Abstract:
A line-replaceable unit for a phased array antenna including a thermally conductive housing having a front face and an opposed rear face, at least one open-ended waveguide extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in a first frequency band; and at least one second radiating element positioned on the front face of the housing and adapted to emit energy in a second frequency band distinct from the first frequency band. The waveguide is dimensioned to pass energy in the first frequency band and is exposed to the environment outside the housing at the front and rear faces to define a cooling duct passing through the housing.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/571,710, filed May 17, 2004, the entirety of which is incorporated herein by reference. 
    
    
     FEDERAL RESEARCH STATEMENT 
     This invention was made with government support under M67854-04-C-2004 awarded by the United States Marine Corps. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to array antenna systems, and more particularly, line-replaceable transmit/receive units for multi-band active phased array systems with forced air cooling. 
     BACKGROUND OF THE INVENTION 
     Next generation radar systems will be required to perform multiple missions and deliver higher levels of performance, while being readily integrated into their host platforms. Providing the ability for the radar system to operate in more than a single frequency band enables realizing optimum multi-mission performance. For example, lower operating frequencies generally provide superior long range surveillance capabilities particularly when the detrimental effects of weather are considered. In contrast, higher operating frequencies, with their associated narrower antenna beamwidths and wider available instantaneous bandwidth waveforms, excel for angular accuracy and target discrimination. 
     To support these multiple missions with high levels of operational flexibility and overall performance, next generation radars will also need to employ active phased array antenna systems. Phased arrays are configured from a multitude of individual radiating elements whose phase and amplitude states can be electronically controlled. The radiated energy from the collection of elements combines constructively (focused) so as to form a beam. The angular position of the beam is electronically redirected by controlling the elements&#39; phases. Controlling both the elements&#39; phases and amplitudes alters the shape of the beam. Each individual radiator of an active phased array antenna includes an initial low noise amplifier for receive mode and a final power amplifier for transmit mode, in addition to the phase and amplitude control circuitry. 
     Juxtaposing multiple single-band array antennas to achieve operation in more than a single frequency band is incompatible with platform limitations, particularly from a size viewpoint. Consequently, the multiple band coverage must be derived from a single antenna system. Previous attempts to do so have comprised performance. Phased arrays have been designed to provide operation on widely separating frequencies by using a common radiating element for the multiple bands. These designs exhibit low efficiencies at the lower operating frequency and lose full control of the beam at the upper frequency extreme. Most of these conventional phased arrays are also passive in that they do not include receive and transmit amplifiers with each radiating element. 
     Dual frequency active arrays have been demonstrated where the frequency bands are contiguous. The array radiating elements and their associated electronics attempt to cover the full frequency range. The drawback with these designs is that the amplifiers exhibit non-optimum performance due to their necessity to cover an extended bandwidth. Additionally, the quantity of elements and electronics is denser than what would generally be required for the lower frequency band, which leads to the array being heavier, having higher heat densities, and being too costly. 
     Most host platform limitations, especially mobile platforms, necessitate that the radar system be assembled with light weight, small volume components and structures. Highly reliable operation with ease of maintenance and component replacement is also required. In addition, the inclusion of active components will require an effective thermal management system, preferably using air to minimize cooling system power consumption and to maximize reliability. To date, no such radar systems are available. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the problems of the prior art by providing a compact, lightweight line-replaceable transmit/receive (T/R) unit for assembling active phased array antenna systems that provide operation in two distinct frequency bands. The line-replaceable T/R unit in accordance with the present invention integrates the radiating elements and their transmit/receive electronics plus the associated DC power supply and control circuitry into a compact, lightweight modular building block for assembling multi-band active phased arrays. The units are constructed using light weight materials having favorable thermal properties. The line-replaceable T/R unit employs air cooling to convectively remove heat from the active electronics where the radiating element waveguide design for one operating frequency band also serves as an air coolant passage. The line-replaceable T/R unit is designed to plug into an array structure, in a manner that promotes ready access for service or replacement as required. This approach also facilitates system growth by either increasing the array size through additional line-replaceable T/R units or by upgrading the line-replaceable T/R units with, for example, higher power transmit amplifiers. The line-replaceable T/R unit is described herein in the context of a dual-band application where the line-replaceable T/R units, when assembled into an antenna array structure, form an active phased array antenna capable of operating on two distinct frequency bands with uncompromised performance. 
     In accordance with one embodiment of the invention, a line-replaceable T/R unit is provided for a phased array antenna, the unit comprising a thermally conductive housing having a front face and an opposed rear face, at least one open-ended waveguide extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in a first frequency band, and at least one second radiating element positioned on the front face of the housing and adapted to emit energy in a second frequency band distinct from the first frequency band. The waveguide is dimensioned to pass energy in the first frequency band and is exposed to the environment outside the housing at the front and rear faces to define a cooling duct passing through the housing. 
     In accordance with another embodiment of the invention, a line-replaceable T/R unit is provided for a phased array antenna, the unit comprising a housing having a front face and an opposed rear face, at least one open-ended waveguide dimensioned to pass energy in a first frequency band extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in the first frequency band, and at least two second radiating elements positioned on the front face of the housing and adapted to emit energy in a second frequency band distinct from the first frequency band. 
     In accordance with yet another embodiment of the invention, a line-replaceable T/R unit is provided for a phased array antenna, the unit comprising a housing having a front face and an opposed rear face, at least one open-ended waveguide dimensioned to pass energy in a first frequency band and attenuate energy in a second frequency band extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in the first frequency band, and at least two second radiating elements positioned on the front face of the housing adjacent to the waveguide and adapted to emit energy in the second frequency band. The radiated electric field polarization direction of the first radiating element is arranged orthogonal to the radiated electric field polarization direction of the second radiating elements. 
     In accordance with another embodiment of the invention there is provided a phased array antenna comprising a plurality of line-replaceable T/R units. Each line-replaceable T/R unit comprises a thermally conductive housing having a front face and an opposed rear face, at least one open-ended waveguide extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in a first frequency band, and at least one second radiating element positioned on the front face of the housing and adapted to emit energy in a second frequency band distinct from the first frequency band. The waveguide is dimensioned to pass energy in the first frequency band and is exposed to the environment outside the housing at the front and rear faces to define a cooling duct passing through the housing. 
     In accordance with another embodiment of the invention, there is provided a phased array antenna comprising a plurality of line-replaceable T/R units. Each line-replaceable T/R unit comprises a housing having a front face and an opposed rear face, at least one open-ended waveguide dimensioned to pass energy in a first frequency band extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in the first frequency band, and at least two second radiating elements positioned on the front face of the housing and adapted to emit energy in a second frequency band distinct from the first frequency band. 
     In accordance with another embodiment of the invention, there is provided a phased array antenna comprising a plurality of line-replaceable T/R units. Each line-replaceable T/R unit comprises a housing having a front face and an opposed rear face, at least one open-ended waveguide dimensioned to pass energy in a first frequency band and attenuate energy in a second frequency band extending through the housing from the front face to the rear face, at least one first radiating element including the waveguide and adapted to emit energy in the first frequency band, and at least two second radiating elements positioned on the front face of the housing adjacent to the waveguide and adapted to emit energy in the second frequency band. The radiated electric field polarization direction of the first radiating element is arranged orthogonal to the radiated electric field polarization direction of the second radiating elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fully understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred mode of practicing the invention, read in connection with the accompanying drawings in which: 
         FIG. 1   a  is a perspective front view of a line-replaceable T/R unit for a phased array antenna in accordance with an embodiment of the present invention; 
         FIG. 1   b  is a perspective rear view of the line-replaceable T/R unit shown in  FIG. 1   a;    
         FIG. 2   a  is an exploded perspective view of the line-replaceable T/R unit shown in  FIGS. 1   a  and  1   b;    
         FIG. 2   b  is a top view of the line-replaceable T/R unit shown in  FIGS. 1   a – 2   a;    
         FIG. 2   c  is a cross-sectional view of the line-replaceable T/R unit taken through line  2   c – 2   c  of  FIG. 2   b;    
         FIG. 2   d  is a bottom view of the line-replaceable T/R unit shown in  FIGS. 1   a – 2   c;    
         FIG. 2   e  is a cross-sectional view of the line-replaceable T/R unit taken through line  2   e — 2   e  of  FIG. 2   d;    
         FIG. 2   f  is a cross-sectional view of the line-replaceable T/R unit taken through line  2   f — 2   f  of  FIG. 2   d;    
         FIG. 3  is a top interior view of the line replaceable T/R unit showing an example of placement of electronic T/R components in accordance with an embodiment of the present invention; 
         FIG. 4  is a block diagram of the transmit and receive circuitry for a line replaceable T/R unit in accordance with an embodiment of the present invention; 
         FIG. 5  is a block diagram showing the relationship between two separate frequency band radiators in accordance with an embodiment of the present invention; and 
         FIG. 6  is a perspective view of a section of a phased array antenna incorporating the line-replaceable T/R unit in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention will now be explained with reference to  FIG. 1   a  and  FIG. 1   b .  FIG. 1   a  is a perspective front view and  FIG. 1   b  is a perspective rear view of a line-replaceable transmit/receive (T/R) unit for a phased array antenna in accordance with one embodiment of the present invention. The housing  201  of line-replaceable T/R unit  200  is fabricated as a one-piece, net-shape casting, for example, which requires minimal, if any, machining and provides thin cross-sections resulting in a low overall weight. Housing  201  can be made from a variety of well-known materials, one example of which is a metal matrix composite, preferably Aluminum Silicon Carbide (AlSiC). AlSiC has a high thermal conductivity to promote heat extraction from heat producing components, and has a thermal coefficient of expansion well matched to the typical component materials, which results in reduced stresses during temperature cycling. Additionally, AlSiC is electrically conductive and contributes to a low overall weight and can be plated to facilitate direct solder attachment of the high heat generating components. 
     First radiating element  239  includes open-ended waveguide  204  which extends fully from the approximate center of rear face  203  to the approximate center of front face  202  of line-replaceable T/R unit  200 . Waveguide  204  of first radiating element  239  is preferably dimensioned to pass energy in a first frequency band and attenuate energy in a second frequency band. In other words, one dimension of the open-ended waveguide  204  of first radiating element  239 , for example width, is dimensioned to pass energy in a first frequency band and a second dimension of open-ended waveguide  204 , for example height, is dimensioned to attenuate energy in a second frequency band. 
     Second radiating elements  205  are positioned in a plane parallel to front face  202  in an upper row  220  and a lower row  221  on the front face  202  of housing  201 . Second radiating elements  205  are formed as printed microstrip patch radiators to emit energy in a second selected frequency band. The microstrip patch radiators are flush to front face  202  of housing  201  to minimize system volume requirements and may be directly connected to the transmit/receive electronics via simple coaxial interfaces as will be described later in more detail. 
     It is preferred that the ratio of the operating frequencies between the two frequency bands is at least 3 to 1. By way of example only, the first frequency band is selected to be S-band and the second frequency band is selected to be X-band. However, the invention is not limited to these frequency bands. In the present embodiment, one dimension of open-ended waveguide  204 , for example width, is dimensioned to pass energy in the S-band (nominally 3 GHz) and a second dimension of open ended waveguide  204 , for example height, is dimensioned to attenuate energy in at least the X-band (nominally 10 GHz). Therefore the height of the open-ended S-band waveguide  204  is dimensioned such that its electrical length is less than one-half of the wavelength of the highest X-band frequency and the width of the open-ended S-band waveguide  204  is dimensioned such that its electrical length is greater than one-half of the wavelength of the lowest S-band frequency. 
     Open-ended waveguide  204  of first radiating element  239  is exposed to the environment outside the housing at the front  202  and rear  203  faces of housing  201 . In accordance with a preferred embodiment, coolant air  206  is ducted through open-ended waveguide  204  from rear face  203  to front face  202  to effectively extract heat from the active T/R components within the housing. Vertical conductive slats  207  act as cooling fins to facilitate the heat transfer from the active T/R components to the coolant air  206 , and further act as an electrical short for the operation of the S-band radiating element  239  as will be described later in more detail. 
     DC connector  209  and plunge-style Radio Frequency (RF) connectors  208   a–c  facilitate mating of the line-replaceable T/R unit  200  to an antenna array system&#39;s RF manifolds and DC/control distribution networks when the line-replaceable T/R unit  200  is placed into an array. Guide pins  210  properly align and locate the line-replaceable T/R unit  200  when installed in an antenna array. 
     Referring now to  FIGS. 2   a – 2   f , front face  202  of housing  201  is formed as a flat panel and functions as a ground plane for the phased array radiating aperture. X-band microstrip patch radiating elements  205  are photo-lithographically printed onto dielectric material  211  that is bonded by an interposed adhesive sheet  212  to the front face  202  of housing  201 . A two-layer patch  205   a  and  205   b , may be employed due to its wide bandwidth properties. Coaxial feed probes  213  penetrate front face  202  so as to directly interconnect each X-band patch radiator  205  with its respective X-band T/R channel circuitry  214 . 
     Open-ended waveguide  204  of S-band radiating element  239  opens at front face  202 , between the rows of X-band patch radiators  205 . Dielectric material  211 , which supports the patches, is removed at the waveguide opening. The bottom and top interior walls of open-ended waveguide  204  of radiating element  239  each have a longitudinal ridge  215 , which is smaller in width than open-ended waveguide  204 . Longitudinal ridges  215  enable the S-band radiator to operate at lower frequencies for a given interior width and contribute to heat transfer between active components  214 ,  216  and coolant air as will be discussed later in more detail. Longitudinal ridges  215  are tapered in height from front face  202  to rear face  203  such that the space between longitudinal ridges  215  increases in a direction moving toward front face  202  of housing  201 . 
     Open-ended waveguide  204  is directly coupled to S-band T/R channel circuitry  216  via a coaxial feed probe  217  to complete S-band radiating element  239 . Coaxial feed probe  217  is embedded in the upper floor of housing  201  and extends downward into open-ended waveguide  204 . 
     Partitioned areas  237 ,  238  are formed in the top of housing  201  for the placement of the electronic components for the S-band channel and each of the three top X-band channels. Similar partitioned areas  237 ,  240  are formed in the bottom of housing  201  for the placement of the electronic components for each of the three bottom X-band channels as well as a DC power supply and controller. The partitions promote electrical isolation and provide energy shielding between the T/R circuits, DC power supply and controller. Cover plates  218  can be laser welded against the top and bottom surfaces of the walls of housing  201  to complete a hermetic package for the components. 
     RF energy is coupled into and out from line-replaceable T/R unit  200  through RF connectors  208 . For example, RF connector  208   a  couples X-band energy into line-replaceable T/R unit  200  for transmission from X-band patch radiators  205  in upper row  220 . The X-band energy propagates through signal combining/dividing network  219  formed in housing  201  to X-band T/R channel circuitry  214  for each of the X-band radiator elements  205  in upper row  220 . Signal combining/dividing network  219  also performs initial beam forming for the X-band signal. X-band T/R channel circuitry  214  processes the X-band energy in accordance with control signals received via DC connector  209  prior to transmission through coaxial feed probes  213  to X-band radiators  205  on upper row  220  as will be described later in more detail. X-band energy received by X-band radiators  205  on upper row  220  propagates through coaxial feed probes  213  to X-band T/R channel circuitry  214  through signal combining/dividing network  219  and out from line-replaceable T/R unit  200  through RF connector  208   a . Similarly, X-band energy is coupled into and out from line-replaceable T/R unit  200  through RF connector  208   c  and X-band radiators  205  on bottom row  221 . 
     S-band energy is coupled into S-band T/R channel circuitry  216  of line-replaceable T/R unit  200  through RF connector  208   b . T/R channel circuitry  216  processes the S-band energy in accordance with control signals received via DC connector  209  prior to transmission through S-band radiating element  239  via coaxial feed probe  217 , as will be described later in more detail. As previously discussed, vertical conductive slats  207  act as an electrical short such that S-band energy from coaxial feed probe  217  is transmitted only from front face  202  of line-replaceable T/R unit  200 . S-band energy that may propagate toward the rear face  203  of line-replaceable T/R unit  200  is significantly attenuated via vertical conductive slats  207 . 
     S-band energy received by radiating element  239  is coupled into S-band T/R channel circuitry  216  via coaxial feed probe  217  and out of line-replaceable T/R unit  200  through RF connector  208   b.    
       FIG. 3  shows representative layouts of the X-band  214  and S-band  216  T/R channel components within the top partitions of housing  201 . High heat generating components of both X-band  214  and S-band  216  T/R channel components are mounted directly to the floor of partitioned areas  237  and  238  of housing  201 , which forms part of an upper inner surface of open-ended waveguide  204 . As previously discussed, housing  201  is made from a material with high thermal conductivity to promote heat extraction from heat producing components. Additionally, the open-ended waveguide  204  of S-band radiating element  239  extends fully from the rear face  203  to the front face  202  of the line-replaceable T/R unit housing  201  and passes directly beneath all of the active components of the S-band T/R electronics  216  and top row X-band T/R electronics  214 . Therefore, coolant air  206 , which is ducted through open-ended waveguide  204 , effectively extracts heat from active X-band  214  and S-band  216  T/R channel components through conduction from the base of each circuit  214 ,  216  through the floor of partitioned areas  237  and  238  of housing  201  and convection by the coolant air  206 . The thermal impedance of this design is low so that the temperature differential between the air coolant and the active components is limited to acceptable values. Similarly, the open-ended waveguide  204  of the S-band radiating element  239  passes directly over all of the active components  214  of the bottom row of X-band radiators as well as the DC power supply and controller which are mounted directly to the ceiling of the bottom partitioned areas (not shown) of housing  201  which forms part of a lower inner surface of open-ended waveguide  204 . As a result the same cooling process occurs with respect to the active components within the bottom partitioned areas of housing  201 . 
       FIG. 4  is a block diagram of the transmit and receive circuitry for a line replaceable T/R unit in accordance with an embodiment of the present invention. The upper row  420  and lower row  421  X-band T/R channel components  414  include RF connectors  408   a  and  408   c , signal combining/dividing networks  419 , X-band amplitude control components  422 , X-band phase control components  423 , final X-band transmit power amplifiers  424 , initial X-band receive low noise amplifiers  425 , X-band directional circulators  426 , coaxial feed probes  413  and X-band radiators  405 . These components are closely located proximate X-band radiators  405  to minimize detrimental signal losses arising from physically long interconnections. 
     The S-band T/R channel components  416  include RF connector  408   b , S-band amplitude control components  427 , S-band phase control components  428 , final S-band transmit power amplifier  432  initial S-band receive low noise amplifier  429 , S-band directional circulator  433 , coaxial feed probe  417  and open-ended waveguide  404 . Again, these components are closely located proximate open-ended waveguide  404  to minimize detrimental signal losses arising from physically long interconnections. 
     DC power supply  430  and controller  431  are provided in line-replaceable T/R unit  400  for deriving the collection of voltages required for the T/R channel components and for setting the states of the phase and amplitude control components and sequencing transmit/receive operation. 
     X-band energy coupled into line-replaceable T/R unit  400  via RF connectors  408   a  and  408   c  is divided into separate signals by signal combining/dividing network  419 . Each X-band signal is then subject to proper amplitude and phase adjustments by X-band amplitude control components  422  and X-band phase control components  423  for proper beam steering of the transmitted energy based on signals provided from controller  431  as is known in the art. The X-band signals, now of proper phase and amplitude are amplified by final X-band transmit power amplifiers  424 , pass through directional circulators  426  and are transmitted out through X-band radiators  405  via coaxial feed probes  413 . 
     X-band signals received through X-band radiators  405  pass through coaxial feed probes  413  and directional circulators  426  and are amplified by initial X-band receive low noise amplifiers  425  to a level where the signals can be phase and amplitude adjusted by X-band phase control components  423  and X-band amplitude control components  422 , respectively. The X-band signals are combined by signal combining/dividing network  419  and coupled out from line-replaceable T/R unit  400  via RF connectors  408   a  and  408   c.    
     S-band energy coupled into line-replaceable T/R unit  400  via RF connector  408   b  is subject to proper amplitude and phase adjustments by S-band amplitude control components  427  and S-band phase control components  428  for proper beam steering of the transmitted energy based on signals provided from controller  431  as is known in the art. The S-band signals, now of proper phase and amplitude are amplified by final S-band transmit power amplifier  432 , pass through directional circulator  433 , and are coupled to open-ended waveguide  404  via coaxial feed probe  417  and subsequently transmitted out the front face of line-replaceable T/R unit  400 . As previously discussed, vertical conductive slats  207  ( FIG. 1   b ) act as an electrical short to prevent S-band energy from exiting the rear face of line-replaceable T/R unit  400 . 
     S-band signals received through open-ended waveguide  404  are coupled out of open-ended waveguide  404  via coaxial feed probe  417  through directional circulator  433  and are amplified by initial S-band receive low noise amplifier  429  to a level where the signals can be phase and amplitude adjusted by S-band phase control components  428  and S-band amplitude control components  427 , respectively. The amplified S-band signals are coupled out from line-replaceable T/R unit  400  via RF connector  408   b . Again, vertical conductive slats  207  ( FIG. 1   b ) ensure that no received S-band energy exits open-ended waveguide  404  through the rear face of line-replaceable T/R unit  400 . 
       FIG. 5  is a block diagram of a portion of a phased array antenna aperture incorporating line-replaceable T/R units in accordance with the present invention showing an interleaving of X-band  505  and S-band  539  radiating elements. The ratio of X-band  505  to S-band  539  radiating elements depicted is six-to-one where two rows of three X-band radiators  505  each are arranged horizontally; one X-band radiator  505  row above the associated S-band radiating element  539  and one X-band radiator  505  row below the associated S-band radiating element  539 . The radiating element ratio is dictated by the relationship of the operating frequencies and the phased array beam angular coverage required in each of the bands. The ratio of six-to-one is appropriate for a typical ground-based radar application. The radiated electric field polarization  534  for the S-band radiating element  539  is vertical while the radiated electric field polarization  535  for the X-band radiators  505  is horizontal. The orthogonal orientation of the electric fields  534 ,  535  promotes isolation of the signals originating from either one of the bands&#39; T/R electronics into the T/R electronics for the other band. In other words, the response of the X-band radiating element  505  to the energy from the S-band radiating element  539  will be significantly lower due to the orthogonal orientation of the electric fields. Further, the height of the S-band waveguide  504  of S-band radiating element  539  is selected so as to effectively “cut-off” the orthogonally polarized X-band electric field. For example, the height of the S-band waveguide  504  is selected such that the electrical length of the height of the waveguide is less than one-half of the wavelength of the highest X-band frequency. This promotes additional isolation of signals between the two bands as is known in the art. 
       FIG. 6  is a perspective view of a section of a phased array antenna  636  incorporating line-replaceable T/R units  200  in accordance with the present invention. Line replaceable T/R units  200  are guided into antenna array structure  636  by aligning grooves  640  in line replaceable T/R unit  200  with ridges  641  in antenna array structure  636  and sliding line replaceable T/R unit  200  into antenna array structure  636  to engage guide pins  210 . As previously discussed, guide pins  210  positively locate and secure the line-replaceable T/R unit  200  in antenna array structure  636 . Additionally, guide pins  210  ensure correct alignment of DC connector  209  ( FIG. 1   b ) and RF connectors  208   a–c  with mating connectors (not shown) within the antenna array structure. Openings in the antenna array&#39;s air supply plenum align to the open-ended waveguide  204  at the rear face of line-replaceable T/R unit  200 . A skeletal design for the antenna array structure  636  permits it to be rigid yet light in weight. 
     It will be understood that various modifications and changes may be made in the present invention by those of ordinary skill in the art who have the benefit of this disclosure. All such changes and modifications fall within the spirit of this invention, the scope of which is measured by the following appended claims.