Patent Publication Number: US-7710335-B2

Title: Dual band loop antenna

Description:
TECHNICAL FIELD 
   The present invention generally relates to antenna assemblies and, more particularly, to a dual band loop antenna. 
   BACKGROUND OF THE INVENTION 
   Automotive vehicles are commonly equipped with dual-band personal communication systems (PCS) and digital/analog mobile phone service (AMPS) antennas. Such antennas have a height, for example, of at least 70 mm, and are implemented for cellular phone usage. Typically, these antennas are mounted exterior to the vehicle to achieve improved antenna performance and reduced radio frequency (RF) emissions to the inside of the vehicle. In many circumstances, height of the antenna may not be reduced because antenna performance may be compromised. 
   Due to high efficiency and ease of construction characteristics, helical wire antennas remain the first choice for many cellular antenna designers. For wire antennas, the optimum operation corresponds to λ/4 wavelength. The height, which is approximately 75-80 mm, is very close to λ/4 of the operation wavelength at the cellular phone lower frequency band (e.g. AMPS). This height may be further reduced using a normal-mode helical antenna. The height may be reduced to as little as 65 mm, however, a height reduction less than 65 mm may degrade the overall performance of the antenna. 
   Other known cellular antennas include a planar inverted circular/rectangular patch antenna having a reduced height, for example, of at least 30 mm. Additionally, the inverted path antenna has a higher linear gain. However, the diameter/width of antenna is undesirably increased to be at least 115 mm, and, are typically difficult to include dual band applications. 
   When antennas having large dimension width, but more often, height, are mounted on the exterior of the vehicle, the antenna becomes very noticeable, and often, unpleasant for vehicle users while introducing manufacturing difficulties for the OEMs. Accordingly, it is therefore desirable to provide an improved antenna assembly that is compact, provides adequate antenna performance, and offers multi-band capabilities. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an antenna assembly. Accordingly, one embodiment of the invention is directed to an antenna assembly including a dual band vertical loop wire antenna extending from a printed circuit board positioned over a ground plane. The wire antenna includes at least one coiled section, at least one straight wire section, and at least one feeding post section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1A  illustrates a perspective view of a dual band loop antenna according to one embodiment of the invention; 
       FIG. 1B  illustrates a front view of the dual band loop antenna according to  FIG. 1A ; 
       FIG. 1C  illustrates a side view of the dual band loop antenna according to  FIG. 1A ; 
       FIG. 2A  illustrates a top view of a dual band loop antenna according to another embodiment of the invention; 
       FIG. 2B  illustrates a side view of a dual band loop antenna according to another embodiment of the invention; 
       FIG. 3  illustrates a perspective view of a dual band loop antenna according to another embodiment of the invention; and 
       FIG. 4  illustrates a perspective view of a dual band loop antenna according to another embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring generally to  FIGS. 1A-4 , the above described disadvantages are overcome and a number of advantages are realized by a dual band loop antenna assembly, which is seen generally at  10 ,  100 ,  200 , and  300 . Each antenna assembly  10 ,  100 ,  200 ,  300  is a low-profile dual band antenna that accommodates operation between the 824-849 MHz band for AMPS uplink, the 869-894 MHz band for AMPS downlink, the 1850-1910 MHz band for PCS uplink, and the 1930-1990 MHz band for PCS downlink. 
   Referring initially to  FIGS. 1A-1C , the antenna assembly  10  includes at least one radiating element, such as, for example, a PCS/AMPS wire antenna  12 , a patch antenna  14 , and associated immediate active circuitry (not shown) within a printed circuit board (PCB)  16 . The patch antenna may provide a combinational antenna assembly if global positioning signals (GPS), satellite digital audio radio system (SDARS) signals, or the like, are to be received. Functionally, antennas, such as the patch antenna  14 , are receiving-only antennas that typically encounter weak satellite signal reception (i.e. by the time the satellite signal reaches the earth&#39;s surface, the received signal is weak). To compensate for the weakened signal reception, the antennas typically employ a known active microwave circuit, such as a low noise amplifier (LNA) that is located inside the PCB  16 , to amplify the received weak signal to a much stronger level so that it can be further processed with the receiver/navigation system. Wire antennas  12 , which are employed for analog and digital telephones bands PCS/AMPS applications, on the other hand, are used for both earth-based-transmitting (i.e. uplink frequencies) and earth-based-receiving (i.e. downlink frequencies) purposes, and therefore do not need an active microwave circuit for immediate amplification. At their transmitting mode, PCS/AMPS antennas are required to emit low electromagnetic energy to the people inside the vehicle so as not to cause any harm to living tissues. To comply with the specific absorption rate (SAR) standards determined by the Federal Communications Commission (FCC), the antenna assembly  10  is mounted exterior to the vehicle, such as the exterior roof  24  of the vehicle, which also acts as the ground plane  24  ( FIG. 1A ), so that radiation to the interior cabin of the vehicle is minimized. 
   Referring now to  FIG. 1A , the antenna assembly  10  includes a single arm vertical half wavelength (λ/2) dual band loop antenna. The wire antenna  12  comprises first and second coiled sections  12   a ,  12   b , straight sections  18 ,  20 , and a feeding post section  22  extending from the straight section  20 . As illustrated, the first coiled section  12   a  is located between the straight sections  18 ,  20 , and is positioned over the patch antenna  14  and printed circuit board  16 . The second coiled section  12   b  is generally perpendicular to and intermediately located between the printed circuit board  16  and the straight section  18 . The feeding structure is located where the feed post  22  meets the printed circuit board  16  and the ground point is located where the second coiled section  12   b  meets the PCB  16 . The feeding post  22  may also include a greater diameter than that of the wire antenna  12  to provide improved impedance matching. 
   As seen in  FIG. 1C , the wire antenna  12  is defined by an overall length, L a , and an overall height, H f . The first coiled section  12   a , and straight sections  18  and  20  are defined by lengths, L b , L c , and L d , respectively. The second coiled section  12   b  is defined by a height, H b , and is spaced from the straight section  18  and PCB  16  by heights, H b  and H c , respectively. The PCB  16  is spaced from the ground plane  24  by a height, H e . The lengths, L a , L b , L c , and L d , may be any desirable length, and the heights, H a , H b , H c , H d , H e , H f , may be any desirable height, such that an overall length, L, width, W ( FIG. 1B ), and height, H, provides a compact structure that is less visible when mounted on the vehicle&#39;s outer ground plane, such as the roof  24 . 
   Accordingly, the utilization of the coiled sections  12   a ,  12   b  provides dual-band operation and the feeding post section  22  provides impedance matching to reduce the overall height, H, of the antenna assembly  10  from the ground plane  24 . The overall height, H, of the antenna assembly  10  may be any desirable minimized height, and is generally determined by the overall wire antenna height, H f , of the PCS/AMPS antenna  12 . According to one embodiment of the invention, the overall wire antenna height, H f , is approximately 15 mm and the overall height, H, of the antenna assembly is approximately 23 mm. Additionally, the height, H, of the antenna may be further reduced by providing material loading to antenna assembly  10 . The material loading provides a longer electrical path so that the antenna assembly will be electrically higher than its physical height, thereby reducing the bandwidth of the antenna assembly. 
   The coiled windings results in an increased wire antenna length, L a , that corresponds to a lower-frequency, such as for AMPS or PCS, while also reducing the overall length, L, thereby providing a shorter antenna for higher frequencies, to allow dual band operations. According to one embodiment of the invention, the overall wire antenna length, L a , is approximately 52.5 mm and the overall length, L, of the antenna assembly is approximately 70 mm. Because only one branch or section, which is defined by the wire antenna  12  is implemented, the overall width, W, is reduced, such as, for example, to as little as approximately 30 mm. 
   In operation, the ground plane  24  introduces an image of the antenna so that the total length becomes a one wavelength (1.0λ) loop antenna (i.e. theoretically, the wire antenna  12  and posts raised from ground plane constitute a λ/2 long loop antenna). Essentially, the ground plane  24 , or any other type of metallization, mirrors the antenna such that the wire antenna  12  resonates over the ground plane, causing two antennas to radiate into space and the ground plane  24 , thereby causing the λ/2 long loop antenna to appears as a 1.0λ long loop antenna. Such loop antennas that have a circumference on the order of one wavelength include radiation patterns both at vertical and horizontal planes (i.e. the loop antenna has two E-planes and one H-plane). Essentially, the loop plane has a loop space that is the vertical plane for the electric field. 
   Referring now to  FIGS. 2A and 2B , another embodiment of the invention is directed to an antenna assembly  100  that includes at least one radiating element, such as, for example, a PCS/AMPS wire antenna  102 , a patch antenna  104 , and associated immediate active circuitry (not shown) within a PCB  106 . The antenna assembly  100  includes a three-branch or Y-shaped vertical λ/2 loop antenna to provide an optimum circular pattern. As illustrated, the antenna assembly  100  comprises six coiled sections  102   a - 102   e , straight sections  108 ,  110 ,  112 ,  114 , and a central feeding post section  122  extending from the straight section  110  into the PCB  106 . Although only 5 coiled sections are visually accounted for in  FIGS. 2A and 2B , the sixth coiled section extends from the straight section  114  perpendicularly towards the PCB  106 , and the visible coiled sections  102   a  and  102   e  are shown extending in a generally perpendicular configuration with respect to the ground plane  24 . The remaining three coiled sections  102   b - 102   d  are shown generally parallel to the ground plane  24 , which are hereinafter referred to as the ‘top coiled sections.’ 
   The top coiled sections  102   b - 102   d  and associated straight sections  108 - 114  of the Y-shaped antenna assembly  100  are positioned at angles, θ 1 -θ 3 , that determine the overall shape of the antenna assembly  100 . According to one embodiment of the invention, angles θ 1 -θ 3  may each be approximately equal to 120°, thereby complementing each other in symmetrical fashion. However, for packaging considerations, the branches may not be separated by 120°. For example, two arms may be separated by 60° at θ 1 , as θ 2 , θ 3  may separate the remaining branch by 150° each to arrive at a symmetrical antenna assembly  100  when viewed from the X-Y plane. 
   In this embodiment, the vertical polarization pattern is nearly uniform in the azimuth plane because interaction between the branches is maintained as a result of the antenna assembly  100  being symmetrical in the X-Y plane ( FIG. 2A ). With three branches extending in a horizontal plane and each elevated from ground at approximately 0.1λ, the radiation pattern in azimuth becomes almost uniform, which thereby achieves relatively high gains due to increased length of the wire antenna  102 . However, by including additional branches beyond the illustrated total of three branches, uniformity of the vertical polarization pattern may be lost. For example, when four, five, or six branches are included in the design, ripples in the signal may occur. 
   As seen in  FIGS. 3 and 4 , another embodiment of the invention is directed to antenna assemblies  200 ,  300  that includes at least one radiating element, such as, for example, a PCS/AMPS wire antenna  202 ,  302 , a patch antenna  204 ,  304 , and associated immediate active circuitry (not shown) within a PCB  206 ,  306 . As illustrated, the antenna assemblies  200 ,  300  respectively include straight wire sections and coiled sections  202   a - 202   c ,  302   a - 302   c , and a feeding post  210 ,  310 . In this embodiment of the invention, each antenna assembly  200 ,  300  includes a diamond-shaped wire section  208 ,  308  integrated with the vertical λ/2 loop antenna to provide improved impedance matching. Although shapes other than a diamond may be used, the diamond-shaped wire section  208 ,  308  provides an inductive load that neutralizes the capacitive impedance of the antenna assembly  200 ,  300 . In operation, the diamond-shaped wire section  208 ,  308  is located over the antenna  204 ,  304  in a generally parallel configuration with respect to the PCB  206 ,  306  such that additional height is not introduced to the antenna assembly  200 ,  300 . If additional impedance matching is needed to provide optimum performance, discrete components may be introduced at the terminals of the antenna. 
   As a result of the present invention, a smaller dual band antenna assembly  10 ,  100 ,  200 ,  300  may be used rather than high-profile dual band antenna assemblies. High profile dual band antennas for purposes of comparison, may be greater than or equal to approximately 65 mm. Additionally, the patch antenna  14 ,  104 ,  204 ,  304  may provide a combinational antenna assembly that permits reception of other signals, such as GPS, SDARS, or the like. Thus, the present antenna assembly is compact, provides adequate antenna performance, and offers multi-band, such as dual-band capabilities. As a result, because the antenna is a compact design, overall packaging of the antenna assembly is reduced and a more aesthetically pleasing antenna when mounted on the exterior of a vehicle is achieved. 
   The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.