Antenna circuit

Abstract

In an antenna circuit including an antenna 1 and an antenna 2 that is connected in series with the antenna 1 and includes inductance, a variable capacitor C.sub.v and a variable resistor R.sub.v connected in parallel with the antenna 2 are provided. This enables controlling of an actual amplitude ratio r and a phase difference between currents I.sub.1 and I.sub.2 flowing through the two antennas 1 and 2 into desired values. Flows of the currents I.sub.1 and I.sub.2 with the phase difference through the antennas 1 and 2 enable forming of a favorable communication area. In addition, setting of the actual amplitude ratio r between the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 to a value other than 1 enables forming of an asymmetric communication area.

Claims

1. An antenna circuit, comprising: a first antenna; a second antenna that is connected in series with the first antenna and includes inductance; an adjustment capacitor that is connected in parallel with the second antenna; and an adjustment resistor that is connected in parallel with the second antenna, wherein desired amplitude ratio r.sub.0 and phase difference .sub.0 are obtained under conditions where a capacitance value of the adjustment capacitor is expressed by: $\frac{1-r_0\cos {}_0}{{}^2{L}_2}$ and a resistance value of the adjustment resistor is expressed by: $\frac{L_2}{r_0\sin {}_0}$ where r.sub.0 is an amplitude ratio between currents flowing through the first antenna and the second antenna, .sub.0 is a phase difference between the currents flowing through the first antenna and the second antenna, is an angular frequency of the currents flowing through the first antenna and the second antenna, and L.sub.2 is the inductance of the second antenna.

2. The antenna circuit according to claim 1, wherein the amplitude ratio r.sub.0 is 1.

3. The antenna circuit according to claim 1, wherein the first antenna and the second antenna are in a same shape.

4. An antenna circuit comprising: a first antenna; a second antenna that is connected in series with the first antenna and includes inductance; a capacitor connected in series with the second antenna; a resistor connected in series with the second antenna; an adjustment capacitor connected in parallel with a circuit including the second antenna, the capacitor, and the resistor, and an adjustment resistor connected in parallel with the circuit including the second antenna, the capacitor, and the resistor.

5. The antenna circuit according to claim 4, wherein desired amplitude ratio r.sub.0 and phase difference .sub.0 are obtained under conditions where a capacitance value of the adjustment capacitor is expressed by $C_2.Math.\frac{C_2{R}_2{r}_0\sin {}_0+\left({}^2{L}_2{C}_2-1\right)\left(1-r_0\cos {}_0\right)}{1+{\left(C_2{R}_2\right)}^2+{}^2{L}_2{C}_2\left({}^2{L}_2{C}_2-2\right)}$ and a resistance value of the adjustment resistor is expressed by $\frac{1}{C_2}.Math.\frac{1+{\left(C_2{R}_2\right)}^2+{}^2{L}_2{C}_2\left({}^2{L}_2{C}_2-2\right)}{C_2{R}_2\left(r_0\cos {}_0-1\right)+r_0\sin {}_0\left({}^2{L}_2{C}_2-1\right)}$ to obtain desired amplitude ratio r.sub.0 and phase difference .sub.0 where r.sub.0 is an amplitude ratio between currents flowing through the first antenna and the second antenna, .sub.0 is a phase difference between the currents flowing through the first antenna and the second antenna, is an angular frequency of the currents flowing through the first antenna and the second antenna, L.sub.2 is the inductance of the second antenna, C.sub.2 is the capacitance value of the adjustment capacitor, and R.sub.2 is the resistance value of the adjustment resistor.

6. The antenna circuit according to claim 5, wherein the amplitude ratio r.sub.0 is 1.

7. The antenna circuit according to claim 4, wherein the first antenna and the second antenna are in a same shape.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram that illustrates a configuration example of an antenna circuit of an embodiment.

(2) FIG. 2 is a diagram that illustrates an equivalent circuit of the antenna circuit in FIG. 1.

(3) FIG. 3 is a diagram that illustrates current waveforms in a simulation on the equivalent circuit in FIG. 2.

(4) FIG. 4 is a diagram that illustrates an arrangement example of antennas.

(5) FIG. 5 is a diagram that illustrates magnetic field distribution obtained when currents with an amplitude ratio of 1 and no phase difference are applied to the antennas in FIG. 4.

(6) FIG. 6 is a diagram that illustrates magnetic field distribution obtained when the currents illustrated in FIG. 3 are applied to the antennas in FIG. 4.

(7) FIG. 7 is a diagram that illustrates other current waveforms in a simulation on the equivalent circuit in FIG. 2.

(8) FIG. 8 is a diagram that illustrates magnetic field distribution obtained when the currents illustrated in FIG. 7 are applied to the antennas in FIG. 4.

(9) FIG. 9 is a diagram that illustrates a configuration example of another antenna circuit of the embodiment.

(10) FIG. 10 is a diagram that illustrates an equivalent circuit of the antenna circuit in FIG. 9.

(11) FIG. 11 is a diagram that illustrates current waveforms in a simulation on the equivalent circuit in FIG. 10.

(12) FIG. 12 is a diagram that illustrates other current waveforms in a simulation on the equivalent circuit in FIG. 10.

MODE FOR CARRYING OUT THE INVENTION

(13) An embodiment of the present invention is described below with reference to the drawings.

(14) FIG. 1 is a diagram that illustrates a configuration example of an antenna circuit of this embodiment.

(15) The antenna circuit illustrated in FIG. 1 includes multiple antennas 1 and 2 connected in series, a signal source 3, and a variable capacitor C.sub.v and a variable resistor R.sub.v connected in parallel with the antenna 2.

(16) In the antenna circuit illustrated in FIG. 1, the antennas 1 and 2 are connected such that currents therethrough flow in different directions from each other. However, the antennas 1 and 2 may be connected such that the currents therethrough flow in the same direction. Complex amplitude of a current I.sub.2 flowing through the antenna 2 varies according to a capacitance value of the variable capacitor C.sub.v and a resistance value of the variable resistor R.sub.v and differs from complex amplitude of a current I.sub.1 flowing through the antenna 1. Appropriate setting of the capacitance value of the variable capacitor C.sub.v and the resistance value of the variable resistor R.sub.v enables controlling of an actual amplitude ratio (r=|I.sub.1/I.sub.2|) and a phase difference (=arg (I.sub.1/I.sub.2)) between the currents flowing through the two antennas 1 and 2 into desired values. Unless otherwise stated, the symbols I.sub.1 and I.sub.2 denoting the currents represent the complex amplitude.

(17) FIG. 2 illustrates an equivalent circuit of the antenna circuit in FIG. 1. Z.sub.1 represents impedance of the antenna 1. L.sub.2 represents inductance of the antenna 2.

(18) The actual amplitude ratio r and the phase difference between the currents are indicated by the following equations (1) and (2).

(19) $\begin{array}{cc}\left[\mathrm{Math}.1\right]& \\ r=.Math.1-{}^2{L}_2{C}_v.Math..Math.\sqrt{1+{\left(\frac{L_2/R_v}{1-{}^2{L}_2{C}_v}\right)}^2}& \left(1\right)\\ \left[\mathrm{Math}.2\right]& \\ =\mathrm{arc}\tan \left(\frac{L_2/R_v}{1-{}^2{L}_2{C}_v}\right)& \left(2\right)\end{array}$

(20) In the equations, is an angular frequency of a signal generated by the signal source 3.

(21) Both the equations (1) and (2) are independent of the impedance Z.sub.1 of the antenna 1. Thus, an arbitrary antenna can be used as the antenna 1. The antenna 2 may be at least an antenna having inductance such as a loop antenna and a bar antenna. In the example of FIG. 1, the antennas 1 and 2 have the same shape; however, they may have different shapes.

(22) Now, a value C.sub.v.sup.opt of the variable capacitor C.sub.v and a value R.sub.v.sup.opt of the variable resistor R.sub.v required to achieve desired actual amplitude ratio r.sub.0 and phase difference .sub.0 between the currents are described.

(23) The actual amplitude ratio r.sub.0 and phase difference .sub.0 are substituted into the equations (1) and (2) to solve for C.sub.v.sup.opt and R.sub.v.sup.opt, and thus the following equations (3) and (4) can be obtained.

(24) $\begin{array}{cc}\left[\mathrm{Math}.3\right]& \\ C_v^{\mathrm{opt}}=\frac{1-r_0\cos {}_0}{{}^2{L}_2}& \left(3\right)\\ \left[\mathrm{Math}.4\right]& \\ R_v^{\mathrm{opt}}=\frac{L_2}{r_0\sin {}_0}& \left(4\right)\end{array}$

(25) The desired actual amplitude ratio r.sub.0 and phase difference .sub.0 can be achieved by setting the value of the variable capacitor C.sub.v of the antenna circuit in FIG. 1 to a value obtained from the equation (3) and setting the value of the variable resistor R.sub.v of the antenna circuit in FIG. 1 to a value obtained from the equation (4).

(26) For example, it is assumed that the desired actual amplitude ratio r.sub.0 and phase difference .sub.0 are values of the following equation (5).

(27) [Math. 5]
r.sub.0=1,0.sub.0=11 deg(5)

(28) In addition, it is assumed that a signal frequency f of the signal source 3 and the inductance L.sub.2 of the antenna 2 are values of the following equation (6).

(29) [Math. 6]
f=1 MHz, L.sub.2=10 H(6)

(30) The equations (5) and (6) are substituted into the equations (3) and (4) to calculate appropriate parameters, and thus the following values C.sub.v.sup.opt and R.sub.v.sup.opt can be obtained.

(31) [Math. 7]
C.sub.v.sup.optT46.5389 pF, R.sub.v.sup.opt=329.292(7)

(32) FIG. 3 illustrates waveforms of the currents I.sub.1 and I.sub.2 in a simulation on the equivalent circuit in FIG. 2 using the parameters of the equations (6) and (7). An amplitude of voltage applied from the signal source 3 is 1V.

(33) According to FIG. 3, it can be seen that the amplitude ratio between the currents I.sub.1 and I.sub.2 is 1 and a time difference therebetween is 31 ns (comparable to 11 deg). In the antenna circuit of this embodiment, employment of the values C.sub.v.sup.opt and R.sub.v.sup.opt calculated from the equations (3) and (4) enables controlling of the amplitude ratio and phase difference between the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 as desired.

(34) Now, effects of the current phase difference on magnetic field distribution are described.

(35) As illustrated in FIG. 4, it is assumed that the antennas 1 and 2 having the same shape are disposed in an x-y plane. It is assumed that directions of the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 are different from each other.

(36) FIG. 5 is a diagram that illustrates magnetic field distribution obtained when currents with the amplitude ratio of 1 and no phase difference are applied to the antennas 1 and 2 illustrated in FIG. 4. FIG. 6 is a diagram that illustrates magnetic field distribution obtained when the currents I.sub.1 and I.sub.2 illustrated in FIG. 3 are applied to the antennas 1 and 2 illustrated in FIG. 4. Each contour in FIGS. 5 and 6 indicate a line plotting points where a magnetic field strength has a certain value at a y-coordinate of 0, 50, 60, 70, or 80 cm.

(37) In FIG. 5, the contour is divided when a value of the y-coordinate is large, and this is a problem in forming a communication area. On the other hand, in FIG. 6, the contour is not divided even when the value of the y-coordinate is large, and this enables forming of a favorable communication area.

(38) Next, an example of forming asymmetric magnetic field distribution is described.

(39) As illustrated in FIG. 5, the magnetic field distribution obtained by the conventional antenna circuit configuration is symmetric about a y-z plane. In addition, as illustrated in FIG. 6, changing of only phases of the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 still makes the magnetic field distribution symmetric about the y-z plane. However, asymmetric magnetic field distribution may be desired in some application.

(40) In the antenna circuit of this embodiment, setting of both the amplitude and the phases of the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 to different values enables creating of magnetic field distribution asymmetric about the y-z plane.

(41) For example, it is assumed that the currents I.sub.1 and I.sub.2 satisfying the following equation (8) flow through the antennas 1 and 2. Note that, it is assumed that the signal frequency f of the signal source 3 and the inductance L.sub.2 of the antenna 2 are the values of the equation (6).

(42) [Math. 8]
r.sub.0=1/1.3,.sub.0=10 deg(8)

(43) The equations (6) and (8) are substituted into the equations (3) and (4) to calculate appropriate parameters, and thus the following values C.sub.v.sup.opt and R.sub.v.sup.opt can be obtained.

(44) [Math. 9]
C.sub.v.sup.optT614.147 pF, R.sub.v.sup.opt=470.384(9)

(45) FIG. 7 illustrates waveforms of the currents I.sub.1 and I.sub.2 in a simulation on the equivalent circuit in FIG. 2 using the parameters of the equations (6) and (9). An amplitude of voltage applied from the signal source 3 is 1V.

(46) According to FIG. 7, it can be seen that the amplitude ratio of the currents I.sub.1 and I.sub.2 is 1/1.3 and a time difference therebetween is 28 ns (comparable to 10 deg).

(47) FIG. 8 is a diagram that illustrates magnetic field distribution obtained when the currents I.sub.1 and I.sub.2 illustrated in FIG. 7 are applied to the antennas 1 and 2 illustrated in FIG. 4. As illustrated in FIG. 8, it can be seen that magnetic field distribution asymmetric about the y-z plane is formed.

(48) Next, another antenna circuit of this embodiment is described.

(49) FIG. 9 is a diagram that illustrates a configuration example of the other antenna circuit of this embodiment.

(50) The antenna circuit illustrated in FIG. 9 is different from the antenna circuit in FIG. 1 in that a resistor R.sub.1 and a capacitor C.sub.1 as well as a resistor R.sub.2 and a capacitor C.sub.2 are connected in series with the antenna 1 and the antenna 2 respectively. The variable capacitor C.sub.v and the variable resistor R.sub.v are connected in parallel with the components including the antenna 2, the resistor R.sub.2, and the capacitor C.sub.2.

(51) When a loop antenna is used as each of the antennas 1 and 2, a capacitor is connected in series with the antenna in order to reduce the impedance due to the inductance of the loop antenna. In some cases, a resistor is intentionally connected in order to decrease a quality factor of the antenna.

(52) FIG. 10 illustrates an equivalent circuit of the antenna circuit in FIG. 9. For the equivalent circuit in FIG. 10, the capacitance value C.sub.v.sup.opt of the variable capacitor C.sub.v and the resistance value R.sub.v.sup.opt of the variable resistor R.sub.v for achieving the desired actual amplitude ratio r.sub.0 and phase difference .sub.0 between the currents can be obtained from the following equations (10) and (11).

(53) $\begin{array}{cc}\left[\mathrm{Math}.10\right]& \\ C_v^{\mathrm{opt}}=C_2.Math.\frac{C_2{R}_2{r}_0\sin {}_0+\left({}^2{L}_2{C}_2-1\right)\left(1-r_0\cos {}_0\right)}{1+{\left(C_2{R}_2\right)}^2+{}^2{L}_2{C}_2\left({}^2{L}_2{C}_2-2\right)}& \left(10\right)\\ \left[\mathrm{Math}.11\right]& \\ R_v^{\mathrm{opt}}=\frac{1}{C_2}.Math.\frac{1+{\left(C_2{R}_2\right)}^2+{}^2{L}_2{C}_2\left({}^2{L}_2{C}_2-2\right)}{C_2{R}_2\left(r_0\cos {}_0-1\right)+r_0\sin {}_0\left({}^2{L}_2{C}_2-1\right)}& \left(11\right)\end{array}$

(54) For example, it is assumed that the desired actual amplitude ratio r.sub.0 and phase difference .sub.0 are values of the following equation (12).

(55) [Math. 12]
r.sub.0=1,.sub.0=11 deg(12)

(56) In addition, it is assumed that the signal frequency f of the signal source 3 and the inductance L.sub.2, the capacitors C.sub.1 and C.sub.2, and the resistors R.sub.1 and R.sub.2 of the antenna 2 have values of the following equation (13).

(57) [Math. 13]
f=1 MHz=, L.sub.1=L.sub.2=10 H, C.sub.1=C.sub.2=2800 pF, R.sub.1=R.sub.2=10(13)

(58) The equations (12) and (13) are substituted into the equations (10) and (11) to calculate appropriate parameters, and thus the following values C.sub.v.sup.opt and R.sub.v.sup.opt can be obtained.

(59) [Math. 14]
C.sub.v.sup.opt=2363.68 pF, R.sub.v.sup.opt=141.645(14)

(60) FIG. 11 illustrates waveforms of the currents I.sub.1 and I.sub.2 in a simulation on the equivalent circuit in FIG. 10 using the parameters of the equations (13) and (14). An amplitude of voltage applied from the signal source 3 is 1V.

(61) According to FIG. 11, it can be seen that the amplitude ratio of the currents I.sub.1 and I.sub.2 is 1 and a time difference therebetween is 31 ns (comparable to 11 deg). In addition, as an effect of connecting the capacitors C.sub.1 and C.sub.2 in series with the antennas 1 and 2 respectively, the amplitude of the currents I.sub.1 and I.sub.2 is greater than that in FIG. 3.

(62) Next, an example where the amplitude ratio r.sub.0 is a value other than 1 (r.sub.01) in the antenna circuit in FIG. 9 is described.

(63) For example, it is assumed that the currents I.sub.1 and I.sub.2 satisfying the following equation (15) flow through the antennas 1 and 2.

(64) [Math. 15]
r.sub.0=1.3,.sub.0=11 deg(15)

(65) Note that, it is assumed that the signal frequency f of the signal source 3 and the inductance L.sub.2, the capacitors C.sub.1 and C.sub.2, and the resistors R.sub.1 and R.sub.2 of the antenna 2 have the values of the equation (13).

(66) The equations (12) and (13) are substituted into the equations (10) and (11) to calculate appropriate parameters, and thus the following values C.sub.v.sup.opt and R.sub.v.sup.opt can be obtained.

(67) [Math. 16]
C.sub.v.sup.opt=967.844 pF, R.sub.v.sup.opt=31.9953(16)

(68) FIG. 12 illustrates waveforms of the currents I.sub.1 and I.sub.2 in a simulation on the equivalent circuit in FIG. 10 using the parameters of the equations (13) and (16). An amplitude of voltage applied from the signal source 3 is 1V.

(69) According to FIG. 12, it can be seen that the amplitude ratio of the currents I.sub.1 and I.sub.2 is 1.3 and a time difference therebetween is 31 ns (comparable to 11 deg). In addition, as an effect of connecting the capacitors C.sub.1 and C.sub.2 in series with the antennas 1 and 2 respectively, the amplitude of the currents I.sub.1 and I.sub.2 is greater than that in FIG. 7.

(70) As described above, in the antenna circuit in FIG. 9, employment of the values C.sub.v.sup.opt and R.sub.v.sup.opt calculated from the equations (10) and (11) enables controlling of the amplitude ratio and phase difference between the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 as desired and also allows large currents to flow through the antennas 1 and 2.

(71) As described above, according to this embodiment, in the antenna circuit including the antenna 1 and the antenna 2 that is connected in series with the antenna 1 and has inductance, the variable capacitor C.sub.v and the variable resistor R.sub.v connected in parallel with the antenna 2 are provided, and this enables controlling of the actual amplitude ratio r and the phase difference between the currents I.sub.1 and I.sub.2 flowing through the two antennas 1 and 2 into desired values. Flows of the currents I.sub.1 and I.sub.2 with the phase difference through the antennas 1 and 2 enables forming of a favorable communication area. In addition, setting of the actual amplitude ratio r between the currents I.sub.1 and I.sub.2 flowing through the antennas 1 and 2 to a value other than 1 enables forming of an asymmetric communication area.

REFERENCE SIGNS LIST

(72) 1, 2 antenna 3 signal source C.sub.v variable capacitor R.sub.v variable resistor C.sub.1, C.sub.2 capacitor R.sub.1, R.sub.2 resistor