GB2439863A

GB2439863A - Antenna structure and radio communication device using the same

Info

Publication number: GB2439863A
Application number: GB0718977A
Authority: GB
Inventors: Satoru Hirano
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2005-05-13
Filing date: 2005-07-13
Publication date: 2008-01-09
Anticipated expiration: 2025-07-13
Also published as: GB0718977D0; GB2439863B; GB2439863C; WO2006120763A1; JP4103936B2; JPWO2006120763A1

Abstract

An antenna structure (1) includes a circuit substrate (5) on which a base (2) having a radiation electrode (3) is mounted. The radiation electrode (3) is arranged on the base (2) so as to oppose to the circuit substrate surface via a gap. On the circuit substrate (5), there is formed an inter-ground capacity loading electrode (7) arranged to oppose to the radiation electrode (3) of the base (2) and having a capacity between itself and the radiation electrode (3). Moreover, on the circuit substrate (5), there is formed a ground electrode (6) with a gap to the inter-ground capacity loading electrode (7). Furthermore, a resonance frequency adjusting element (8) is arranged to make a connection between the inter-ground capacity loading electrode (7) and the ground electrode (6). The resonance frequency adjusting element (8) has a capacity or an inductance for adjusting the resonance frequency of the antenna structure (1) to a predetermined resonance frequency.

Description

DESCRIPTION

ANTENNA STRUCTURE AND WIRELESS COMMUNICATION DEVICE INCLUDING

THE SAME

Technical Field

The present invention relates to an antenna structure in which a substrate having a radiating electrode formed thereon is mounted on a circuit board and to a wireless communication device including the antenna structure.

Background Art

As a type of antenna provided on a wireless communication device, a surface-mount antenna that is mounted on a circuit board of a wireless communication device and contained and disposed within a case of the wireless communication device exists. In the surface-mount antenna, for example, a radiating electrode that performs an antenna operation is formed on a dielectric substrate.

Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-209733 Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-141739 Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-335117

Disclosure of Invention

problems to be Solved by the Invention The frequency characteristics (return-loss characteristics) of radiowavesof awirelesscommunicatiofldevice in which a surface-mount antenna is mounted on a circuit board antenna, but are determined by various factors such as a ground electrode or parts of the circuit board having mounted the surface-mount antenna thereon. Thus, the resonant frequency of radio waves for wireless communications carried out by a wireless communication device differs from the resonant frequency of the radiating electrode of the surface-mount antenna. Thus, evenwhen the same surface-mount antenna is mounted, for example, when the type of wireless communication device varies, problematically, the resonant frequency of radio waves for wireless communications carried out by the wireless communication device (hereinafter referred to as the resonant frequency of the antenna) varies.

That is, the peripheral conditions of the surface-mount antenna vary when the type of wireless communication device varies.

For example, the size and shape of a ground electrode formed on thecircuitboardvarieS, thetypesofpartsdisPoSediflthePeriPherY of the surface-mount antenna or the gaps between the surface-mount antenna and peripheral parts vary, or the material of the case of the wireless communication device varies. Since the resonant frequency of the antenna is determined by complex effects of such peripheral conditions of the surface-mount antenna, when the type of the wireless communication device having the surface-mount antenna mounted thereon varies so that the peripheral conditions of the surface-mount antenna varies, the resonant frequency of the antenna varies even when the same surface-mount antenna is provided.

As described above, even when the same surface-mount antenna isprovided, of the antenna when the type of wireless communication device varies. Thus, even when the desired resonant frequency of the antenna is the same, for example, when the type of wireless communication device varies, it is not allowed to provide the same surface-mount antenna, and it is needed to prepare custom design of factors such as the size of the radiating electrode device, which is laborious.

According to another method that has been proposed (e.g., refer to Patent Documents 1. to 3), the resonant frequency of an antenna is adjusted to a predetermined resonant frequency by preparing custom designs not for a surface-mount antenna but for partsotherthantheSurfacem0Unttmas forexarnple, bychanging a circuit on a circuit board electrically connected to the surface-mount antenna for each type of wireless communication device.

However, according to the method that has been used in which the resonant frequency of an antenna is adjusted by a circuit on a circuit board, problematically, current loss increases and antenna gain decreases. Furthermore, when a part having a

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capacitance or an inductance is used for adjusting the resonant frequency of an antenna, if a generic part is used, for example, in view of cost, it is only possible to prepare parts (capacitor parts or inductor parts) having several predetermined values of the magnitude of capacitance or the value of inductance. Thus, in many cases, it is not possible to obtain a capacitor part or an inductor part having an optimal value, so that it has been difficult to adjust the resonant frequency of an antenna precisely to a predetermined resonant frequency.

Means for Solving the Problems The present invention provides the following constructions as means for solving the problems. That is, a construction of an antenna structure according to the present invention is an antenna structure in which a radiating electrode that performs an antenna operation is provided on a substrate, and in which the substrate ismountedonacircuitboard, theradiatingelectrode being provided on the substrate so as to oppose a board surface of the circuit board via a gap, wherein an inter-ground-capacitor loading electrode is formed on the circuit board so as to oppose the radiating electrode a ground electrode is formed on the circuit board via a gap with the inter-ground-capacitor loading electrode so as to avoid a region where the inter-groundcapacitor loading electrode is formed, a resonant-frequency adjusting element connecting the inter-ground-capacitor loading electrode with the ground electrode is further provided, and the resonant-frequency adjusting element has a capacitance or an inductance for adjusting a resonant frequency of the antenna structure to a predetermined resonant frequency. Furthermore, awirelesscommUflicationdevice according to the present invention includes an antenna structure described above.

Advantages of the Invention According to the present invention, a substrate having a radiating electrode formed thereon is mounted on a circuit board, and an inter-ground-capacitor loading electrode disposed so as to oppose the radiating electrode on the substrate and to form a capacitor with the radiating electrode is formed on the circuit board. Furthermore, on the circuit board, a ground electrode is formedwithagap from the inter-ground-capacitor loading electrode, and a resonant-frequency adjusting element connecting the inter-groundcapacitor loading electrode with the ground electrode is further provided. The resonant-frequency adjusting element has a capacitance or an inductance. That is, according to the present invention, the radiating electrode is connected to the ground electrode via the capacitor between the radiating electrode and the inter-ground-capacitor loading electrode and the capacitance or inductance of the resonantfrequency adjusting element. The impedance of a circuit (hereinafter referred to as a resonant-frequency adjusting circuit) formed by a series connection of the capacitor between the radiating electrode and the inter-ground-capacitor loading electrode and the capacitance or inductance of the resonant-frequency adjusting element affects the electrical length of the radiating electrode, which determines the resonant frequency of the radiating electrode. Thus, by changing and adjusting the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element, the impedance of the resonant-frequency adjusting circuit can be changed so that the electrical length of the radiating electrode can be changed. Accordingly, it is possible to change and adjust the resonant frequency of the radiating electrode (i.e., the resonant frequency of the antenna structure) As described above, it is possible to change and adjust the resonant frequency of an antenna structure simply by changing the magnitude of capacitance or the value of inductance of a resonant-frequency adjusting element without changing design of factors such as the shape of a radiating electrode on a substrate.

Thus, it is possible tocommonlyuseacomPoflent (antenna component) in which a radiating electrode is formed on a substrate for a plurality of types of wireless communication device, and this serves for common use of components. Accordingly, cost of an antenna component or a wireless communication device can be readily reduced.

Furthermore, as the electrode area of the inter-ground-capacitor loading electrode is increased to increase

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the capacitance between the inter-ground-capacitor loading electrode and the radiating electrode, the amount of change in the resonant frequency of the antenna structure in relation to the amount of change in the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element can be increased. That is, as the electrode area of the intergroundcapacitOrlOadiflgelectr0 isdecreasedtodeCreaSe the capacitance between the inter-ground-capacitor loading electrode and the radiating electrode, the amount of change in the resonant frequency of the antenna structure in relation to the amount of change in the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element can be decreased.

Thus, when a generic capacitor part or inductor part is used as the resonant-frequency adjusting element, for example, forthepurposeOfredUCiflgCosti evenif themagnitudeof capacitance or the value of inductance of the resonant-frequency adjusting element can only be changed non-continuously, by changing and adjusting the electrode area of the interground-capacitor loading and the inter-ground-capacitor loading electrode, it is possible to decrease the step size of change in the resonant frequency of the antenna structure, so that it is possible to delicately adjust the resonant frequency of the antenna structure.

Accordingly, it is possible to achieve a desired resonant frequency of a radiating electrode, so that it is possible to improve the reliabilityof wireless communications byawireless communication device including the antenna structure according to the present invention.

Furthermore, according to the present invention, the inter-ground-capacitor loading electrode is provided on the circuit board, not on the substrate having the radiating electrode formed thereon. Thus, when it is desired to change the capacitance between the radiating electrode and the inter-ground-capacitor loadingelectrode, for example, due toadesign change, it suffices to change the electrode area of the inter-ground-capacitor loading electrode formed on the circuit board, and it is not needed to changedesignofacomPoflent (antennacomponent) inwhicharadiating electrode is formed on a substrate. That is, the same antenna component used before the design change can be used after the design change. As described above, the construction where the inter-ground-capacitor loading electrode is provided on the circuit board is an important factor that facilitates use of common antenna parts.

Furthermore, when the radiating electrode is provided on the substrate so as to oppose a board surface of the circuit board viaagap, assumingacasewhere the intergroundcapacitOr loading electrode is provided ona side surface of the substrate, avirtual plane extending along an electrode surface of the radiating electrode and a virtual plane extending along an electrode surface

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of the inter-ground-capacitor loading electrode has, for example, an orthogonal relationship or a substantially orthogonal relationship, so that the capacitance between the radiating electrode and the inter-ground-capacitor loading electrode is small. Thus, when a large capacitance between the radiating electrode and the inter-groundcapacitor loading electrode is required, the inter-ground-capacitor loading electrode must be formed so as to have an enlarged size. This causes the problem of increase in the size of the substrate (i.e., the antenna component) In contrast, according to the present invention, the inter-ground-capacitor loading electrode is formed in a region of the circuit board surface opposing the radiating electrode.

Thus, the are of the opposing region between the inter-ground-capacitor loading electrode and the radiating electrode can be increased, so that it is readily possible to provide a large capacitance between the inter-ground-capacitor 10djngelectrodeandtheradiatingeectr0de. Furthermore, since the inter-ground-capacitor loading electrode is not provided on the substrate, it is possible to reduce the size of the substrate (antenna component) in accordance with the absence of the inter-ground-capacitor loading electrode on the substrate.

Furthermore, since the inter-ground-capacitor loading electrode is formed in a region of the circuit board opposing the radiating electrode and a large capacitance can be provided -10 -between the inter-ground-capacitor loading electrode and the radiatingelectrode, it ispossib1etochangeandadiUSttheres0nt frequency of the antenna structure while avoiding deterioration of antenna gain.

Brief Description of the Drawings

[Fig. la] Fig. 1j55hematicplanviewforeXpla1ninganantenh structure according to a first embodiment.

[Fig. ib] Fig. lb is a schematic perspective view of the antenna structure shown in Fig. la.

[Fig. id Fig. ic j55hematicp1anvieWfOreXp1a1ningemP].e construction of a circuit board constituting the antenna structure shown in Fig. la.

[Fig. 21 Fig. 2 is a graph for explaining an example of change in return-loss characteristics of the antenna structure according to the first embodiment in relation to change in the magnitude of capacitance of a resonant-frequency adjusting element constituting the antenna structure.

[Fig. 3] Fig. 2 is a graph f or explaining an example of change in return-loss characteristics of the antenna structure according to the first embodiment in relation to change in the value of inductance of resonantfrequencyadjUStingelem1t constituting the antenna structure.

[Fig. 4] Fig. 4 of a slit in a ground electrode constituting the antenna structure according to the first embodiment.

-11 - [Fig. 5] Fig. 5 is a graph for explaining an example of change to change in the length of the slit for adjusting the resonant frequency, provided in the ground electrode.

[Fig. Ga] Fig. Ga is a model diagram for explaining advantages achieved by the construction according to the first embodiment.

[Fig. 6b] Fig. 6b is a model diagram for explaining problems of the related art.

[Fig. Gd Fig. 6c is a model diagram for explaining problems of the related art together with Fig. Gb.

[Fig. 7a1 Sectional view for explaining another embodiment, together with Fig. 7b.

[Fig. 7a] Sectional view for explaining another embodiment, together with Fig. 7a.

Reference Numerals i antenna structure 2 dielectric substrate 3 radiating electrode circuit board 6 ground electrode 7 inter-ground-capacitor loading electrode 8 resonant-frequency adjusting element 13 slit

Best Mode for Carrying Out the Invention

Now, embodiments of the present invention will be described -12 -with reference to the drawings.

Fig. la is a schematic plan view showing a first embodiment of an antenna structure according to the present invention. Fig. lb is a schematic perspective view of the antenna structure shown in Fig. la. Fig. ic is a schematic plan view showing an example form of a conductor pattern on a circuit board constituting the antenna structure shown in Fig. la.

An antenna structure 1 according to the first embodiment includesasubstrate2COmPoSed0fadjelectr1cmater aradiating formedonthedielectric substrate 2, a circuit board 5 on which the dielectric substrate 2 is surface-mounted, a ground electrode 6 and an inter-ground-capacitor loading electrode 7 formed on the circuit board 5, a resonant-frequency adjusting element 8 connecting the ground electrode 6 with the inter-ground-capacitor loading electrode 7, and a power-feeding line 9 formed on the circuit board 5 and electrically connected to the power-feeding electrode 4 of the dielectric substrate 2.

That is, in the first embodiment, the dielectric substrate Fig. 6ashowsasChematiC sectional view of the dielectric substrate 2. The radiating electrode 3 is formed so as to extend from a top surface to an edge portion of the bottom surface of the dielectric substrate 2, for example, via the right-end surface as viewed in Fig. lb. Furthermore, the power-feeding electrode 4 is formed so as to -13 -extend from an edge portion of the bottom surface of the dielectric substrate 2 to a position opposing the radiating electrode 3 with agaponthe top surfaceof thedielectric substrate 2, forexample, via the left-end surface as viewed in Fig. lb. AcornerpOrtiOflOftheCirCUitb0a5c0nstitutesananta constituting part, and the inter.groufld-caPacitor loading electrode 7 and the power-feeding line 9 are formed on the board surface of the corner portion of the circuit board 5. On the board surface of the circuit board 5, the ground electrode 6 is formed substantially over the entire region except for the regions where the intergroufld-caPaCit0r loading electrode 7 and the powerfeediflg1ifle9aref0rm the radiating electrode 3 and the power-feeding electrode 4 formed thereon is mounted (surface-mounted) on the antenna constituting part of the corner portion of the circuit board 5 so that the bottom surface thereof faces the circuit board 5 and so that, for example, the right-end portion where the radiating electrode 3 is formed as viewed in Fig. la is disposed on the ground electrode 6. By mounting the dielectric substrate 2 on the circuit board as described above, of both ends of the radiating electrode 3, the end remoter from the power-feeding electrode 4 (e.g., the right-end portion as viewed in Fig. la) is directly bonded to the ground electrode 6, and the radiating electrode 3 provided on the top surface of the dielectric substrate 2 is disposed so as to oppose the board surface of the circuit board 5.

-14 -One endof thepower-feeding line 9 is electricallyconnected to the power-feeding electrode 4. The other end of the power-feeding line 9 is electrically connected to, for example, a high-frequency circuit 10 for wireless communications by a wireless communication device. That is, the power-feeding line 10 forwireless communications with the power-feeding electrode 4. On the power-feeding line 9, amatchingelement iiconstitutingarnatching circuit The power-feeding electrode 4 is formed with a gap from the radiating electrode 3, and the power-feeding electrode 4 is electromagnetically coupled with the radiating electrode 3 via a capacitor. That is, for example, when signals for wireless transmission are transmitted from the high-frequency circuit 10 to the power-feeding electrode 4 via the power-feeding line 9, the signals for wireless transmission are transmitted from the power-feeding electrode 4 to the radiating electrode 3 by the capacitive coupling between the powerfeeding electrode 4 and the radiating electrode 3. That is, the radiating electrode 3 constitutesacapacitivePower feedingtyperadiatinge1eCtr0de.

In the first embodiment, on the circuit board 5, the inter-ground-capacitor loading electrode 7 is formed with a gap from the ground electrode 6 in a region opposing the radiating electrode 3 (refertoFig. ic) . The interground-capacitor10athng -15 -electrode 7 has a leadout portion 7a that is formed so as to extend from a region where the dielectric substrate 2 is mounted to the outside of the region. The resonant-frequency adjusting element B is implemented by a capacitor part or an inductor part, and is mounted on the circuit board 5 so as to connect the leadout portion 7a of the inter-ground-capacitor loading electrode 7 with the ground electrode 6. In the first embodiment, the dielectric isof aground-mounting type, so that the ground electrode 6 is supposed to be formed on a part of the circuit board 5 having the dielectric substrate 2 mounted thereon. However, in order to form the inter-ground-capacitor loading electrode 7 and the power-feeding line 9, the ground electrode 6 is not formed on the board surface in the regions where the inter-ground-capacitor loading electrode 7 and the powerfeeding line 9 are formed.

In the first embodiment, the inter-ground-capacitor loading electrode 7 is disposed so as to oppose the radiating electrode 3, wherebyacapacitoriS formedbetweenthe inter-ground-capacitor loading electrode 7 and the radiating electrode 3. The inter-ground-capacitor loading electrode 7 is connected to the ground electrode 6 via the resonant-frequency adjusting element B. That is, the radiating electrode 3 is connected to the ground electrode 6 via a circuit (resonant-frequency adjusting circuit) formed bya series connection of the capacitor between the radiating electrode 3 and the inter-ground-capacitor loading electrode 7

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-16 -and the capacitance or inductance of the resonant-frequenCY adjusting element 8. The impedance of the resonant-frequency adjusting circuit affects the electrical length of the radiating electrode 3, i.e., the resonant frequency. Thus, it is possible to change and adjust the resonant frequency of the radiating electrode 3 (the resonant frequency of the antenna structure 1) by changing and adjusting the impedance of the resonant-frequency adjusting circuit through changing and adjusting the magnitude of capacitance or the value of inductance of the resonant -frequency adjusting element 8.

When the resonant-frequency adjusting element 8 is implemented by a capacitor part, compared with a case where the resonant-frequency adjusting element 8 is not disposed, the resonant frequency of the antenna structure 1 can be decreased.

The amount of decrease in the resonant frequency increases as the magnitude of capacitance of the resonant-frequency adjusting element (capacitor part) 8 increases.

Fig. 2 shows examples of return-loss characteristics of five types of the antenna structure 1 that are constructed the same except for the arrangement of the resonant-frequencyadjusting element 8. More specifically, a dotted line A in a graph in Fig. 2 represents an example of return-loss characteristics of the antenna structure 1 element 8 is not provided. Solid lines B to E in the graph in Fig. 2 represent examples of return-loss characteristics of the -17 -antenna structure 1 in cases where a capacitor part is provided as the resonant-frequency adjusting element 8. The solid line B represents an example in a case where the capacitance of the resonant-frequency adjusting element 8 is 0.5 pF, the solid line C represents an example in a case where the capacitance of the resonant-frequency adjusting element 8 is 1 pF, the solid line D represents an example in a case where the capacitance of the resonant-frequency adjusting element 8 is 3 pF, and the solid line E represents an example in a case where the capacitance of the resonant-frequency adjusting element 8 is 6 pF. As will be understood from the graph in Fig. 2, when the resonant-frequency adjusting element (capacitor part) 8 is provided, the resonant frequency of the antenna structure 1 decreases compared with the case where the resonant-frequency adjusting element 8 is not provided. Furthermore, the amount of decrease M3, M, MD, and in the resonant frequency of the antenna structure 1 increases as the capacitance of the resonant-frequency adjusting element 8 increases.

On the other hand, when the resonant-frequency adjusting element 8 is implementedbyaninduCtOrpart, the resonant frequency of the antenna structure 1 can be increased compared with the case where the resonant-frequency adjusting element 8 is not provided. TheeffectOf theresonantfrequeflCyadjuStiflgelemeflt 8 on the amount of increase in the resonant frequency increases as the value of inductance of the resonant-frequency adjusting -18 -element (inductorpart) 8 decreases, sothat the resonant frequency of the antenna structure 1 increases.

Fig. 3 shows examples of return-loss characteristics of five types of the antenna structure 1 that are constructed the same except for the arrangement of the resonant frequency adjusting element 8. More specifically, a dotted line a in a graph in Fig. 3 represents an example of return-loss characteristics of the antennastructUrel element 8 is not provided. Solid lines b to e in the graph in Fig. 3 represent examples of return-loss characteristics of the antenna structure 1 in cases where an inductor part is provided as the resonant-frequency adjusting element 8. The solid line b represents an example in a case where the value of inductance of the resonant-frequency adjusting element 8 is 6.8 nH, the solid line c represents an example in a case where the value of inductance of the resonantfrequency adjusting element 8 is 4.7 nH, the solid line d represents an example in a case where the value of inductance of the resonant-frequency adjusting element 8 is 3.9 nH, and the solid line e represents an example in a case where the value of inductance of the resonant-frequency adjusting element 8 is 2.7 nH. As will be understood from the graph in Fig. 3, when the resonantfrequencyadjU5tiT1gelemen1t (inductorpart) 8 isprovided, the resonant frequencyof the antenna structure 1 increases compared with the case where the resonant-frequency adjusting element 8 is not provided. Furthermore, the amount of increase Mb, Mc, -19 -d' and e in the resonant frequency of the antenna structure 1 increases as the value of inductance of the resonant-frequency adjusting element 8 decreases.

Regarding the resonant-frequency adjusting circuit formed by a series connection of the capacitor between the radiating electrode 3 and the inter-ground-capacitor loading electrode 7 and the capacitance or inductance of the resonant-frequency adjusting element 8, depending on the capacitance between the radiating electrode 3 and the inter-ground-capacitor loading electrode 7, the amount of change in the impedance of the resonant-frequency adjusting circuit relative to the amount of change in the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8 differs. Thus, by changing and adjusting the amount of change in the impedance of the resonant-frequency adjusting circuit relative to the amount of change in the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8 through changing and adjusting the capacitance between the radiating electrode i.e., through changing and adjusting the electrode area of the inter-ground-capacitor loading electrode 7, it is possible to change and adjust the resonant frequency of the antenna structure 1.

More specifically, as the electrode area of the inter-ground-capacitor loading electrode 7 is decreased to -20 -decrease the capacitance between the inter-ground-capacitor loading electrode 7 and the radiating electrode 3, the amount of change in the resonant frequency of the antenna structure 1 relative to the amount of change in the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8 decreases. That is, as the electrode area of the inter-ground-capacitor loading electrode 7 is increased to increase the capacitance between the inter-ground-capacitor loading electrode 7 and the radiating electrode 3, the amount of change in the resonant frequency of the antenna structure 1 relative to the amount of change in the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8 increases. Thus, for example, when the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8 is changed in the same way, the amount of change in the resonant frequency of the antenna structure 1 changes depending on the electrode area of the inter-ground-capacitor loading electrode 7. Accordingly, the electrode area of the inter-ground-capacitor loading electrode 7 is decreased whenit is desired to delicately control the resonant frequency of the antenna structure 1. conversely, the electrode area of the inter-ground-capacitor loading electrode 7 is increased when the resonant frequency of the antenna structure 1 is to be changed roughly.

In the first embodiment, a slit 13 is formed in the ground -21 -electrode 6 soas toextend fromaportiOn joinedwiththe radiating electrode 3. With the slit 13, a part of the ground electrode 6 joined with the radiating electrode 3 functions as a part of theradiatingeleCtrode3.

the part of the ground electrode 6 that functions as the part of the radiating electrode 3 from the remaining part of the ground electrode 6.

By the formation of the slit 13, for example, a part of the groundelectrode 6 enclosedbya chain line Z in Fig. 4 functions as a part of the radiating electrode 3. Thus, changing the shape or length of the slit 13 to change the electrical length of the portion Z of the ground electrode 6 that functions as the part length of the radiating electrode 3, so that it is possible to change the resonant frequency of the antenna structure 1. More specifically, forexarnple, whenthelengthOf theslitl3 isextended from the length indicated by a solid line in Fig. 4 as indicated by a dotted line, the return-loss characteristics of the antenna structure 1 change as indicated in order by solid line La, solid line Lb, solid line Lc, solid line Ld, solid line Le, and solid line Lf, as shown in the graph in Fig. 5. That is, the resonant frequency of the antenna structure 1 can be decreased as the length of the slit 13 increases so that the equivalent electrical length of the radiating electrode 3 increases.

By changing and adjusting the magnitude of capacitance or -22 -thevalueof 8 or by changing and adjusting the electrode area of the inter-ground-capacitor loading electrode 7, the resonant frequency of the antenna structure 1 can be changed and adjusted by a step size of, for example, approximately 10 MHz, or by a step size of 1 MHz or several MHz depending on the electrode area of the inter-ground-capacitor loading electrode 7. on the other hand, by changing and adjusting the slit 13, the resonant frequency of the antenna structure 1 can be changed and adjusted by a step size of, for example, approximately 100 MHz. As described above, according to the first embodiment, the resonant frequency of the antenna structure 1 is roughly adjusted by the slit 13, and the resonant frequencyof the antennastructurel isdelicatelyadjusted by the resonant-frequency adjusting element 8 and the electrode area of the inter-ground-capacitor loading electrode 7. Thus, the resonant frequency of the antenna structure 1 can be adjusted precisely.

According to the first embodiment, it is possible to change and adjust the resonant frequency of the antenna structure 1 by changing and adjusting the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8, the electrode area of the inter-ground-capacitor loading electrode 7 (the capacitance between the inter-ground-capacitor loading electrode 7 and the radiating electrode 3), and the length or shape of the slit 13 as described above. Thus, the magnitude of -23 -capacitance or the value of inductance of the resonant-frequency adjusting element 8, the electrode area of the inter-ground-capacitor loading electrode 7, and the length or shape of the slit 13 are adjusted and determined appropriately so that the resonant frequency of the antenna structure 1 is set to a predetermined resonant frequency.

With the construction of the first embodiment, advantageously, it is possible to adjust the resonant frequency of the antenna structure 1 to a predetermined resonant frequency simply by changing and adjusting the magnitude of capacitance or the value of inductance of the resonant-frequency adjusting element 8, the electrode area of the inter-ground-capacitor loading electrode 7, or the length or shape of the slit 13 without changing factors such as the size or shape of the radiating electrode 3.

Furthermore, since the inter-ground-capacitor loading electrode 7 is formed in a region of the board surface of the advantageously, it is possible to suppress increase in the size of the dielectric substrate 2. More specifically, for example, as shown in a schematic sectional view in Fig. 6b, when an electrode 14 for forming a capacitor between the radiating electrode 3 and the ground electrode 6 is formed on an end surface of the dielectric substrate 2, the electrode 14 is formed on the end surface of the dielectric substrate 2 in addition to the powerfeeding electrode 4, as shown in Fig. 6c, in which the configuration of -24 -the end surface of the dielectric substrate 2 is viewed from the left in Fig. 6b. Thus, the size of the dielectric substrate 2 must be increased, so that the size of the antenna structure 1 is increased. Furthermore, since the electrode 14 is not disposed so as to oppose the radiating electrode 3, the capacitance between the electrode 14 and the radiating electrode 3 is small. Thus, an approach for increasing the capacitance between the electrode 14 and the radiating electrode 3 is to extend the electrode 14 toward the radiating electrode 3 and thereby narrow the gap between is increased.

However, as the gap between the electrode 14 and the radiating electrode 3 becomes narrower, variation inthe capacitance between the electrode 14 and the radiating electrode 3 due to variation in the gap increases, which is unfavorable. Thus, when the electrode area of the electrode 14 is to be increased in order to increase the capacitance between the electrode 14 and the radiating electrode 3, the size of the dielectric substrate 2 mustbenecessarilyincreaSed. ThiscausestheprOblemOf increase in the size of the antenna structure 1.

In contrast, according to the first embodiment, as shown aschematicsectioflalVieWiflFig. 6a, theinter-ground-capacitOr loading electrode 7 is formed on a board circuit of the circuit board 5 opposing the radiating electrode 3. Thus, the inter-ground-capacitor loading electrode 7 is disposed so as to

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-25 -oppose the radiating electrode 3, so that it is readily possible to provide a large capacitance between the inter_ground- capacitor loading electrode 7 and the radiating electrode 3. Furthermore, the inter-ground-capacitor loading electrode 7 is provided on a board surface of the circuit board 5, not on the dielectric substrate 2, and the region of the circuit board where the inter-ground-capacitor loading electrode 7 is formed is a dead space where the dielectric substrate 2 is mounted that has not hitherto been used. From what has been described above, when the size of the inter-ground-capacitor loading electrode 7 is increased in order to increase the capacitance between the inter-ground-capacitor loading electrode 7 and the radiating electrode 3, increase in the size of the dielectric substrate 2 (i.e., increase in the size of the antenna structure 1) can be suppressed.

Furthermore, even when the resonant-frequency adjusting element 8 is provided as in the first embodiment, variation in antenna gain can be maintained small. This has been verified by experiments by the inventor. In the experiments, three types of the antenna structure 1 (samplesa, 3, andy) withthe same conditions except for the arrangement of the resonant-frequency adjusting element 8 were prepared. More specifically, in the sample a, the resonant-frequency adjusting element 8 was not provided. In the sample, a capacitor part having a capacitance of, for example, 6 pF was provided as the resonant-frequency adjusting element 26 - 8. In the sample y, an inductor part having an inductance value of, for example, 3.9 nH was provided as the resonant-frequency adjusting element 8. Antenna gains for linearly polarized waves were obtained for each of these samples a, i, and y. Tables 1 to 3 show the results of the experiments. Table 1 relates to the sample a, Table 2 relates to the sample, and table 3 relates to the sample y.

-27 - [Table 1] ______________________ ______________________ YZ plane ZX plane Horizontally Vertically Horizontally vertically Antenna Sample cx polarized polarized polarized polarized gain waves waves waves waves Maximum value -0.6 -4.6 0.3 -0.9 2400 (dBi) -1.2 MHz Average (dB) value -4.0 -9.3 -4.8 -2.3 (dB i) ____________ ________ Max i mum value -0.5 -4.7 0.5 -0.3 2442 (dBi) -0.9 MHz Average (dB) value -4.0 -9.1 -4.7 -1.9 (dB 1) ___________ ________ Maximum value -0.2 -5.0 0.2 -0.1 2484 (dBi) -0.7 MHz Average (dE) value -3.9 -9.3 -4.9 -1.7 (dBi) ____________ __________ _______ -28 - [Table 2) _____________________ YZ plane ZX plane Horizontally Vertically Horizontally vertically Antenna Sample 3 polarized polarized polarized polarized gain waves waves waves waves Maximum value -1.0 -4.5 0.1 -1.0 2400 (dBi) -1.4 MHz Average (dE) value -4.3 -9.2 -5.0 -2.5 (dBi) ____________ __________ _______ Maximum value -0.5 -4.1 0.6 -0.3 2442 (dBi) -0.8 MHz Average (dB) value -3.9 -8.6 -4.7 -1.7 (dB i) _____________ ___________ ________ Maximum value -0.3 -4.6 0.2 -0.1 2484 (dBi) -0.7 MHz Average (dB) value -4.0 -8.9 -5.1 -1.7 (dBi) ______________________ _______ -29 - [Table 3] _______________________ _______________________ _______ YZ plane ZX plane Horizontally Vertically Horizontally Vertically Antenna Sample y polarized polarized polarized polarized gain waves waves waves waves Maximum value -0.7 -4.6 0.0 -0.9 2400 (dBi) -1.3 MHz Average (dB) value -4.0 -9.4 -5.1. -2.3 (dBi) ________ Maximum value -0.8 -4.8 0.1 -0.5 2442 (dBi) -1.0 MHz Average (dB) value -4.1 -9.3 -5.1 -2.0 (dBi) ________ Maximum value -0.5 -5.1 -0.2 -0.2 2484 (dBi) -0.9 MHZ Average (dB) value -4.1 -9.5 -5.3 -1.8 (dBi) ________ As will be understood from the comparison between Table 1 representing antenna gains in the sample a (where the resonant-frequency adjusting element 8 is not provided) and Tables 2 and 3 representing antenna gains in the samples and y (where the resonant-frequency adjusting element 8 is provided), even when the resonant-frequency adjusting element 8 is provided, -30 -antenna gains similar to those in the case where the resonant-frequency adjusting element 8 is not provided can be achieved.

Now, a second embodiment will be described. The second embodiment relates to a wireless communication device. In the wireless communication device according to the second embodiment, The construction of the wireless communication device may vary, and any constructionmay be employed for the wireless communication device except for the antenna structure 1, and description of the construction will be omitted herein. Furthermore, since the construction of the antenna structure 1 has been described in the context of the first embodiment, repeated description thereof will be refrained.

The present invention is not limited to the first and second embodiments, and may be embodied in various forms. For example, although the slit 13 is provided so that a part of the ground electrode 6 functions as a part of the radiating electrode 3 in the slit 13 maybe omitted when it is possible to adjust the resonant frequency of the antenna structure 1 to a predetermined frequency without the slit 13.

Furthermore, in addition to the constructions of the first and second embodiments, an inter-ground-capacitor loading electrode 7' may be provided in a region of the bottom surface -31 -of the dielectric substrate opposing the inter-ground-capacitor loading electrode 7 on the circuit board 5, as shown ma schematic sectional view in Fig. 7a, and in Fig. 7b, which is an exploded view thereof. The inter-ground-capacitor loading electrode 7' on the dielectric substrate 2 is bonded with the inter-ground-capacitor loading electrode 7 on the circuit board by a conductive bonding material such as solder. This construction exhibits the following advantages. The dielectric substrate 2 is mounted on the circuit board 5 with a conductive bonding material such as solder. Part of the conductive bonding material intervenes between the dielectric substrate 2 and the circuitboardS. The interveningamOUfltvarieSdepefldiflg0flvar]0Us conditions, such as the state of heating or the state of melting of the conductive bonding material at the time of mounting the dielectric substrate 2 on the circuit board 5 with the conductive bonding material. Thus, the gap between the dielectric substrate 2 and the circuit board 5 varies. Accordingly, the gap between the radiating electrode 3 on the dielectric substrate 2 and the inter-ground-capacitor loading electrode 7 on the circuit board varies. For example, in the construction shown in Fig. 6a, the capacitance between the radiating electrode 3 on the dielectric substrate 2 and the inter-ground-capacitor loading electrode 7 on the circuit board 5 varies. In contrast, when the inter-ground-capacitor loading electrode 7' is provided on the bottom surface of the dielectric substrate 2, the gap between

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-32 -the radiating electrode 3 and the inter-ground-capacitor loading electrode 7' can be precisely formed substantially in accordance withdesign. Thus, bybonding the inter-ground-capacitor loading electrode 7' with the inter-ground-capacitor loading electrode 7 on the circuit board 5 with a conductive bonding material, even when the gap between the dielectric substrate 2 and the circuit board5varies, variation in the capacitance between the radiating electrode 3 arid the inter-ground-capacitor loading electrodes 7 and 7' can be suppressed. This serves to further improve antenna performance.

Furthermore, although the dielectric substrate 2 has a rectangular parallelepiped shape in the first and second embodiments, the dielectric substrate 2 may have other shapes, suchasacylindrical shapeorapolygOnalPriSmShaPe. Furthermore, the radiating electrode 3 may have shapes other than, for example, the shape shown in Fig. 1, as long as it is a capacitivepowerfeedingtyPe radiating electrode.

Furthermore, although the ground electrode 6 is not formed in the region of the circuit board where the inter-ground-capacitor loading electrode 7 is formed in the first and second embodiment, for example, the inter-ground-capacitor loading electrode 7 may be formed on the top surface of the circuit board 5, and the ground electrode 6 may be formed on the bottom surface or an inner layer of the circuit board 5 corresponding to the region where the inter-ground-capacitor loading electrode 7 is formed.

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-33 -Furthermore, although the dielectric substrate 2 having the radiating electrode 3 is mounted in a ground region, the present invention can be applied to constructions in which a dielectric substrate having a radiating electrode is mounted in a non-ground region.

Industrial Applicability

According to the present invention, it is readily possible to adjust the resonant frequency of an antenna structure to a predetermined resonant frequency while suppressing increase in the size of the antenna structure deterioration in antenna gain.

This is effective for application to an antenna structure or a wireless communication device requiring miniaturization.

Claims

-34 -

CLAIMS

1. An antenna structure in which a radiating electrode that performs an antenna operation is provided on a substrate, and in which the substrate is mounted ona circuit board, the radiating electrode being provided on the substrate so as to oppose a board surface of the circuit board via a gap, wherein an inter-ground-capacitor loading electrode is formed on the circuit board so as to oppose the radiating electrode a ground electrode is formed on the circuit board via a gap with the inter-ground-capacitor loading electrode so as to avoid a region where the inter-ground-capacitor loading electrode is formed, a resonant-frequency adjusting element connecting the inter-ground-capacitor loading electrode with the ground electrode is further provided, and the resonant-frequency adjusting element has a capacitance or an inductance for adjusting a resonant frequency of the antenna structure to a predetermined resonant frequency.

2. The antenna structure according to Claim 1, wherein the substrate is mounted on the circuit board so that a part of the substrate is disposed on the ground electrode, the radiating electrode is formed so as to extend to the ground electrode through the part of the substrate disposed on the ground electrode so that the radiating electrode is connected directly to the ground electrode, and a slit is formed fri the ground electrode so that

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-35 -a part of the ground electrode extends continuously with the radiating electrode from a portion where the radiating electrode is joined and so that the part of the ground electrode functions as a part of the radiating electrode, the slit being formed so as to distinguish the part of the radiating electrode that functions as the part of the radiating electrode from the remaining part of the ground electrode.

3. A wireless communication device comprising the antenna structure according to Claim 1 or
2.