Patent Publication Number: US-6987483-B2

Title: Effectively balanced dipole microstrip antenna

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to wireless communication antennas and, more particularly, to an effectively balanced dipole, formed from an unbalanced microstrip antenna, and suitable for use in a wireless communications device telephone. 
     2. Description of the Related Art 
     The size of portable wireless communications devices, such as telephones, continues to shrink, even as more functionality is added. As a result, the designers must increase the performance of components or device subsystems while reducing their size, or placing these components in less desirable locations. One such critical component is the wireless communications antenna. This antenna may be connected to a telephone transceiver, for example, or a global positioning system (GPS) receiver. 
     Wireless communications devices, a wireless telephone or laptop computer with a wireless transponder for example, are known to use simple cylindrical coil antennas as either the primary or secondary communication antennas. The resonance frequency of the antenna is responsive to its electrical length, which forms a portion of the operating frequency wavelength. The electrical length of a wireless device helical antenna is often an odd multiple of a quarter-wavelength, such as 3λ/4, 5λ/4, or λ/4, where λ is the wavelength of the operating frequency, and the effective wavelength is responsive to the dielectric constant of the proximate dielectric. 
     Wireless telephones can operate in a number of different frequency bands. In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band, at around 1900 MHz, are used. Other frequency bands include the PCN (Personal Communication Network) at approximately 1800 MHz, 
     the GSM system (Groupe Speciale Mobile) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are global positioning satellite (GPS) signals at approximately 1575 MHz and Bluetooth at approximately 2400 MHz. 
     Typically, better communication results are achieved using a whip antenna, as opposed to the above-mentioned helical antennas. Using a wireless telephone as an example, it is typical to use a combination of a helical and a whip antenna. In the standby mode with the whip antenna withdrawn, the wireless device uses the stubby, lower gain helical coil to maintain control channel communications. When a traffic channel is initiated (the phone rings), the user has the option of extending the higher gain whip antenna. Some devices combine the helical and whip antennas. Other devices disconnect the helical antenna when the whip antenna is extended. However, the whip antenna increases the overall form factor of the wireless telephone. 
     It is known to use a portion of a circuitboard, such as a dc power bus, as an electromagnetic radiator. This solution eliminates the problem of an antenna extending from the chassis body. However, these radiators are extremely inefficient “antennas”, typically providing poor gain and directionality. These types of radiators are also susceptible to crosstalk from other signals on the board. Further, these types of radiators can also propagate signals that interfere with digital or radio frequency (RF) on the circuitboard. Electromagnetic communications through these radiators can also be shielded by other circuits, circuit groundplanes, the chassis, or other circuitboards in the chassis. 
     Regardless of whether the antenna is formed as a helical coil, a whip, or a microstrip (printed circuitboard) antenna, a conventional dipole is fabricated in a balanced configuration. That is, the radiator and counterpoise are 180 degrees out of phase. The balanced transmission line provides the optimal interface for a balanced dipole antenna. However, the typical radio frequency (RF) electrical circuit, including wireless telephones, use unbalanced transmission lines. When an unbalanced transmission line is interfaced with a balanced antenna, a mismatch occurs, as the antenna counterpoise processes a different RF voltage potential than the transmission line ground. As a result, the transmission line ground radiates. Alternately stated, the transmission line ground becomes part of the antenna. This unintentional radiation degrades the intended electromagnetic radiation pattern, and may radiate into other sensitive electrical circuits. 
     Likewise, when an unbalanced dipole antenna is interfaced with a transmission line, a mismatch occurs. Without an antenna counterpoise, the transmission line ground radiates. Alternately stated, the transmission line ground becomes part of the antenna. This unintentional radiation degrades the intended electromagnetic radiation pattern, and may radiate into other sensitive electrical circuits. 
       FIG. 10  is a schematic diagram of a balun used for interfacing an unbalanced transmission line to a balanced antenna (prior art). A balanced-to-unbalanced balun is used to minimize RF current in the transmission line ground. The balun induces a current choke at a low impedance point. Alternately stated, a current is induced in the balun that is equal and opposite is phase to the current in the ground. Shown is a so-called bazooka balun that uses λ/4 decoupling stubs. 
     Baluns, such as the balun shown in  FIG. 10 , are typically used with coaxial cable or coax hardlines. Alternately for lower frequency applications, toroidal baluns made with wire can be formed around loaded or unloaded core materials. However, these types of baluns are not practical for use with microstrip transmission lines. Conventionally, when microstrip transmissions lines are interfaced with a balanced antenna, for example in applications where space is critical, the overall electromagnetic performance suffers due to the lack of a balanced-to-unbalanced balun. These same problems also exist with the use of coplanar and stripline transmission lines. Likewise, when a microstrip antenna is fabricated without a counterpoise, when space is a pressing concern for example, and interfaced to a microstrip transmission line, a mismatch will occur. 
     It would be advantageous if a practical balun could be developed for use in interfacing an unbalanced microstrip, coplanar, or stripline transmission line to an unbalanced microstrip antenna. 
     SUMMARY OF THE INVENTION 
     Microstrip, coplanar, and stripline baluns are provided for interfacing unbalanced transmission lines to an unbalanced antenna. These baluns are especially advantageous when the interfacing antenna is a microstrip antenna, so that the transmission line, balun, and antenna can all be formed on the same substrate. 
     Accordingly, an effectively balanced dipole antenna is provided comprising an unbalanced microstrip antenna having a transmission line interface, and a planar balun connected to the transmission line interface of the antenna. The balun can be coplanar or multi-planar. For example, a coplanar balun includes an unbalanced coplanar transmission line, with a signal line interposed between a pair of coplanar grounds, and a pair of planar stubs plan-wise adjacent the coplanar grounds. The coplanar grounds are connected to the plane stubs with conductive lines proximate to the antenna transmission line interface. 
     A microstrip planar balun includes an unbalanced microstrip signal line, a microstrip ground formed on the dielectric layer underlying the signal line, and a pair of planar stubs, plan-wise adjacent the microstrip ground. The planar stubs can be located on the same dielectric layer as the signal line or the ground. 
     A stripline planar balun includes two dielectric layers, an unbalanced stripline signal line between the dielectric layers, stripline grounds formed overlying and underlying the stripline signal line, and a pair of planar stubs formed plan-wise adjacent the stripline signal line. 
     Additional details of the above-described planar balun and an unbalanced microstrip antenna, that when combined form an effective balanced dipole antenna, are provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective drawing of one aspect of the present invention planar balun. 
         FIGS. 2   a  and  2   b  are perspective drawings featuring a multi-planar aspect of the present invention planar balun. 
         FIG. 3  is a perspective drawing featuring another multi-planar aspect of the present invention planar balun. 
         FIG. 4  is a perspective view showing a combination of the present invention planar balun with an unbalanced microstrip antenna. 
         FIGS. 5   a  and  5   b  are plan view details of an unbalanced microstrip antenna. 
         FIGS. 6   a  and  6   b  are plan views illustrating a two-sided circuitboard aspect of the unbalanced microstrip antenna. 
         FIGS. 7   a  and  7   b  are plan views illustrating a two-sided circuit aspect of the unbalanced microstrip with side-alternating first and second radiator sections. 
         FIGS. 8   a  and  8   b  are plan views illustrating a two-sided circuit aspect of the unbalanced microstrip antenna with side-alternating first and second radiator section combinations. 
         FIG. 9  is a schematic block diagram of the present invention wireless communications telephone system. 
         FIG. 10  is a schematic diagram of a balun used for interfacing an unbalanced transmission line to a balanced antenna (prior art). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective drawing of one aspect of the present invention planar balun. As explained in more detail below, the planar balun is connected to the transmission line interface of an unbalanced microstrip antenna (not shown is this figure). The combination of planar balun, with the unbalanced microstrip antenna, results in a balanced antenna. That is, the overall result is a balanced antenna, referred to herein as an effectively balanced dipole antenna that minimized transmission line radiation. 
     More specifically,  FIG. 1  depicts a coplanar balun  100 . The coplanar balun  100  includes a dielectric layer  102  with a first side  104  and a second side  106  that cannot be seen in this view. An unbalanced coplanar transmission line is shown, with a signal line  108  (cross-hatched lines) interposed between a pair of coplanar grounds  110  and  112 , on the dielectric layer first side  104 . A pair of planar stubs  114  and  116  is formed in the dielectric layer first side  104 . Each stub  114 / 116  is plan-wise adjacent the coplanar grounds  110  and  112 , respectively. As used herein, plan-wise adjacent means that elements are adjacent when viewed from the plan perspective. It should be understood that elements can be plan-wise adjacent when they are located on the same, or different dielectric layer sides. As shown, the coplanar grounds  110 / 112  are interposed between the planar stubs  114 / 116  on the dielectric layer first side  104 . 
     The planar stubs  114 / 116  each have an effective electrical length  118  approximately equal to a quarter-wavelength odd multiple of the antenna operating frequency. That is, a wavelength of (2n+1) (λ/4), where n=0, 1, 2, . . . . The length of the stubs  114 / 116  must be considered in light of the dielectric constant of the circuitboard dielectric layer, as is well known in the art. The antenna interface is depicted with reference designator  120 . As shown, the planar stubs  114 / 116  are lines oriented parallel to the coplanar transmission line ( 108 / 110 / 112 ). The coplanar grounds  110 / 112  are connected to the planar stubs  114 / 116  with conductive lines  122  and  124 , respectively, proximate to the antenna transmission line interface  120 . 
       FIGS. 2   a  and  2   b  are perspective drawings featuring a multi-planar aspect of the present invention planar balun. More specifically, the figures depict a microstrip balun. As seen in both figures, the microstrip balun  200  includes a dielectric layer  202  with a first side  204  and a second side  206  that cannot be seen in this view. An unbalanced microstrip signal line  208 , depicted with cross-hatched lines, is located on the dielectric layer first side  204 . A microstrip ground  210  formed on the dielectric layer second side  206  underlying the signal line  210 . The microstrip ground  210  cannot be seen in this view but is depicted with dotted lines. A pair of planar stubs  212  and  214  are plan-wise adjacent the microstrip ground  210 . 
     In  FIG. 2   a , the planar stubs  212 / 214  are located on the dielectric layer first side  204 . The microstrip ground  210  is connected to the planar stubs  212 / 214  formed on the dielectric layer first side  204  through vias  216  and  218 , respectively, located proximate to the antenna transmission line interface  220 . Note that the present invention is not limited to any particular number of vias. 
     In  FIG. 2   b , the planar stubs  212 / 214  are located on the dielectric layer second side  206  and depicted with dotted lines, as they cannot be seen from this view. 
     Regarding either  FIG. 2   a  or  2   b , each of the planar stubs  212 / 214  has an effective electrical length  222  approximately equal to a quarter-wavelength odd multiple of the antenna operating frequency. The planar stubs  212 / 214  are lines oriented parallel to the microstrip signal line  208 . 
     As seen in  FIG. 2   b , the microstrip ground  210  is connected to the planar stubs  212 / 214  formed on the dielectric layer second side  206  with conductive lines  224  and  226 , respectively, located proximate to the antenna transmission line interface  220 . With respect to  FIG. 2   a , although the conductive lines  224  and  226  are shown on the dielectric first side between the vias  216 / 218  and the stubs  212 / 214 , in other aspects of the invention (not shown) the conductive lines  224 / 226  are formed on the dielectric second side  206  between the ground  210  and vias  216 / 218  directly connected to the stubs  212 / 214 . 
     The microstrip balun  200  of  FIGS. 2   a  and  2   b  is shown formed on a dielectric with only two layers for simplicity. However, in other aspects of the invention the microstrip balun can be formed on a dielectric with multiple layers. For example, a dielectric where either digital or radio frequency signal traces are formed in layers intervening between the first side  204  and the second side  206 . Alternately, there may be no intervening layers between side  204  and  206 , but other dielectric layers may be formed either overlying the first side  204  and/or underlying the second side  206 . 
       FIG. 3  is a perspective drawing featuring another multi-planar aspect of the present invention planar balun. More specifically, the figure depicts an exploded view of a stripline balun. The stripline balun  300  includes a first dielectric layer  302  with a first side  304  and a second side  306  that cannot be seen in this view. A second dielectric layer  308  has a first side  310  and a second side  312  that cannot be seen in this view. An unbalanced stripline signal line  314 , depicted with cross-hatched lines, is formed on the second dielectric layer first side  310 . Alternately but not shown, the stripline signal line  314  can be formed on the first dielectric layer second side  306 . Stripline grounds  316  and  318 , respectively, are formed on the first dielectric layer first side  304  overlying the stripline signal line  314  and the second dielectric second side  312  underlying the stripline signal line  314 . The stripline ground  318  cannot be seen in this view but is depicted with dotted lines. A pair of planar stubs  320  and  322  are formed between the first dielectric layer second side  304  and the second dielectric layer first side  310 , plan-wise adjacent the stripline grounds  316 / 318 , respectively. Note that the planar stubs  320 / 322  are shown formed on the second dielectric layer first side  310 , but they can alternately be formed on the first dielectric second side  306 . 
     The planar stubs  320 / 322  each have an effective electrical length  324  approximately equal to a quarter-wavelength odd multiple of the antenna operating frequency. The planar stubs  320 / 322  are lines oriented parallel to the stripline signal line  314 . The planar stubs  320 / 322  are connected to the stripline grounds  316  through vias  326  and  328  located proximate to the antenna transmission line interface  330 . Likewise, planar stubs  320 / 322  are connected to the stripline grounds  318  through vias  332  and  334  located proximate to the antenna transmission line interface  330 . Note that although four vias are shown, the present invention is not limited to any particular number of vias. Also note that connecting lines  336 ,  338 ,  340 , and  342  are used to join the vias  326 ,  328 ,  332 , and  334 , respectively, to grounds  316  and  318 . The connecting lines are shown formed on the same dielectric sides as the stripline grounds, but in other aspects of the invention not shown, the connecting lines can be formed in the first dielectric second side  306  and the second dielectric first side  310 . 
     The stripline balun  300  of  FIG. 3  is shown formed on a dielectric with only four sides for simplicity. However, in other aspects of the invention the stripline balun can be formed on a dielectric with more layers. For example, a dielectric where either digital or radio frequency signal traces are formed in layers intervening between the first side  304  and the second side  306 . Alternately, there may be no intervening layers between side  304  and  306 , but other dielectric layers may be formed either overlying the first side  304  and/or underlying the second side  306 . A similar analysis applies to sides  310  and  312 . 
       FIG. 4  is a perspective view showing a combination of the present invention planar balun with an unbalanced microstrip antenna  400 . Again, it should be noted that the above-mentioned combination results in an effectively balanced dipole antenna  402 . Specifically, a microstrip balun  200  is shown (see  FIG. 2   b ) with an unbalanced microstrip antenna  400  having a radiator formed in a zig-zag pattern on two sides of the dielectric layer. Note that the dotted lines represent conductive traces on the dielectric layer underside. Below, are presented many variations of the unbalanced antenna. Although not every combination is specifically depicted, it should be understood that any of the unbalanced microstrip antenna variations can be combined with any of the above-mentioned planar balun designs to form an effectively balanced dipole antenna. 
     The microstrip antenna  400  is considered to be unbalanced because there is no counterpoise section. The missing counterpoise could be a groundplane, in which case the antenna would be a monopole. Alternately, the missing counterpoise could be another radiator section formed to have an effective electrical length, in which case the antenna would be a dipole. The balun can be considered to be an emulation of a monopole or dipole antenna counterpoise. Hence, the invention is called an effectively balanced dipole antenna. In other aspects, the invention could equally well be called an effectively balanced monopole antenna. The present invention balun could also be considered a choke device that prevents transmission line radiation from occurring when an unbalanced transmission line is interfaced to an unbalanced microstrip antenna. 
       FIGS. 5   a  and  5   b  are plan view details of an unbalanced microstrip antenna. Considering either  FIG. 5   a  or  5   b , the antenna  400  includes a radiator  500  formed from a printed conductive line  502  overlying the circuitboard second portion dielectric layer with a first end  504  for connection to a transmission line and a second, unterminated end  506 . The lines can be formed from an etching process that selectively removes portions of a metal cladding overlying the circuitboard. Alternately, the conductive lines can be formed through a metal deposition process. 
     Typically, the antenna radiator  500  has an effective electrical length of approximately a quarter-wavelength odd multiple at the operating frequency. That is, a wavelength of (2n+1) (λ/4), where n=0, 1, 2, . . . . The length of the radiator is a combination of the various meandering sections considered in light of the dielectric constant of the circuitboard dielectric layer, as is well known in the art. In other aspects, the antenna  400  can be different length than a quarter-wavelength odd multiple. Such a situation may occur, for example, when the antenna is expected to operate over a wide bandwidth or multiple bandwidths. 
     The antenna radiator  500  includes a plurality of first sections  508  with a first orientation  510  and a plurality of second sections  512  oriented with a second orientation  514 , that can be orthogonal, or approximately orthogonal to the first orientation  510 . When the first and section sections  508 / 512  are orthogonal, coupling between the sections can be minimized, permitting the antenna to be made “stubby” without substantially degrading the antenna performance. The sections can also be oriented so that they are not orthogonal, further reducing the form factor of the antenna at the expense of performance, which is degraded by increased coupling between radiator first and second sections. 
     As shown in  FIG. 5   a , the antenna radiator  500  is formed in a pattern of meandering rectangular lines. As shown in  FIG. 5   b , the antenna radiator  400  is shown in a pattern of meandering zig-zag lines. The invention can be enabled with other patterns or shapes,  FIGS. 5   a  and  5   b  are merely exemplary. 
       FIGS. 6   a  and  6   b  are plan views illustrating a two-sided circuitboard aspect of the unbalanced microstrip antenna. The circuitboard dielectric layer  102  has a first side  600  that can be seen and a second, opposite side that cannot be seen in the figures. Further, at least one connection via  602  exists between the dielectric layer first side  600  and the dielectric layer second side. The vias can be formed through a process that drills holes through the dielectric and plates the holes with a conductive material. Alternately, the vias can be any means that pass through the dielectric layer to electrically connect to the first and second sides. The antenna radiator  500  includes sections overlying the dielectric layer first side  600  connected to sections on the dielectric layer second side through the via  602 . The overall size of the antenna  400  can be reduced by printing the radiator on both sides of the circuitboard. 
     As shown in  FIG. 6   a , the antenna radiator  500  is formed from a meandering rectangular line overlying the dielectric layer first side  600 , and connected through the via  602  to a meandering rectangular line overlying the dielectric layer second side (represented as a dotted line). As shown, the first sections  508  on the circuitboard second side minimally underlie first section  508  on the first side  600  while the second sections  512  on the circuitboard second side minimally underlie second sections  512  on the first side  600 . However, other arrangements are possible. For example (not shown), the first sections  508  of the circuitboard second side may underlie the first sections  508  on the circuitboard first side  600 . Likewise, the radiator second sections  512  on the circuitboard second side can underlie the second sections  512  on the circuitboard first side  600 . 
     As shown in  FIG. 6   b , the antenna radiator  500  is formed from a meandering zig-zag line overlying the dielectric layer first side  600 , and connected through the via  602  to a meandering zig-zag line overlying the dielectric layer second side (represented as a dotted line). As shown, the first sections  508  minimally underlie first section  508  on the first side  600  while the second sections  512  on the circuitboard second side minimally underlie second sections  512  on the first side  600 . Alternately but not shown, the first sections  508  of the circuitboard second side may underlie the first sections  508  on the circuitboard first side  600 . As another alternate (not shown), the second sections  512  of the circuitboard second side may underlie the second sections  512  on the circuitboard first side  600 . 
     Both figures represent the radiator length to be approximately evenly divided between the dielectric layer first and second sides. However, the lengths need not necessarily be equal. The invention can be enabled with other patterns or shapes,  FIGS. 6   a  and  6   b  are merely exemplary. 
       FIGS. 7   a  and  7   b  are plan views illustrating a two-sided circuit aspect of the unbalanced microstrip antenna with side-alternating first and second radiator sections  508 / 512 . The antenna radiator first sections  508  overlie the dielectric layer first side  600  and the radiator second sections  512  overlie the dielectric layer second side. The antenna radiator first and second sections  508 / 512  are connected with a plurality of vias  602 . While not always as space efficient as the antennas of  FIGS. 6   a  and  6   b , the antennas of  FIGS. 7   a  and  7   b  promote decoupling between the first and second sections  508 / 512  by forming them on opposite sides of the circuitboard. In some aspects, as shown in  FIG. 7   b , this increased decoupling permits the first and second sections to be aligned non-orthogonally, to reduce the form factor while minimally impacting the antenna performance. 
     As shown in  FIG. 7   a , the antenna radiator first sections  508  and second sections  512  (represented as dotted lines) form a meandering rectangular line. As shown in  FIG. 7   b , the antenna radiator first sections  508  and second sections (represented as dotted lines) form a meandering zig-zag line. 
     Both figures represent the radiator length to be approximately evenly divided between the dielectric layer first and second sides. However, the lengths need not necessarily be equal. The invention can be enabled with other patterns or shapes,  FIGS. 7   a  and  7   b  are merely exemplary. 
       FIGS. 8   a  and  8   b  are plan views illustrating a two-sided circuit aspect of the unbalanced microstrip antenna with side-alternating first and second radiator section combinations. As above, the radiator  500  includes sections overlying the dielectric layer first side connected to sections on the dielectric layer second side through a plurality of vias  602 . More specifically, the radiator  500  includes a plurality of first and second section combinations overlying the dielectric layer first side and the radiator includes a plurality of first and second section combinations overlying the dielectric layer second side. 
     As shown, the combinations each include one first section and one second section, however, the invention is not limited to just this type of combination. The radiator combinations on the dielectric layer first side are connected to the radiator combinations on the dielectric layer second side (shown as dotted lines) with a plurality of vias  602 . 
     In  FIG. 8   a , the antenna radiator first sections  508  and second section  512  combinations form a meandering rectangular line. As shown in  FIG. 8   b , the antenna radiator first sections  508  and second section combinations form a meandering zig-zag line. 
     Both figures represent the radiator length to be approximately evenly divided between the dielectric layer first and second sides. However, the lengths need not necessarily be equal. The invention can be enabled with other patterns or shapes,  FIGS. 8   a  and  8   b  are merely exemplary. 
       FIG. 9  is a schematic block diagram of the present invention wireless communications telephone system. The system  900  comprises a transceiver  902  with an antenna port on line  904  and an effectively balanced dipole antenna  906 . The effectively balanced dipole antenna  906  includes an unbalanced microstrip antenna  908  having a transmission line interface on line  910  and a planar balun  912  having a first port connected to the transmission line interface of the antenna  908  and a second port connected to the transceiver antenna port on line  904 . The dipole antenna  908  communicates at one, or more of the following frequencies: 824 to 894 megahertz (MHz), 1850 to 1990 MHz, 1565 to 1585 MHz, and 2400 to 2480 MHz. Detailed descriptions of the balun and antenna are provided in the explanations of  FIGS. 1 through 8   b  above. 
     An effectively balanced dipole antenna has been provided comprising an unbalanced microstrip antenna and a planar balun. Some examples have been given of balun types, antenna types, and balun/antenna combinations. However, other variations and embodiments of the present invention will occur to those skilled in the art.