Abstract:
An antenna device comprises an antenna section and a filter section which are formed in an integrated manner in a dielectric substrate, wherein the antenna section and the filter section are coupled to one another via a capacitance. Further, 0.3×Lr≦Lt≦1.2×Lr is satisfied provided that an antenna length of the antenna section is Lt, and an antenna length measured for a single antenna is Lr. Accordingly, it is possible to realize a small size of the antenna device while avoiding the decrease in gain and the disadvantage of narrow band.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna device comprising an antenna pattern based on an electrode film formed on a dielectric substrate. 
     2. Description of the Related Art 
     In order to realize a small size of the antenna device and realize a small size of the communication apparatus, a large number of devices have been hitherto suggested, in which, for example, an antenna pattern based on an electrode film is formed on the surface of a dielectric substrate (see, for example, Japanese Laid-Open Patent Publication Nos. 10-41722, 9-162633, and 10-32413). 
     Most of the antenna devices can be used by being directly mounted on a circuit board. This fact is an advantage of such antenna devices. 
     However, the antenna device, which includes the antenna pattern based on the electrode film formed on the surface of the dielectric substrate, involves the following inconvenience. That is, usually, when the device is made compact, then the gain is decreased, and the band is consequently narrowed. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the points as described above, an object of which is to provide an antenna device which makes it possible to realize a small size while avoiding the decrease in gain and the disadvantage of narrow band. 
     According to the present invention, there is provided an antenna device comprising an antenna section and a filter section which are formed integrally in a dielectric substrate, wherein the antenna section and the filter section are coupled to one another via a capacitance. 
     When the antenna section and the filter section are integrated into one unit with the capacitance intervening therebetween, the antenna length is theoretically determined in conformity with the center frequency of the filter section. 
     The size of the antenna section is dominant as compared with the size of the filter section, in the antenna device in which the antenna section and the filter section are integrated into one unit. Therefore, it is clear from the form or shape thereof that the size of the antenna device greatly depends on the antenna length (wavelength). 
     Further, it is known for the antenna that the small size causes the decrease in gain and the disadvantage of narrow band. 
     However, according to the present invention, it has been revealed that the input impedance of the antenna device is not changed even if the antenna length is changed when the antenna device is produced by integrating the antenna section and the filter section into one unit with the capacitance intervening therebetween. 
     Accordingly, for example, when the antenna length of the antenna section is shortened, it is possible to suppress the decrease in gain to be minimum. The advantage that the input impedance of the antenna device is not changed even when the antenna length is changed results in the successful improvement in yield by adjusting the antenna length during the production step. 
     It is also preferable for the device constructed as described above that 0.3×Lr≦Lt≦1.2×Lr is satisfied provided that an antenna length of the antenna section is Lt, and an antenna length measured for a single antenna is Lr. 
     The reason why the antenna length Lt of the antenna section includes the portion in the range in which it is longer than the antenna length Lr of the single antenna is as follows. That is, although the effect of realization of the compact size is reduced, another effect is obtained such that the margin for mass production is increased when the device is designed, because the change of gain is small even when the antenna length is changed. 
     The antenna length Lt of the antenna section preferably satisfies 0.6×Lr≦Lt≦1.2×Lr, and more preferably 0.75×Lr≦Lt≦Lr. 
     An antenna for constructing the antenna section may be a monopole antenna, or it may be an antenna having a meander line configuration. Alternatively, the antenna may be an antenna having a helical configuration. 
     It is also preferable that a length of a resonator disposed on an input side of the filter section is different from a length of a resonator disposed on an output side. 
     Accordingly, it is possible to counteract the difference in resonance frequency between the respective resonators, which would be otherwise caused by any mismatch between respective impedances on the antenna side and the external circuit side of the filter section. Thus, it is possible to obtain the filter section which has good attenuation characteristics. This results in the high quality of the antenna device. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view illustrating an antenna device according to an embodiment of the present invention; 
     FIG. 2 shows an exploded perspective view illustrating the antenna device according to the embodiment of the present invention; 
     FIG. 3 shows an equivalent circuit diagram illustrating the antenna device according to the embodiment of the present invention; 
     FIG. 4 illustrates a method for measuring the frequency characteristic of a single antenna; 
     FIG. 5 shows a representative frequency characteristic of a single antenna; 
     FIG. 6 shows a characteristic curve illustrating the change of the center frequency depending on the difference in antenna length of the single antenna; 
     FIG. 7 shows characteristic curves illustrating the change of the antenna gain obtained by varying the antenna length in the antenna device according to the embodiment of the present invention; 
     FIG. 8 shows a characteristic curve illustrating the relationship between the antenna gain and the antenna length in the pass band (2400 to 2500 MHz) of a filter section of the antenna device according to the embodiment of the present invention; 
     FIG. 9 shows a perspective view illustrating an antenna device according to a first modified embodiment; 
     FIG. 10 shows a perspective view illustrating an antenna device according to a second modified embodiment; 
     FIG. 11 shows an exploded perspective view illustrating an antenna device according to a third modified embodiment; 
     FIG. 12 shows an equivalent circuit diagram illustrating the antenna device according to the third modified embodiment; 
     FIG. 13 shows an exploded perspective view illustrating an antenna device according to a second embodiment; and 
     FIG. 14A illustrates an impedance as viewed from an arrow A concerning the equivalent circuit shown in FIG. 3, and FIG. 14B illustrates an impedance as viewed from an arrow B concerning the equivalent circuit shown in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Illustrative embodiments of the antenna device according to the present invention will be explained below with reference to FIGS. 1 to  14 B. 
     As shown in FIGS. 1 and 2, an antenna device  10 A according to a first embodiment is composed of a dielectric substrate  12  comprising a plurality of stacked and sintered plate-shaped dielectric layers, which is formed with, in an integrated manner, a filter section  18  including an input/output electrode  14  disposed on the circuit side and an input/output electrode  16  disposed on the antenna side (see FIG.  2 ), and an antenna section  20  connected via the capacitance to the input/output electrode  16  disposed on the antenna side of the filter section  18 . In the following description, the input/output electrode  14 , which is disposed on the circuit side, is referred to as “first input/output electrode  14 ”, and the input/output electrode  16 , which is disposed on the antenna side, is referred to as “second input/output electrode  16 ”. 
     The filter section  18  comprises two one-end-open type ¼ wavelength resonator elements  22   a ,  22   b  which are formed in parallel to one another respectively. The antenna section  20  has an antenna  24  which is composed of an electrode film formed to have a meander line configuration on the upper surface of the dielectric substrate  12 . 
     As shown in FIGS. 1 and 2, the antenna device  10 A according to the first embodiment is formed with an input/output terminal  26  which is connected to the first input/output electrode  14  of the filter section  18 . Ground electrodes  28  are formed at portions corresponding to the filter section  18 , on the right side surface and the left side surface of the dielectric substrate  12  respectively. 
     Specifically, as shown in FIG. 2, the dielectric substrate  12  comprises first to tenth dielectric layers S 1  to S 10  which are stacked and superimposed in this order from the top. Each of the first to tenth dielectric layers S 1  to S 10  is composed of one layer or a plurality of layers. 
     The antenna section  20  and the filter section  18  are formed in regions which are separated from each other as viewed in a plan view. The antenna section  20  is formed on the upper surface of the first dielectric layer S 1 . The filter section  18  is formed over a range from the third dielectric layer S 3  to the tenth dielectric layer S 10 . 
     As shown in FIG. 2, the antenna device  10 A according to the first embodiment comprises two resonator elements (first and second resonator elements  22   a ,  22   b ) which are formed in parallel to one another on the first principal surface of the seventh dielectric layer S 7 . Respective first ends of the resonator elements  22   a ,  22   b  are open, and respective second ends thereof form the short circuit with the ground electrode  28 . 
     The components, which are formed on the first principal surface of the sixth dielectric layer S 6 , are the first input/output electrode  14  which has a first end connected to the input/output terminal  26  and which is capacitively coupled to the first resonator element  22   a , and the second input/output electrode  16  which has a first end connected to the antenna section  20  via the capacitance and which has a second end capacitively coupled to the second resonator element  22   b.    
     Two inner layer ground electrodes  30   a ,  30   b , which are opposed to the respective open ends of the two resonator elements  22   a ,  22   b , are formed on the first principal surface of the fifth dielectric layer S 5  respectively. 
     An inner layer ground electrode  32 , which is connected to the ground electrode  28  disposed on the outer surface, is formed of a portion of the first principal surface of the third dielectric layer S 3  corresponding to the filter section  18 . 
     A coupling-adjusting electrode  34 , which is in a potentially floating state, for example, with respect to the ground electrode  28  and the input/output terminal  26  of the filter section  18 , is formed on the first principal surface of the eighth dielectric layer S 8 . 
     The coupling-adjusting electrode  34  is shaped such that a first main electrode body  34   a  opposed to the first resonator element  22   a  and a second main electrode body  34   b  opposed to the second resonator element  22   b  are electrically connected to one another with a lead electrode  34   c  formed therebetween. 
     Two inner layer ground electrodes  36   a ,  36   b , which are opposed to the respective open ends of the two resonator elements  22   a ,  22   b , are formed on the first principal surface of the ninth dielectric layer S 9  respectively. 
     As shown in FIGS. 1 and 2, the antenna device  10 A according to the first embodiment includes an electrode  38  which is formed on the first principal surface of the second dielectric layer S 2  to form the capacitance between the second input/output electrode  16  and the first end of the antenna  24 . The electrode  38  is electrically connected to the second input/output electrode  16  via a through-hole  40 . 
     The electric coupling among the respective electrodes of the antenna device  10 A according to the first embodiment will now be explained with reference to an equivalent circuit diagram shown in FIG.  3 . 
     Two resonators  50   a ,  50   b , which are based on the first and second resonator elements  22   a ,  22   b , are connected in parallel between the input/output terminal  26  and the ground respectively. The adjoining resonators  50   a ,  50   b  are inductively coupled to one another. Accordingly, on the equivalent circuit, an inductance L is consequently inserted between the adjoining resonators  50   a ,  50   b.    
     A combined capacitance C, which is based on the coupling-adjusting electrode  34 , is formed between the first resonator element  22   a  and the second resonator element  22   b . An LC parallel resonance circuit, which is based on the inductance L and the capacitance C, is consequently connected between the respective resonators  50   a ,  50   b.    
     Capacitances (combined capacitances) C 1 , C 2  are formed between the respective open ends of the first and second resonator elements  22   a ,  22   b  and the corresponding inner layer ground electrodes  30   a ,  36   a  and  30   b ,  36   b  respectively. 
     A capacitance C 3  is formed via the first input/output electrode  14  between the first resonator element  22   a  and the input/output terminal  26 . A capacitance C 4  is formed between the second resonator element  22   b  and the second input/output electrode  16  for constructing a contact CN. A capacitance C 5  is formed via the electrode  38  between the contact CN (second input/output electrode  16 ) and the antenna section  20 . A capacitance C 6  is formed between the contact CN (second input/output electrode  16 ) and the ground (ground electrode  32 ). 
     That is, the antenna device  10 A according to the first embodiment is constructed such that the filter section  18  and the antenna section  20  are coupled to one another via the capacitance C 5  (and C 4 ). Especially, the circuit is constructed such that an impedance-matching circuit  52 , which is composed of the capacitances C 5 , C 6 , is inserted and connected between the filter section  18  and the antenna section  20 . It is also possible to realize the impedance matching by changing the length of the resonators  50   a ,  50   b , or varying the capacitances C 1 , C 2  shown in FIG. 3, in place of the capacitance C 6 . 
     It has been revealed for the antenna device  10 A according to the first embodiment that the input impedance of the antenna device  10 A is not changed even when the antenna length of the antenna section  20  is changed. 
     This fact results in the following advantages. That is, the decrease in gain can be suppressed to be minimum, for example, even when the antenna length of the antenna section  20  is shortened. Further, it is possible to consequently improve the yield by adjusting the antenna length in the production step. 
     An experiment was carried out for the antenna device  10 A according to the first embodiment in order to clarify the contents of the necessary antenna length. An illustrative experiment will be explained below. 
     At first, a single antenna  60  was evaluated in accordance with a measuring method shown in FIG.  4 . As shown in FIG. 4, the measuring method was carried out as follows. That is, a hole  68  for allowing a connector  66  of a network analyzer  64  was bored through a central portion of a copper plate  62  having a planar square configuration. The single antenna  60  (antenna length=L) as a measurement objective was attached to a dielectric substrate  70  extending in the vertical direction of the connector  66 . The length m of one side of the copper plate  62  was not less than 1.5 of the wavelength at the measurement frequency in vacuum. 
     The network analyzer  64  was used to measure the way of the change of center frequency when the antenna length L of the single antenna  60  was changed. FIG. 5 shows a representative frequency characteristic of the single antenna  60 , and FIG. 6 shows the change of the center frequency depending on the difference in antenna length L. 
     In the case of an ordinary high frequency circuit, i.e., in the case of a circuit in which the antenna and the filter are not integrated into one unit, as shown in FIG. 5, the antenna length L is determined so that the frequency corresponding to the smallest reflection amount conforms to the frequency necessary for the circuit. Otherwise, as clarified from FIG. 5, the antenna would be used in a region in which the reflection amount is large, which would cause the output loss (loss of transmission of any transmission signal to the antenna) and the unnecessary oscillation. 
     On the contrary, in the case of the antenna device  10 A according to the first embodiment, even when the antenna length is changed, the antenna gain (gain to indicate the degree of transmission of the signal (output) from the antenna to the outside) is not changed. 
     This phenomenon will be explained with reference to FIGS. 7 and 8. In this example, it is assumed that the center frequency of the filter section  18  is 2450 MHz in the antenna device  10 A according to the first embodiment (see FIGS.  1  and  2 ). 
     At first, the frequency characteristic was evaluated with only the single antenna before integrating the filter section  18  and the antenna section  20  into one unit. As a result, it was revealed that the antenna length L was required to be 21 mm in order to obtain the center frequency of 2450 MHz. 
     On the other hand, the antenna gain was measured while changing the antenna length L, after the filter section  18  and the antenna section  20  were integrated into one unit. An obtained result of the measurement is shown in FIG.  7 . The relationship between the antenna gain and the antenna length L was investigated concerning the pass band (2400 to 2500 MHz) of the filter section  18  of the antenna device  10 A. An obtained result is shown in FIG.  8 . 
     When the single antenna having the antenna length L of 21 mm was shortened to have a length of 15.3 mm, the gain was deteriorated by about 8 dB. However, in the case of the antenna device  10 A according to the first embodiment, even when the antenna length L of the antenna section  20  was shortened from 21 mm to 15.3 mm, the gain was deteriorated by only about 3 dB. Further it was revealed that when the antenna length L was shortened to 12.6 mm, the deterioration of the gain was suppressed to be 6 dB. 
     As described above, in the antenna device  10 A according to the first embodiment, for example, even when the antenna length L of the antenna section  20  is shortened, it is possible to suppress the decrease in gain to be minimum. Further, the antenna length L can be adjusted during the production step, and hence it is possible to improve the yield of the antenna device  10 A. 
     The embodiment described above is illustrative of the case in which the antenna  24 , which has the meandering configuration with the width smaller than the width of the dielectric substrate  12 , is formed on the upper surface of the dielectric substrate  12 . Alternatively, as in an antenna device  10   a  according to a first modified embodiment shown in FIG. 9, it is also preferable to form an antenna  24  having a meandering configuration with approximately the same width as the width of the dielectric substrate  12 . Further alternatively, as in an antenna device  10   b  according to a second modified embodiment shown in FIG. 10, it is also preferable that an antenna  24  may be overlapped with the both side surfaces of the dielectric substrate  12 . Although not shown in the drawing, it is also preferable to use an antenna having a simple strip-shaped configuration. 
     In the embodiments described above, the connection between the first resonator element  22   a  and the input/output terminal  26  is made by means of the capacitive coupling via the first input/output electrode  14  which is formed on the sixth dielectric layer S 6 , and the connection between the second resonator element  22   a  and the electrode  38  is made by means of the capacitive coupling via the second input/output electrode  16  which is formed on the sixth dielectric layer S 6  as well. Alternatively, it is also possible to adopt an arrangement as shown in FIG. 11 (antenna device  10   c  according to a third modified embodiment). 
     That is, in the antenna device  10   c  according to the third modified embodiment, the first and second input/output electrodes  14 ,  16  are not formed on the sixth dielectric layer S 6 . In this preferred embodiment, the connection between the first resonator element  22   a  and the input/output terminal  26  is made by means of direct connection via a first connecting electrode  80  which is formed on the seventh dielectric layer S 7 , and the connection between the second resonator element  22   a  and the electrode  38  is made by means of direct connection via a second connecting electrode  82  which is formed on the seventh dielectric layer S 7  as well. In this embodiment, it is possible to obtain a wide band width. FIG. 12 shows an equivalent circuit of the antenna device  10   c  according to the third modified embodiment. 
     Next, an antenna device  10 B according to a second embodiment will be explained with reference to FIGS. 13 to  14 B. Components or parts corresponding to those shown in FIG. 2 are designated by the same reference numerals, duplicate explanation of which will be omitted. 
     As shown in FIG. 13, the antenna device  10 B according to the second embodiment is constructed in approximately the same manner as the antenna device  10 A according to the first embodiment described above (see FIG.  2 ). However, in this embodiment, the length of the resonator element  11   a  disposed on the input side of the filter section  18  is different from the length of the second resonator element  22   b  disposed on the output side. 
     Specifically, the length of the second resonator element  22   b  is designed to be shorter than the length of the first resonator element  22   a . As a result, with reference to FIG. 3, the impedance, which is estimated when the left side (side of the input/output terminal  26 ) is viewed from the arrow A, is a characteristic impedance (50Ω) of an external circuit connected to the input/output terminal  26  as shown in FIG.  14 A. On the other hand, the impedance, which is estimated when the right side (side of the antenna section  20 ) is viewed from the arrow B, is equivalent to an impedance obtained by connecting a capacitance C 10  to the characteristic impedance (50Ω) in parallel as shown in FIG.  14 B. 
     The capacitance C 10  is added in parallel to the second resonator  50   b  based on the second resonator element  22   b . Therefore, the resonance frequency differs between the first and second resonators  50   a ,  50   b . In order to compensate the difference, the second resonator element  22   b  is made to be shorter than the first resonator element  22   a  as shown in FIG.  13 . Thus, it is possible to set the first and second resonators  50   a ,  50   b  to have an identical resonance frequency. 
     As described above, in the antenna device  10 B according to the second embodiment, it is possible to counteract the difference in resonance frequency between the respective resonators  50   a ,  50   b , which would be otherwise caused by the mismatch between the respective impedances on the side of the antenna section  20  and the side of the external circuit of the filter section  18 . Thus, it is possible to obtain the filter section  18  having a good attenuation characteristic. This results in realization of a high quality of the antenna device  10 B. 
     Next, explanation will be made for a method for producing the antenna devices  10 A and  10 B according to the first and second embodiments. The antenna devices  10 A and  10 B according to the first and second embodiments include the various electrodes which are internally mounted (contained) in the substrate  12 . Therefore, it is preferable that those used for the electrodes have little loss with a low specific resistance. 
     Those preferably used as the dielectric are highly reliable with a wide range of selection of dielectric constant. That is, it is preferable to use a ceramic dielectric. In this case, it is possible to effectively realize a small size of each filter. 
     The following production method is desirably adopted. That is, a conductive paste is applied to a ceramic powder green sheet to form an electrode pattern. After that, various green sheets are stacked with each other, followed by sintering to obtain a dense structure which is integrated with a ceramic dielectric in a state in which the conductor is stacked at the inside. 
     When a conductor based on Ag or Cu is used, it is difficult to perform the simultaneous sintering together with the ordinary dielectric material, because such a conductor has a low melting point. Therefore, it is necessary to use a dielectric material which can be sintered at a temperature lower than the melting point (not more than 110° C.) of such a conductor. 
     In view of the feature of the device to be used as a microwave filter, it is preferable to use a dielectric material with which the temperature characteristic (temperature coefficient) of the resonance frequency of the resonance circuit to be formed is not more than ±50 ppm/° C. 
     Those usable as such a dielectric material include, for example, those based on glass such as a mixture of cordierite-based glass powder, TiO 2  powder, and Nd 2 Ti 2 O 7  powder, those obtained by adding a slight amount of glass-forming component or glass powder to BaO—TiO 2 —Re 2 O 3 —Bi 2 O 3 -based composition (Re: rare earth component), and those obtained by adding a slight amount of glass powder to barium oxide-titanium oxide-neodymium oxide-based dielectric magnetic composition powder. 
     For example, a powder mixture is obtained by sufficiently mixing 73 wt % of glass powder having a composition of MgO (18 wt %)-Al 2 O 3  (37 wt %)-SiO 2  (37 wt %)-B 2 O 3  (5 wt %)-TiO 2  (3 wt %), 17 wt % of commercially available TiO 2  powder, and 10 wt % of Nd 2 Ti 2 O 7  powder. 
     The material used as the Nd 2 Ti 2 O 7  powder is obtained by calcining Nd 2 O 3  powder and TiO 2  powder at 1200° C., followed by pulverization. 
     In the method for producing the antenna devices  10 A and  10 B according to the first and second embodiments, an acrylic organic binder, a plasticizer, and a solvent based on toluene and alcohol are added to the powder mixture described above, followed by sufficient mixing with alumina cobblestone to obtain a slurry. The slurry is used to produce a green tape having a thickness of 0.2 mm to 0.5 mm in accordance with the doctor blade method. 
     Subsequently, the green tape is punched and processed into a desired shape. After that, the conductor patterns shown in FIGS. 1 and 2 are printed with a silver paste as the conductive paste respectively. Subsequently, necessary green tapes, which are required to adjust the thickness of the green tapes printed with the conductor patterns, are stacked and superimposed to give the structure as shown in FIGS. 1 and 2, and they are laminated with each other, followed by sintering, for example, at 900° C. to produce the dielectric substrate  12 . 
     The pattern of the antenna  24  is printed on the upper surface of the dielectric substrate  12  constructed as described above. The patterns of the ground electrodes  28  are printed on the both side surfaces of the dielectric substrate  12 . The printed patterns are heat-treated at 850° C. 
     When the production method described above is adopted, it is possible to easily produce the antenna device  10  comprising the filter section  18  and the antenna section  16  which are integrated into one unit with the capacitance intervening therebetween in the single dielectric substrate  12 . 
     It is a matter of course that the antenna device according to the present invention is not limited to the embodiments described above, which may be embodied in other various forms without deviating from the gist or essential characteristics of the present invention. 
     As explained above, according to the antenna device concerning the present invention, it is possible to suppress the decrease in gain to be minimum, for example, even when the antenna length of the antenna section is shortened. Further, the antenna length can be adjusted in the production step. Therefore, it is possible to improve the yield of the antenna device.