Patent Publication Number: US-8120542-B2

Title: Notch antenna and wireless device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to notch antennas having multiple resonant frequency bands and wireless devices using the same. 
     2. Description of the Related Art 
     With the current trend toward multifunctional mobile wireless terminals, various functions, such as GPS (Global Positioning System) and Bluetooth™ (a short range radio communication system), are implemented in mobile wireless terminals. In addition, mobile wireless terminals each incorporating a plurality of wireless communication systems using different frequency bands, or what is called multiband mobile wireless terminals, have been developed and have appeared on the market. 
     From the viewpoint of downsizing and weight reduction of wireless terminals each employing a plurality of wireless communication systems, it is preferable to share a built-in antenna among the plurality of wireless communication systems. 
     Japanese Patent No. 3916068 discloses a notch antenna provided by forming a notch (or a slit), which is a slim groove having an open end, in a ground plate (ground conductor). The notch antenna is relatively simple in structure and therefore suitable as an antenna device built in a mobile or small wireless device. 
     The resonant frequency of a notch antenna is determined by the length of the slit, and the length is usually set to approximately 0.2 times the wavelength of a working frequency. For working frequency bands used by PDC (Personal Digital Cellular) systems (approximately 800 MHz) or GSM (Global System for Mobile communications) systems (approximately 800 to 900 MHz) in the past, the slit is usually set as long as 70 to 80 mm, and it is not easy to incorporate such a long-slit notch antenna in mobile phone terminals supporting these systems. On the other hand, the notch antennas can be easily adapted to third generation mobile phone systems (e.g., W-CDMA system operating at approximately 2 GHz), GPS-installed mobile phone terminals (approximately 1.575 MHz) and systems operating in higher frequency bands, such as Bluetooth (2.5 GHz). 
     Proposed in Japanese Patent No. 3844717 and Japanese Unexamined Patent Application Publication No. 2004-274445 are notch antennas in which a plurality of slits are formed in a ground plate to obtain multiband operation. 
     Japanese Unexamined Patent Application Publication No. 2004-32303 proposes a technique of producing resonance in a plurality of frequency bands by providing a resonator on a short-circuited end (closed end) side of a slit of a notch antenna to make the slit behave as if it is short at high frequencies. 
     Moreover, Japanese Unexamined Patent Application Publication No. 2004-336328 proposes a technique of obtaining broadband characteristics (double resonance characteristics) by inserting a parallel resonant circuit in the vicinity of a closed end of a slit in parallel, the parallel resonant circuit including a capacitor arranged in parallel with inductance occurring between the feed point and short-circuited end. 
     SUMMARY OF THE INVENTION 
     The formation of a plurality of notches, as disclosed in Japanese Patent No. 3844717 and Japanese Unexamined Patent Application Publication No. 2004-274445, prevents miniaturization of the notch antenna. 
     The related art, disclosed in Japanese Unexamined Patent Application Publication No. 2004-32303, makes the electrical slit-length short in high frequencies; however, the shorter slit-length lowers the efficiency of the antenna. The related art, therefore, may not make full use of the physical size of the entire notch, in other words, it may not obtain antenna efficiency proportional to the size of the antenna. 
     Since the Q factor inherent in the parallel resonant circuit determines the interval between the two resonant frequencies and the inductance value is not adjustable in the related art disclosed in Japanese Unexamined Patent Application Publication No. 2004-336328, the bandwidths of the resonant frequencies become narrower with the increase in the interval between the frequencies. To prevent this, a limit is necessarily imposed on the interval between the two frequencies. In addition, the capacitor placed in the vicinity of the closed end of the slit also increases the match loss caused by the resistance component of the capacitor, thereby impairing the antenna efficiency. 
     Against these backdrops, an embodiment of the present invention provides a relatively simple-structured one-slit notch antenna capable of producing resonance in a plurality of frequency bands, and a wireless device using the notch antenna. 
     The notch antenna according to the embodiment of the present invention includes a ground conductor having a slit and a reactance circuit containing a capacitive reactance element and an inductive reactance element, the reactance circuit being placed at an open end of the slit so as to bridge the slit and being connected to the ground conductor. The slit has a closed end to which power is supplied. The capacitance of the capacitive reactance element and the inductance of the inductive reactance element are set so that the reactance circuit has a capacitance desired to obtain a first antenna resonance point at a first frequency and a capacitance desired to obtain a second antenna resonance point at a second frequency. 
     Since the reactance circuit presenting capacitive reactance in at least two frequency bands is placed at the open end of the slit so as to bridge the slit and is connected to the ground conductor, the notch antenna can operate as a capacity loaded antenna in the plurality of bands, and obtain multi-resonance characteristics. 
     The wireless device according to the embodiment of the present invention includes a notch antenna and a feeding unit that supplies power to the notch antenna. The notch antenna includes a ground conductor having a slit and a reactance circuit containing a capacitive reactance element and an inductive reactance element, the reactance circuit being placed at an open end of the slit so as to bridge the slit and being connected to the ground conductor. The slit has a closed end to which power is supplied. The capacitance of the capacitive reactance element and the inductance of the inductive reactance element are set so that the reactance circuit has a capacitance desired to obtain a first antenna resonance point at a first frequency and a capacitance desired to obtain a second antenna resonance point at a second frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are illustrations of a notch antenna according to an embodiment of the present invention,  FIG. 1A  showing the configuration of the notch antenna and  FIG. 1B  being a graph showing the resonance characteristics; 
         FIGS. 2A ,  2 B and  2 C are illustrations showing a schematic configuration of a notch antenna according to another embodiment of the present invention; 
         FIGS. 3A and 3B  are graphs showing the relationship between frequency and return loss and the relationship between frequency and antenna efficiency of the notch antenna shown in  FIGS. 2A to 2C , respectively, those results are obtained through simulation; 
         FIG. 4  illustrates a circuit diagram of another example of the reactance circuit shown in  FIGS. 2A to 2C ; 
         FIGS. 5A ,  5 B and  5 C are illustrations of a modification of the notch antenna shown in  FIGS. 2A to 2C ; 
         FIG. 6  illustrates an example of drive circuits for dynamically changing the capacitance of a variable capacitor VC; 
         FIG. 7  is a graph showing frequency responses, obtained by simulation, of notch antennas with variable capacitors VC whose capacitances are different; 
         FIGS. 8A ,  8 B and  8 C illustrate yet another embodiment of the present invention; 
         FIGS. 9A and 9B  illustrate an exemplary configuration of the second modification according to the embodiment of the present invention; 
         FIG. 10  illustrates the configuration of a notch antenna which is a combination of the techniques featured in Japanese Unexamined Patent Application Publication No. 2004-336328 and the techniques featured in the embodiment of the present invention; 
         FIG. 11  depicts graphs each showing impedance characteristics and frequency response of the notch antenna shown in  FIG. 10 ; and 
         FIG. 12  is a block diagram showing a schematic hardware configuration of a mobile phone terminal to which the notch antenna according to the embodiment of the present invention can be applied. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, a preferred embodiment of the present invention will be described in detail below. 
       FIGS. 1A and 1B  are illustrations of a notch antenna according to an embodiment of the present invention.  FIG. 1A  illustrates the configuration of the notch antenna, while  FIG. 1B  is a graph showing the resonance characteristics of the notch antenna. The horizontal axis of the graph in  FIG. 1B  represents frequency and the vertical axis represents return loss (reflection coefficient: S 11  of S-parameters). 
     As shown in  FIG. 1A , a notch antenna  10  is made up of a ground plate (ground conductor)  11  with a notch or slit  12  formed in the ground plate  11 , the slit  12  being a slim groove having a short-circuited end  15 , and a feed point  16  placed in the vicinity of the short-circuited end  15 . The resonant frequency of the notch antenna  10  is determined by the length of the slit  12  (hereinafter referred to as “slit length”). The characteristic impedance of the antenna is determined by the distance B from the short-circuited end  15  of the slit  12  to the feed point  16  (the characteristic impedance is 50Ω in this description). 
     In addition to the above components the notch antenna is provided with a capacitor  14  bridging the slit at an open end  13  as shown in  FIG. 1A . Replacement of the capacitor with a capacitor having a different capacitance enables adjustment of the resonant frequency of the notch antenna without changing its physical shape (especially, the slit length) as shown in  FIG. 1B . 
     In the example in  FIGS. 1A and 1B , the ground plate  11  has a size of 80 mm×40 mm, a thickness of 1 mm, a slit length A of 15 mm, and a slit width of 1 mm, and the distance B is 4 mm; however, these concrete values are merely examples and the present invention is not limited to these specific values. 
       FIGS. 2A to 2C  illustrate schematic configurations of a notch antenna  100  according to another embodiment of the present invention. The feed point  16  is placed on the side of the closed end of the slit  12  formed in the ground conductor  11 . As shown in  FIGS. 2A and 2B , the capacitor  14  placed at the open end  13  of the slit is replaced with a reactance circuit  17 . This reactance circuit  17  is an LC resonant circuit, including an inductor (inductive reactance element) L and capacitors (capacitive reactance elements) C, whose capacitance varies in accordance with frequencies (frequency L and frequency H). For these elements, small components, such as chip parts (surface-mount devices) can be used. Although the elements are shown as being leaded components in  FIG. 2B , the form of the parts making up each element is not limited in the present invention. The capacitance of each capacitive reactance element and the inductance of the inductive reactance element are set so that the reactance circuit  17  has a capacitance desired to obtain a first antenna resonance point at a first frequency and a capacitance desired to obtain a second antenna resonance point at a second frequency. 
     In the embodiment, the slit length A is 21 mm, slit width is 1 mm, impedance matching is 50Ω, and the distance B is 4 mm. The configuration allows the notch antenna to produce resonance in a plurality of frequency bands without replacing the element placed at the open end  13  of the slit. Since electric field strength changes most at the open end  13  of the slit, it is preferable to arrange the reactance circuit  17  in the vicinity of the open end  13  of the slit. In this embodiment, the reactance circuit  17  is placed approximately 2 to 3 mm inward from the open end  13 . 
       FIG. 2C  illustrates an exemplary reactance circuit  17 . This reactance circuit  17  includes a series circuit with an inductor L 1  and a first capacitor C 1  connected to each other in series and a second capacitor C 2  connected to the series circuit in parallel. The reactance circuit  17 , which has capacitive reactance, serves as a dual-band matching circuit in two frequency bands (e.g., 3 pF in 800 MHz band and 1.5 pF in 2 GHz band). 
       FIGS. 3A and 3B  are graphs showing the relationship between frequency and return loss and between frequency and antenna efficiency of the notch antenna shown in  FIGS. 2A to 2C , respectively, the data being obtained by simulation. The simulation is an electromagnetic field simulation using an FDTD (Finite Difference Time Domain) method. These graphs show that the notch antenna produces resonance in two frequency bands and offers high antenna efficiencies in each band. In both bands, two different current modes are shown, proving that the resonance is not a single resonance spread across a wide band. Note that the graph in  FIG. 3B  shows resonance around 950 MHz and 2.2 GHz, which do not exactly agree with the frequency bands, “3 pF in the 800 MHz band and 1.5 pF in the 2 GHz band”, shown in  FIGS. 2A to 2C . This is because the graphs in  FIGS. 3A and 3B  show computing results based on reactance including actual resistance components. 
     The configuration of the reactance circuit  17  is not limited to the one shown in  FIG. 2C . For example, the reactance circuit  17   a  in  FIG. 4  includes a parallel circuit with a first inductor L 1  and a first capacitor C 1  connected to each other in parallel and a second capacitor C 2  connected to the parallel circuit in series. 
       FIGS. 5A to 5C  show a modification of the notch antenna shown in  FIGS. 2A to 2C . Like components are denoted by like numerals of the embodiment shown in  FIGS. 2A to 2C  and will not be further explained. The modification in  FIGS. 5A to 5C  is made by replacing the capacitor C 2  in the reactance circuit  17  of  FIG. 2C  with a variable capacitor VC, which is a capacitive reactance element whose capacitance is controllable by control signals. Controlling the capacitance in each frequency band can adaptively modulate the resonant frequency for desired bands. 
       FIG. 6  shows an exemplary drive circuit that can dynamically change the capacitance of the variable capacitor VC. In this example, a digital signal output from a digital circuit (D)  61 , such as a processor, is converted by a digital-analog (D/A) converter  62  into an analog voltage, and then the analog voltage is applied through a bias circuit  63  to the variable capacitor VC. A capacitor  66  is interposed between the voltage-applied point and ground in order to block direct current. The bias circuit  63  includes an inductor  64  (for blocking alternating current) connected to the digital-analog converter  62  in series and a resistor  65  connected between the output of the digital-analog converter  62  and ground. In practical implementation to mobile phone terminals, the amount of bias voltage for the variable capacitor VC is preferably set to change within a range approximately from 0 to 3.0 V. 
     With this drive circuit, the capacitance of the variable capacitor VC can be dynamically and variably controlled. 
       FIG. 7  is a graph showing the frequency responses of notch antennas with variable capacitors VC whose capacitance is different. The frequency responses are obtained by simulations. Variations of the bias voltage for the variable capacitors VC from 0 to 3 V change the capacitance from 1.2 pF to 5 pF stepwise (10 steps in the description). With the notch antenna according to the modifications, it is found that the resonant frequency can be modulated from 700 MHz to 900 MHz in a low band and from 1.5 GHz to 2.2 GHz in a high band. 
     This configuration can realize not only a diversity antenna available in 850 M/1.9 G/2.1 G, but also a multiband antenna available in the bands for GPS, Bluetooth and other systems through the use of a single antenna device. 
     Furthermore, with this configuration, the capacitance of the variable capacitor VC and also the resonant frequency of the notch antenna can be adjusted to adapt to certain cases. The certain cases include, for example, a case where the default resonant frequency is shifted in accordance with specific communication systems, a case of a frequency drift caused by the human body, a case of a frequency drift in a flip-style terminal caused by a user opening the terminal, and some other cases. To deal with these cases, a predetermined sensor is designed to detect whether the user interferes with the antenna part. 
       FIGS. 8A to 8C  are illustrations showing yet another embodiment of the present invention. An existing technique is made by combining a transmission line, or stripline  18 , having a characteristic impedance (50Ω in this example) matching with the characteristic impedance of the notch antenna and a capacitor  19  to move a feed point, thereby allowing the notch antenna to operate in a wider band. The notch antenna in  FIG. 8A  has components that are substantially the same as those of the notch antenna in  FIG. 1A . Although the notch antennas in  FIGS. 8A and 8C  have a slit bent at some midpoint, they are in principle the same as a notch antenna having a straight slit. In other words, the slit is bendable to meet the layout constraints and requirements. In the notch antenna in  FIG. 8C , an end of a stripline  18  having a predetermined length (approximately 3 mm in this example) is connected to the slit in the vicinity of the closed end of the slit, the distance from the closed end to the feed point being shorter than the above-described predetermined distance (approximately 1.5 mm from the closed end in this example). The other end of the stripline  18  is grounded via a capacitor  19  and is supplied with power. 
     The notch antenna in  FIG. 8A  exhibits impedance characteristics and frequency responses represented by a VSWR (Voltage Standing Wave Ratio) as shown in  FIG. 8A . The VSWR is an index representing a ratio of reflected-to-input waves, and although being expressed in a different unit, it can be mutually converted into the above-described S 11 . The graph in  FIG. 8A  indicates that the antenna produces resonance in a single frequency band. 
     In the notch antenna with the configuration in  FIG. 8A , at first, the feed point is displaced. Specifically, in this example, the distance B is changed from 3.5 mm to 1.5 mm. This change shifts the locus of the impedance of the antenna toward +j as shown in the Smith chart in  FIG. 8B , and therefore the antenna impedance is no longer matched to 50Ω. From this configuration, the feed point is further moved along with a stripline  18  and the end of the stripline  18  adjacent to the feed point is grounded via a capacitor  19 . As shown in  FIG. 8C , the locus created on the Smith chart for representing impedance characteristics has a large loop so as to surround the center of the Smith chart. This allows the antenna to obtain a matched impedance of 50Ω again. As can be seen from the graph of frequency response, the single resonant frequency band is widened. 
     On the contrary to the embodiment in  FIGS. 8A and 8B , the notch antenna  100   a  according to the second modification has a reactance circuit  17   b , as shown in  FIG. 9A , instead of the capacitor  14 , and can make the resonant frequency bands wider as well as variably controlling the resonant frequencies. In short, the second modification can produce resonance in a plurality of broad frequency bands. The technique in  FIGS. 8A to 8C  can of course use not only the reactance circuit  17   b  but also the reactance circuit  17  or  17   a . The antenna with the reactance circuit  17  or  17   a  does not control the resonant frequency, but can make the resonant frequency bands wider. 
     Next, the third modification of the embodiment of the present invention will be described. 
       FIG. 10  is an illustration showing the configuration of a notch antenna formed using the techniques featured in the above embodiment and the techniques featured in Japanese Unexamined Patent Application Publication No. 2004-336328. Specifically, a parallel resonant circuit is placed in the vicinity of the closed end  15  of the slit in parallel, the parallel resonant circuit including a capacitor C 3  arranged in parallel with inductance occurring around the slit between the feed point  16  and short-circuited end  15 . This inductance is associated with the actual circuit, but is not caused by an external device. 
     The notch antenna  100   b  in  FIG. 10  exhibits impedance characteristics and frequency responses as shown in  FIG. 11 . In the Smith chart in  FIG. 11 , an impedance locus revolves three times around the center of the plot in accordance with changes in frequency. In addition, the graph for the frequency response shows clearly that the antenna produces resonance in the 800 MHz band and 2 GHz band, and furthermore that the resonance in the 2 GHz band is made wide (double resonance). According to the third modification, the antenna obtains frequency response with three resonances in total. 
     It is also possible to replace the reactance circuit  17  in  FIG. 10  with a reactance circuit  17   a  or  17   b.    
       FIG. 12  is a block diagram showing a schematic hardware configuration of a mobile phone terminal in which the notch antenna according to the embodiments of the present invention can be implemented. 
     A mobile phone terminal  101  includes an antenna  102 , a radio-frequency circuit, or an RF circuit  103 , a baseband signal circuit  104 , a CODEC  105 , a memory  106 , a display  107 , a key entry unit  108 , a speaker  109 , a microphone  110 , a GPS circuit  112 , a Bluetooth (BT) circuit  114 , and a controller (CPU)  111  controlling these components. The RF circuit  103 , GPS circuit  112 , BT circuit  114  are provided with antennas  102 ,  113  and  115 , respectively. In this embodiment, at least two of these antennas  102 ,  113 ,  115  can be any of the above-described notch antennas. 
     The CODEC  105  encodes a voice signal input through the microphone  110  to transmit it to the baseband signal circuit  104 , while decoding a signal received from the baseband signal circuit  104  into a voice signal to send it to the speaker  109 . 
     The baseband signal circuit  104  modulates the signal received from the CODEC  105  into a baseband signal to transmit it to the RF circuit  103 , while retrieving a signal, which is processable by the CODEC  105 , from the baseband signals decoded by the RF circuit  103 . 
     The RF circuit  103  appropriately modulates the baseband signal received from the baseband signal circuit  104  into an RF signal (radio frequency signal) to supply it to the antenna  102 , while decoding the RF signal received through the antenna  102  into a baseband signal to send it to the baseband signal circuit  104 . 
     The memory  106  may be, for example, a ROM (Read Only Memory), a RAM, a flash memory or the like, and stores programs to be executed by the controller  111  and various setting data. 
     The display  107  may be, for example, a liquid crystal display for displaying various types of information. 
     The key entry unit  108  includes input means, such as a numeric-key pad, used by a user to input instructions and information into the controller  111 . 
     The speaker  109  is used to output sound corresponding to the voice signals sent from the CODEC  105 . On the other hand, the microphone  110  captures sound, converts it into a voice signal and then sends it to the CODEC  105 . 
     The notch antenna according to the embodiments of the present invention can produce a plurality of resonance points with a single slit, while maintaining the existing advantages including small size and thinness. The components to be added to the antenna are only passive components, which can reduce the cost. In addition, the notch antenna produces resonance in a plurality of frequency bands by sharing the physical length of the single slit, thereby achieving high antenna efficiency. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2008-228002 filed in the Japan Patent Office on Sep. 5, 2008, the entire content of which is hereby incorporated by reference. 
     It should be understood that in addition to the above-described modifications, various modifications and alterations can be made to the preferred embodiments of the present invention. For instance, the present invention includes not only the aforementioned embodiments and the plurality of modifications thereof but also any possible combination of the modifications.