Antenna unit and electronic device

A first antenna element includes a first end connected to a feedpoint, a second end connected to a connection point on a ground conductor, and a fold disposed between the first and the second ends. A part of a segment between the first end and the fold of the first antenna element is disposed along the ground conductor. A second antenna element branches off the first antenna element. The second antenna element is disposed between the part of the first antenna element disposed along the ground conductor and the ground conductor. The segment between the first end and the fold of the first antenna element resonates at a first frequency. The second antenna element and a segment between the first end and a branch point of the first antenna element resonate at a second frequency that is higher than the first frequency.

TECHNICAL FIELD

The present disclosure relates to an antenna unit designed to operate on multiple bands of frequencies. The present disclosure relates to an electronic device equipped with such an antenna unit.

BACKGROUND ART

Antenna units for electronic devices that serve as portable wireless communication tools are required to be compact and operate on multiple bands of frequencies.

PTL 1 discloses a wireless communication tool equipped with antenna components including a first antenna element and a second antenna element that resonate at different frequencies, for example.

CITATION LIST

Patent Literature

SUMMARY

An antenna unit according to an aspect of the present disclosure includes a conductive ground plate, a first antenna element, and a second antenna element. The first antenna element includes a first end connected to a feedpoint, a second end connected to a first connection point on the conductive ground plate, and a fold disposed between the first and the second ends. A part of a segment between the first end and the fold of the first antenna element is disposed along the conductive ground plate. The second antenna element branches off the first antenna element at a branch point that is disposed on the segment between the first end and the fold of the first antenna element. The second antenna element is disposed between the part of the first antenna element disposed along the conductive ground plate and the conductive ground plate. The segment between the first end and the fold of the first antenna element resonates at a first frequency. The second antenna element and a segment between the first end and the branch point of the first antenna element resonate at a second frequency that is higher than the first frequency.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments will now be described in detail with reference to the drawings as appropriate. However, description in more detail than is necessary may be omitted. For example, detailed descriptions of well-known matters and redundant descriptions of substantially identical structural elements may be omitted so as to avoid unnecessarily redundant description and enable those of skill in the art to readily understand the exemplary embodiments herein.

The inventor(s) have provided the accompanying drawings and the following description to allow those skilled in the art to fully understand the present disclosure. Accordingly, these examples should not be construed to limit the spirit and scope of the appended claims.

The exemplary embodiments will be described with reference to XYZ Cartesian coordinates shown on the drawings.

1. First Exemplary Embodiment

With reference toFIGS. 1 to 5, an antenna unit according to a first exemplary embodiment will now be described.

FIG. 1is a schematic view illustrating a configuration of an antenna unit according to the first exemplary embodiment. The antenna unit ofFIG. 1includes ground conductor GND, first antenna element1, and second antenna element2.

Ground conductor GND is a conductive ground plate. In this specification, ground conductor GND is also referred to as a “conductive ground plate”. In an example ofFIG. 1, ground conductor GND is disposed parallel to an xy plane.

First and second antenna elements1and2are each made from a linear or strip-shaped conductor. First antenna element1is a folded monopole antenna. Second antenna element2is an open-ended monopole antenna. First antenna element1includes a first end connected to feedpoint11, a second end connected to connection point P4on ground conductor GND, and fold P3disposed between the first and the second ends. The antenna unit ofFIG. 1is fed with power in an unbalanced state via a feed line, e.g. coaxial cable at feedpoint11. An inner conductor of the feed line is connected to the first end of first antenna element1, whereas an outer conductor of the feed line is connected to connection point P1on ground conductor GND. A part of a segment between the first end and fold P3of first antenna element1is disposed along ground conductor GND. Likewise, a part of a segment between the second end and fold P3of first antenna element1is disposed along ground conductor GND. In the example ofFIG. 1, first antenna element1has bends midway, and the portions of predetermined lengths from the respective bends to fold P3are disposed along an x-axis. Second antenna element2branches off first antenna element1at branch point P2that is disposed on the segment between the first end and fold P3of first antenna element1. Second antenna element2is disposed between the portions of first antenna element1disposed along ground conductor GND and ground conductor GND. In the example ofFIG. 1, second antenna element2is disposed along the x-axis.

The antenna unit ofFIG. 1may further include matching circuit12between feedpoint11and branch point P2on first antenna element1.

The segment between the first end (adjacent to feedpoint11) and fold P3of first antenna element1resonates at frequency f1(which is also referred to as a “first frequency” in this specification). Thus, the segment between the first end and fold P3of first antenna element1has an electrical length of substantially one quarter of wavelength λ1corresponding to frequency f1. Second antenna element2and the segment between the first end and branch point P2of first antenna element1resonate at frequency f2(which is also referred to as a “second frequency” in this specification) that is higher than frequency f1. Thus, second antenna element2and the segment between the first end and branch point P2of first antenna element1have an electrical length of substantially one quarter of wavelength λ2corresponding to frequency f2.

FIG. 2is a schematic graph illustrating a profile of VSWR versus frequency of the antenna unit ofFIG. 1. The antenna unit ofFIG. 1includes first and second antenna elements1and2, and thus can operate on two frequency bands. With reference toFIG. 2, in a frequency band containing frequency f1, the antenna unit resonates at two frequencies, and a frequency band containing frequency f2is relatively narrow.

A decrease in the specific absorption rate (SAR) for the antenna unit ofFIG. 1will now be described.

Electronic devices that serve as portable wireless communication tools are used near the human body. As a result, some radiation power from the antenna of the device is absorbed by the human body. The SAR is a measure of the amount of this absorption and is represented by the following equation (1) using electrical conductivity σ, density ρ, and magnetic field intensity E.
SAR=σ/(2ρ)×|E|2(1)

The SAR is high in a vicinity of an area (a peak) where high-frequency currents crowd on a conductor. Thus, distributing peak high-frequency currents over the conductor leads to a decrease in the SAR. Since wavelength decreases with an increase in frequency, currents crowd in a small area on a conductor, and the SAR is high especially in the vicinity of the area. Consequently, in the antenna unit ofFIG. 1, electric currents tend to be locally concentrated on second antenna element2rather than first antenna element1that resonates at frequency f1because second antenna element2resonates at higher frequency f2.

Magnetic field intensity E is in inverse proportion to distance. Thus, according to the equation (1), the SAR comes down with an increase in distance between the antenna and the human body. Presumably, when the antenna unit ofFIG. 1is used in an electronic device, other internal components of the electronic device are disposed at a negative side relative to ground conductor GND in the Z-threction, and a casing of the electronic device and a part external to the casing (i.e. the human body of a user of the electronic device) are disposed at a positive side relative to first antenna element1in the Z-threction. In the antenna unit ofFIG. 1, second antenna element2is disposed between first antenna element1and ground conductor GND and is thereby kept away from the human body. This configuration can limit a rise in SAR even if electric currents of frequency f2are flowing into second antenna element2.

FIG. 3is a schematic view illustrating a configuration of an antenna unit according to a variation of the first exemplary embodiment. The second end of first antenna element1is connected to connection point P4on ground conductor GND via trap circuit13. Trap circuit13exhibits low impedance at frequency f1and high impedance at frequency f2.

FIG. 4is a circuit diagram illustrating a configuration of trap circuit13inFIG. 3. Trap circuit13includes capacitor C1and inductor L1in parallel, as well as another capacitor C2connected in series with these parallel-connected parts.FIG. 5is a schematic graph illustrating a profile of reactance versus frequency of trap circuit13inFIG. 3. Reactance in trap circuit13represents impedance in trap circuit13. With reference toFIG. 5, trap circuit13exhibits high impedance over frequency band F2containing frequency f2and low impedance over frequency band F1containing frequency f1. The antenna unit ofFIG. 3includes trap circuit13and thereby disconnects the second end of first antenna element1from connection point P4at frequency f2in terms of high frequencies. This causes first antenna element1, a folded monopole antenna, to show self-balancing behavior and limits the influence of first antenna element1on second antenna element2. This in turn enables second antenna element2to cover a wide frequency band.

1-4. Effects and Others

The antenna unit according to the first exemplary embodiment includes ground conductor GND, first antenna element1, and second antenna element2. First antenna element1includes the first end connected to feedpoint11, the second end connected to connection point P4on ground conductor GND, and fold P3disposed between the first and the second ends. A part of a segment between the first end and fold P3of first antenna element1is disposed along ground conductor GND. Second antenna element2branches off first antenna element1at branch point P2that is disposed on the segment between the first end and fold P3of first antenna element1. Second antenna element2is disposed between the parts of first antenna element1disposed along ground conductor GND and ground conductor GND. The segment between the first end and fold P3of first antenna element1resonates at frequency f1. Second antenna element2and the segment between the first end and branch point P2of first antenna element1resonate at frequency f2that is higher than frequency f1.

The antenna unit according to the first exemplary embodiment includes first and second antenna elements1and2, and thus can operate on two frequency bands. In the antenna unit according to the first exemplary embodiment, second antenna element2is disposed between first antenna element1and ground conductor GND. This configuration can limit a rise in SAR during operation on any of the frequency bands.

In the antenna unit according to the first exemplary embodiment, second antenna element2is disposed between first antenna element1and ground conductor GND. This configuration allows the antenna unit to come down in size, especially profile (height) along a z-axis inFIG. 1.

In the antenna unit according to the first exemplary embodiment, first antenna element1, which resonates at low frequencies and thus needs to have large dimensions, is relatively remote from ground conductor GND. This configuration enables the antenna unit to achieve high radiation efficiency both at frequencies f1and f2.

In the antenna unit according to the first exemplary embodiment, the second end of first antenna element1may be connected to connection point P4on ground conductor GND via trap circuit13. Trap circuit13exhibits low impedance at frequency f1and high impedance at frequency f2.

In the first exemplary embodiment, the antenna unit that includes trap circuit13can exhibit wideband characteristics both at frequencies f1and f2.

2. Second Exemplary Embodiment

With reference toFIGS. 6 and 7, an antenna unit according to a second exemplary embodiment will now be described.

FIG. 6is a schematic view illustrating a configuration of the antenna unit according to the second exemplary embodiment. The antenna unit ofFIG. 6is equivalent to the antenna unit ofFIG. 1that includes short-circuit element14as a replacement for matching circuit12. Short-circuit element14connects a point (connection point P5) on a segment between a first end (adjacent to feedpoint11) and fold P3of first antenna element1with a point (connection point P6) on a segment between a second end (adjacent to connection point P4) and fold P3of first antenna element1. A closed-loop circuit formed of first antenna element1and short-circuit element14has electrical length λa that differs both from integer multiples of wavelength λ1for frequency f1and integer multiples of wavelength λ2for frequency f2.

The antenna unit ofFIG. 6includes the closed-loop circuit formed of first antenna element1and short-circuit element14and thus has an effect of matching the impedance of first antenna element1. This allows the antenna unit to remove matching circuit12ofFIG. 1, i.e. a lumped parameter circuit, and provide improved radiation efficiency. The closed-loop circuit has electrical length λa that differs both from integer multiples of wavelength λ1and integer multiples of wavelength λ2. This configuration prevents the closed-loop circuit from resonating and reduces the occurrence of a decrease in radiation efficiency at frequencies f1and f2.

FIG. 7is a schematic view illustrating a configuration of an antenna unit according to a variation of the second exemplary embodiment. Short-circuit element14inFIG. 6may include reactance element15. The antenna unit ofFIG. 7includes short-circuit element14, which is omitted for the sake of simplified illustration, and reactance element15inserted between connection points P5and P6. Reactance element15is selected such that the closed-loop circuit has a desired electrical length. Reactance element15may be selected such that the impedance of first antenna element1is matched. Reactance element15may exhibit different characteristics at different frequencies f1and f2.

With reference toFIG. 7, the antenna unit may have the closed-loop circuit and matching circuit12in combination to match the impedance of first antenna element1.

2-4. Effects and Others

The antenna unit according to the second exemplary embodiment further includes short-circuit element14that connects the point on the segment between the first end and fold P3of first antenna element1with the point on the segment between the second end and fold P3of first antenna element1. The closed-loop circuit formed of first antenna element1and short-circuit element14has an electrical length that differs both from integer multiples of the wavelength for frequency f1and integer multiples of the wavelength for frequency f2.

In the antenna unit according to the second exemplary embodiment, short-circuit element14may include reactance element15.

The antenna unit according to the second exemplary embodiment includes the closed-loop circuit and thus displays improved radiation properties.

The antenna unit according to the second exemplary embodiment includes the closed-loop circuit and thus prevents reverse phase currents from flowing between first and second antenna elements1and2. This configuration allows the antenna unit to come down in size by having these antenna elements adjacent to each other.

3. Third Exemplary Embodiment

With reference toFIGS. 8 to 11, an antenna unit according to a third exemplary embodiment will now be described.

FIG. 8is a schematic view illustrating a configuration of the antenna unit according to the third exemplary embodiment. The antenna unit ofFIG. 8is equivalent to the antenna unit ofFIG. 1that further includes parasitic element3. In this specification, parasitic element3is also referred to as a “first passive element”. Parasitic element3has a first end and a second end. Parasitic element3is disposed relative to first antenna element1such that the first end is electromagnetically coupled to fold P3of first antenna element1and the second end is remoter from feedpoint11than the first end. Parasitic element3has no electrical connection with ground conductor GND and other conductors.

Parasitic element3may be disposed along ground conductor GND. In the example ofFIG. 8, parasitic element3is disposed along an x-axis.

Parasitic element3resonates at frequency f3(which is also referred to as a “third frequency” in this specification) that is other than frequencies f1and f2. Thus, parasitic element3has an electrical length that is between substantially one quarter and substantially one half of wavelength λ3corresponding to frequency f3. Frequency f3is close to frequency f2such that parasitic element3resonates at frequency f2as well to some extent. Frequency f3is higher than frequency f1.

FIG. 9is a schematic graph illustrating a profile of VSWR versus frequency of the antenna unit ofFIG. 8. The antenna unit ofFIG. 8includes first and second antenna elements1and2as well as parasitic element3, and thus can operate on three frequency bands. With reference toFIG. 9, in a frequency band containing frequency f1, the antenna unit resonates at two frequencies, and a frequency band containing frequencies f2and f3is wide.

A decrease in the SAR for the antenna unit ofFIG. 8will now be described.

FIG. 10is a drawing illustrating a flow of electricity in the antenna unit ofFIG. 8operating at frequency f2. Electric currents of frequency f2flow into first antenna element1as well as second antenna element2from feedpoint11via branch point P2. If frequencies f2and f3are close to each other, electric currents of frequency f2that have flowed into first antenna element1flow to parasitic element3by means of electromagnetic coupling between first antenna element1and parasitic element3. As described above, parasitic element3is disposed relative to first antenna element1such that one of the ends of parasitic element3is remote from feedpoint11. This configuration allows electric currents of frequency f2to flow from feedpoint11to the remote end of parasitic element3and thus distributes electric currents of frequency f2to a wider range than another configuration without parasitic element3. The antenna unit ofFIG. 8allows electric currents of frequency f2to flow into parasitic element3, and thereby lowers the level of current crowding on second antenna element2and limits a rise in SAR more effectively than the antenna unit ofFIG. 1.

The distance between parasitic element3and second antenna element2is larger than the distance between parasitic element3and first antenna element1in order that second antenna element2resonates at frequency f2without interference. This configuration hinders electric currents of frequency f2from directly flowing from second antenna element2to parasitic element3.

FIG. 11is a drawing illustrating a flow of electricity in the antenna unit ofFIG. 8operating at frequency f3. Electric currents of frequency f3flow from feedpoint11to first antenna element1and then flow to parasitic element3by means of electromagnetic coupling between first antenna element1and parasitic element3. As a result, parasitic element3resonates at frequency f3. Electric currents of frequency f3not only crowd on parasitic element3but also flow to first antenna element1. This configuration distributes peak electric currents over the elements and thus limits a rise in SAR.

While the antenna unit ofFIG. 8is operating at frequency f1, electric currents of frequency f1flow from feedpoint11into first antenna element1. If frequencies f1and f3are not close to each other, parasitic element3does not resonate at frequency f1, and electric currents of frequency f1that have flowed into first antenna element1do not flow to parasitic element3. Nevertheless, since wavelength λ1corresponding to frequency f1is longer than wavelengths λ2, λ3corresponding to frequencies f2, f3, electric currents are less inclined to crowd on first antenna element1resonating at frequency f1than second antenna element2resonating at frequency f2and parasitic element3resonating at frequency f3. Thus, the antenna unit can limit a rise in SAR even if electric currents of frequency f1are flowing into first antenna element1.

3-3. Effects and Others

The antenna unit according to the third exemplary embodiment further includes parasitic element3. Parasitic element3has the first and the second ends, and is disposed relative to first antenna element1such that the first end is electromagnetically coupled to fold P3of first antenna element1and the second end is remoter from feedpoint11than the first end. Parasitic element3has no electrical connection with ground conductor GND. Parasitic element3resonates at frequency f3other than frequencies f1and f2.

The antenna unit according to the third exemplary embodiment includes first and second antenna elements1and2, as well as parasitic element3, and thus can operate on three frequency bands despite small size. The antenna unit according to the third exemplary embodiment has parasitic element3and thus can limit a rise in SAR during operation on any of the frequency bands.

With reference toFIGS. 12 to 16, an antenna unit according to a fourth exemplary embodiment will now be described.

FIG. 12is a schematic view illustrating a configuration of the antenna unit according to the fourth exemplary embodiment. The antenna unit ofFIG. 12is equivalent to the antenna unit ofFIG. 1that further includes ground element4. In this specification, ground element4is also referred to as a “second passive element”. Ground element4has a first end that is electrically connected to connection point P7on ground conductor GND and a second end that is disposed so as to be electromagnetically coupled to fold P3of first antenna element1. Ground element4is disposed relative to first antenna element1such that the first end of ground element4is remoter from feedpoint11than the second end of ground element4.

Ground element4may be partly disposed along ground conductor GND. In the example ofFIG. 12, ground element4has a bend midway, and a portion of a predetermined length containing the bend and the second end of ground element4(the end electromagnetically coupled to fold P3of first antenna element1) is disposed along an x-axis.

Ground element4may be connected to ground conductor GND via reactance circuit16that exhibits high impedance at frequency f1.

Ground element4resonates at frequency f4(which is also referred to as a “fourth frequency” in this specification) that is other than frequencies f1and f2. Thus, ground element4has an electrical length of substantially one quarter of wavelength λ4corresponding to frequency f4. Frequency f4is close to frequency f2such that ground element4resonates at frequency f2as well to some extent. Frequency f4is higher than frequency f1.

Frequency f4is approximately one and a half times as high as frequency f1, for example.

FIG. 13is a schematic graph illustrating a profile of VSWR versus frequency of the antenna unit ofFIG. 12. The antenna unit ofFIG. 12includes first and second antenna elements1and2as well as ground element4, and thus can operate on three frequency bands. With reference toFIG. 13, in a frequency band containing frequency f1, the antenna unit resonates at two frequencies, and a frequency band containing frequencies f2and f4is wide.

A decrease in the SAR for the antenna unit ofFIG. 12will now be described.

FIG. 14is a drawing illustrating a flow of electricity in the antenna unit ofFIG. 12operating at frequency f2. Electric currents of frequency f2flow into first antenna element1as well as second antenna element2from feedpoint11. If frequencies f2and f4are close to each other, electric currents of frequency f2that have flowed into first antenna element1flow to ground element4by means of electromagnetic coupling between first antenna element1and ground element4. Electric currents of frequency f2that have flowed into ground element4flow to ground conductor GND via connection point P7. As described above, ground element4is disposed relative to first antenna element1such that one of the ends of ground element4is remote from feedpoint11. This configuration allows electric currents of frequency f2to flow from feedpoint11to the remote end of ground element4and thus distributes electric currents of frequency f2to a wider range than another configuration without ground element4. The antenna unit ofFIG. 12allows electric currents of frequency f2to flow into ground element4and ground conductor GND, and thereby lowers the level of current crowding on second antenna element2and limits a rise in SAR more effectively than the antenna units ofFIGS. 1 and 8.

The distance between ground element4and second antenna element2is larger than the distance between ground element4and first antenna element1in order that second antenna element2resonates at frequency f2without interference. This configuration hinders electric currents of frequency f2from directly flowing from second antenna element2to ground element4.

FIG. 15is a drawing illustrating a flow of electricity in the antenna unit ofFIG. 12operating at frequency f4. Electric currents of frequency f4flow from feedpoint11to first antenna element1and then flow to ground element4by means of electromagnetic coupling between first antenna element1and ground element4. The electric currents of frequency f4also flow through ground conductor GND and into ground element4via connection point P7. As a result, ground element4resonates at frequency f4. The electric currents of frequency f4not only crowd on ground element4but also flow to first antenna element1and ground conductor GND. This configuration distributes peak electric currents over the elements and thus limits a rise in SAR.

While the antenna unit ofFIG. 12is operating at frequency f1, electric currents of frequency f1flow from feedpoint11into first antenna element1. If frequencies f1and f4are not close to each other, ground element4does not resonate at frequency f1, and the electric currents of frequency f1that have flowed into first antenna element1do not flow to ground element4. In the case of frequencies f1and f4that are close to each other, ground element4does not resonate at frequency f1but resonates only at frequency f4if ground element4is connected to ground conductor GND via reactance circuit16that exhibits high impedance at frequency f1. As described above, the antenna unit can limit a rise in SAR even if the electric currents of frequency f1are flowing into first antenna element1.

FIG. 16is a schematic view illustrating a configuration of an antenna unit according to a variation of the fourth exemplary embodiment. The antenna unit ofFIG. 16is equivalent to the antenna unit ofFIG. 12that further includes capacity coupler18between first antenna element1and ground element4. A section of ground element4containing the second end of ground element4(the end electromagnetically coupled to fold P3of first antenna element1) is capacitively coupled to a section of first antenna element1containing fold P3through capacity coupler18over a predetermined length. Reactance circuit17works to shorten the electrical length of ground element4at frequency f4. Thus, reactance circuit17includes a capacitor.

Since capacity coupler18is disposed between first antenna element1and ground element4, electromagnetic coupling between first antenna element1and ground element4is strengthened. This configuration facilitates the flow of electric currents of frequencies f2and f4between first antenna element1and ground element4and reduces the occurrence of current crowding.

The length of ground element4may be increased to distribute electric currents of a certain frequency over a wider range rather than letting the electric currents crowd in a narrow range. The length of ground element4may be increased to extend the distance from feedpoint11to the first end of ground element4(the end electrically connected to connection point P7on ground conductor GND) and enable the antenna unit to cover a wide frequency band. The length of ground element4needs to be increased to have capacity coupler18installed. Even if the length of ground element4is longer than substantially one quarter of wavelength λ4corresponding to frequency f4for any of these purposes, ground element4resonates at frequency f4because of reactance circuit17that shortens the electrical length of ground element4.

4-4. Effects and Others

The antenna unit according to the fourth exemplary embodiment further includes ground element4. Ground element4has the first end that is electrically connected to connection point P7on ground conductor GND and the second end that is disposed so as to be electromagnetically coupled to fold P3of first antenna element1. Ground element4is disposed relative to first antenna element1such that the first end of ground element4is remoter from feedpoint11than the second end of ground element4. Ground element4resonates at frequency f4other than frequencies f1and f2.

In the antenna unit according to the fourth exemplary embodiment, ground element4may be connected to ground conductor GND via reactance circuit16or17. Reactance circuit16or17exhibits high impedance at frequency f1.

In the antenna unit according to the fourth exemplary embodiment, a section of ground element4containing the second end of ground element4may be capacitively coupled to a section of first antenna element1containing fold P3over a predetermined length. Reactance circuit16or17works to shorten the electrical length of ground element4at frequency f4.

The antenna unit according to the fourth exemplary embodiment includes first and second antenna elements1and2, as well as ground element4, and thus can operate on three frequency bands despite small size. The antenna unit according to the fourth exemplary embodiment has ground element4and thus can limit a rise in SAR during operation on any of the frequency bands. The antenna unit that further includes reactance circuit16or17allows ground element4to longitudinally overlap first antenna element1. This configuration encourages the flow of electric currents to ground element4and distributes peak electric currents over the elements more effectively, leading to a further reduction in SAR.

With reference toFIGS. 17 and 18, an antenna unit according to a fifth exemplary embodiment will now be described.

FIG. 17is a schematic view illustrating a configuration of the antenna unit according to the fifth exemplary embodiment. The antenna unit ofFIG. 17is a combination of the third and fourth exemplary embodiments and includes both parasitic element3inFIG. 8and ground element4inFIG. 12.

Parasitic element3is disposed along ground conductor GND, whereas a part of ground element4is disposed along ground conductor GND. Parasitic element3is disposed between the part of ground element4disposed along ground conductor GND and ground conductor GND.

FIG. 18is a schematic graph illustrating a profile of VSWR versus frequency of the antenna unit ofFIG. 17. Frequencies f2and f4are higher than frequency f1. Frequency f3is higher than frequencies f2and f4. The antenna unit ofFIG. 17includes first and second antenna elements1and2, as well as parasitic element3and ground element4, and thus can operate on four frequency bands.

While the antenna unit ofFIG. 17is operating at frequency f2, electric currents of frequency f2flow into parasitic element3and ground element4. This configuration lowers the level of current crowding on second antenna element2and limits a rise in SAR.

As described in the third and fourth exemplary embodiments, the antenna unit ofFIG. 17limits a rise in SAR while the antenna unit is operating at any of frequencies f3and f4.

As described above, parasitic element3has an electrical length of substantially one half of wavelength λ3corresponding to frequency f3, whereas ground element4has an electrical length of substantially one quarter of wavelength λ4corresponding to frequency f4. Thus, if parasitic element3and ground element4have similar dimensions, parasitic element3resonates at a frequency higher than (about twice) a frequency at which ground element4resonates, and electric currents are more crowded on parasitic element3. In the antenna unit ofFIG. 17, parasitic element3is disposed between the part of ground element4disposed along ground conductor GND and ground conductor GND such that the antenna unit limits a rise in SAR even if electric currents of frequency f3are flowing into parasitic element3. This configuration has parasitic element3kept away from the human body and thus can limit a rise in SAR even if electric currents of frequency f3are flowing into parasitic element3.

5-3. Effects and Others

The antenna unit according to the fifth exemplary embodiment includes both parasitic element3and ground element4.

In the antenna unit according to the fifth exemplary embodiment, parasitic element3may be disposed along ground conductor GND, and a part of ground element4may be disposed along ground conductor GND. Parasitic element3is disposed between the part of ground element4disposed along ground conductor GND and ground conductor GND.

The antenna unit according to the fifth exemplary embodiment includes first and second antenna elements1and2, as well as parasitic element3and ground element4, and thus can operate on four frequency bands despite small size. The antenna unit according to the fifth exemplary embodiment has parasitic element3and ground element4, and thus can limit a rise in SAR during operation on any of the frequency bands.

With reference toFIG. 19, an antenna unit according to a sixth exemplary embodiment will now be described.

FIG. 19is a schematic view illustrating a configuration of the antenna unit according to the sixth exemplary embodiment. The antenna unit ofFIG. 19is equivalent to the antenna unit ofFIG. 1that further includes passive element5. Passive element5has a first end that is electrically connected to connection point P7on ground conductor GND via reactance circuit19and a second end that is disposed so as to be electromagnetically coupled to fold P3of first antenna element1. Passive element5is disposed relative to first antenna element1such that the first end of passive element5is remoter from feedpoint11than the second end of passive element5. Reactance circuit19exhibits high impedance at frequency f3other than frequencies f1and f2, and low impedance at frequency f4other than frequencies f1, f2and f3. Reactance circuit19acts as a filter circuit that substantially blocks electrical currents of frequency f4.

Passive element5resonates at frequencies f3and f4. Thus, passive element5has an electrical length that is between substantially one quarter and substantially one half of wavelength λ3corresponding to frequency f3and an electrical length of substantially one quarter of wavelength λ4corresponding to frequency f4.

While the antenna unit ofFIG. 19is operating at frequency f3, passive element5acts as parasitic element3in the third exemplary embodiment. While the antenna unit ofFIG. 19is operating at frequency f4, passive element5acts as ground element4in the fourth exemplary embodiment. The antenna unit ofFIG. 19includes first and second antenna elements1and2, as well as passive element5, and thus can operate on four frequency bands in like manner with the antenna unit of the fifth exemplary embodiment.

While the antenna unit ofFIG. 19is operating at frequency f2, electric currents of frequency f2flow into passive element5. This configuration lowers the level of current crowding on second antenna element2and limits a rise in SAR.

As described in the third and third exemplary embodiments, the antenna unit ofFIG. 19limits a rise in SAR while the antenna unit is operating at any of frequencies f3and f4.

6-3. Effects and Others

The antenna unit according to the sixth exemplary embodiment further includes passive element5. Passive element5has the first end that is electrically connected to connection point P7on ground conductor GND via reactance circuit19and the second end that is disposed so as to be electromagnetically coupled to fold P3of first antenna element1. Passive element5is disposed relative to first antenna element1such that the first end of passive element5is remoter from feedpoint11than the second end of passive element5. Reactance circuit19exhibits high impedance at frequency f3other than frequencies f1and f2, and low impedance at frequency f4other than frequencies f1, f2and f3. Passive element5resonates at frequencies f3and f4.

The antenna unit according to the sixth exemplary embodiment includes first and second antenna elements1and2, as well as passive element5, and thus can operate on four frequency bands despite small size. The antenna unit according to the sixth exemplary embodiment has passive element5and thus can limit a rise in SAR during operation on any of the frequency bands. The antenna unit according to the sixth exemplary embodiment has single passive element5that integrates parasitic element3in the third exemplary embodiment with ground element4in the fourth exemplary embodiment. This configuration allows the antenna unit to come down in size.

With reference toFIGS. 20 to 22, an electronic device according to a seventh exemplary embodiment will now be described.

FIG. 20is a schematic view illustrating a configuration of the electronic device according to the seventh exemplary embodiment. The electronic device ofFIG. 20includes an antenna unit according to any of the first to sixth exemplary embodiments, casing21having a first surface and a second surface opposite to each other, and display22provided on the first surface of casing21. Hereafter, the first surface of casing21(at a negative side in the Y-direction inFIG. 20) is referred to as a “front surface”, and the second surface of casing21(at a positive side in the Y-direction inFIG. 20) is referred to as a “rear surface”. In the example ofFIG. 20, the electronic device includes the antenna unit of the fifth exemplary embodiment.

If the electronic device is, for example, a tablet-type electronic device equipped with a display, the rear surface of the electronic device is presumably held by a user's hand or other body part while the device is in use. Consequently, the necessity to reduce the occurrence of a rise in SAR is greater at the rear surface than at the front surface of the electronic device. Since frequency f2at which second antenna element2resonates is higher than frequency f1at which first antenna element1resonates, second antenna element2is closer to the front surface than to the rear surface of casing21. This configuration keeps second antenna element2away from the human body.

Feedpoint11may be closer to the first surface than to the second surface of casing21, and connection point P4may be closer to the second surface than to the first surface of casing21. Feedpoint11and connection point P4that are away from each other enable the antenna unit to cover a wide frequency band.

A part of a segment between a first end (adjacent to feedpoint11) and fold P3of first antenna element1is disposed at a distance from ground conductor GND, whereas a part of a segment between a second end (adjacent to connection point P4) and fold P3of first antenna element1is disposed at a distance from ground conductor GND. Both the distances may be substantially equal to each other. In other words, out of the segments of first antenna element1, the respective parts disposed along ground conductor GND (along an x-axis) may be disposed at a substantially identical distance from ground conductor GND. In this configuration, the segment between the first end and fold P3of first antenna element1is closer to the first surface than to the second surface of casing21, whereas the segment between the second end and fold P3of first antenna element1is closer to the second surface than to the first surface of casing21. This configuration, in which the parts disposed along ground conductor GND are disposed at a substantially identical distance from ground conductor GND, allows the antenna unit to come down in size, especially profile along a z-axis inFIG. 20.

If the electronic device is equipped with the antenna unit of the third exemplary embodiment, parasitic element3may be closer to the front surface than to the rear surface of casing21.

If the electronic device is equipped with the antenna unit of the fourth exemplary embodiment, ground element4may be closer to the front surface than to the rear surface of casing21.

If the electronic device is equipped with the antenna unit of the fifth exemplary embodiment, any one of parasitic element3and ground element4, whichever resonates at a higher frequency, may be closer to the front surface than to the rear surface of casing21. If the interior of casing21has dimensional allowance, both parasitic element3and ground element4may be closer to the front surface than to the rear surface of casing21.

If the electronic device is equipped with the antenna unit of the sixth exemplary embodiment, passive element5may be closer to the front surface than to the rear surface of casing21.

FIG. 21is a perspective view illustrating example implementation of the electronic device according to the seventh exemplary embodiment. The electronic device ofFIG. 21includes the antenna unit of the fourth exemplary embodiment. First and second antenna elements1and2, and ground element4of the antenna unit are constituted as patterns of copper foil formed on dielectric substrate31, i.e. a flexible substrate, for example. The patterns of copper foil formed on dielectric substrate31may further include a part of ground conductor GND. Dielectric substrate31is fasten to casing21with screws32,33. If casing21is metallic, ground element4and ground conductor GND on dielectric substrate31are electrically connected to casing21via screws32,33. If casing21is dielectric, ground element4and ground conductor GND on dielectric substrate31are connected to a metallic chassis or any conductive part inside casing21of the electronic device. Connection point P1is connected to a wireless communications circuit in casing21of the electronic device via coaxial cable34. Casing21has a recess for accommodating the antenna unit. The antenna unit provided on the recess is covered with covering21amade from a dielectric (e.g. a synthetic resin).

In the electronic device ofFIG. 21, dielectric substrate31may be bent. This configuration, if the antenna unit ofFIG. 12is put in the electronic device of the seventh exemplary embodiment, allows the parts of first antenna element1disposed along the x-axis to be disposed at a substantially identical distance from ground conductor GND. This configuration also allows second antenna element2and ground element4to be closer to the front surface than to the rear surface of casing21.

FIG. 22is a schematic graph illustrating a profile of VSWR versus frequency of the antenna unit in the electronic device ofFIG. 21. The antenna unit ofFIG. 21includes first and second antenna elements1and2, as well as ground element4, and thus can operate on three frequency bands in like manner with the antenna unit ofFIG. 12.

7-3. Effects and Others

The electronic device in the seventh exemplary embodiment includes any one of the antenna units according to the first to sixth exemplary embodiments.

The electronic device according to the seventh exemplary embodiment may include casing21having a first surface and a second surface opposite to each other and display22provided on the first surface of casing21. Second antenna element2may be closer to the first surface than to the second surface of casing21.

In the electronic device according to the seventh exemplary embodiment, feedpoint11may be closer to the first surface than to the second surface of casing21, and connection point P4may be closer to the second surface than to the first surface of casing21.

In the electronic device according to the seventh exemplary embodiment, the part of the segment between the first end and fold P3of first antenna element1and the part of the segment between the second end and fold P3of first antenna element1may be disposed at a substantially identical distance from ground conductor GND.

In the electronic device according to the seventh exemplary embodiment, parasitic element3may be closer to the first surface than to the second surface of casing21.

In the electronic device according to the seventh exemplary embodiment, ground element4may be closer to the first surface than to the second surface of casing21.

In the electronic device according to the seventh exemplary embodiment, passive element5may be closer to the first surface than to the second surface of casing21.

The electronic device in the seventh exemplary embodiment includes any of the antenna units according to the first to sixth exemplary embodiments and thus can operate on a plurality of frequency bands despite small size.

The electronic device according to the seventh exemplary embodiment can limit a rise in SAR during operation on any of the frequency bands since at least one of second antenna element2, parasitic element3, ground element4, and passive element5is closer to the front surface than to the rear surface of casing21. The electronic device according to the seventh exemplary embodiment can limit a rise in SAR especially at the rear surface of casing21.

In the electronic device according to the seventh exemplary embodiment, feedpoint11and connection point P4are away from each other. This configuration enables the antenna unit to cover a wide frequency band.

In the electronic device according to the seventh exemplary embodiment, the parts disposed along ground conductor GND are disposed at a substantially identical distance from ground conductor GND. This configuration allows the antenna unit to come down in size.

Example

FIG. 23is a schematic view illustrating a configuration of an antenna unit according to a comparative example. The antenna unit ofFIG. 23includes ground conductor GND, first antenna element101, and second antenna element102.

First and second antenna elements101and102are each an open-ended monopole antenna. First antenna element101is an inverted-L antenna. Second antenna element102branches off first antenna element101at branch point P2. In the antenna unit ofFIG. 1, second antenna element2is disposed between the parts of first antenna element1disposed along ground conductor GND and ground conductor GND. In the antenna unit ofFIG. 23, second antenna element102is disposed on an opposite side of first antenna element101from ground conductor GND.

The antenna unit ofFIG. 23further includes matching circuit12between feedpoint11and branch point P2on first antenna element101.

First antenna element101resonates at frequency f1. Second antenna element102and a segment between feedpoint P1and branch point P2of first antenna element101resonate at frequency f2.

FIG. 24is a schematic graph illustrating profiles of antenna efficiency versus frequency of the antenna units ofFIGS. 1, 16, and 23. The antenna efficiency is a ratio of the power radiated from an antenna unit relative to the power delivered to the antenna unit and literally represents the efficiency of the antenna unit. Generally, antenna units with higher antenna efficiencies are preferable. The antenna units ofFIGS. 1 and 16had higher antenna efficiencies than the antenna unit ofFIG. 23at frequency band f1. In the antenna unit ofFIG. 16, the short circuit between connection points P5, P6contributes to improvement in antenna efficiency, while a loss generated on trap circuit13at frequency f1impairs performance. Consequently, the antenna units ofFIGS. 1 and 16exhibited comparable antenna efficiencies. At frequency band f2, the antenna unit ofFIG. 23exhibited a higher antenna efficiency than the antenna unit ofFIG. 1, and the antenna unit ofFIG. 16exhibited a higher antenna efficiency than the antenna unit ofFIG. 23. This is because of ground element4enhancing radiation efficiency, as well as trap circuit13enabling the antenna unit to deliver performance on a par with a monopole antenna.

FIG. 25is a schematic graph illustrating SAR values for the antenna units ofFIGS. 1, 16, and 23.FIG. 25shows SAR values measured for the antenna units ofFIGS. 1, 16, and 23when a human body phantom comes nearer to each of the units from upward (a positive side in the Z-direction). As a general trend, SAR values were higher (worse) at frequency f2, a higher frequency, than at frequency f1. The antenna units ofFIGS. 1, 16, and 23showed no large difference in SAR at frequency f1. At frequency f2, the antenna unit ofFIG. 1exhibited a low SAR than the antenna unit ofFIG. 23. This is because in the antenna unit ofFIG. 1, second antenna element2is disposed between the parts of first antenna element1disposed along ground conductor GND and ground conductor GND. At frequency f2, the antenna unit ofFIG. 16exhibited a substantially low SAR than the antenna unit ofFIG. 1. This is because electric currents on first antenna element1are distributed into ground element4.

Other Exemplary Embodiments

The first to seventh exemplary embodiments described above are provided to illustrate technologies disclosed in this patent application. Technologies according to the present disclosure, however, can be applied to any variations to which change, replacement, addition, omission, or the like are appropriately made, other than the exemplary embodiments. A new exemplary embodiment can be made by combining some structural elements in any of the first to seventh exemplary embodiments described above.

In light of this, other exemplary embodiments will now be shown.

The first to sixth exemplary embodiments each have flat-shaped ground conductor GND as a conductive ground plate, for example. The conductive ground plate may be a predetermined area on a metallic chassis of an electronic device, however, and may have any shape other than the flat shape, with proviso that a second antenna element is disposed between a first antenna element and the conductive ground plate.

FIG. 9shows an instance in which frequency f3is higher than frequency f2. But frequency f3may be lower than frequency f2. Likewise,FIG. 13shows an instance in which frequency f4is higher than frequency f2. But frequency f4may be lower than frequency f2.FIG. 18shows an instance in which frequency f3is higher than frequency f4. But frequency f4may be higher than frequency f3.

The exemplary embodiments described above are provided to illustrate technologies according to the present disclosure. For that purpose, the accompanying drawings and detailed description are provided.

Consequently, the accompanying drawings and detailed description provided to illustrate the technologies described above may include structural elements that are not essential for resolving problems as well as those essential for resolving problems. Thus, these non-essential structural elements, if they are included in the accompanying drawings or detailed description, should not be construed as essential structural elements.

Since the exemplary embodiments described above are provided to illustrate technologies according to the present disclosure, various kinds of change, replacement, addition, omission, or the like may be made to these exemplary embodiments without departing from the scope of the claims and equivalents thereof.

INDUSTRIAL APPLICABILITY

An antenna unit according to the present disclosure can operate on multiple bands of frequencies and is very effective among multiband antennas if the antenna unit is required to operate on a wider range of frequencies. The antenna unit according to the present disclosure can limit a rise in SAR and readily satisfy SAR-specific regulatory requirements. The present disclosure can provide an epoch-making antenna unit that enables a tablet-type terminal equipped with a multiband antenna, for example, to fulfill contradicting demands, i.e. improvement in wireless performance and a decrease in SAR, at high level and achieve space savings.

REFERENCE MARKS IN THE DRAWINGS

1first antenna element

2second antenna element

GND ground conductor