Patent Publication Number: US-6982674-B2

Title: Antenna apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to circularly polarized antenna apparatuses suitable for a variety of wireless communication systems using two different frequency bands. 
   2. Description of the Related Art 
   Wireless communication systems using 2.4 and 5.2 GHz wireless local area networks (LANs) are used these days to transmit various types of audiovisual (AV) data including television (TV) pictures. 
   To successfully receive such AV data, a communication error rate in wireless transmission needs to be minimized. Since communication errors become apparent on the screen in the forms of block noises and stops of moving images, the requirement for minimizing the communication errors in transmitting AV data is higher than that in general wireless data transmission such as transmission over the Internet. 
   In wireless transmission of AV data, therefore, circularly polarized antennas are used to minimize the communication error rate. The reasons for use include circularly polarized waves being resistant to multipath interference and being not dependent on the antenna orientation. 
   First, the reason for the circularly polarized waves being resistant to multipath interference will be described. 
   When a single type of linearly polarized waves are used, the waves cancel each other out due to reflection from obstructions, resulting in a plurality of dead spots. 
   On the other hand, when the circularly polarized waves are used and reflected by obstructions, the traveling direction instead of the rotational direction is reversed. In other words, with respect to the traveling direction, the rotational direction of the circularly polarized waves is reversed, and thus, multipath interference caused by reflected waves is minimized. Therefore, the circularly polarized waves are more resistant to multipath interference compared to the linearly polarized waves. 
   Next, the reason for the circularly polarized waves not being dependent on the antenna orientation will be described. 
   When a single type of linearly polarized waves are used and antennas of a transmitter and receiver are different in orientation, the communication error rate increases due to a reduced gain. 
   On the other hand, when the circularly polarized waves are used, the communication error rate remains substantially unchanged regardless of the changes in antenna orientation, since a gain remains unchanged even though antennas of a transmitter and receiver rotate about the traveling direction. 
   Examples of known circularly polarized antennas include single band antennas designed for 2.4 GHz and 5.2 GHz, but no circularly polarized antenna for dual band exists. 
   Therefore, two independent circularly polarized antennas are required, as shown in  FIG. 3 , for use in dual-band wireless communication units. 
   The following describes the circularly polarized antennas in  FIG. 3 . 
     FIG. 3  illustrates a circularly polarized antenna A  301  for a first frequency band and a circularly polarized antenna B  302  for a second frequency band. 
   A flat antenna  301   b  is disposed on a dielectric plate  301   a  to form the circularly polarized antenna A  301 , which is disposed on a surface of a substrate  303 . The flat antenna  301   b  is connected via a wiring pattern on the surface of the substrate  303  to a coaxial cable  305 . 
   Similarly, a flat antenna  302   b  is disposed on a dielectric plate  302   a  to form the circularly polarized antenna B  302 , which is disposed on a surface of a substrate  304 . The flat antenna  302   b  is connected via a wiring pattern on the surface of the substrate  304  to a coaxial cable  306 . 
   The circularly polarized antennas A  301  and B  302 , which are different in size and frequency band characteristics, have hemispherical radiation patterns and are disposed such that their waves are opposite in direction. 
   Furthermore, to provide a diversity antenna apparatus using the above-described circularly polarized antennas, at least two antennas are required for each of two frequency bands, that is, a total of four or more independent antennas need to be mounted on the dual-band wireless communication unit (see, for example, Japanese Unexamined Patent Application Publication No. 2002-43994). 
   In a known circularly polarized antenna, as described above, a combination of two single-band antennas is required for providing a dual-band antenna apparatus, and further, a combination of four single-band antennas is required for providing a diversity antenna apparatus. This interferes with the reduction in size of the wireless communication unit. 
   Moreover, when coaxial cables connected to antennas are mounted on the wireless communication unit, the coaxial cables function as antennas and might emit and detect noises to and from the wireless communication unit and other peripheral devices. The noises caused by the coaxial cables seriously affect the dual-band wireless communication unit, which requires four coaxial cables. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide an antenna apparatus that can reduce the size of a wireless communication unit incorporating the apparatus by combining circularly polarized antennas for a plurality of frequency bands. Another object is to provide an antenna apparatus that can reduce noises by reducing the number and length of the coaxial cables. 
   To achieve the above-described objectives, the antenna apparatus of the present invention includes a first circularly polarized antenna, a second circularly polarized antenna, and a substrate having these two antennas on both the front and rear surfaces. The first circularly polarized antenna and the second circularly polarized antenna, which are oriented in the opposite direction, efficiently radiate high-frequency signals in a first frequency band and a second frequency band, respectively, in substantially hemispherical shapes. 
   In the antenna apparatus of the present invention, the first and second circularly polarized antennas for different frequency bands are disposed on the front and rear surfaces of the same substrate. Since two circularly polarized antennas are combined together, the size of the entire wireless communication unit can be reduced. 
   Moreover, the first and second circularly polarized antennas are connected, via wiring patterns on the substrate and via a common frequency synthesizer, to a common coaxial cable. The number and length of the coaxial cable are thus reduced, and noise reduction and ease of wiring can be achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a dual-band circularly polarized antenna apparatus according to a first embodiment of the present invention; 
       FIG. 2  is a side view of a dual-band circularly polarized antenna apparatus according to a second embodiment of the present invention; 
       FIG. 3  illustrates a known dual-band circularly polarized antenna apparatus; 
       FIG. 4  is a schematic diagram showing radiation characteristics of the antenna apparatuses in  FIGS. 1 and 2 ; 
       FIG. 5  is a sectional side view showing details of the dual-band circularly polarized antenna apparatus in  FIG. 1 ; 
       FIGS. 6A and 6B  are a front view and a rear view, respectively, of the dual-band circularly polarized antenna apparatus in  FIG. 1 ; 
       FIGS. 7A ,  7 B, and  7 C illustrate the structures ( FIGS. 7A and 7B ) and characteristics ( FIG. 7C ) of a frequency synthesizer (diplexer) incorporated in the dual-band circularly polarized antenna apparatus in  FIG. 1 ; 
       FIG. 8  is a perspective view showing an example of a wireless communication system incorporating the dual-band circularly polarized antenna apparatuses in  FIG. 1 ; 
       FIG. 9  is a block diagram showing circuitry of the wireless communication system in  FIG. 8 ; 
       FIG. 10  is a flowchart showing the operation of the wireless communication system in  FIG. 8 ; and 
       FIG. 11  is a flowchart showing the operation of the wireless communication system in  FIG. 8 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of an antenna apparatus according to the present invention will now be described. 
     FIG. 1  is a side view of an antenna apparatus according to a first embodiment of the present invention. 
   The antenna apparatus of this embodiment includes a circularly polarized antenna A  101  that efficiently radiates high-frequency signals in a first frequency band in a substantially hemispherical shape, a circularly polarized antenna B  102  that efficiently radiates high-frequency signals in a second frequency band in a substantially hemispherical shape, and a substrate  104  that is common to these two antennas. The circularly polarized antennas A  101  and B  102  are arranged back to back on the front and rear surfaces of the substrate  104 . 
   The circularly polarized antennas A  101  and B  102 , which are different in size and frequency band, have hemispherical radiation patterns and radiate radio waves in the opposite direction as indicated by arrows A and B in  FIG. 1 . 
   A flat antenna A  112  is disposed on a dielectric plate A  111  to form the circularly polarized antenna A  101 , which is disposed on the front surface of the substrate  104 . A wiring pattern  104 A, which is provided on the front surface of the substrate  104  and is connected to the flat antenna A  112 , then leads to the lower end of the substrate  104 , further to the rear surface via a through hole  113  of the substrate  104 , and is connected to a frequency synthesizer (diplexer)  103  via a wiring pattern  104 B on the rear surface. 
   Similarly, a flat antenna B  115  is disposed on a dielectric plate B  114  to form the circularly polarized antenna B  102 , which is disposed on the rear surface of the substrate  104 . A wiring pattern  104 B, which is provided on the rear surface of the substrate  104  and is connected to the flat antenna B  115 , then leads to the lower end of the substrate  104 , and is connected to the frequency synthesizer (diplexer)  103 . 
   High-frequency signals in the first frequency band and the second frequency band received at the circularly polarized antenna A  101  and the circularly polarized antenna B  102 , respectively, are output, via the frequency synthesizer  103  and a coaxial cable  105 , to an antenna connector of a wireless communication apparatus, which is described below. High-frequency signals transmitted from the wireless communication apparatus pass through the coaxial cable  105  and the frequency synthesizer  103  and are output to the circularly polarized antenna A  101  or B  102  depending on whether the signals are in the first or second frequency band. 
   As described above, the use of the frequency synthesizer  103  reduces the number of coaxial cables  105  connected to the wireless communication apparatus, from two to one. 
   In the first embodiment, the frequency synthesizer  103  synthesizes the high-frequency signals from the circularly polarized antenna A  101  or B  102  to be provided to the coaxial cable  105 , or separates the signals from the coaxial cable  105  to be provided to the circularly polarized antenna A  101  or B  102 . However, the frequency synthesizer may be removed and each of the two circularly polarized antennas may be provided with a coaxial cable, instead. 
     FIG. 2  is a side view of an antenna apparatus according to a second embodiment of the present invention. In this embodiment, each of two circularly polarized antennas is provided with a coaxial cable. 
   Similarly to the first embodiment shown in  FIG. 1 , a circularly polarized antenna A  201  for a first frequency band and a circularly polarized antenna B  202  for a second frequency band are disposed back to back on the front and rear surfaces of a common substrate  203 . In this embodiment, however, each of the circularly polarized antenna A  201  and the circularly polarized antenna B  202  is provided with a coaxial cable  204  and a coaxial cable  205 , respectively. The detailed descriptions of each component will be omitted here, as they are similar to those in the first embodiment ( FIG. 1 ) described above. 
   Although two coaxial cables are required in this embodiment, the removal of a frequency synthesizer simplifies the structure of the antenna apparatus. Furthermore, similarly to the first embodiment, the size of the wireless communication unit is reduced since the circularly polarized antennas A  201  and B  202  are disposed on the same substrate  203 . 
   The details of the antenna apparatuses according to the above-described embodiments will now be described. 
     FIG. 4  illustrates radiation characteristics of each circularly polarized antenna of the antenna apparatus in  FIG. 1 . 
   As shown, the circularly polarized antennas A  101  and B  102  have hemispherical radiation characteristics  401  and  402  with different frequency bands. Radiation directions A and B of the circularly polarized antennas A  101  and B  102 , respectively, are opposite one another. 
     FIG. 5  is a cross-sectional view showing the arrangement of the circularly polarized antennas and the substrate that are included in the antenna apparatus illustrated in  FIG. 1 . 
     FIGS. 6A and 6B  are external views of the antenna apparatus in  FIG. 5 .  FIG. 6A  shows the first circularly polarized antenna A side, and  FIG. 6B  shows the second circularly polarized antenna B side. 
   Referring to  FIG. 5 , a four-layer substrate  518  is used here. The substrate  518  includes two ground layers A  516  and B  517  that are interposed between conductive pattern layers A  514  and B  517 , each having a wiring pattern. 
   A frequency synthesizer (diplexer)  511  leading to a coaxial cable  512  is connected to a 90-degree phase shifter A  509  and a 90-degree phase shifter B  510  that are adjacent to a flat antenna A  501  and a flat antenna B  502 , respectively. 
   The flat antenna A  501  disposed on a dielectric plate A  503  is fed from feeding points A  505  and B  506 , and radiates circularly polarized waves. 
   Similarly, the flat antenna B  502  is fed from feeding points C  507  and D  508 , and radiates circularly polarized waves. 
   The 90-degree phase shifter A  509  is connected via a through hole  513  to the frequency synthesizer (diplexer)  511 . 
   In each circularly polarized antenna of the first embodiment, the square shape of the flat antenna and a 90-degree phase shift between the two feeding points, which is provided by the 90-degree phase shifter, allow the circularly polarized waves to be radiated. 
   Further, the size of the flat antenna can be reduced, since wavelengths are reduced in the dielectric plate by disposing the flat antenna on the dielectric plate. 
   The frequency synthesizer (diplexer)  511  is provided on the side of the second frequency band (the second circularly polarized antenna B) that is higher than the first frequency band (the first circularly polarized antenna A). The reasons include high space efficiency and less degradation of signals, that is, when transmitting signals via a though hole, low frequency signals suffer less degradation compared to that suffered by high frequency signals. 
     FIGS. 7A ,  7 B, and  7 C illustrate the structures and characteristics of the frequency synthesizer (diplexer) according to the first embodiment. 
   As shown in  FIGS. 7A and 7B , the frequency synthesizer  103  is formed of a low pass filter (LPF)  701  for the first frequency band and a high pass filter (HPF)  702  for the second frequency band. A terminal A of the LPF  701  and a terminal B of the HPF  702  are connected to the circularly polarized antenna A  101  and the circularly polarized antenna B  102 , respectively. A terminal C that is common to both the LPF  701  and the HPF  702  is connected to the coaxial cable  105 . 
   As shown in  FIG. 7C , different frequency signals, that is, the first frequency signals and the second frequency signals are transmitted. 
     FIG. 8  shows an example of wireless communication systems incorporating the dual-band circularly polarized antenna apparatus according to the first embodiment.  FIG. 9  is a block diagram showing the inner structure of the wireless communication system shown in  FIG. 8 . 
   Referring to  FIG. 8 , data of a source unit  801  is transmitted from a base unit  802  to a portable unit  803  by wireless transmission. The data of the source unit  801  is thus viewed on a liquid crystal display  808  of the portable unit  803 . 
   Since one circularly polarized antenna has a hemispherical radiation pattern, two circularly polarized antennas are required for implementing antenna diversity to spherically transmit signals in wireless transmission. 
   Therefore, each of the portable unit  803  and the base unit  802  is provided with the two dual-band antenna apparatuses of this embodiment, the two apparatuses, each having two circularly polarized antennas for two different frequencies and being disposed back to back. 
   In the portable unit  803 , two dual-band circularly polarized antenna apparatuses  804  and  805  are disposed in the opposite direction to form a spherical radiation pattern of each of two frequency bands. Similarly, in the base unit  802 , two dual-band circularly polarized antenna apparatuses  806  and  807  are disposed in the opposite direction to form a spherical radiation pattern of each of two frequency bands. 
   Radiation patterns of two different frequency bands that are opposite in direction thus form a spherical radiation pattern. 
   Referring to  FIG. 9 , the source unit  801  is a supply source for supplying various types of image data. Examples of the source unit  801  include equipment for handling pictures of TV., video, and digital versatile disc (DVD), and networks such as the Internet. The source unit  801  supplies the image data to the base unit  802  via wires. 
   The portable unit  803  has a selector switch  914  and a wireless communication apparatus  915 , and the base unit  802  similarly has a selector switch  918  and a wireless communication apparatus  919 . The selector switches  914  and  918  select the most suitable antennas depending on the locations of the portable unit  803  and the base unit  802 , and the radio wave conditions such as the presence of interference. The wireless communication apparatuses  915  and  919  are capable of sending and receiving data in the first and second frequency bands via wireless transmission. Each of the wireless communication apparatuses  915  and  919  is connected to two dual-band circularly polarized antenna apparatuses. That is, the wireless apparatus  915  is connected to the dual-band circularly polarized antenna apparatuses  804  and  805 , and the wireless apparatus  919  is connected to the dual-band circularly polarized antenna apparatuses  806  and  807 . 
   Although the circularly polarized antenna apparatus shown in  FIG. 1  is adopted in the wireless communication system in  FIG. 9 , it may be replaced with the antenna apparatus shown in  FIG. 2 . 
     FIGS. 10 and 11  are flowcharts showing the steps of selecting antennas in the wireless communication systems illustrated in  FIGS. 8 and 9 . The following descriptions refer to the positions of antennas A to H shown in  FIG. 9 . 
   First, the operation starts ( 1001 ) and communication in the second frequency band is prepared ( 1002 ). Here, the second frequency band is selected as the initial setting because of its higher throughput and less interference compared to that in the first frequency band. 
   As the initial settings of antennas, moreover, the selector switch  914  of the portable unit  803  selects the rear antenna (antenna B), and the selector switch  918  of the base unit  802  selects the front antenna (antenna G) ( 1003 ). 
   The next step determines whether communication between the portable unit  803  and the base unit  802  is possible ( 1004 ), and when possible, determines whether the signal level is high enough ( 1005 ). Further, when the signal level is high enough, the antennas are fixed and communication starts ( 1006 ). 
   When the signal level is not high enough, other combinations of antennas in the portable unit  803  and the base unit  802  are examined. When an antenna combination with a high signal level is found, the antennas are fixed and communication starts ( 1010 – 1015 ). 
   When no antenna combination with a high signal level is found, communication between the portable unit  803  and the base unit  802  is assumed to be impossible due to the distance therebetween. Then communication in the first frequency band, where attenuation of radio waves tends not to occur, is prepared ( 1016 ). 
   Referring back to  1004 , when communication is impossible in the second frequency band, presence of interference in the frequency channel is examined ( 1007 ). 
   When no interference occurs in the frequency channel, other combinations of antennas in the portable unit  803  and the base unit  802  are examined ( 1010 – 1015 ). 
   When interference occurs in the frequency channel, presence of interference in all the frequency channels in the second frequency band is examined ( 1008 ). 
   When interference occurs in all the frequency channels, communication in the first frequency band is prepared ( 1016 ). As the initial setting of the antennas in the first frequency band, the selector switch  914  of the portable unit  803  selects the rear antenna (antenna D), and the selector switch  918  of the base unit  802  selects the front antenna (antenna E) ( 1017 ). After the procedure similar to that in the second frequency band, antennas are fixed and communication starts ( 1018 – 1020 ,  1024 – 1030 ). 
   When interference occurs in all the frequency channels in the first frequency band ( 1022 ), an “out of range” message is displayed on the liquid crystal display ( 1023 ) and communication in the second frequency band is prepared ( 1002 ). 
   When no interference occurs in the channel in the first frequency band ( 1021 ), and when no combination of antennas in the portable unit  803  and the base unit  802  with a high signal level is found, the portable unit  803  and the base unit  802  are assumed to be distant, an “out of range” message is displayed on the liquid crystal display ( 1031 ), and communication in the second frequency band is prepared ( 1002 ). 
   In the above embodiments, the antenna apparatus of the present invention is applied to a diversity-antenna wireless communication system. The present invention is not limited to the embodiments shown, but is widely applied to dual-band antenna apparatuses in various types of wireless communication systems. 
   While effective, application of the circularly polarized antenna apparatus of the present invention is not limited to 2.4 and 5.2 GHz wireless LAN systems.