Patent Publication Number: US-2006001574-A1

Title: Wideband Patch Antenna

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     BACKGROUND OF INVENTION  
      Microstrip patch antennas are very popular for a wide variety of applications. They have several advantages such as low profile, low cost, simple fabrication and light weight that make them very suitable in fixed and mobile communication systems. A typical microstrip patch antenna comprises a patch above a ground plane and separated from the ground plane by a dielectric. A typical patch is fed by means of a coaxial feed, where the center conductor pin is physically connected to the patch. One drawback with such microstrip patch antennas is that they have a relatively narrow bandwidth and thus, are not generally suitable for applications requiring broad bandwidth. The bandwidth can be increased by increasing the substrate thickness and decreasing the substrate permittivity. Relatively large bandwidth is obtained by suspending the patch in air and increasing the antenna thickness: distance from patch to ground plane. However, the increase in thickness increases the coaxial probe inductance due to increased probe length, thus, limiting the antenna bandwidth. Several methods have been disclosed that reduce or compensate for this additional probe inductance while increasing the bandwidth of relatively thick microstrip patch antennas. These methods are described below.  
      Method 1: (Sabbin A. “A new broadband stacked two-layer microstrip antenna”, IEEE AP-S Int. Sym. Digest, 1983, 63-66) and (Lee, R. Q., Lee, K. F., Bobinchak, J. “Two-layer electromagnetically coupled rectangular patch antenna”, Antennas and Propagation Society International Symposium, 1988, AP-S. Digest, pp 948-951). A second parasitic patch on top of the driven patch electromagnetically coupled to the driven patch. The use of a parasitic patch on top or next to the driven patch increases the overall thickness and volume of the antenna, and cost.  
      Method 2: (Fong K. S., Pues H. F., Withers M. J. “Wideband multiple layer coaxial fed microstrip antenna element”, Electron Lett, 1985, 21, pp 497-499.) This method utilizes a capacitively coupled feed where a conductive disk, etched on a substrate, is attached to the top section of the feed and spaced a small distance below the patch. The capacitively coupled feed, although neutralizing the extra probe inductance, is a high-cost complex structure and requires high precision, thus increasing cost.  
      Method 3: (Pozar D. “A reciprocity method of analysis for printed slot and slot-coupled microstrip antennas”, IEEE Transactions on Antennas and Propagation, Vol. 34, 1986, Pp 1439-1446) and (Pozar D. M., Targonski, S. D. “Improved coupling for aperture coupled microstrip antennas”, Electronics Letters, 27, 13, 1991, pp 1129-1131). This approach utilizes aperture coupling using a slot and a microstrip line for feeding the patch. The slot/microstrip line approach requires an additional substrate where the slot and microstrip line are etched. This solution also increases cost and assembly time.  
      Method 4: (Hall P. S. “Probe Compensation in Thick Microstrip Patches” Electronic Letters, vol. 23, No. 11, 1987, pp 606-607). A conductive disk is attached at the end of the feed just like method 2. In this case, the disk and driven patch are located on the same layer forming an annular gap between them, thus forming a capacitor. This annular gap increases the probe capacitance required to reduce the extra probe inductance. However, the antenna radiation pattern exhibits cross-polar components (Garg et al., “Microstrip Antenna Design Handbook, ISBN 0-89006-513-6, 2001 Artech House, Inc. page 19). In addition, this arrangement results in a complex structure, especially when the patch and disk are suspended in air, thus, increasing cost.  
      Method 5: (Hall P. S., Dahele J. S., Haskins P. M. “Microstrip patch antennas on thick microstrip patches”, Antennas and Propagation Society International Symposium, 1989, AP-S. Digest, 1, June 1989, pp 458462). A capacitor is formed by placing a small conductive disk at the end of the feed, just like methods 2 and 4. The conductive disk is placed on top of the patch and separated from the top surface of the patch by a small gap, thus, creating the required capacitance. This extra capacitance compensates for the additional probe inductance, thus increasing the antenna bandwidth. However, this approach also results in a complex structure and high cost.  
      Method 6: (Luk K. M., Chow Y., Mak L., U.S. Pat. No. 6,593,887, Jul. 15, 2003). The inventors describe a patch antenna using an L-shaped feed probe. The L-shaped probe has a first portion normal to ground plane and patch, and a second portion parallel to ground plane and patch. The L-shaped probe is electromagnetically coupled to the patch. This arrangement is also effective in reducing the extra inductance of the probe. However, the total physical length of the probe is relatively large, approximately ¼ of the wavelength (i.e., 8.72 cm at 860 MHz). This large size can cause interference and EMI problems with RF circuits located in the vicinity of the probe. In addition, since the horizontal component of the L-shaped probe is much longer than the vertical section of the probe, it will be difficult to implement a two-feed circularly polarized patch antenna: the two probes may interfere with each other due to their close proximity. Another disadvantage is that the long horizontal probe requires means of mechanical support. This increases the cost and design complexity of the structure.  
      Thus, for the reasons mentioned above, a need exists of a simple, compact, and low-cost probe resulting in wide frequency bandwidth.  
     SUMMARY OF INVENTION  
      According to the present invention there is provided an antenna comprising a patch which may be of pure metallic form or may be etched on a dielectric and is disposed by a dielectric a distance above a ground plane, and a helix-shaped or meandering probe disposed between said patch and said ground plane, said probe is normal to said ground plane, said antenna further comprising means for connecting said probe to means for transmitting a signal to or from said antenna, and said helical probe is adapted to be electromagnetically coupled to said patch. The patch may be rectangular, elliptical, triangular, or any other geometric shape.  
      In one preferred embodiment of the invention, an antenna is presented comprising a rectangular patch suspended in air above a ground plane by a distance h, and a helical probe disposed between said patch and said ground plane, said helical probe is normal to said ground plane and said patch, and spaced from one edge of the patch by a distance d, said antenna further comprising means for connecting said helix probe to means for transmitting a signal to or from said antenna. The helix may consist of several turns or a fractional turn depending of its diameter. Generally, smaller diameters result in more turns.  
      In another embodiment of the invention, an antenna is presented comprising a rectangular patch suspended in air above a ground plane by a distance h, and a meandering-wire probe disposed between said patch and said ground plane, said meandering-wire probe is normal to said ground plane and said patch, and spaced from the top of the patch by a distance d, said antenna further comprising means for connecting said meandering-wire probe to means for transmitting a signal to or from said antenna.  
      All probes according to the present invention, do not exhibit the additional inductance problem, resulting in wideband patch antenna structures. For example, the capacitance between neighboring helix turns cancels the additional inductance. For example, in the case of helix probe, the capacitance between neighboring wires neutralizes the extra inductance caused by the increase of wire length. The same effect is observed in meandering-wire structures.  
      The antenna may be a single antenna with one patch and one probe according to the present invention. However, viewed from another aspect, a plurality of antennas according to the present invention can form an antenna array comprising a plurality of patches disposed above a ground plane, each said patch having a respective probe disposed between said patch and said ground plane, said antenna array further comprising a transmission network connecting said probes to each other and to means for transmitting a signal to or from said antenna array. Such an antenna array may take several forms. One simple structure is an array that comprises two patches with their respective probes being connected by a single transmission line. The arrays can use one type or combination of the probes according to the present invention. Antenna arrays such as a two-by-two or four-by-four array may be formed. More complicated arrays may also be formed.  
      Another example is a dual band antenna structure. A preferred particular example of such structure comprises two rectangular patches and two respective probes, said both patches and probes are of different dimensions. The said patches are disposed above a ground plane and spaced at different distances from the ground plane. The dual band antenna structure further comprises a transmission line connecting said probes to each other and to means for transmitting a signal to or from said antenna structure, said transmission line being parallel to said ground plane.  
      It will also be understood that the patch antennas may be spaced from the ground plane by any form of dielectric material (including air) or by multiple layers of differing dielectric materials. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a diagram that illustrates a conventional patch antenna with a feed probe directly connected to the patch.  
       FIG. 2  is a diagram of a stacked patch antenna arrangement.  
       FIG. 3  is a diagram of a wideband patch antenna using a disk-loaded feed probe spaced below the patch.  
       FIG. 4  is a diagram of a conventional aperture-coupling patch antenna.  
       FIG. 5  is a diagram of a wideband patch antenna using a disk-loaded feed probe.  
       FIG. 6  is a diagram of a wideband patch antenna using a disk-loaded feed probe spaced above the patch.  
       FIG. 7  is a diagram of a wideband patch antenna using an L-shaped feed probe spaced below the patch.  
       FIG. 8  is a diagram of a wideband patch antenna according to the present invention, using a helix-shaped probe.  
       FIG. 9  is a diagram of a wideband patch antenna according to the present invention, using a meandering probe.  
       FIG. 10  is a diagram according to the present invention, showing two types of helical feed probes.  
       FIG. 11  is a diagram according to the present invention, showing two additional probe embodiments: vertical planar meandering-wire feed probes.  
       FIG. 12  is a diagram of a two-antenna array of wideband patch antennas according to the present invention.  
       FIG. 13  is a diagram of an alternative embodiment of dual-band arrangement of wideband patch antennas according to the present invention. 
    
    
     DETAILED DESCRIPTION OF DRAWINGS  
      Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.  
       FIG. 1  shows a standard patch antenna  10 , where patch  11  is suspended in air a distance h 1  above ground plane  12 . Patch  11  is fed by means of a coaxial connector  13 , where its center conductor probe  14  is physically connected to the patch at feed point  15 .  
       FIG. 2  illustrates method 1 described above. It shows a stacked patch antenna assembly  20 . Patch  21  is suspended in air a distance h 1  above ground plane  22 . Patch  21  is fed by means of a coaxial connector  23 , where its center conductor probe  24  is physically connected to patch  21 . A second patch  25  is placed a distance h 2  above patch  21 .  
       FIG. 3  illustrates method 2. It shows a wideband patch antenna assembly,  30 , that utilizes a disk-loaded feed probe. Patch  31  is suspended in air a distance h 1  above ground plane  32 . Conductive disk  33 , etched on a substrate, is attached to the top section of center conductor probe  34  of coaxial connector  35 . Disk  33  has diameter d 1  and is spaced at a height h 2  above ground plane  32  and a small distance d below patch  31 . Typical dimensions at a frequency of 1900 MHz are: patch 301: 6.5 cm×6.5 cm, h 1 =1 cm, d 1 =1 cm, d=0.08 cm.  
       FIG. 4  shows an aperture coupling patch assembly,  40 , according to method 3. The microstrip feed line is etched on the feed substrate and can be connected to a coaxial connector or cable. The coupling aperture is on ground plane  41 . The patch antenna is etched on a dielectric and placed above ground plane  41 .  
       FIG. 5  shows a patch antenna assembly,  50 , according to method 4. Conductive disk  51  is attached to the end of the coaxial center conductor probe  52  of coaxial connector  53 . A hole in the patch allows disk  51  to be located on the same plane as patch  54  and same distance h from ground plane  56 . An annular gap  55  is formed between disk  51  and patch  54 .  
       FIG. 6  illustrates a patch antenna assembly,  60 , according to method 5. Conductive disk  61  is attached to the end of the coaxial center conductor probe  62  of coaxial connector  63 , identical to methods 2 and 5. In this case, disk  61  is spaced above patch  64  by a distance d.  
       FIG. 7  shows a patch antenna assembly,  70 , according to method 6. The L-shaped probe has a first portion  71  normal to ground plane  72  and patch  73 , and a second portion  74  parallel to ground plane and patch. Horizontal section  74  of the probe is at a distance h 2  above ground plane  72  and distance d from patch  73 .  
       FIG. 8  is a diagram of a wideband patch antenna assembly,  80 , according to the present invention. Patch  81  is suspended in air a distance h 1  above ground plane  82 . Patch  81  has dimensions of d 1  by d 2  and is fed by means of a coaxial connector  83 , where its center conductor is connected to a helix  84 . Helix  84  is placed underneath patch  81 . The tallest point of helix  84  is at a small distance d from the bottom surface of patch  81 . Helix  84  is at a distance d 3  from one edge of patch  81  and d 4  from the other edge of patch  81 . Distance d 4  is significantly larger than distance d 3 . Distance d 3  can also be 0 mm. In some cases the helix probe does not have to be directly below patch  81  but can be placed outside the patch antenna. The height of helix  84  is h 2  and it may consist of several turns or a fractional turn depending of its diameter. Typical dimensions for a patch antenna according to the present invention and operating at 1840 MHz are: ground plane=12 cm×12 cm, patch=6.7×6.7 cm, patch height=0.95 cm, helix height=0.75 cm, distance d=2 mm, helix diameter=1 cm, number of turns=1.3, wire diameter=0.5 mm. These particular dimensions result in a 12.6% bandwidth.  
       FIG. 9  is a diagram of a wideband patch antenna assembly,  90 , according to the present invention, using a meandering-wire probe. Patch  91  is suspended in air a distance h 1  above ground plane  92 . Patch  101  has dimensions of d 1  by d 2  and is fed by means of a coaxial connector  93 , where its center conductor is connected to the meandering wire probe  94 . Probe  94  is placed underneath patch  91  at a distance d from the bottom surface of patch  91 . The probe  94  is at a distance d 3  from one edge of patch  91  and d 4  from the other edge. Distance d 4  is significantly larger than distance d 3 . Distance d 3  can also be 0 mm. In some cases the probe  94  can be placed outside the patch antenna  91 . The height of probe  94  is h 2 . It should be noted that meandering wire probe  94  can be realized using a substrate on which the meandering pattern is etched.  
       FIG. 10  ( a ) shows a standard helix probe  100 . Helix  101  is placed above ground plane  102  and is connected to a short vertical section  103  of height h 1  that is an extension of the center conductor of coaxial connector  104 . The helix parameters are: diameter d, pitch p, number of turns, and height h 2 . The height of the total probe assembly is h.  
       FIG. 10  ( b ) shows a conical helix probe  105 . Conical helix  106  is placed above ground plane  107  and is connected to a short vertical section  108  of height h 1  that is an extension of the center conductor of coaxial connector  109 . The helix parameters are: bottom helix diameter d, top helix diameter D, pitch p, number of turns, and height h 2 . The height of the total probe assembly is h.  
       FIG. 11  ( a ) shows one type of vertical meandering or zigzag wire probe  110 . Meandering wire section  111  is placed above ground plane  112  and is connected to a short vertical section  113  of height h 1  that is an extension of the center conductor of coaxial connector  114 . The meandering wire section parameters are: width d, pitch p, number of turns, and height h 2 . The height of the total probe assembly is h. Parameter h 2  can be multiples of dimension p.  
       FIG. 11  ( b ) shows another type of vertical meandering or zigzag wire probe  115 . Meandering wire section  116  is placed above ground plane  117  and is connected to a short vertical section  118  of height h 1  that is an extension of the center conductor of coaxial connector  119 . The meandering wire section parameters are: width d, distance between horizontal wires p, number of turns, and height h 2 . The height of the total probe assembly is h. Parameter h 2  can be multiples of dimension p.  
       FIG. 12  is a diagram of a two-antenna array  120  of wideband patch antennas according to the present invention. Two substantially identical rectangular patches  121  and  122  each of dimensions d 1  and d 2  are suspended in air a distance h 1  above ground plane  123 . The distance between the patches is D. Helical probe  124 , of height h 2 , is placed below patch  121  and is connected to the center conductor of connector  125  and transmission line  126 . Helical probe  127  is substantially identical to probe  124  and is placed below patch  122  and is connected to transmission line  126 .  
       FIG. 13  is a diagram of of dual-band arrangement,  130 , of wideband patch antennas according to the present invention. The assembly  130  is similar to assembly  120  of  FIG. 12 . One difference between assemblies  120  and  130  is that the patches and their respective probes are of different dimensions. Patch  131  of dimensions of d 1  by d 3  is placed a distance h 1  above ground plane  132  and is fed by means of a coaxial connector  133  where its center conductor is connected to helix probe  134  of height h 2 , and transmission line  135 . The tallest point of helix probe  134  is at a small distance d 7  from the bottom surface of patch  131 . Helix probe  134  is at a distance d 5  from one edge of patch  131  and d 9  from the other edge of patch  131 . Patch  136  of dimensions of d 2  by d 4  is placed a distance h 3  above ground plane  132  and is fed by means of transmission line  135 . End of transmission line  135  is connected to helix probe  137  of height h 4 , located below patch  136 . The tallest point of helix probe  137  is at a small distance d 8  from the bottom surface of patch  136 . Helix probe  137  is at a distance d 6  from one edge of patch  136  and d 10  from the other edge of patch  136 . The distance between patches  131  and  136  is D. Patch  131  and helix probe  134  are designed to work in one frequency band, while patch  136  and probe  137  are designed to work in the other frequency band.  
      It should be noted that the embodiments described herein should not limit the scope of the invention. The description above is intended by way of example only and is not intended to limit the present invention in any way except as set forth in the following claims. For example, the probes according to the present invention can be connected to a transmission line or coaxial cable in addition to a coaxial connector. The meandering-shape probes can be etched on substrates like FR-4 and can be connected to the center conductor of a coaxial cable or connector, or can be connected to a microstrip transmission line. The coaxial cable shield may be soldered to the bottom side of the ground plane and the cable center conductor can connect to the probe through a hole on the ground plane. The coaxial cable shield may also be soldered to the top side of the ground plane and the cable center conductor can connect to the probe without the need for a hole on the ground plane. It shall be understood, that the patch antennas may be of pure metallic form or etched on any type of dielectric. In addition, as mentioned above, the patch antennas may be spaced from the ground plane by any form of dielectric material (including air) or by multiple layers of differing dielectric materials.