Patent Grant
7053847
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.
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.
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.
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.
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.
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.
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.
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%.
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%.
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.
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.
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.
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.
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.
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
By way of further example, and as best shown in
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.
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
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.
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 60° 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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5539415 | Metzen et al. | Jul 1996 | A |
| 5539420 | Dusseux et al. | Jul 1996 | A |
| 5703601 | Nalbandian et al. | Dec 1997 | A |
| 5818391 | Lee | Oct 1998 | A |
| 6160522 | Sanford | Dec 2000 | A |
| 6166692 | Nalbandian et al. | Dec 2000 | A |
| 6184828 | Shoki | Feb 2001 | B1 |
| 6219002 | Lim | Apr 2001 | B1 |
| 6492949 | Breglia et al. | Dec 2002 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20060033671 A1 | Feb 2006 | US |
