Abstract:
A phased array antenna structure capable of operation at millimeter-wave frequencies and having multiple ring slot radiator elements ( 10 ). The RF feed structure for each radiator element includes a feed via ( 28 ) extending part-way through a multi-layer structure (FIG.  3 ) on which the radiator elements ( 10 ) are formed and a strip line feed probe ( 30 ) extending from the via ( 28 ) toward the radiator element. A key feature facilitating high-frequency operation is the inclusion of multiple mode suppressors ( 32 ) surrounding the via ( 28 ) and providing a smooth transition from a coaxial mode of RF transmission to a strip line mode of RF transmission. The feed probe ( 30 ) is tailored to provide either a narrow-band or a wideband frequency characteristic.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to phased array antennas and, more particularly, to phased array systems using ring slot radiator elements. Phased array antenna systems provide a convenient technique for steering antenna beams electrically. Each phased array system consists of a relatively large number of antenna elements that are separately fed with a radio-frequency (RF) signal to be transmitted. By controlling the relative phase of the RF signal in the separate antenna elements of the array, one can effectively steer a beam emanating from the array. If the array is two-dimensional, the beam may be steered about two axes. It will be understood, of course, that although such antennas are often described in terms pertaining to a transmitting antenna, the same principles also apply to steering a receiving antenna.  
         [0002]     Although such antenna systems are well known, in radar and communications systems they have typically employed conventional radiator elements, such as horn antennas, helical antennas, or open-ended waveguide elements. These conventional radiator elements are prohibitively large in size and weight, and are relatively costly to manufacture, especially for operation at millimeter wave frequencies (30-300 GHz). There is a requirement in some applications for phased array antenna systems that have very closely spaced radiator elements, to provide fast scanning of pencil beams over a large search or coverage volume without forming a grating lobe. A grating lobe is an unwanted lobe in the antenna radiation pattern, caused by steering the beam too far in relation to the element spacing.  
         [0003]     Use of ring slot radiator elements in phased array systems has been proposed for low frequency applications. For example, U.S. Pat. No. 5,539,415, issued in the name of Phillip L. Metzen et al., discloses an antenna system with an array of ring slot radiators. The same system is disclosed in a paper by Phillip L. Metzen et al., entitled “The Globalstar cellular satellite system,” IEEE Trans. Vol AP-46, no. 6, June 1998, pp. 935-942. The antenna array and associated feed probe structure disclosed in these publications is designed for operation in the L-band (1.61 GHz to 1.6265 GHz) and provides a very narrow (1%) bandwidth. Unfortunately, antenna systems of the type disclosed by Metzen et al. do not work at millimeter-wave frequencies, such as 35 GHz or higher. Moreover, the narrow 1% bandwidth is so narrow as to render the design very sensitive to manufacture, resulting in high production costs.  
         [0004]     More specifically, one important reason that prior designs worked well at lower frequencies but not at millimeter-wave frequencies has to do with the difficulty of impedance matching a coaxial feed to a strip line mode for coupling to a ring slot radiator. At low frequencies, the thickness of a substrate on which the antenna array is formed is electrically quite thin (less than 2% of the operating wavelength). The feed probe, therefore, exhibits a negligibly small self-reactance, and transition from coaxial mode to the strip line mode requires little or no impedance matching. At millimeter-wave frequencies, a substrate of the same physical thickness has a significantly increased electrical thickness (about 12% of the operating wavelength). The self-reactance of the feed probe is, therefore, much larger, causing a serious impedance mismatch problem in the transition from coaxial mode to strip line mode.  
         [0005]     Therefore, there is still a need for an antenna system using an array of ring slot radiators that can be operated at millimeter-wave frequencies, and preferably at a greater bandwidth. The present invention satisfies this need.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention resides in a phased array antenna system operable at millimeter-wave frequencies, and in a ring slot radiator structure for use in a phased array antenna system. Briefly, and in general terms, the ring slot radiator structure of the invention comprises a dielectric substrate, having a top face and a bottom face; a conductive layer formed over the top face of the substrate and having an annular gap that in part defines a radiator element; a conductive feed via extending part-way through the substrate in a direction normal to the conductive layer, to transmit radio-frequency (RF) energy from a location located below the substrate to transition point located outside the annular gap in the conductive layer and spaced beneath the conductive layer; a strip line feed probe extending from the transition point in a generally radial direction parallel to the conductive layer and at least partially across the annual gap; and a plurality of mode suppressor posts extending through the substrate in a direction parallel to the conductive feed via and spaced in a generally uniform array around the conductive feed via. The plurality of mode suppressor posts effect a smooth transition from a coaxial mode of transmission through the conductive feed via to a strip line mode of transmission along the strip line feed probe that couples RF energy to the ring slot radiator.  
         [0007]     The ring slot radiator structure may further comprise a plurality of mode suppressors, also extending in a direction normal to the conductive surface, and spaced uniformly around the annular gap to effect better isolation of the ring slot radiator element from other neighboring radiator elements.  
         [0008]     In one disclosed embodiment of the invention, the strip line feed probe is generally uniform in width and extends fully across the annular gap toward the geometric center of the annular gap. In this configuration, the ring slot radiator structure has a relatively narrow bandwidth in the order of 1%.  
         [0009]     In another disclosed embodiment of the invention, the strip line feed probe comprises a first section of uniform width extending from the transition point to a point near the outer diameter of the annular gap, and a contiguous transition section of increased width extending part-way across the annular gap. In this configuration, the ring slot radiator structure has an increased bandwidth in the order of 10%.  
         [0010]     The invention may also be defined as a miniature phased array antenna system capable of operation at millimeter-wave frequencies and formed as a unitary structure. The antenna system comprises a multilayer structure having an upper face from which radiation is transmitted in a transmit mode of operation and which receives radiation in a receive mode of operation, and a lower face to accommodate radio-frequency (RF) feed and control circuitry; a conductive layer formed over the top face of the substrate and having a plurality of annular gaps formed in a geometric array, wherein each annular gap in part defines one of a plurality of ring slot radiator elements; an equal plurality of conductive feed vias extending part-way through the multi-layer structure in a direction normal to the conductive layer, each capable of transmitting radio-frequency (RF) energy from a location located at the bottom of the substrate to transition point located outside one of the annular gaps in the conductive layer and spaced beneath the conductive layer; an equal plurality of strip line feed probes, each extending from the transition point associate with one of the plurality of radiator elements in a generally radial direction with respect to its annular gap, parallel to the conductive layer and at least partially across the annual gap; an RF divider/combiner, integrated into the multi-layer structure and coupled to each of the conductive feed vias and to an RF transmitter/receiver connector; and an equal plurality of sets of mode suppressor posts, each set being associated with a corresponding one of the conductive feed vias, and extending through the multi-layer structure in a direction parallel to the conductive feed via and spaced in a generally uniform array around the conductive feed via. Each set of mode suppressor posts effects a smooth transition from a coaxial mode of transmission through the conductive feed via to a strip line mode of transmission along the strip line feed probe that couples RF energy to the ring slot radiator.  
         [0011]     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of miniature phase array antennas capable of operation at millimeter-wave frequencies. In particular, the invention provides a ring slot radiator structure that facilitates smooth RF coupling from a coaxial mode of transmission to a strip line mode for transmission and coupling to each ring slot radiator. The invention also provides alternate configurations for narrow-band and wideband operation. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is simplified isometric view showing a plurality of ring slot radiators and radio-frequency (RF) feed structures;  
         [0013]      FIG. 2  is an enlarged plan view of a single ring slot radiator and its associated RF feed structure.  
         [0014]      FIG. 3  is a fragmentary cross-sectional view of a ring slot radiator antenna structure in accordance with the invention.  
         [0015]      FIG. 4  is a simplified plan view of an antenna array in accordance with the invention;  
         [0016]      FIG. 5  is a set of graphs showing the variation of return loss with scan angle in one axis and pointing angle in an orthogonal axis.  
         [0017]      FIG. 6  is a graph showing the variation of predicted return loss with frequency for the embodiment of the invention depicted in  FIG. 3 .  
         [0018]      FIG. 7  is a graph similar to  FIG. 6 , but pertaining to an alternate embodiment of the invention.  
         [0019]      FIG. 8  is a plan view of a single ring slot radiator similar to  FIG. 3 , but depicting an alternate embodiment providing a wider bandwidth as illustrated in  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     As shown in the drawings for purposes of illustration, the present invention pertains to a phased array antenna system having ring slot radiator elements, operable at millimeter-wave frequencies. Millimeter-wave frequencies are usually defined to be in the range 30-300 GHz. The present invention has important applications with a need for operation at frequencies in the vicinity of 35 GHz, and this description is consistent with a goal of operation at approximately this frequency. Prior to the present invention, arrays of ring slot radiators have been developed for operation at much lower frequencies but have not been capable of operation at millimeter-wave frequencies. One reason for this is that making a transition from a coaxial mode of transmission to a strip line mode for low profile coupling to a ring slot radiator is subject to an increasing impedance mismatch as the frequency is increased.  
         [0021]     In accordance with one aspect of the present invention, operation at millimeter-wave frequencies is facilitated by a novel structure for effecting the transition from the coaxial mode to the strip line mode of transmission. In particular, the invention provides an antenna feed with a characteristic impedance equivalent to that of a 50-ohm coaxial circuit. The structural details relating to implementation of the transition to the strip line feed probe, while minimizing any impedance mismatch, will be best understood from the accompanying drawings and the following description.  
         [0022]      FIG. 1  is an isometric view depicting three ring slot radiators, indicated by reference numeral  10 , and their associated feed structures. Various dielectric layers and ground planes have been omitted from the figure for clarity. Each ring slot radiator  10  is formed as an annular slot  12  in a metal layer  14 . The radiators  10  are integrated into a monolithic structure with many identical others, each with its own amplifier and control circuitry, shown in the figure as a millimeter wave integrated circuit (MMIC) 16 . A millimeter-wave radio-frequency (RF) signal for transmission is input to the structure over a common feed  20 , is divided into multiple signals in a power divider  22 , and then distributed to the individual radiator modules by transmission lines  24 . It will be understood that, although the antenna functions are described in terms of a transmit function, the antenna operates equally well to receive millimeter-wave signals. For example, in the receive-mode the power divider  22  functions as a power combiner.  
         [0023]     Each RF signal on a transmission line  24  is transmitted to the MMIC  16  through a via  26 . After amplification and phase control in the MMIC  16 , the RF signal is transmitted over a. feed via  28  to a feed probe  30 . The vias  26  and  28  are oriented generally perpendicular to the plane of the ring slot radiators  10  and the MMICs  16 . The feed probe  30  is a strip line waveguide that is oriented in a plane parallel with and slightly below the ring slot radiator  10 , and extends radially across the annular slot  12  of the radiator, to overlap the circular region of the metal layer  14  inside the slot.  
         [0024]     An important aspect of this feed structure is that the feed via is almost surrounded by five parallel mode suppressors  32 . In the illustrative embodiment of the invention, the mode suppressors  32  are metal posts of the same diameter as the feed via  28 . As best shown in  FIG. 2 , the mode suppressors  32  and the feed via  28  are, for example, 0.010 inch (0.25 mm) diameter and are centered on a circle of 0.046 inch (1.17 mm) diameter. The five mode suppressors  32  are angularly spaced at approximately 600 intervals, except that there is a larger angular space of approximately 120° in the region of the feed probe  30 .  
         [0025]     By way of further example, and as best shown in  FIG. 2  in relation to a radiator element  10 , the feed via  28  is located outside the radiator annular gap  12 , at a radius of 0.091 inch (2.31 mm). In this example, the radiator slot  12  has an outer boundary diameter of 0.128 inch (3.25 mm) and an inner boundary diameter of 0.094 inch (2.39 mm). It will be understood that these dimensions are provided by way of example only and are not intended to be limiting. As also shown in  FIG. 2 , each ring slot radiator  10  also includes a plurality of mode suppressors  36  spaced uniformly around the annular slot  12 . For example, the mode suppressors may be 0.010 inch (0.25 mm) diameter and positioned with their centers on a circle of 0.165 inch (4.19 mm) diameter. The number of mode suppressors  36  is not critical but in the example shown in  FIG. 2  there are fifteen of them at an angular spacing of 20° to 22.5° , with a larger angular space in the region of the feed probe  30 .  
         [0026]     The mode suppressors  32  and  36  provide sufficient suppression for surface modes that would otherwise be transmitted between adjacent radiator elements  10 . In addition, the five mode suppressors  32  carry an induced current that results in a negative reactance, which significantly neutralizes the self-reactance of the feed probe  28 , allowing a smoother transition between the coaxial mode and the strip line mode of transmission. From a different perspective, the five plated-through vias forming the mode suppressors  32  and the centrally located feed probe  28  may be considered to form a coaxial-like transmission line that smoothes the transition or RF energy to the strip line mode.  
         [0027]      FIG. 3  is a simplified cross-sectional view depicting multiple layers used to manufacture the antenna array of the invention in a structure that minimizes mechanical interconnections. The fabrication technique is often referred to as “connectionless.” Where appropriate, components in this figures are identified by the same respective reference numerals used to identify components that were described above with reference to  FIGS. 1 and 2 .  
         [0028]     The multiple layers of the structure include a radiator layer  40 , which is further detailed in the table to the right of the figure. On the top face of the radiator layer  40  is the conductive (typically copper) layer  14  in which the ring slots  12  are etched. (The “top” face referred to in the previous sentence is shown at the bottom of  FIG. 3 .) The mode suppressors  36  are formed as plated through vias in the radiator layer  40 . The other mode suppressors  32  surrounding the via  28  are omitted for clarity, but are impliedly present around all the RF vias. The feed probe  30  is formed within the radiator layer  40  by etching a copper layer  42  formed within the radiator layer. More specifically, the radiator layer  40  comprises a first board  44  and a second board  46  joined by a bonding film  48 . The first board  44  includes a dielectric board  50  on which the copper layer  14  is formed. The second board  46  is another dielectric board  52 , on the top of which the copper layer  42  is formed and etched to define the feed probe(s)  30 , and on the bottom of which is formed another copper layer  54 , which is etched to provide openings for the probe via(s)  28 .  
         [0029]     The radiator layer  40  is bonded to a silicon motherboard  60 , on the reverse side of which are located a MMIC layer  62 , RF processing layers  64  and  66  and, lastly, a digital control board  68 . An RF input/output connector  70  on the bottom of the digital control board  68  couples RF signals to (or from) the MMIC layer  62 , and the RF processing layers  64  and  66  perform the signal dividing or combining function. Control signals are applied through an input connector  72 , and eventually coupled through a via  74  to the MMIC layer  54 . The control signals are translated into phase control signals applied to the radiator  10 , and collectively comprise a beam forming network that controls the angular direction of the beam transmitted from or received by antenna array.  
         [0030]      FIG. 4  shows an example of a 738-element antenna array. Each of the small circles is a ring slot radiator  10  having the structure described above with reference to  FIGS. 1-3 . Because the array is not perfectly symmetrical in all directions, it exhibits slightly different characteristics depending on the azimuth angle of the desired beam direction. For example, the return loss characteristics of the antenna array vary slightly with the azimuth angle (ø) and also vary with the scan angle, which is the angle of beam deflection from the normal direction to the array. The return loss, usually expressed in decibels (dB), is the ratio of the power reflected back into the antenna to the total power fed to the antenna.  FIG. 5  shows the predicted radiator return loss for scan angles of 0° to 60° and for beam deflection in azimuth directs of 0° 45° and 90°.  
         [0031]      FIG. 6  is a graph showing the variation, with frequency, of the predicted return loss of the an antenna ring slot element in accordance with the invention.  FIG. 7  is a similar graph, but for an alternate embodiment of the invention providing a wider bandwidth or approximately 10% of the resonant frequency of the element (approximately 3 GHz). It is known that most of the RF coupling between the strip line feed probe  30  and the radiator slot  12  takes place through the open-end region of the probe, where the strip line becomes discontinuous. A 50-ohm strip line makes a very narrow coupling aperture (approximately equal to the width of the strip plus fringing effects), which results in a very narrow-band radiator. (For strip or microstrip radiators, bandwidth is typically proportional to the aperture size.) To improve the bandwidth, a larger aperture size strip line is used for the probe  30 . This necessitated a taper transition to match the low-impedance strip line with the 50-ohm coaxial probe feed.  FIG. 8  is a fragmentary plan view of the wideband version of the ring slot radiator  10 . The modified feed probe  30 ′ is widened at the end region  30   a , where coupling with the slot occurs, and extends over the slot  12  but beyond it. The modified feed probe  30 ′ also has a tapered section  30   b , between the widened end region  30   a  and the transition to the feed via  28 .  
         [0032]     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of miniature phased array antenna systems. In particular, the invention provides a compact phased array antenna that produces a beam at millimeter-wave frequencies, steerable over at least 600 in each direction, with no unwanted grating lobe and a good directivity pattern. The manufacturing process employed to fabricate the antenna array uses standard printing circuit fabrication and lamination techniques, and produces the product at relatively low cost and at high yield. The process is fully automatic and, therefore, not labor intensive. It will also be appreciated that, although embodiments of the invention have been described in detail, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.