JP2010081370A - Antenna circuit and radio apparatus using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized antenna circuit and a radio apparatus using the same which controls an increase of a radiation electrode as low as possible and varies a resonance frequency of a fundamental frequency band by a variable reactance element reducing an influence giving to a resonance state of a higher order frequency band. <P>SOLUTION: A multi-band antenna having at least the radiation electrode corresponding to the fundamental frequency band and the radiation electrode corresponding to the higher order frequency band, a first matching circuit to connect to a power feeding side of the multi-band antenna, a variable matching circuit to connect through the first matching circuit and a first transmission line, and a second matching circuit to connect through the variable matching circuit and a second transmission line are provided. The variable matching circuit includes a variable capacity element connected to the first matching circuit in series and a power feeding circuit toward the variable capacity element. The first matching circuit and the second matching circuit are comprised of reactance elements, change electrostatic capacity by feeding power toward the variable capacity element, and varies the resonance frequency of the fundamental frequency band. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an antenna circuit used in a radio apparatus, and more particularly to a multiband antenna circuit that can be used in a plurality of different frequency bands and a radio apparatus using the same.

In recent years, wireless devices such as mobile phones have rapidly spread, and the bandwidth used for communication is also wide-ranging. In particular, in recent mobile phones, there are an increasing number of examples in which a plurality of transmission / reception bands are provided in a communication device such as a single mobile phone as called a dual band method, a triple band method, a quad band method, or the like.
The frequency band of the communication system used in the quad-band mobile phone is, for example, GSM850 / 900 band (824 to 960 MHz), DCS band (1710 to 1850 MHz), PCS band (1850 to 1990 MHz), UMTS band (1920 to 2170 MHz). If the GSM band is the basic frequency band, the three consecutive frequency bands, DCS band, PCS band, and UMTS band, are approximately 2 to 2.5 times the frequency of the GSM band. Sometimes called the next frequency band.

Under such circumstances, a multiband antenna capable of supporting a plurality of transmission / reception bands is required as an antenna constituting an antenna circuit built in a wireless device such as a mobile phone.
Usually, the radiation electrode constituting the antenna has a basic resonance frequency, and further has a higher-order resonance frequency. Here, the frequency band including the resonance frequency includes the frequency f1 (sometimes referred to as a main resonance point) that resonates at the lowest frequency and can be matched with the high frequency circuit, that is, the voltage standing wave ratio VSWR is predetermined. A frequency band that is equal to or less than the numerical value is a basic frequency band, and a frequency band that includes a frequency f2 (which may be referred to as a higher-order resonance point) that causes higher-order resonance than that is a higher-order frequency band.
The frequency band required for multiband antennas is particularly wide in the higher-order frequency band than the fundamental frequency band, and sufficient bandwidth cannot be secured by higher-order resonance alone, making it difficult to use. It was. In the communication system exemplified above, the frequency bandwidth covered by the basic frequency band is 136 MHz, and the frequency bandwidth is 460 MHz in the higher frequency band.

In order to deal with such a problem, Patent Document 1 discloses that a variable reactance element (varicap diode) is connected in series with an antenna, and the capacitance value is changed to adjust the resonance frequency according to the frequency to be used. Thus, it has been proposed to enable wireless communication at different frequencies.
However, this method cannot broaden the fundamental frequency band or higher-order frequency band, and the resonance frequency in the fundamental frequency band or higher-order frequency band changes, so that the fundamental frequency as in GSM and UMTS. A mobile phone that uses a multimode that simultaneously uses bands and higher-order frequency bands cannot be used due to a problem that matching cannot be obtained in a necessary frequency band.

As another method, it is possible to broaden the high-order frequency band by adding to the antenna another radiation electrode that obtains the resonance frequency f3 at a frequency close to the higher-order resonance frequency f2 generated by one radiation electrode. (For example, Patent Document 2). According to this method, a multiband antenna that can be used for a wireless device that operates in a multimode can be obtained, but there remains a problem that the bandwidth of the fundamental frequency band is narrow.
Based on the same technical idea, by adding another radiation electrode that obtains the resonance frequency f4 at a frequency close to the frequency f1 to the antenna, the bandwidth of the fundamental frequency band is widened, and the higher order resonance causes the higher order frequency. Although it is possible to widen the band, the bandwidth obtained by higher-order resonance is narrower than that at low-frequency resonance, so the desired higher-order frequency band may still not be obtained. there were.
Although other radiation electrodes may be added, the antenna is usually disposed in a limited space, for example, a limited space in a mobile phone casing, so that it is difficult to add additional radiation electrodes. There are cases. In addition, when using a feed radiation electrode and a feed radiation electrode, there may be a case where the electromagnetic coupling between the radiation electrodes is used to make a double resonance state, but increasing the radiation electrode complicates the coupling state, There may be a problem that desired characteristics cannot be obtained.
JP 2002-208771 A Special table 2001-522152

The present inventors are diligently researching a multiband antenna that can be used for a multimode / multiband radio apparatus, and combine the conventional techniques to widen a high-order frequency band by using a plurality of radiation electrodes and to change the band. A multiband antenna was conceived in which a reactance element was connected and the resonance frequency in the fundamental frequency band was made variable by changing the capacitance value and the inductance value.
However, even if the known techniques are simply combined, the higher-order resonance frequency changes with the resonance frequency in the fundamental frequency band, resulting in a problem that the VSWR characteristic in the higher-order frequency band deteriorates and the multi-band antenna does not function. did.
Therefore, the present invention provides a multi-band antenna having a plurality of radiation electrodes, suppressing the increase of radiation electrodes as much as possible, and reducing the influence on the resonance state of the higher-order frequency band, while reducing the resonance frequency of the fundamental frequency band by a variable reactance element. It is an object of the present invention to provide a small antenna circuit that can change the frequency and a wireless device using the antenna circuit.

  The present invention is an antenna circuit comprising a multiband antenna having a radiation electrode corresponding to at least a fundamental frequency band and a radiation electrode corresponding to a higher order frequency band, and a variable matching circuit connected to a power feeding side of the multiband antenna. The variable matching circuit includes a first matching circuit connected to a multiband antenna, a variable capacitance element connected in series with the first matching circuit, and a power feeding circuit to the variable capacitance element. The circuit is constituted by a reactance element including a grounded inductance element, and an electrostatic capacity is changed by feeding power to the variable capacitance element so that a resonance frequency in a fundamental frequency band is variable.

The first matching circuit includes a grounded inductance element and a capacitance element connected in parallel thereto, and the second matching circuit includes an inductance element connected in series with the variable capacitance element.
In the present invention, with respect to the impedance (characteristic impedance is 50Ω) when the multiband antenna is viewed from the connection point between the first matching circuit and the first transmission line, the impedance resistance from the start frequency fa1 to the end frequency fa2 in the fundamental frequency band. The component is preferably 15Ω or more and 150Ω or less. The resistance component of the impedance at the center frequency fa3 exceeds 50Ω, and the resistance component of the impedance at the start frequency fa1 and the end frequency fa2 is less than 50Ω. Preferably, the resistance component of the impedance at the center frequency fa3 is 100Ω or more, and the resistance component of the impedance at the start frequency fa1 and the end frequency fa2 is less than 25Ω.

  The resistance component of the impedance from the start frequency fb1 to the end frequency fb2 in the higher frequency band is 15Ω or more and 400Ω or less, the resistance component of the impedance fb3 at the center frequency fb3 is less than 50Ω, and the impedance components at the start frequency fb1 and the end frequency fb2 The resistance component exceeds 50Ω. Preferably, the resistance component in the impedance at the start frequency fb1 and the end frequency fb2 is 100Ω or more.

  The first transmission line connected to the first matching circuit and the second transmission line connected to the second matching circuit function as a phase circuit. The phase rotation in the fundamental frequency band is less than the phase rotation in the higher-order frequency band, and the reactance change amount due to the change in the capacitance value of the variable matching circuit is inversely proportional to the frequency. By appropriately configuring the first transmission line, the variable matching circuit, the second transmission line, and the second matching circuit by a method described later in accordance with such general rules, even if the capacitance value is changed by the variable matching circuit, The resonance frequency in the fundamental frequency band can be easily adjusted without greatly affecting the resonance frequency in the higher-order frequency band, and the VSWR is 4 or less, preferably 3 or less, more preferably 2 in the desired frequency band. It can be as follows.

The impedance when the multiband antenna is viewed from one end of the second matching circuit on the high frequency circuit side is such that the resistance component of the impedance at the start frequency fb1 and the end frequency fb2 is lower than 50Ω in the high-order frequency band, and the variable capacitance element It is preferable that the phase θlb of the start frequency is adjusted to a region of −135 ° to −180 ° and the phase θhb of the impedance ZH at the end frequency is adjusted to a region of + 135 ° to + 180 °.
According to such a configuration, the impedance on the Smith chart shows a trajectory that rotates around the standardized impedance of 50Ω, and a plurality of resonance points are generated in the high-order frequency band, and the band where the VSWR is low is expanded. I can do it.

In addition, in a state where the variable capacitance element has a relatively low capacitance value, the phase θla of the impedance Zl at the start frequency fa1 in the fundamental frequency band is in the region of + 135 ° to + 180 °, and the variable capacitance device has a relatively high capacitance value. In this state, it is preferable to adjust the phase θha of the impedance Zha at the end frequency fa2 in the fundamental frequency band to a range of + 135 ° to + 180 °.
At the center frequency fa3, as described above, the impedance of the multiband antenna viewed from the first matching circuit is in a high impedance state. For this reason, even when the second matching circuit composed of the inductance elements is connected in series and the capacitance value of the variable capacitance element is further changed, the phase change of the impedance when the multiband antenna side is viewed from one end on the high frequency circuit side is small.
On the other hand, since the phase change between the start frequency fa1 and the end frequency fa2 is large depending on the capacitance value of the variable capacitance element, the capacitance value of the variable capacitance element is changed to change between the center frequency fa3 and the start frequency fa1 or between the end frequency fa2. By setting the phase difference to approximately 180 °, it is possible to change the resonance frequency of one resonance point generated between the start frequency fa1 and the end frequency fa2.

In the present invention, the multiband antenna has at least two radiation electrodes. The first radiating electrode that resonates at the first frequency and the second radiating electrode that resonates at the second frequency are formed of reverse L-shaped, inverted F-shaped, meander-shaped, spiral-shaped, strip-shaped conductors, Arranged in a positional relationship. All of the radiation electrodes may be power supply radiation electrodes, or a part of the radiation electrodes may be composed of parasitic radiation electrodes. Further, a λ / 4 antenna that short-circuits one end side or a λ / 2 antenna that opens both ends may be used.
Each radiation electrode is composed of a conductive thin plate made of Cu or phosphor bronze, printed circuit board such as FR4 (glass epoxy resin board), or ceramic substrate made of alumina or other dielectric ceramic material, such as printing or etching. It is preferable to use a good conductor such as Au, Ag, or Cu having a low resistance by the above method. If the radiation electrode is formed of an alloy such as phosphor bronze which is easy to process but is not easily deformed by an external force, it is possible to form the radiation electrode in a free shape regardless of the support. Further, a radiation electrode may be formed on the ceramic body with the good conductor and mounted on a printed board.

  Each radiation electrode is preferably configured so that most of it does not face the ground plate. When a printed circuit board or the like is used as a support, a ground board (ground electrode) non-forming portion is provided on the printed circuit board, or preferably separated from other high-frequency circuits, and the ground board (ground electrode) is substantially As a printed circuit board (sub board) which does not have, a ground board and a radiation electrode are formed apart. When the ground plate is disposed in the vicinity of the radiation electrode, the current flowing through the ground plate increases, and the radiation characteristics are deteriorated, so that the antenna gain is lowered. In either case, the distance from the ground plate is determined so as to be a value that does not cause a problem in performance. The sub-board and the main board on which another high-frequency circuit is formed may be connected by a coaxial line (first transmission line). In addition, the reactance element constituting the first matching circuit is preferably grounded to the ground pattern of the main board via a connection pattern formed on the sub board.

The multiband antenna used in the present invention is configured such that the first radiation electrode has a resonance frequency at a frequency relatively lower than that of the second radiation electrode. FIG. 8A shows an example of the VSWR characteristic of the first radiation electrode in a state where impedance matching with the feeder circuit is achieved. The VSWR value is small in the frequency band in the vicinity of the main resonance point f1, and the VSWR value is also small in the vicinity of the high-order resonance point f2. Similarly, an example of the VSWR characteristic by the second radiation electrode is shown in FIG. The second radiation electrode has a resonance frequency f3 close to the higher-order resonance frequency f2 of the first radiation electrode. FIG. 8C shows the VSWR characteristics of a multiband antenna configured by combining radiation electrodes having such characteristics. By setting the double resonance state in the high-order frequency band by the first radiation electrode and the second radiation electrode, the high-order frequency band can be widened.
Note that the higher-order resonance frequency in the first radiation electrode varies depending on the capacitance (including parasitic capacitance and coupling capacitance c ′ with a conductor pattern including another radiation electrode) added to the radiation electrode. The change in capacitance has a greater effect at the higher-order resonance point f2 than at the main resonance point f1, and when the adjustment is made, the higher-order resonance point f2 is changed and combined with other radiation electrodes. The bandwidth over which the VSWR value can be obtained can be adjusted.

The variable capacitance element is preferably a varicap diode or an RF-MEMS variable capacitance element. An RF-MEMS variable capacitance element can be selected if a large change in capacitance value is necessary, and a varicap diode can be selected if a relatively small change in capacitance value is acceptable.
Further, if the variable capacitance element is a varicap diode, a first matching circuit is connected to the cathode side of the varicap diode, and a power feeding circuit is connected to the anode side, the grounded inductance element of the first matching circuit is used as a current. Therefore, the number of parts of the variable capacitance circuit can be reduced, and the antenna circuit can be downsized.

  According to a second aspect of the present invention, there is provided a radio apparatus including the antenna circuit according to the first aspect of the present invention.

  ADVANTAGE OF THE INVENTION According to this invention, the multiband antenna which makes the resonance frequency of a fundamental frequency band variable while reducing the influence which it has on the resonance state of a high-order frequency band, and a radio | wireless apparatus using the same can be provided.

FIG. 1 is a block diagram of an antenna circuit according to an embodiment of the present invention, and FIG. 2 shows an equivalent circuit. The antenna circuit connects the first matching circuit 20 connected to the multiband antenna ANT, the first transmission line φ1 connecting the first matching circuit 20 and the variable matching circuit 10, and the variable matching circuit 10 and the second matching circuit 30. Configured as a circuit including the second transmission line φ2.
In order to simplify the description below, the start frequency fa1 of the fundamental frequency band is 824 MHz, the end frequency fa2 is 960 MHz, the center frequency fa3 is 894 MHz, the start frequency fb1 of the higher-order frequency band is 1710 MHz, and the end frequency fb2 is 2182. Although it is set to 96 MHz, it is not limited to this.

  The antenna circuit of the present invention includes a multiband antenna ANT, a first matching circuit 20 connected to the power feeding side of the multiband antenna ANT, and a variable matching connected via the first matching circuit 20 and the first transmission line φ1. The circuit 10 includes a second matching circuit 30 connected to the variable matching circuit 10 via the second transmission line φ2.

  FIG. 3 is a perspective view of a multiband antenna according to an embodiment of the present invention. 4A shows impedance characteristics of the multiband antenna alone, and FIG. 4B shows VSWR (voltage standing wave ratio) characteristics. FIG. 5A shows impedance characteristics of the multiband antenna including the first matching circuit, and FIG. 5B shows its VSWR (voltage standing wave ratio) characteristics.

The multiband antenna shown here has a first feeding radiation electrode ANT1 and a second feeding radiation electrode ANT2 mainly composed of sheet metal.
The first feeding radiation electrode ANT1 and the second feeding radiation electrode ANT2 are arranged on a printed board 200 (sometimes referred to as a sub-board) made of glass epoxy. On the printed circuit board 200, there are provided an electrode portion 260 connected to the casing of the mobile phone and the ground of the main substrate, and a feed line 250 to each radiation electrode. In this example, a portion extending in the same direction as the radiation electrode of the feed line 250 is used as a part of the radiation electrode.

The total length of the first feeding radiation electrode ANT1 is set to about ¼ of the wavelength of the fundamental frequency band (center frequency fa3 to end frequency fa2), and the total length of the second feeding radiation electrode ANT2 is set to a high-order frequency band (start frequency). It is set to about 1/4 of the wavelength of fb1 to end frequency fb2).
For the first and second feeding radiation electrodes ANT1 and ANT2, a conductor thin plate (also referred to as a sheet metal) made of Cu or phosphor bronze is used. Two U-shaped sheet metals are used for the first feeding radiation electrode ANT1 and are arranged on both sides of the printed circuit board 200 at a predetermined interval, and are electrically connected to each other by via-hole electrodes provided on the printed circuit board 200. One end side is an open end, and the other end side is connected to the power supply electrode 250 via another via hole electrode. Further, one U-shaped sheet metal is used for the second feeding radiation electrode ANT2, and is arranged at a predetermined interval on one surface side of the printed circuit board 200, with one end side being an open end and the other end side being a feeding electrode. 250.
The open end sides of the first feed radiation electrode ANT1 and the second feed radiation electrode ANT2 are arranged so as to be capacitively coupled to each other, and the resonance frequency in the higher frequency band can be adjusted by the formed capacitance c ′. ing.
Although not shown, if a dielectric or magnetic ceramic formed in a predetermined shape such as a rectangle in contact with the sheet metal is disposed on the printed circuit board 200, the actual length of the radiation electrode is shortened by the wavelength shortening effect. Can be configured.

  FIG. 4A shows the impedance characteristics of the multiband antenna in this state, and FIG. 4B shows the VSWR characteristics. FIG. 4A shows a change in impedance frequency on a Smith chart. The impedance shows a trajectory that rotates around 50Ω from the fundamental frequency band to the higher frequency band. In the figure, each marker indicated by a triangle indicates a predetermined frequency, marker 1 is a start frequency fa1 in the fundamental frequency band, marker 2 is a center frequency fa3, marker 3 is an end frequency fa2, and marker 4 is a higher frequency band. The start frequency fb1 and the marker 5 indicate the end frequency fb2 of the higher-order frequency band.

  As shown in the VSWR characteristics, the multiband antenna that is not matched at all exhibits a large reflection characteristic in the fundamental frequency band and the higher-order frequency band. There is a frequency band with a VSWR of 4 or less between the end frequency fa2 (marker 3) in the fundamental frequency band and the start frequency fb1 (marker 4) in the higher-order frequency band. On the Smith chart, the end frequency fa2 (marker 3) To a high-order frequency start frequency fb1 (marker 4), and appears as a rotation locus with a small impedance. This is due to the multiple resonance of the first feeding radiation electrode ANT1. When the radiating electrode is formed by being folded repeatedly, resonance appears due to the length of the radiating electrode between the folds, parasitic capacitance, etc. If this resonance is used well, a frequency band with a low VSWR can be obtained. A wide band is also possible.

FIG. 5A shows the impedance characteristics when the first matching circuit is connected to such a multiband antenna, and FIG. 5B shows the VSWR characteristics. The first matching circuit is a parallel resonant circuit of a grounded inductance element L1 and a capacitance element C1, and is a chip component that connects between the electrode portion 260 on the printed circuit board 200 and the feed line 250 to each radiation electrode. Composed.
The parallel resonance circuit gives a positive susceptance value at a higher frequency than the resonance frequency and a negative susceptance value at a lower frequency. Therefore, the resonance frequency of the first matching circuit is sufficiently higher than that of the higher-order frequency band so that the impedance characteristics of the multiband antenna including the first matching circuit are higher in the higher-order frequency band than the fundamental frequency band. In this embodiment, the inductance element is 1.8 nH, the capacitance element is 0.75 pF, a negative susceptance value is given in the fundamental frequency band and the higher frequency band, and the impedance locus Was made as shown in FIG. As shown in the VSWR characteristics of FIG. 5B, a desired bandwidth (VSWR value of 4 or less) cannot be obtained in the high-order frequency band only by connecting the first matching circuit.

The first transmission line φ1 is connected to the first matching circuit 20. Since the first transmission line φ1 functions as a phase circuit, the locus of impedance in the Smith chart rotates clockwise around the center.
The impedance characteristics of the multiband antenna viewed from point c including the first transmission line φ1 are such that the impedance moves to the lower right region of the Smith chart at the start frequency fb1 and end frequency fb2 in the higher-order frequency band. The first transmission line φ1 is rotated so that the reactance is negative so that the start frequency fb1 and the end frequency fb2 are capacitive. The phase change at the start frequency fa1 and the end frequency fa2 in the fundamental frequency band is slight compared to the higher frequency band, and the change in the impedance locus on the Smith chart is small.

  The first transmission line φ1 is configured by a coaxial line or a line pattern formed by etching on a printed circuit board (main board). When the multiband antenna is configured on the sub-board 200 as in this embodiment, a coaxial line or a shielded flexible printed wiring board is used. When a multiband antenna is provided on the main board, it is formed with a line pattern on the main board. In this embodiment, since the main board and the sub board are separated in order to reduce the influence of the ground on the radiation characteristics, the first transmission line φ1 is soldered using a coaxial cable. It is preferable to provide a connector on each substrate and connect it to the coaxial cable because not only the connection is easy, but also the occurrence of misalignment of the connection position for each connection is small, and the variation in characteristics can be reduced.

The variable matching circuit 10 connected to the first matching circuit 20 includes a DC cut capacitor C3, a voltage dividing resistors R1 and R2, a power feeding circuit 50 including a choke coil L2, and a varicap capacitor D1 that is a variable capacitance element. . The varicap diode D1 is connected in series with the multiband antenna ANT, and has an anode connected to the first transmission line φ1 and a cathode connected to the second transmission line φ2. A power supply circuit 50 is connected to the cathode to apply a voltage to the varicap diode.
Chip components are used as elements such as a DC cut capacitor C3, resistors R1 and R2 for voltage division, and a choke coil L2, and are mounted on the main board and soldered.
When a positive voltage is applied to the cathode of the varicap diode D1, its capacitance value decreases. For example, a varicap diode RKV606KL manufactured by Renesas Technology Corporation has a capacitance value of about 5 pF at a voltage of 0V and a capacitance value of about 1.8 pF at a voltage of 3V. In the antenna circuit of the present invention, when a voltage of 0V is applied, a resonance point is generated between the end frequency of the fundamental frequency band between 960 MHz and the center frequency fa3, and the resonance point is started by applying a voltage of 3V. The frequency band is moved between the frequency fa1 and the center frequency fa3 so that the frequency band where the VSWR becomes a predetermined value can be changed.

  The connection state of the varicap diode D1 functions similarly even if the anode is connected to the second transmission line φ2 and the cathode is connected to the first transmission line φ1. However, this connection further requires a grounded resistor or inductance element between the varicap diode D1 and the first matching circuit 20 and between the DC cut capacitor C2. With the connection method described above, since the inductance element L1 of the first matching circuit 20 can be used as a current path, the number of parts of the antenna circuit can be reduced and the size can be reduced.

The impedance of the multiband antenna viewed from the point c including the first transmission line φ1 is higher than the start frequency fa1 in the fundamental frequency band and the impedance in the end frequency fa2, the center frequency fa3 in the fundamental frequency band, and the start frequency in the higher order frequency band The relationship in which the resistance components at fb1 and end frequency fb2 are large is maintained.
In such a state, even if a capacitance component is added by the variable capacitance element, the change in reactance is inversely proportional to the capacitance value and the frequency. Therefore, the change in the locus of impedance on the Smith chart viewed from the point d is the fundamental frequency. The frequency is slight at the center frequency fa3 of the band, the start frequency fb1 of the higher frequency band, and the end frequency fb2.
On the other hand, since the impedance of the start frequency fa1 and the end frequency fa2 in the fundamental frequency band has a small resistance component and is relatively lower in frequency than the higher-order frequency band, the phase changes greatly depending on the magnitude of the capacitance component. Such a behavior of impedance enables the resonance frequency in the fundamental frequency band to be adjusted by the variable capacitance element.

  The impedance of the multiband antenna including the second transmission line φ2 viewed from the point E further rotates in phase clockwise. At this time, by adjusting the line length of the second transmission line φ2, the center frequency fa3 in the fundamental frequency band is set to a higher impedance state than the start frequency fb1 and the end frequency fb2 in the higher frequency band. Further, the impedance change in the fundamental frequency band is less affected by the connection of the second transmission line φ2, and the change in the impedance locus on the Smith chart is small. The second transmission line φ2 was formed by a line pattern on the main substrate.

  The second matching circuit 30 includes an inductance element L3 connected in series with the second transmission line φ2. Furthermore, a capacitance element that is grounded may be included. A positive reactance is given by the inductance element L3, and the VSWR is matched to 4 or less in the fundamental frequency band and the higher-order frequency band. As the inductance element L3, a chip component having an inductance value of 4.7 nH was used.

As described above, the impedance of the start frequency fa1 and the end frequency fa2 in the fundamental frequency band is adjusted by the variable capacitance element D1.
In a state in which a voltage of 0 V is applied to the variable capacitance element D1 (capacitance value is relatively large), the impedance in the fundamental frequency band on the Smith chart is the upper right region at the start frequency fa1, the lower right region at the center frequency fa3, The region is the upper left region at the end frequency fa2. The start frequency fa1 and the center frequency fa3 are higher in impedance than the end frequency fa2, the inductive impedance is indicated at the start frequency fa1 and the end frequency fa2, and the capacitive impedance is indicated at the center frequency fa3. At this time, it is preferable that the impedance phase difference between the center frequency fa3 and the end frequency fa2 is set to be 130 ° to 200 °, preferably 170 ° to 190 °.

  FIG. 6A shows the impedance characteristics in a state where a voltage of 0 V is applied to the variable capacitance element D1, and FIG. 6B shows the VSWR characteristics. The impedances at the start frequency fa1 and the center frequency fa3 in the fundamental frequency band are higher than the impedances of the end frequency fa2, the start frequency fb1 and the end frequency fb2 in the higher frequency band, and the center frequency fa3 and the end frequency fa2 in the fundamental frequency band. The phase difference is about 170 °. The resonance frequency in the fundamental frequency band is between the center frequency fa3 and the end frequency fa2, and the bandwidth in which the VSWR in the higher-order frequency band is 4 or less satisfies the band from the start frequency fb1 to the end frequency fb2. The bandwidth has been increased.

FIG. 7A shows the impedance characteristics in a state where a voltage of 3 V is applied to the variable capacitance element D1, and FIG. 7B shows the VSWR characteristics. In this case, the capacitance value becomes relatively small, and a negative reactance is given.
The impedance characteristics on the Smith chart change little at the center frequency fa3 in the fundamental frequency band, the start frequency fb1 and the end frequency fb2 in the higher frequency band, but the phases of the start frequency fa1 and the end frequency fa2 in the fundamental frequency band are the same. The phase difference between the center frequency fa3 and the start frequency fa1 in the fundamental frequency band was about 180 °.
The bandwidth in the higher-order frequency band is almost unchanged before and after the change of the capacitance value, but the resonance frequency in the fundamental frequency band has moved between the start frequency fa1 and the center frequency fa3.

  As described above, by using a multi-band antenna having a plurality of radiation electrodes and using a variable matching circuit having a variable capacitance element D1, the influence on the higher-order frequency band is suppressed and the fundamental frequency band in the fundamental frequency band is obtained. The resonance frequency of the can be made variable.

According to the present invention, by adjusting the voltage applied to the variable capacitance element constituting the variable matching circuit, the capacitance component of the antenna circuit is adjusted, and the resonance frequency of the fundamental frequency band is made variable, while the higher frequency band It is possible to obtain an antenna circuit that hardly changes the resonance frequency. Therefore, the gain in the higher frequency band is not changed.
In addition, since the resonance frequency in the fundamental frequency band can be adjusted, it is possible to support a wide range of frequency bands while suppressing an increase in the number of radiation electrodes, and wireless devices that transmit and receive in a wide range of frequencies, especially the fundamental frequency band and higher-order frequencies. It can be used for a multi-mode wireless device that uses bands simultaneously.
Further, since the variable matching circuit and the first and second matching circuits can be configured by chip elements, the antenna circuit can be configured in a small size.

It is a block diagram which shows the structure of the antenna circuit which concerns on one Example of this invention. It is an equivalent circuit diagram which shows the structure of the antenna circuit (except a multiband antenna) concerning one Example of this invention. It is a perspective view which shows the structure of the multiband antenna part used for the antenna circuit which concerns on one Example of this invention. (A) It is a Smith chart which shows the impedance characteristic in the multiband antenna single-piece | unit used for the antenna circuit which concerns on one Example of this invention. (B) It is the frequency characteristic figure of the VSWR. (A) It is a Smith chart which shows the impedance characteristic of the multiband antenna used for the antenna circuit which concerns on one Example of this invention, and a 1st matching circuit. (B) It is the frequency characteristic figure of the VSWR. (A) It is a Smith chart which shows the impedance characteristic at the time of applying the voltage of 0V to the variable capacitance element of the antenna circuit which concerns on one Example of this invention. (B) It is the frequency characteristic figure of the VSWR. (A) It is a Smith chart which shows the impedance characteristic at the time of applying the voltage of 3V to the variable capacitance element of the antenna circuit which concerns on one Example of this invention. (B) It is the VSWR frequency characteristic figure. (A) It is a frequency characteristic figure which shows an example of the VSWR characteristic of the 1st radiation electrode of the multiband antenna used for the antenna circuit of this invention. (B) It is a frequency characteristic figure which shows an example of the VSWR characteristic of the 2nd radiation electrode of the multiband antenna used for the antenna circuit of this invention. (C) It is a frequency characteristic figure which shows an example of the VSWR characteristic at the time of combining a 1st radiation electrode and a 2nd radiation electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Variable matching circuit 20 1st matching circuit 30 2nd matching circuit 200 Printed circuit board (sub board | substrate)
250 Feed line φ1 First transmission line φ2 Second transmission line ANT1 First radiation electrode ANT2 Second radiation electrode ANT Multiband antenna

Claims (6)

A multiband antenna having a radiation electrode corresponding to at least a fundamental frequency band and a radiation electrode corresponding to a higher-order frequency band; a first matching circuit connected to a power feeding side of the multiband antenna; the first matching circuit; A variable matching circuit connected via a transmission line; and a second matching circuit connected to the variable matching circuit via a second transmission line;
The variable matching circuit includes a variable capacitance element connected in series with the first matching circuit and a power feeding circuit to the variable capacitance element, wherein the first matching circuit and the second matching circuit are configured by reactance elements, An antenna circuit characterized in that a capacitance is changed by feeding power to a capacitive element, and a resonance frequency in a fundamental frequency band is variable.   2. The first matching circuit includes a grounded inductance element and a capacitance element connected in parallel thereto, and the second matching circuit includes an inductance element connected in series with the variable capacitance element. The antenna circuit described.   The variable capacitance element is a varicap diode, wherein a first matching circuit is connected to a cathode side of the varicap diode, a second matching circuit is connected to an anode side, and a power feeding circuit is connected. The antenna circuit according to claim 2.   The main circuit board on which the high-frequency circuit is formed is formed with a variable capacitance circuit element and a feeding circuit that constitute the second matching circuit and the variable matching circuit, and a multi-band antenna and a first matching circuit are formed on the sub-board, and the first matching circuit is formed. 4. The antenna circuit according to claim 1, wherein the circuit and the variable capacitance element are connected by a coaxial line.   5. The sub-board is not provided with a ground pattern, and the reactance element constituting the first matching circuit is grounded to the ground pattern of the main board through a connection pattern formed on the sub-board. The antenna circuit described in 1.   A radio apparatus comprising the antenna circuit according to claim 1.

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