Patent Publication Number: US-7586451-B2

Title: Beam-tilted cross-dipole dielectric antenna

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of provisional patent application Ser. No. 60/868,452 filed Dec. 4, 2006. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an antenna for radiating electromagnetic waves. 
   2. Description of the Related Art 
   Satellite Digital Audio Radio Service (SDARS) providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to Earth. SDARS providers use multiple satellites in a geostationary orbit or in an inclined elliptical constellation. The elevation angle between the respective satellite and the antenna is variable depending on the location of the satellite and the location of the antenna. Within the continental United States, this elevation angle may be as low as 20 degrees. Accordingly, specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20 degrees. 
   The automotive industry is increasingly including antennas with SDARS applications in vehicles, and specifically mounted to automotive glass. However, certain parts of the vehicle, such as a roof, may block RF signals and prevent the RF signals from reaching the antenna at certain elevation angles. Even if the roof does not block the RF signals, the roof may mitigate the RF signals, which may cause the RF signal to degrade to an unacceptable quality. When this happens, the antenna is unable to receive the RF signals at those elevation angles and the antenna is unable to maintain its intrinsic radiation pattern characteristic. Thus, antenna performance is severely affected by the roof obstructing reception of the RF signals, especially for elevation angles below 30 degrees. In order to overcome this, a radiation beam tilting technique can be used to compensate for signal mitigation caused by the vehicle body. Since antennas capable of receiving RF signals in SDARS frequency bands are typically physically smaller than those antennas receiving signals in lower frequency bands, it becomes challenging to tilt the antenna radiation main beam from the normal direction to the antenna plane, which is substantially parallel to the glass where the antenna is mounted. 
   One such antenna implementing a radiating beam tilting technique is disclosed in U.S. Pat. No. 7,126,539 (the &#39;539 patent). The &#39;539 patent discloses an antenna having a ground plane and a first dielectric layer disposed on the ground plane. A second dielectric layer having a relative permittivity different than that of the first dielectric layer is disposed adjacent to the first dielectric layer. A feeding element is embedded in the first dielectric layer adjacent to the second dielectric layer. The antenna of the &#39;539 patent produces a directional radiation beam with a highest gain portion at a certain elevation angle. Due to the difference between the relative permittivity of the second dielectric layer compared to the first dielectric layer, the radiation beam tilts from a higher to lower elevation angle, thus tilting the highest gain portion, accordingly. However, the antenna of the &#39;539 patent is only able to tilt the radiation beam in one direction. At lower elevation angles, the roof of the vehicle causes too much signal mitigation. 
   Although the antennas of the prior art may receive a relatively high gain at relatively low elevation angles, an antenna is needed for SDARS applications that provides a radiation beam with omnidirectionality at lower elevation angles when mounted on a tilted pane (i.e., a window) of a vehicle while maintaining acceptable gain, polarization, and directionality properties. 
   SUMMARY OF THE INVENTION AND ADVANTAGES 
   The subject invention provides an antenna comprising a ground plane and a first dielectric layer disposed on the ground plane. A second dielectric layer disposed on the first dielectric layer. The antenna further includes at least one feeding element embedded in the first dielectric layer, and a radiating element extending from the feeding element and embedded within the first dielectric layer adjacent to the second dielectric layer. A beam steering element is embedded in the second dielectric layer and electromagnetically coupled to the at least one radiating element. 
   Embedding the beam steering element in the second dielectric layer and electromagnetically coupling the beam steering element to the radiating element allows the antenna to tilt a radiation beam as much as 20 degrees. When mounted on a tilted pane, tilting the beam with the beam steering element reduces signal mitigation or blocking of a signal, and thus, maintains acceptable gain, circular polarization, and directional properties for SDARS applications at lower elevation angles. Therefore, the beam steering element is suitable for SDARS applications and provides a radiation beam with substantial omnidirectionality at lower elevation angles when mounted on a tilted pane (i.e., a window) of a vehicle while maintaining acceptable gain, polarization, and directionality properties. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
       FIG. 1  is a perspective view of a vehicle having an antenna disposed on a non-conductive pane; 
       FIG. 2  is a perspective view of the antenna disposed on the non-conductive pane and having a beam steering element and a plurality of feeding elements and a plurality of radiating elements arranged in a cross-dipole configuration; 
       FIG. 3  is a top view of the antenna of  FIG. 2 ; 
       FIG. 4  is a cross-sectional side view of the antenna of  FIG. 2  taken along the line  4 - 4  in  FIG. 2 ; 
       FIG. 5  is a perspective view of another embodiment of the antenna disposed on the non-conductive pane and having the beam steering element, an impedance matching element, and the plurality of feeding elements and the plurality of radiating elements arranged in a cross-dipole configuration; 
       FIG. 6  is a top view of the antenna of  FIG. 5 ; and 
       FIG. 7  is a cross-sectional side view of the antenna of  FIG. 5  taken along the line  7 - 7  in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, an antenna for radiating an electromagnetic field is shown generally at  10 . In the illustrated embodiments, the antenna  10  is utilized to receive a circularly polarized radio frequency (RF) signal from a satellite. Those skilled in the art realize that the antenna  10  may also be used to transmit the circularly polarized RF signal. Specifically, the antenna  10  receives a left-hand circularly polarized (LHCP) RF signal like those produced by a Satellite Digital Audio Radio Service (SDARS) provider, such as XM® Satellite Radio or SIRIUS® Satellite Radio. However, it is to be understood that the antenna  10  may also receive a right-hand circularly polarized (RHCP) RF signal. 
   As shown in  FIG. 1 , the antenna  10  may be mounted to a window  12  of a vehicle  13 . The window  12  may be a rear window  12  (backlite), a front window  12  (windshield), or any other window  12  or tilted pane of the vehicle  13 . The antenna  10  may also be implemented in other situations completely separate from the vehicle  13 , such as on a building or integrated with a radio receiver. Additionally, the antenna  10  may be disposed at other locations of the vehicle  13 , such as on a side mirror. 
   Multiple antennas may be implemented as part of a diversity system of antennas. For instance, the vehicle  13  of the preferred embodiment may include a first antenna on the windshield and a second antenna on the backlite. These antennas would both be electrically connected to a receiver (not shown) within the vehicle  13 . Those skilled in the art realize several processing techniques may be used to achieve diversity reception. In one such technique, a switch (not shown) may be implemented to select the antenna  10  that is currently receiving a stronger RF signal from the satellite. 
   The preferred window  12  includes at least one non-conductive pane  14 . The term “non-conductive” refers to a material, such as an insulator or dielectric, that when placed between conductors at different potentials, permits only a small or negligible current in phase with the applied voltage to flow through material. Typically, non-conductive materials have conductivities on the order of nanosiemens/meter. 
   In the illustrated embodiments, the non-conductive pane  14  is implemented as at least one pane of glass. Of course, the window  12  may include more than one pane of glass. Those skilled in the art realize that automotive windows, particularly windshields, may include two panes of glass sandwiching an adhesive interlayer. The adhesive interlayer may be a layer of polyvinyl butyral (PVB). Of course, other adhesive interlayers would also be acceptable. The non-conductive pane  14  is preferably automotive glass and more preferably soda-lime-silica glass. The pane of glass defines a thickness between 1.5 and 5.0 mm, preferably 3.1 mm. The pane of glass also has a relative permittivity between 5 and 9, preferably 7. Those skilled in the art, however, realize that the non-conductive pane  14  may be formed from plastic, fiberglass, or other suitable non-conductive materials. Furthermore, the non-conductive pane  14  preferably functions as a radome for the antenna  10 . That is, the non-conductive pane  14  protects the other components of the antenna  10  from moisture, wind, dust, etc. that are present outside the vehicle  13 . 
   As best shown in  FIGS. 2 ,  4 ,  5 , and  7 , the antenna  10  includes a ground plane  16  for reflecting energy received by the antenna  10 . The ground plane  16  is disposed substantially parallel to and spaced from the non-conductive pane  14  and is typically formed of a generally flat electrically conductive material like copper or aluminum having at least one planar surface. The ground plane  16  generally defines a rectangular shape, and specifically a square shape, although those skilled in the art realize the ground plane  16  may have different shapes or configurations. 
   A first dielectric layer  18  is disposed on the ground plane  16 . The first dielectric layer  18  provides support to the antenna  10  and may generally define a rectangular shape, specifically a square shape. Those skilled in the art realize that other shapes of the first dielectric layer  18  may be implemented. A second dielectric layer  20  is disposed on the first dielectric layer  18 . When mounted to the vehicle  13 , the second dielectric layer  20  is disposed between the first dielectric layer  18  and the non-conductive pane  14 . Like the first dielectric layer  18 , the second dielectric layer  20  may also generally define a rectangular shape, and specifically a square shape. Those skilled in the art realize that other shapes of the second dielectric layer  20  may be implemented. 
   The first and second dielectric layers  18 ,  20  each have a relative permittivity between 1 and 100. Preferably, the relative permittivity of the second dielectric layer  20  is different than the relative permittivity of the first dielectric layer  18 . For example, the first dielectric layer  18  may be a plastic and, as shown in the Figures, the second dielectric layer  20  may be an air gap. In this example, a spacer  21  may be used to establish a proper thickness of the second dielectric layer  20  (i.e., the air gap). Alternatively, an antenna housing or radome (not shown) may be used to establish the thickness of the second dielectric layer  20 . It is to be appreciated that the first and second dielectric layers  18 ,  20  may be formed from other materials. The difference between the relative permittivity of the first and second dielectric layers  18 ,  20  may be dependent upon the SDARS application and the characteristics of the signal received by the antenna  10 . 
   The antenna  10  further includes at least one feeding element  24  that is electrically isolated from the ground plane  16 . Preferably, the feeding element  24  is formed from an electrically conductive wire, or alternatively, the feeding element  24  may be formed from a strip. In one embodiment, the at least one feeding element  24  is further defined as a plurality of feeding elements  24 . Each of the at least one feeding elements  24  is embedded in the first dielectric layer  18 . Preferably, the feeding element  24  is partially surrounded by the first dielectric layer  18 , and/or substantially perpendicular to the ground plane  16 . The feeding elements  24  are spaced from one another in the first dielectric layer  18 . For instance, the feeding elements  24  may be approximately 1 mm apart. However, it is to be appreciated that the feeding elements  24  may be spaced from one another at different distances. 
   A radiating element  26  extends from the feeding element  24  and acts as the primary radiating element for the antenna  10 . The radiating element  26  is embedded within the first dielectric layer  18  adjacent to the second dielectric layer  20 , and preferably, the radiating element  26  is flush with a top surface of the first dielectric layer  18  while in physical contact with the second dielectric layer  20 . The at least one radiating element  26  may be further defined as a plurality of radiating elements  26 . The plurality of radiating elements  26  are embedded in the first dielectric layer  18  preferably perpendicular to the feeding elements  24  and coplanar relative to one another. 
   To achieve circular polarization, it is preferred that the plurality of feeding elements  24  and the plurality of radiating elements  26  are arranged in a cross-dipole configuration. The cross-dipole configuration of the feeding elements  24  and the radiating elements  26  is best illustrated in  FIGS. 2 ,  3 , and  5 . Those skilled in the art realize that the term “cross-dipole” is a term of art in the field of antennas. Preferably, in the cross-dipole configuration, the antenna  10  includes four feeding elements  24  and four radiating elements  26  to establish the cross-dipole configuration. The feeding elements  24  are embedded in the first dielectric layer  18  substantially perpendicular to the ground plane  16  and the non-conductive pane  14 . The radiating elements  26  are embedded in the first dielectric layer  18  parallel to and spaced from the ground plane  16 . The four feeding elements  24  and the four radiating elements  26  form a first dipole  28  and a second dipole  30  spaced from the first dipole  28 . The first and second dipoles  28 ,  30  transmit or receive at least one first dipole signal and at least one second dipole signal, respectively. In other words, the signal transmitted or received by the first dipole  28  is the first dipole signal, and the signal transmitted or received by the second dipole  30  is the second dipole signal. The first and second dipole signals have equal amplitudes relative to one another and a phase difference of 90 degrees respectively, to facilitate circular polarization characteristics. Preferably, the first dipole  28  is formed from two of the feeding elements  24  and two of the radiating elements  26 . Likewise, the second dipole  30  is formed from two of the feeding elements  24  and two of the radiating elements  26 . The radiating elements  26  in the first dipole  28  extend in a direction transverse to the radiating elements  26  in the second dipole  30 . Specifically, the radiating elements  26  in the first dipole  28  are orthogonal to the radiating elements  26  in the second dipole  30 , thus establishing the cross-dipole configuration. 
   Referring now to  FIGS. 2-6 , the antenna  10  further includes a beam steering element  32  for disturbing a current flow to control a radiation direction of the antenna  10 . The beam steering element  32  is embedded in the second dielectric layer  20  and electromagnetically coupled to the at least one radiating element  26 . In other words, the beam steering element  32  is at least partially disposed inside the second dielectric layer  20  and spaced from and electromagnetically coupled to the radiating element  26 . Embedding the beam steering element  32  in the second dielectric layer  20  and electromagnetically coupling the beam steering element  32  to the radiating element  26  allows the antenna  10  to tilt a radiation beam as much as 20 degrees. Titling the beam with the beam steering element  32  reduces signal mitigation or blocking of the signal, such that, when mounted on the window  12  or other tilted pane of the vehicle  13  will result in the antenna  10  receiving the SDARS signal in a substantially omnidirectional pattern. Thus, the antenna  10  maintains acceptable gain, polarization, and directional properties for SDARS applications at lower elevation angles. Therefore, the beam steering element  32  is suitable for SDARS applications. Preferably, the beam steering element  32  is disposed on the non-conductive pane  14  and embedded in the second dielectric layer  20  parallel to the first dielectric layer  18  and the ground plane  16 . The beam steering element  32  is embedded in the second dielectric layer  20  typically in a direction transverse to and spaced from the radiating element  26 . Preferably, the beam steering element  32  is embedded in the second dielectric layer  20  in a direction orthogonal to and spaced from the radiating element  26 . 
   In a preferred embodiment, the beam steering element  32  is printed on the non-conductive pane  14 . In this embodiment, all exposed surfaces of the beam steering element  32  are surrounded by the second dielectric layer  20 . Although shown in  FIGS. 2-4  as having a rectangular configuration (i.e., uniform width), it is to be appreciated that the beam steering element  32  may have other configurations. For instance, as shown in  FIGS. 5-6 , the beam steering element  32  may be tapered to gradually change the impedance of the beam steering element  32 . 
   Referring now to  FIGS. 5-7 , an impedance matching element  34  may be embedded in the second dielectric layer  20  and electromagnetically coupled to the at least one radiating element  26  to adjust the input impedance of the antenna  10 . Preferably, the impedance matching element  34  is disposed on the non-conductive pane  14  and embedded in the second dielectric layer  20  parallel to the first dielectric layer  18  and the ground plane  16 . However, the impedance matching element  34  does not necessarily have to be disposed on the non-conductive pane  14 . The impedance matching element  34  also radiates with the at least one radiating element  26  to provide greater efficiency without signal loss. The impedance matching element  34  may include a first impedance matching section  36  and a second impedance matching section  38  integrally formed with the first impedance matching section  36 . The first impedance matching section  36  has a uniform width. For example, the first impedance matching section  36  may have a rectangular configuration from a top view. The second impedance matching section  38  may be tapered from a top view to allow for gradual impedance matching. 
   In one embodiment, the impedance matching element  34  may have a plurality of impedance matching portions  40  each having the first impedance matching section  36  and the second impedance matching section  38 . Furthermore, each impedance matching section is electromagnetically coupled to one of the plurality of radiating elements  26 . Specifically, when the plurality of radiating elements  26  are arranged in the cross-dipole configuration, the plurality of impedance matching portions  40  are also arranged in a cross-dipole configuration spaced from the plurality of radiating elements  26 . In this embodiment, it is preferred that each of the impedance matching portions  40  are positioned over one of the plurality of radiating elements  26 . 
   The impedance matching element  34  is spaced from the beam steering element  32 ; however, positioning the impedance matching portion  40  over the radiating element  26  may cause the beam steering element  32  to come into physical contact with the impedance matching element  34 . To prevent this, as shown in  FIGS. 5 and 6 , the beam steering element  32  may include a first beam steering portion  42  and a second beam steering portion  44  electromagnetically coupled to the first beam steering portion  42 . In other words, the beam steering element  32  may be split into a first beam steering portion  42  and a second beam steering portion  44  spaced from the first beam steering portion  42 . The first and second beam steering portions  42 ,  44  are further spaced from the impedance matching element  34 . In order to allow for a gradual change in impedance, the first and second beam steering portions  42 ,  44  may be tapered from a top view. 
   Additionally, an amplifier  46  may be disposed on the ground plane  16 . As illustrated in one embodiment, the amplifier  46  may be integrated with the ground plane  16 . Furthermore, the ground plane  16  may be used to ground the amplifier  46 . The amplifier  46  is electrically connected to the at least one feeding element  24  to amplify the RF signal received by the antenna  10 . The amplifier  46  is preferably a low-noise amplifier (LNA) such as those well known to those skilled in the art. 
   The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. As is now apparent to those skilled in the art, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.