is operating at a
 frequency of about 5 GHz, the maximum allowable temperature gradient
 across the array is about 20.degree. C. In contrast, when the array is
 operating at a frequency of about 80 GHz, the maximum allowable
 temperature gradient across the array is only about 1.3.degree. C. If the
 maximum temperature gradient across the array cannot be kept below the
 maximum allowable gradient, then it is necessary to provide additional
 circuitry in the antenna system to effect dynamic phase error control
 compensation, which increases the complexity, cost and weight of the
 antenna system. Thus, it is important to have an efficient technique for
 cooling the circuitry of the antenna system, so that a substantially
 uniform temperature is maintained across the array.
 One traditional phased array antenna system has a configuration commonly
 known as an array slat arrangement, and uses forced flow of a liquid
 coolant. However, the thickness, volume and weight of this arrangement are
 greater than desirable, and the forced flow of the liquid coolant requires
 hardware for handling the coolant, which increases the effective volume
 and weight of the overall antenna system.
 A different approach, which is more recent, is commonly known as a tile
 array, and uses a multi-layer circuit board. The circuit board has the
 antenna elements and the circuit components of the antenna system mounted
 thereon, and cooperates with a relatively thin cooling arrangement. This
 has the advantage of being ultra thin and low in weight, and also provides
 shorter conductors for radio frequency signals than the traditional array
 slat approach. However, while this known approach has been generally
 adequate for its intended purpose, it has not been satisfactory in all
 respects.
 More specifically, the ultra thin configuration makes it difficult or
 impossible to use radio frequency circulators and/or isolators, as a
 result of which a given antenna system is typically configured to either
 send or receive signals, but not both. Further, only a limited amount of
 circuitry can be provided directly on the multi-layer circuit board within
 the size limits of the antenna element array, even where some of the
 circuit components are mounted in a stacked or "piggy-back" arrangement.
 As a result, it is difficult to provide multi-beam capability in an
 antenna system. A further consideration in such ultra thin antenna
 configurations is that it is typically difficult to separately optimize
 the cooling system and the packaging of the radio frequency circuitry,
 because the compactness of the system causes various design aspects to
 become interdependent. A further consideration in these ultra thin antenna
 systems is that, since various circuit components are provided directly on
 the multi-layer board, problems can occur as a result of different
 coefficients of thermal expansion.
 SUMMARY OF THE INVENTION
 From the foregoing, it may be appreciated that a need has arisen for an
 antenna system, and a method of operating it, which involve a compact and
 lightweight system, which allow the use of isolators or circulators, which
 allow the implementation of multiple beam capability within the size
 limits of the antenna element array, which permit independent optimization
 of the cooling section and the radio frequency circuitry, which avoid
 problems due to different coefficients of thermal expansion, which provide
 relatively short conductors for radio frequency signals, and/or which
 effect cooling in a manner sufficiently efficient to maintain a
 substantially uniform temperature across the entire array.
 According to the present invention, an antenna system and a method of
 operation are provided in order to address this need, and involve:
 providing an electrical interconnection section which is thin and
 generally planar; arranging on one side of the interconnection section an
 antenna section which includes a plurality of antenna elements that are
 each electrically coupled to the interconnection section; supporting a
 cooling section at a location which is spaced from the interconnection
 section on a side thereof opposite from the antenna section; providing
 between the interconnection section and the cooling section a module which
 has electronic components thereon; transmitting electrical signals between
 the electronic components and the interconnection section; and
 transferring to the cooling section the heat emitted by the electronic
 components.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 is a diagrammatic perspective view of an antenna system 10,
 including an antenna unit 12, and circuitry 13 which is operatively
 coupled to the antenna unit 12. The antenna unit 12 is shown in an
 exploded form in FIG. 1 for purposes or clarity. The exemplary embodiment
 disclosed in FIG. 1 is an antenna system of the type commonly known as an
 active phased array antenna system.
 The antenna unit 12 includes a multi-layer circuit board 21. The circuit
 board 21 is generally planar, and in the disclosed embodiment is generally
 circular, although it could have some other shape, such as a square. The
 circuit board 21 serves as an interconnection section for providing
 electrical interconnections between various component parts of the system,
 as discussed in more detail later. In the disclosed embodiment, the
 circuit board 21 has about twenty-nine layers, but it could have a larger
 or smaller number of layers without departing from the subject matter of
 the present invention.
 An array of antenna elements 23 is provided on one side of the circuit
 board 21. In the disclosed embodiment, the antenna elements 23 are patch
 antenna elements formed by an appropriate patterned etch of a metalization
 film disposed directly on one side of an outer layer of the circuit board
 21. However, the antenna elements 23 do not have to be supported directly
 on the circuit board 12, and could instead be provided thereon in some
 other manner. Further, the antenna elements 23 could have some other
 configuration. For example, they could be waveguide elements, rather than
 patch elements.
 The circuit board 21 has four radio frequency (RF) connectors 26, three of
 which are visible in FIG. 1. These four connectors 26 are each provided
 adjacent the peripheral edge of the circuit board 21, at locations which
 are uniformly circumferentially spaced. The RF connectors 26 each project
 outwardly on a side of the circuit board 21 opposite from the antenna
 elements 23. The antenna unit 12 is a four-beam antenna system, and each
 of the RF connectors 26 corresponds to a respective one of the four beams.
 The circuit board 21 also has mounted thereon a power and logic connector
 27, at a location on the periphery which is between two of the connectors
 26.
 The circuitry 13 includes an RF circuit 31, which is coupled by four
 separate cables to each of the four respective RF connectors 26, one of
 these cables being represented diagrammatically in FIG. 1 by a broken
 line. The circuitry 13 further includes a control circuit 32 and a power
 circuit 33, which are both coupled to the power and logic connector 27.
 The power circuit 33 provides the antenna unit 12 with direct current (DC)
 power, and the control circuit 32 provides the antenna unit 12 with
 digital logic signals that represent control information such as commands
 which indicate directions in which the antenna 12 should scan.
 The circuit board 21 has a plurality of small openings 36 therethrough at
 uniformly spaced locations along its peripheral edge. Adjacent the circuit
 board 21 is an annular housing 37, which has at the end nearest the
 circuit board a radially inwardly projecting annular flange. The circuit
 board 21 is secured to the annular housing 37 by a plurality of bolts 38,
 which each extend through a respective one of the openings 36, and each
 engage a respective threaded opening 41 provided in the annular flange,
 the openings 41 being located at uniformly circumferentially spaced
 locations along the annular flange 42.
 The housing 37 is made of a material having a low coefficient of thermal
 expansion (CTE), for reasons discussed later. In the disclosed embodiment,
 the housing 37 is an epoxy graphite material, but it could be some other
 material with a low CTE. The housing 37 is disposed on a side of the
 circuit board 21 opposite from the antenna elements 23. A cooling section
 51 is fixedly secured to the axial end of the housing 37 remote from the
 circuit board 21. For example, the housing 37 may be brazed or welded to a
 face plate 52 of the cooling section 51. The face plate 52 is made from a
 material which is highly thermally conductive, such as aluminum.
 The particular cooling section shown at 51 in FIG. 1 represents one type of
 cooling section which is suitable for use according to the present
 invention. This particular cooling section 51 is a phase change module. It
 includes a circular base member 56 which is made of a thermally conductive
 material such as aluminum, and which has a plurality of approximately
 triangular recesses 57 machined into one side thereof. The recesses 57
 define radially extending ribs 58, which are disposed between the recesses
 57.
 An approximately triangular piece of porous material 61 is disposed in each
 of the recesses 57. The porous material 61 is a thermally conductive
 material, and may for example be a material of the type commonly known as
 aluminum foam. A suitable porous material is available commercially as
 DUOCEL foam (40 PPI, 6-8%, 6101-T6) from ERG Materials and Aerospace
 Corporation of Oakland, Calif. The remaining space in each of the recesses
 57 is filled with a not-illustrated phase change material, which in the
 disclosed embodiment is a commercially available material, commonly known
 as a phase change wax. The circular face plate 52 is physically secured in
 a suitable, thermally conductive manner to the base member 56 and to each
 of the porous elements 61, for example by vacuum brazing. The base member
 56 has a plurality of radial openings 63 drilled therein, each of the
 openings 63 extending through a respective one of the radial ribs 58. Each
 of the radial openings 63 has therein a respective heat pipe, one of which
 is shown at 64. The heat pipes 64 are commercially available devices, and
 may for example be a heatpipe available as DYNATHERM 0476-1000 from
 Dynathern Corporation of Hunt Valley, Md. The heat pipes 64 are each
 surrounded within the associated opening 63 by a suitable thermal grease
 or thermal epoxy, which is not illustrated. In this regard, the disclosed
 embodiment uses a thermally conductive epoxy, which should be a
 non-degassing or low degassing epoxy, such as that available commercially
 from Dow Corning Corporation of Midland, Mich., as DOW CORNING 3140,
 MIL-A-46146 RTV Coating.
 The antenna unit 12 includes four beam steering modules, one of which is
 visible at 71. Each of the beam steering modules 71 carries electronic
 circuitry, and may alternatively be referred to as a carrier. Each module
 71 has a microprocessor on it, as well as other digital circuitry that
 facilitates steering of a respective one of the four beams which are each
 associated with a respective one of the four RF connectors 26.
 The antenna unit 12 also includes a plurality of RF modules, one of which
 is designated by reference numeral 72. In the disclosed embodiment, the
 number of RF modules 72 is one-half the number of antenna elements 23. The
 beam steering module 71 and the RF module 72 each have approximately the
 same physical configuration. Accordingly, the physical structure of only
 one of the modules 71 and 72 is described in detail below.
 More specifically, FIG. 2 is a diagrammatic exploded perspective view of
 one of the RF modules 72 from the antenna unit 12 in FIG. 1. The module 72
 includes a circuit board 81, which carries various electrical components.
 The end of the circuit board 81 nearest the cooling section 51 (FIG. 1)
 has a plurality of monolithic microwave integrated circuits (MMICS). These
 are high power components, which generate a substantial amount of heat.
 The MMICs 82 carry out functions such as implementing phase shifts in RF
 signals, amplifying RF signals, and effecting associated logic functions.
 The same end of the circuit board 81 has some additional integrated
 circuits 83, which provide control and support for the MMICs 82.
 The opposite end of the circuit board 81 has several circuits 86 which, in
 the disclosed embodiment, are circuits of a type commonly known as
 circulators. The disclosed embodiment uses circulators 86 because the
 antenna unit 12 of the disclosed embodiment has the capability to both
 transmit and receive electromagnetic signals through the antenna elements
 23. On the other hand, the present invention could be used in an antenna
 unit which was configured only to send or only to receive signals, and in
 that case the circulators 86 could be replaced with components commonly
 known as isolators.
 The module 72 further includes a rectangular frame 91, which includes four
 wall portions 87, 88, 89 and 90. It would be possible for the circuit
 board and frame 91 to be formed as an integral unit. In the disclosed
 embodiment, however, this is not the case. In particular, the circuit
 board 81 is formed by starting with a flat plate of a material such as
 berillium oxide, which is thermally conductive but electrically
 non-conductive. Multiple layers of thick film conductors and insulators
 are then printed thereon, using known techniques. Through holes are then
 drilled or laser cut, and subsequently filled with a conductive material,
 in order to make electrical connections between the layers. The various
 integrated circuits, including those at 82-83 and 86 are then soldered in
 place on the board 81.
 In a similar manner, the wall portion 87 is a circuit board formed by
 starting with a flat plate of a material such as glass or a ceramic.
 Multiple layers of thick film conductors and insulators are then printed
 on. Through holes are then drilled or laser cut, and subsequently filled
 with a conductive material, in order to make electrical connections
 between the layers. The wall portion 87 has on the outer side thereof a
 plurality of approximately hemispherical metal "ball" elements 92, which
 are collectively referred to as a ball grid interface. The elements 92 are
 each soldered to a respective pad provided on the outer surface of the
 wall portion 87, using a high-temperature solder which will not melt
 during subsequent soldering operations. At least some of these pads are
 electrically coupled to the conductors within the wall portion 87. In the
 disclosed embodiment, about twenty of the pads are coupled to conductors
 within the wall portion 87. The circuit board 81 and wall portion 87 are
 then mechanically coupled to each other, for example by an appropriate
 brazing technique.
 FIG. 3 is a diagrammatic fragmentary perspective view of a portion of the
 circuit board 81 and the wall portion 87 after they have been coupled
 together. The circuit board 81 has on the surface thereof a plurality of
 conductive strips, two of which are shown at 101 and 102. The conductive
 strips 101 and 102 are each coupled to conductors within the circuit board
 81, and each extend to a location adjacent the end of circuit board 81
 nearest wall portion 87. Similarly, the wall portion 87 has on the inner
 surface thereof a plurality of conductive strips, two of which are shown
 at 103 and 103. The conductive strips 103 and 104 are each coupled to
 conductors within the wall portion 87, and each extend to a location
 thereon adjacent the circuit board 81. A thin rectangular conductive
 ribbon element 108 is bent to an L-shape, and has each end soldered to a
 respective one of the strips 101 and 103, using a high temperature solder
 that will not melt during subsequent soldering. A similar ribbon element
 109 has each end soldered to a respective one of the strips 102 and 104,
 using a high temperature solder. The ribbon element 108 thus electrically
 couples the strips 101 and 103, and the ribbon element 109 electrically
 couples the strips 102 and 104. Alternatively, the elements 108 and 109
 could be thin rectangular gold ribbons welded to 101 and 103 and to 102
 and 104, respectively.
 The other three wall portions 88-90 of the frame 91 do not have conductors
 therein. In the disclosed embodiment, each is made of a ceramic or metal
 material, and they are each brazed to the circuit board 81 and to the
 edges of two other immediately adjacent wall portions. A platelike metal
 lid 94 has its edges each brazed or laser welded to an edge of a
 respective one of the wall portions 87 to 90, in order to hermetically
 seal the integrated circuits within the module 72. The lid is attached
 when the module 72 is in a dry nitrogen atmosphere, so that there is no
 air or moisture trapped within the module 72. As an alternative approach,
 the lid 94 can optionally be omitted and the integrated circuits can be
 coated with a sealant that hermetically seals them.
 The module 72 further includes an L-shaped element 96, which has a high
 thermal conductivity. In the disclosed embodiment, the element 96 is made
 of a graphite material, but some other material with a high thermal
 conductivity would also be suitable. The L-shaped element 96 has two legs
 97 and 98. The leg 97 is disposed against the back side of a portion of
 the circuit board 81, in particular the portion which has thereon the
 MMICs 82 and the control components 83. The leg 97 is fixedly secured to
 the circuit board 81 by a layer of a not-illustrated thermal epoxy which
 is disposed therebetween, and which helps to maximize the transfer of heat
 from the circuit board 81 to the leg 97. A suitable thermal epoxy is the
 epoxy discussed above in association with the heat pipes 64. The other leg
 98 of the element 96 is disposed against the outer side of the end of the
 frame 91 opposite from the ball elements 92.
 After the module 72 has been assembled, each of the ball elements 92 is
 soldered to a respective not-illustrated pad that is provided on a side of
 the circuit board 21 opposite from the antenna elements 23. This provides
 electrical connections between the circuit board 21 and the integrated
 circuits in the module 72, and also mechanically supports the module 72 on
 the circuit board 21.
 The outer surface of the leg 98 of the L-shaped element 96 is in contact
 with, or at least closely adjacent, the face plate 52 of the cooling
 section 51. As mentioned above, the housing 37 is made of a material
 having a low coefficient of thermal expansion. This helps to minimize the
 extent to which the housing 37 will tend to move the cooling section 51
 toward or away from the circuit board 21, and thus toward or away from the
 leg 98 of the element 96.
 A material such a thermal grease or a thermal epoxy may be provided between
 the leg 98 and the cooling section 51, in order to maximize the transfer
 of heat from the leg 98 to the cooling section 51. The disclosed
 embodiment uses a thermal epoxy, such as the epoxy discussed above in
 association with the heat pipes 64. Use of a thermal epoxy has the added
 benefit of fixedly securing the leg 98 to the cooling section 51, which
 helps to physically support the modules 71 and 72, so that they are not
 supported solely by the solder connections to the ball elements 92
 provided at the opposite end thereof.
 During operation of the antenna system of FIG. 1, the MMICs 82 (FIG. 2) on
 the modules 72 generate a substantial amount of heat while the antenna
 system is operating. This heat is transferred rapidly and directly through
 legs 97 and 98 of the element 96 to the cooling section 51. The antenna
 unit 12 of FIG. 1 is intended for use in a satellite, in a situation where
 the antenna unit is intermittently operated and idle. When the antenna
 unit is operational, the cooling section 51 continuously absorbs heat from
 the circuitry and therefore begins to heat up. The not-illustrated phase
 change material in the recesses 57 absorbs the heat, and in the process
 changes phase by melting so as to change from a solid to a liquid. When
 the antenna unit 12 is thereafter turned off, the cooling unit slowly
 discharges the heat which it absorbed, such that the phase change material
 cools off and changes phase back to its original state, for example by
 changing from a liquid to a solid. In some other application, where the
 antenna unit 12 had to be operated continuously, or at least for long
 periods of time, the disclosed cooling section 51 could be replaced with a
 different type of cooling section, which is capable of effecting
 continuous cooling, examples of which are a convection cooling arrangement
 such as a heat sink, a liquid cooled arrangement, or a conduction cooling
 arrangement.
 The present invention provides a number of technical advantages. One such
 advantage is that the disclosed antenna unit is relatively thin in
 comparison to traditional antenna units that have an array slat
 configuration. It has a smaller physical volume, lower weight, and lower
 cost than the traditional array slat configuration. A further advantage,
 in comparison to ultra-thin antenna units, is that RF circulators and/or
 isolators can be used between the RF power output amplifiers and the
 antenna elements, thereby permitting the system to both send and receive
 signals. Another advantage is that the antenna unit can provide multiple
 beam capability within the transverse size limits defined by the antenna
 element array, which is difficult with existing antenna units of ultra
 thin configuration, which are commonly referred to as tile arrays.
 Still another advantage is that putting the cooling section at the very
 rear of the antenna unit allows both the RF circuit packaging and the
 cooling system to be optimized separately and independently of each other.
 Yet another advantage is that putting circuitry on the modules, rather
 than on the main circuit board, reduces problems due to different
 coefficients of thermal expansion. Consequently, the disclosed antenna
 system is capable of operation at relatively high frequencies, such as 80
 GHz, while keeping thermal gradients within the circuitry low so that
 there is no need for the complexity and cost of providing extra circuitry
 in order to effect phase error compensation. Still another advantage is
 that the length of most RF conductors is less than in pre-existing antenna
 units having the traditional array slat configuration.
 Although one embodiment has been illustrated and described in detail, it
 should be understood that various substitutions and alterations can be
 made thereto without departing from the invention. For example, the
 disclosed embodiment has a cooling section which uses phase change
 technology, but it will be recognized that various other types of cooling
 sections could also be used. As another example, the antenna elements in
 the disclosed embodiment are patch elements fabricated directly on the
 main circuit board, but other types of antenna elements could
 alternatively be used, such as a configuration of waveguide elements. As
 still another example, the modules of the disclosed embodiment are
 electrically coupled to the main circuit board by solder connections
 between the circuit board and an array of ball elements, but it will be
 recognized that there are other ways in which the modules and circuit
 board could be electrically coupled.
 Still another example is that each of the modules in the disclosed
 embodiment includes an L-shaped element which is made of material with a
 high thermal conductivity, in order to facilitate the transfer of heat
 from the module to the cooling section. However, it will be recognized
 that there are other physical configurations which could facilitate a heat
 transfer from the modules to the cooling section. Yet another example
 involves the fact that the disclosed antenna unit has a multi-beam
 capability, but it will be recognized that the invention can be utilized
 in antenna units configured with single beam capability. Other
 substitutions and alterations are also possible without departing from the
 spirit and scope of the present invention, as defined by the following
 claims.