Patent Publication Number: US-7215298-B1

Title: Extendable/retractable antenna calibration element

Description:
GOVERNMENTAL INTEREST 
   This invention was made with government support under Contract/Grant SBAR 1TL405P01T11. The United States Government has a non-exclusive, non-transferable, paid-up license in this invention. 

   FIELD OF THE INVENTION 
   This invention relates to antennas and arrays thereof, and more particularly to the calibration of antennas and arrays of antennas by means of retractable and extendable elements. 
   BACKGROUND OF THE INVENTION 
   Antennas are widely used for remote sensing, communications, and for industrial/therapeutic purposes. An antenna is simply a transducer between guided and unguided electromagnetic fields. An antenna, seen as a transducer, includes a “feed” port and a “radiating aperture” unguided or free-space radiation port. The feed port is so termed for historical reasons; early receiving antennas were simply pieces of electrically conductive wire which received little attention, while transmitting antennas received sophisticated attention because of the great effect that they could have on the high-power transmitters of the age. Thus, antennas were originally viewed as being transmitting devices defining one or more feed points. Only later was it discovered that antennas have the same radiation patterns and other characteristics in both the transmitting and receiving modes of operation. The feed port is ordinarily coupled to a “transmission line,” which is simply an electrical conducting arrangement having a defined (or at least controlled) surge or characteristic impedance. The electromagnetic energy flowing in the transmission line is guided by the line, and the radiation at the free-space port is in directions controlled by the “electrical field distribution” at the radiating aperture of the antenna. When the antenna operates in a receiving mode, free-space or unguided energy impinging thereon is transduced to become guided energy in the transmission line, and in the transmitting mode, guided energy applied to the feed port from the transmission line is radiated as unguided radiation (subject to certain limitations). 
   The field distribution characteristics of the radiating aperture of an antenna determine the “far-field” radiation pattern. One of the salient generalizations which can be made about antennas is that the radiating beam width is inversely related to the dimensions of the radiating aperture. That is, a highly directive antenna or radiation beam (a beam subtending a small sector of space) requires a large radiating aperture in terms of wavelength, and conversely a small radiating aperture results in a low-directivity or broad radiation beam. There are two popular ways to achieve a large radiating aperture in order to form directive antenna beams, namely (a) reflectors and (b) arrays. 
   An antenna array is an array including a plurality of antennas. When antennas are arrayed and properly phased, the overall radiation pattern is determined as the result of an “array factor” which multiplies the radiation pattern of the underlying antenna element of the array. Array antennas are of two general types, namely line (one-dimensional) arrays and surface (two-dimensional) arrays. The salient difference between these two is that the line array produces an array factor which multiplies the pattern of the underlying array only in the direction of the array, while a surface array produces a useful array factor in two mutually orthogonal directions. Thus, when a three-dimensional “pencil” beam is desired, it is likely that a two-dimensional surface array will be required. A plurality of line arrays can be juxtapositioned and fed so as to form a surface array, and a surface array can be viewed as being a plurality of interconnected line arrays. 
   As mentioned, it is necessary to feed the elements of an array antenna with signals of controlled phase in order to achieve the desired radiation pattern. The distribution of signals from a common feed point to the individual antenna elements of the array is often accomplished by a beamformer, which divides the available signal among the antenna elements, and which may include a phase shifter associated with each antenna element, or at least with subgroups of antenna element. The phase shifters are controlled in well-known manner in order to achieve the desired antenna beam direction. Each antenna element (or subgroup of antenna elements) of an array antenna may be associated with controllable attenuators and amplifiers as well as with phase shifters. In order to route the signals between and among the antenna elements, their amplifiers, phase shifters, and attenuators, if any, and possibly other elements, the array antenna will also include transmission lines. These transmission lines may take the form of hollow conductive waveguides, coaxial transmission lines, andor any one of various forms of “printed-circuit” transmission lines, such as finline, stripline or microstrip, all known in the art. Each transmission line must maintain its proper impedance to prevent the introduction of unwanted phase shifts andor attenuation, and remain electrically connected to its signal source and load. 
   Considering the complexity of array antennas, and all the potential problems which can arise due to degradation or failure of one or more of the amplifiers, phase shifters, attenuators, and transmission lines, it may be desirable to provide for some means for calibrating the antenna array in order to allow monitoring of its condition. One way to calibrate an array antenna is to compare it with a standard antenna, such as a horn antenna. That is, a source is coupled to the array and then to the horn, and the radiated power or energy at a substantial distance (the “far field”) in a particular direction is determined for each. The difference between the two represents the “gain” difference. As mentioned, this technique is quite suitable to a laboratory, but may not be easy to accomplish where the array is installed or located. 
   Another way to calibrate an antenna is to mount a test or calibration antenna near the antenna to be tested. Such calibrations are often known as “near-field” calibrations, and have the advantage of improved signal-to-noise ratio over the far-field technique. Signals are transmitted between the antenna being tested and the test/calibration antenna. Such a technique is described in U.S. Pat. No. 6,084,545, issued Jul. 4, 2000 in the name of Lier et al. In this patent, a calibration antenna or probe is placed in front of the array antenna to be tested, and the test signals are transmitted from the probe to the antenna being tested in the receive mode or from the antenna being tested to the probe in the transmit mode. 
   Another calibration method is described in U.S. Pat. No. 6,356,233, issued Mar. 12, 2002 in the name of Miller et al. This arrangement deems certain antenna elements of the antenna under test to be “kernel” elements, and uses mutual coupling between the kernel elements and the remainder of the antenna elements of the array to determine characteristics of the antenna. A system of switches and directional couplers routes test signals through the various kernel elements and their mutually coupled array elements. 
   Improved array antenna calibration arrangements are desired. 
   SUMMARY OF THE INVENTION 
   An antenna arrangement according to an aspect of the invention comprises an electrically conductive ground sheet defining at least a first broad side, and possibly a second broad side. A principal antenna arrangement or “principal antenna” is provided for at least one of transmitting and receiving. The principal antenna arrangement coacts with the conductive ground sheet for transducing electromagnetic signals flowing in space in that half-space adjacent to, or facing the first broad side of the ground sheet. The ground sheet may be flat, curved or generally nonplanar. The principal antenna includes at least one principal antenna port accessible from the other half-space remote from the first broad side of the ground sheet. The antenna arrangement also includes a retractable/extensible calibration radiation or antenna element, which may be a monopole, capable of mechanically extending through the first broad side of the ground sheet, and also being capable of assuming (a) a retracted position in which the calibration radiation or antenna element is retracted below the first broad side of the ground sheet (that is, having no part extending into the half-space adjacent the first broad side of the ground sheet) and (b) an extended position in which the calibration radiation or antenna element extends from the first side of the ground sheet into the adjacent half-space. A calibration antenna feed port is associated with the calibration radiation or antenna element. A calibration arrangement may be coupled to the calibration antenna feed port and to the at least one port of the principal antenna, for applying signals to one of (a) at least a portion of the principal antenna and (b) the calibration radiation or antenna element, for causing signals to flow between the calibration radiation or antenna element and the principal antenna. This signal flow may be in either direction. 
   In a particular embodiment of the invention, the principal antenna is an array antenna including a beamformer to which the principal antenna port is coupled. In this particular embodiment, the array antenna may be (a) an array of electromagnetic radiators, each of which is flush with the first side of the ground sheet, (b) an array of horn aperture elements, (c) an array of patch antenna elements. 
   In a particular hypostasis of the invention, the principal antenna is an array of antenna elements, each of which extends into the half-space. Such antennas may include axial-mode helical antennas. 
   In another hypostasis, the principal antenna is an array of antenna elements, each of which has a radiating aperture which is flush with the local portion of the first side of the ground sheet. The antenna elements may be monolithic, printed-circuit or patch antennas, or they may be horn antennas. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a simplified perspective or isometric view of a portion of an array antenna including a conductive ground plane, a plurality of antenna elements, and an extendible/retractable calibration antenna shown in its retracted position; 
       FIG. 2  is a simplified cross-sectional view of the structure of  FIG. 1  in the region of the calibration antenna, with the calibration antenna in its retracted position; 
       FIG. 3  is a simplified cross-sectional view of the structure of  FIG. 2  in its extended position; 
       FIG. 4  is a simplified block diagram of an array antenna and a calibration monopole antenna according to an aspect of the invention, in which the array antenna is associated with a beamformer and a calibration device is coupled to the array and the calibration monopole; 
       FIG. 5  is a simplified illustration of the radiating side of a patch antenna array; and 
       FIG. 6  is a simplified illustration of the radiating side of an antenna array where the array elements project into the half-space. 
   

   DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , antenna arrangement  10  includes a portion of an array antenna  14 . Antenna arrangement  10  includes a generally planar electrically conductive ground plane, ground conductor or sheet  12 , and a plurality of individual antenna elements of the array, illustrated as a set  14  of generally rectangular radiating apertures (array antenna elements)  14   a ,  14   b ,  14   c ,  14   d ,  14   e , and  14   f  facing a half-universe or half-space  16   u  lying “above” upper surface  12   us  of ground plane  12 . Such radiating apertures transduce electromagnetic energy between the antenna element and half-space  16   u . For definiteness, the radiating antenna elements are assumed to be rectangular horns including tapered electrically-conductive hollow waveguide structures designated generally as  18  extending into the lower half-space  16   l  lying “below” conductive ground plane  12 . Thus, structure  18   d  represents the waveguide horn structure associated with radiating aperture  14   d , structure  18   e  represents the waveguide horn or feed structure associated with radiating aperture  14   e , and structure  18   f  represents the waveguide horn structure associated with radiating aperture  14   f . The waveguide horn structure includes the “feed” for the associated antenna element. As illustrated in  FIG. 1 , no details of the horn structure of the set  14  if antenna elements are shown. This may be viewed as being attributable to an electromagnetically transparent or dielectric “radome” overlying each radiating aperture of set  14  of radiating apertures, each flush with upper surface  12   us  of ground plane  12 . 
   The description herein includes relative placement or orientation words such as “top,” “bottom,” “up,” “down,” “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” as well as derivative terms such as “horizontally,” “downwardly,” and the like. These and other terms should be understood as to refer to the orientation or position then being described, or illustrated in the drawing(s), and not to the orientation or position of the actual element(s) being described or illustrated. These terms are used for convenience in description and understanding, and do not require that the apparatus be constructed or operated in the described position or orientation. 
   Terms concerning mechanical attachments, couplings, and the like, such as “connected,” “attached,” “mounted,” refer to relationships in which structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable and rigid attachments or relationships, unless expressly described otherwise. 
   In  FIG. 1 , a “ridge” portion of the ground plane  12  lies between each mutually adjacent pair of radiating apertures of set  14 . In  FIG. 1 , that ridge portion of the ground plane lying between radiating apertures  14   a  and  14   b  is designated  20   ab , that ridge portion lying between apertures  14   b  and  14   c  is designated  20   bc , and that ridge portion lying between radiating apertures  14   b  and  14   e  is designated  20   be.    
   According to an aspect of the invention, a retractable and extensible calibration element  22  is mounted at a location designated  22  in  FIG. 1 , at the junction of ground plane  12  ridge portions  20   be ,  20   bc , and  20   cf . The selected location for calibration element  22  provides substantial coupling or mutual coupling between the calibration antenna  22  and the four adjacent radiating apertures  14   b ,  14   c ,  14   e , and  14   f  of the set  14  of radiating apertures (antenna elements) of the array. Depending upon the characteristics of the calibration element  22  and the antenna elements of array  10 , each calibration element may provide significant coupling to antenna elements more remote than the four immediately adjacent apertures or antenna elements. It is contemplated that a plurality of calibration elements will be associated with each array antenna, with the locations of the calibration antennas being selected to provide suitable coupling to a set or subset of the array antenna elements. 
     FIG. 2  is a simplified cross-sectional view of retractable and extensible calibration antenna element  22  of  FIG. 1  in its retracted position. In  FIG. 2 , electrically conductive ground plate or sheet  12  defines a lower surface  12   ls . Side and bottom edges of the cavity of horn element  14   b  are illustrated as dash lines  14   se  and  14   bbe , respectively, side and bottom edges of the cavity of horn element  14   c  are illustrated as dash lines  14   cse  and  14   cbe , respectively. A stepped bore designated generally as  214  extends through ground plane  12 . Stepped bore  214  includes a first, relatively small-diameter portion  218  extending from the upper surface of ground plane  12  to a transverse plane  216 , and a second, larger-diameter portion  220  extending from plane  216  to lower surface  12   ls  of ground sheet  12 . An elastomeric weatherseal  219  protects the upper end of bore  218 . An electrically conductive rod or monopole  230  extends through portions of bores  216  and  218 , and is supported away from the walls of bore  218  by a pair of elastic O-rings  232  and  234 , which also tend to prevent dirt or unwanted matter from leaking into the interior of the calibration antenna arrangement. The bottom of monopole  230  defines an enlarged flange  230   f , which, in the illustrated retracted position, bears against an inwardly-projecting ridge or collar  214   s  of the large portion  220  of bore  214 , and is prevented thereby from retracting to a lower position. In the retracted position of the calibration antenna, the contact of flange  230   f  with the ridge or collar  214   s  of bore  214  tends to “ground” the feed end of the calibration antenna monopole  230 , and to thereby prevent effective feeding of the calibration antenna, thereby tending to prevent unwanted calibration signal leakage. An electrically nonconductive stow spring  232  bears at its upper end against the step  219  between the small-diameter bore portion  218  and the large-diameter bore portion  220 , and at its lower end against flange  230   f , and urges the rod or monopole  230  toward the illustrated stowed position. Flange  230   f  bears a circumferential electrically nonconductive seal  230   fs  which bears against the walls of bore portion  220  of stepped bore  214 , to provide an axially movable seal against leakage of fluid such as air, and to maintain the calibration monopole antenna  230  centered in the bore. 
   One or more vacuum channels  240  extend through portions of block  12  to a remote control location (not illustrated). At the remote control location, vacuum can be selectively applied to or removed from (that is, returned to atmospheric pressure) the various vacuum channels, for control of the extension or state of the rod  230  of monopole calibration element  22 . More particularly, the various vacuum channels  240  to which vacuum is selectively applied communicate by way of slant channels, two of which are illustrated as  242 , with that portion of large portion  220  of bore  214  lying immediately below step  219 . Application of vacuum to the various channels  240  results in application of vacuum to the upper side of flange  230   f . The pressure difference between the atmospheric pressure applied to the lower side of flange  230   f  and the vacuum applied to the upper side results in an upwardly-directed force which is sufficient to overcome the downwardly-directed force of stow spring  232 , with the result that the rod or monopole element  230  tends to rise or move upward toward the extended position. The extended position is reached when stow spring  232  is fully compressed. 
   A coaxial feed transmission line or “cable”  250 , including a center conductor  250   c  and an outer conductor  250   o , is coupled by way a connector arrangement  252  to the bottom end of monopole element  230 . More particularly, the center conductor  250   c  of cable  250  is electrically connected to the flange  230   f , and the outer conductor  250   o  is electrically connected, by means of electrically conductive springs (not illustrated) to the surrounding electrically conductive bore and to conductive ground sheet  12 . Electrical isolation is maintained between flange  230   f  and outer conductor  250   o  of coaxial feed cable  250 . 
   Application of vacuum to vacuum channels  240  and slant channels  242  of  FIG. 2  results in extension of the monopole calibration antenna element  230 , to a position illustrated in  FIG. 3 . As illustrated in  FIG. 3 , application of vacuum by way of the vacuum channels  240  and  242  to the bore  214  and the upper surface of flange  230   f , sealed by flange seal  230   fs , creates forces which oppose the force of stow spring  232 . As a result, the calibration antenna monopole  230  rises until the stow spring  230  is fully compressed and the distal end of the calibration antenna monopole  230  extends above the plane of the upper surface  12   us  of the ground plane  12 . In the illustrated embodiment, the projection is one-quarter free-space wavelength (λ/4). 
   Once the calibration antenna  230  (and any other calibration antennas which may be present) are extended by application of vacuum to their respective vacuum ports, calibration signals may be passed by mutual coupling between the antenna of the antenna array and the calibration antennas. As an alternative, the calibration antennas may be extended individually, with the other calibration antennas retracted, so as to minimize the effects of mutual coupling between the extended calibration antennas themselves. 
   More particularly, signals may be applied to the extended calibration antenna  230  for reception by one or more of the array antenna elements, such as nearby array antenna elements  14   b ,  14   c ,  14   e , and  14   f  of  FIG. 1 . Depending upon how many array antenna elements are associated with each calibration monopole antenna, calibration signals may be transmitted in either direction between the calibration monopole antenna and any number of array antenna elements, individually or as groups or subgroups. 
     FIG. 4  illustrates an array  14  of symbolic antenna elements, which are connected to the antenna ports  418   a ,  418   b ,  418   c , . . . ,  418   n  of a beamformer  414 . The (or possibly a) common beamformer port  420  feeds a subgroup or all of the antennas of array  14 . Common port  420  is connected to a calibration device  416 . Calibration device  416  is also connected at a feed port  252  to calibration monopole antenna element  230 . Calibration device  416  measures the coupling magnitude andor phase between the calibration monopole antenna element  230  and the group of antenna elements of array  14 , or possibly subgroups or individual elements of the array, depending upon the connections available in beamformer  414 . Naturally, if there are additional calibration monopole antenna elements equivalent to  230 , it is connected to them, too, as by selection switches. It will seldom be necessary to determine the mutual coupling between one calibration monopole antenna element and another. 
     FIG. 5  illustrates an array  514  of patch antenna elements including coplanar conductive patches  514   a ,  514   b ,  514   c , and  514   d  mounted on a support  512 . Those skilled in the art know that such patch antennas must be electrically isolated from ground in most applications, so the upper surface  512   us  of support  512  cannot be electrically conductive. However, such patch antenna arrays are often associated with a ground plane which lies below the upper surface  512   us  of support  512 . A calibration monopole antenna element can be associated with this “hidden” ground plane. A possible location for a calibration monopole antenna element is indicated as  522 . 
     FIG. 6  illustrates a portion of an array  614  of helical antennas, which may be axial-mode helical antennas. As known to those skilled in the antenna arts, such antennas are fed “against” ground  612 , so that the feed is isolated from ground plane  612 . As illustrated, each feed point is associated with an aperture in the ground plane  612 . Each aperture may be connected to the outer conductor of a coaxial cable, as known, and the center conductor of the coaxial cable can be connected to the bottom of the helical element. A possible location for a calibration monopole antenna element is indicated as  622 . 
   While the extension of the calibration monopole antenna as described uses vacuum power, any type of extension/retraction mechanism could be used. For example, an electrically powered motor similar to those used for automobile monopole antennas could be used instead of a vacuum/spring mechanism. If desired, a manually-operated mechanical device, such as a handwheel-operated gear arrangement, could also be used to raise and lower the calibration antenna. Those skilled in the antenna arts know that various types of end loading could be used with the monopole, as for example a capacitive top cap, which could retract into a correspondingly dimensioned aperture in the ground plane. 
   An antenna arrangement ( 10 ) according to an aspect of the invention comprises an electrically conductive ground sheet ( 12 ) defining a first ( 12   us ), and possibly a second ( 12   ls ) broad side. A principal antenna arrangement (array  14 ) or “principal antenna” is provided for at least one of transmitting and receiving. The principal antenna arrangement ( 14 ) coacts with the conductive ground sheet ( 12 ) for transducing electromagnetic signals flowing in space in that half-space ( 16   u ) adjacent to, or facing, the first broad side ( 12   us ) of the ground sheet ( 12 ). The ground sheet may be flat, curved or generally nonplanar. The principal antenna ( 14 ) includes at least one principal antenna port ( 18   d ,  18   e ) accessible from that half-space remote from the first broad side of the ground sheet ( 12 ). The antenna arrangement ( 10 ) also includes a retractable or retractable/extensible calibration radiation element or antenna ( 230 ), which may be a monopole, capable of mechanically extending through the first side ( 12   us ) of the ground sheet ( 12 ), and also being capable of assuming (a) a retracted position ( FIG. 2 ) in which the monopole calibration antenna element ( 230 ) is retracted, possibly completely retracted, below the first side ( 12   us ) of the ground sheet ( 12 ), which is to say that it has no part extending into the upper half-space  16   u , and (b) an extended position ( FIG. 3 ) in which the calibration radiation element or antenna ( 230 ) extends from the first side ( 12   us ) of the ground sheet ( 12 ) into the half-space ( 16   u ). A calibration radiation element or antenna feed port ( 252 ) is associated with the calibration radiation or antenna element ( 230 ). A calibration arrangement may be coupled to the calibration antenna feed port ( 252 ) and to the at least one port ( 18   d ,  18   e ) of the principal antenna ( 14 ), for applying signals to one of (a) at least a portion of the principal antenna ( 14 ) and (b) the calibration radiation or antenna element ( 230 ), for causing signals to flow between the calibration radiation or antenna element and the principal antenna. 
   In a particular embodiment of the invention, the principal antenna ( 14 ) is an array antenna including a beamformer ( 514 ) to which the principal antenna port ( 418   a ,  418   b , . . .  418   n )) is coupled. In this particular embodiment, the array antenna ( 14 ) may be (a) an array of electromagnetic radiators, each of which is flush with the first side ( 12   us ) of the ground sheet ( 120 , (b) an array of horn aperture elements, or (c) an array ( 514 ) of patch antenna elements ( 514   a ,  514   b ,  514   c ,  514   d , . . . .) 
   In a particular hypostasis of the invention, the principal antenna ( 614 ) is an array of antenna elements, each of which extends into the half-space. Such antennas may include helical antennas, including axial-mode helical antennas. 
   In another hypostasis, the principal antenna is an array of antenna elements, each of which has a radiating aperture which is flush with the local portion of the first side of the ground sheet. The antenna elements may be monolithic, printed-circuit or patch antennas ( FIG. 5 ), or they may be horn antennas ( FIG. 1 ).