JPH0227842B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a small loop antenna.

Conventionally, there is a loop antenna as shown in FIG. As shown in the figure, the configuration is such that a capacitive element 2 is attached to one end of a loop-shaped conductor 1, and points a and b of this conductor are used as feeding ends. The capacitive element 2 and the conductor 1 are tuned at the design frequency, and the positions of points a and b are selected to match the reference impedance of 50Ω, 75Ω, etc.

However, in the case of FM broadcasting or general radio frequency broadcasting, it is desirable that a single antenna can receive radio waves from any broadcasting station. For example, in our country's channel plan, FM broadcasting is 76-90MHz,
VHF television broadcasting is allocated to 90-108MHz and 170-222MHz, respectively, and radio waves are emitted from many broadcasting stations. However, due to differences in the distance from each broadcasting station to the receiving point and the transmission power, the received electric field strength varies from weak electric field to strong electric field. Therefore, even if an amplifier is necessary in a weak electric field, it is not necessary for strong electric field reception.On the other hand, if an amplifier is inserted, a new problem arises in that in a strong electric field, cross-modulation occurs at the input stage. In addition, in order to be able to receive many of these broadcasts with one antenna, a technology is required that can change the center frequency of the relatively narrow matching band of a small antenna over a very wide frequency range. . For receiving antennas used in various locations, the above-mentioned problems must be solved.

The present invention was made in view of the above-mentioned drawbacks and demands of conventional antennas, and its first purpose is to provide a small loop antenna that can operate stably without problems such as cross-modulation in any location. It is about providing. A second object of the present invention is to provide a small loop antenna that can change the center frequency of the matching band while maintaining matching over a wide frequency range by simply changing the capacitance without moving the position of the feeding point. Then there it is.

In the present invention, a capacitive element is connected in series to a part of a loop-shaped conductor, and a plurality of power supply terminals are provided on this loop conductor, thereby making it possible to connect to a plurality of external circuits. Furthermore, in the present invention, the loop area, perimeter, and equivalent radius of a loop-shaped conductor are
By adjusting and matching the antenna so that it becomes 0.5f ₀ /f _n 4.0, receiving outputs can be obtained independently from the plurality of feeding terminals, and multiple stable signals can be extracted from the same antenna. . Therefore, a multi-feeding point small loop antenna with improved functionality so as to avoid problems such as cross-modulation, for example, by passing one of the plurality of signals through an amplifier or taking them out directly from another terminal. is obtained.

Hereinafter, a small loop antenna with multiple feeding points according to the present invention will be described with reference to the drawings. First, the configuration of this antenna will be described. FIGS. 2 and 3 show the configuration of a small loop according to each embodiment of the present invention. 11 is a loop-shaped conductor, 1
2 is a variable capacitance element, 13 is an amplifier, and 1, 2 and 3 are power supply terminals for power supply. For these antennas, A and B and B and C are the feeding points, respectively.
It constitutes a small loop antenna with two feeding points.

FIG. 2 shows a case where the conductor loop has a circular cross section with a radius b, and FIG. 3 shows a case where the loop is constructed of a conductor plate with a width W. A specific example of the variable capacitance element 12 is shown in FIGS. 4A and 4B. FIG. 4A shows an air variable condenser. Figure B shows a variable capacitance circuit using a variable capacitance diode D, and as the reverse bias DC applied voltage in the figure increases, the junction capacitance of the diode D decreases. A specific circuit example of the amplifier 13 in FIGS. 2 and 3 is shown in FIG. In FIG. 2, a is the loop radius of the conductor 11, and b is the radius of the conductor 11 itself. If the loop is a conductor plate having a width W as shown in FIG. 3, it is replaced by the radius when this plate is regarded as a cylinder, that is, the equivalent radius, and the equivalent radius is set as b. l _s is the length of the circular arc at the center of the conductor between adjacent power supply terminals A, B, and C.

With these symbols, the input admittance Y _iof of the small loop antenna shown in FIG.
When resonance is achieved at f ₀ , it is expressed by the following equation.

Y _iof0 ≒ R _r + R _l / (L _s + M _sp ) ²・1/w ₀ ² []……(1
) Here, w ₀ =2πf ₀ , f ₀ [Hz] is the resonant frequency, λ ₀ [m] is the free space wavelength at frequency f ₀ ,
Ls [H] is the self-inductance of section length ls, L _sp
[H] is the mutual inductance between the section lengths l _s and l _p . Also, the radiation resistance Rr is given by the following formula.

Rr=320π ⁴ A ² /λ ₀ ⁴ [Ω] (2) Here, A [m ² ] is the loop area, and for example, in the case of a circular loop, A=πa ² [m ² ].

Moreover, when the other terminals are open, the loss resistance R _l is given by the following formula.

Here, S [m] represents the loop perimeter, and in the case of a circular loop, S = 2πa [m]. or,
b [m] is the radius or equivalent radius of the loop conductor,
σ[/m] is conductivity, and μ[H/m] is magnetic permeability.

However, if R ₁ is connected to another terminal, the loss resistance R ₁ is approximately expressed by the following equation.

By substituting the above equations (2), (3), and (4) into equation (1) and rearranging, the following equation is obtained.

Y _iof0 =K{f ₀ ² +7.24×10 ²⁹ /m・f ₀ +10 ³² /π ² A ²・
1/R} ...(5) Here, 10 ^-32 π ² A ² /(L _s +M _sp ) ² , and m is defined by the following formula.

m=A ² b/S・σ/μ ...(6) If the structure, dimensions, and conductivity of the material of the antenna are fixed, equation (5) is a function of frequency,
It can be seen from the same equation that the curve is convex downward.
Therefore, equation (5) has a minimum value determined by the following equation.

∂Y _iof0 / ∂f ₀ = 0 ...(7) Now, the frequency at which the minimum value is obtained is expressed as f _n ,
The relationship between m introduced earlier in equation (6) and the above frequency f _n can be expressed as
By calculating from equation (7), the following equation is obtained.

m=1.5×10 ³² /16π ⁴ π・f _n ^-3.5 ...(8) The conditional expression for designing the antenna to achieve the desired f _n is obtained by eliminating m from equations (6) and (8). It is expressed by the following relational expression.

When a small loop antenna is designed to satisfy the condition of equation (9), the input admittance is expressed by the following equation.

Y _iof0 =K・f _n ² {f ₀ /f _n ) ² +1.33(f ₀ /f _n ) ^-15 +10 ³² /π ² A ² f _n ²・1/R ₁ }
...(10) Equation (10) expressing input admittance is an equation that exhibits a value close to the reference admittance over a very wide frequency range, and therefore represents matching over a wide frequency range. This can be intuitively understood by graphing equation (10). Regardless of the value of the loop area A of the antenna, K can be set to an appropriate value by adjusting _Ls , that is, the section length _Ls , so that the reference admittance at any frequency, e.g. It is possible to match this antenna to Y ₀ =0.02.

Based on the above idea, the operation of the small two-feed point antenna shown in FIGS. 2 and 3 will be explained. Second
In the figure and FIG. 3, when the switch _SW1 is connected to the contact point H, the received output amplified by the amplifier 13 appears at the terminal 14. Also, switch
When SW ₁ is connected to contact RI, the received output of the loop antenna appears directly at terminal 14.
Therefore, when using a signal that has passed through the amplifier 13, the feed terminal loha is open, and the output signal of the loop antenna is efficiently input to the amplifier 13. Since the amplifier 13 is used in a place where the electric field is weak, the other feed terminal loha is left open, which is desirable for efficient use as a loop antenna.

Next, under strong electric fields, cross-modulation occurs at the input stage of amplifier 13. In particular, the lower the noise amplifier is, the more likely cross modulation will occur. In this case, the switch SW ₁ of FIGS. 2 and 3 is switched to the contact position. At this time, with the input resistor _R1 of the amplifier 13 connected to the power supply terminal E-LO, the received signal appearing at the power supply terminal ROHA is guided to the terminal 14. The antenna efficiency in this case is slightly lower than that at terminals E and E on the amplifier side for reasons described later. However, since it is used in a strong electric field, there is no problem even if the efficiency decreases slightly.

Efficiency 〓 is generally defined by the following formula.

〓=R _r /R _r +R _l ...(11) Here, R _r is the radiation resistance and R _l is the loss resistance. By substituting equations (2) and (4) into equation (10) and using the relationship in equation (9), equation (10) can be rewritten as follows.

〓=1/1+1.35(f ₀ /f _n ) ^-3.5 + [Wo ² (L
s+M _SP ) ² /Rr〕1/R ₁ Therefore, if R ₁ is not connected, i.e.
The efficiency in the case of R ₁ = is as follows from equation (12): 〓=1/1+1.35(f ₀ /f _n ) ^-3.5 (13). If R ₁ is equal to the reference impedance, the efficiency of equation (12) is
It only decreases by 3 dB, and as mentioned above, it does not cause any problem at strong electric field locations.

In order to explain the effect of an antenna provided with a plurality of feeding points as shown in FIGS. 2 and 3, in comparison with a conventional method, FIG. 6 will be described. There are 11 power feeding points on E-Ro, and switches SW ₁ and SW ₂ are installed. The connection position of the switch shown in the diagram is amplifier 13.
This is a case where the received signal is guided to the terminal 14 via the terminal 14. If you turn switches SW ₁ and SW ₂ in the opposite direction as shown, the signal will be sent directly from the loop antenna to terminal 1.
It can be taken out at 4.

Comparing this method with the method of the present invention, the number of switches is increased from one to two, and therefore the number of contact parts is two.
Not only does electrical stability and reliability deteriorate due to the presence of the metal, but also contact resistance is added, resulting in a reduction in characteristics. Furthermore, in a small antenna, the increase in the number of switch mechanisms has the disadvantage of impairing the miniaturization of the antenna. On the other hand, the multi-feeding point small loop antenna of the present invention shown in FIGS. 2 and 3 can realize the desired antenna function with one switch, and can also be made smaller in size. As mentioned above, the present invention has extremely effective effects in small antennas.

Although a small die ball with a top load as shown in FIG. 7 is well known, it is structurally difficult to provide a plurality of feeding points in this type of small antenna. However, regarding the small loop antenna handled in this specification, it is structurally easy to provide multiple feeding points, and furthermore, a multi-feeding point small loop antenna having a structure like the one of the present invention has not been used in the past. Ta.

In the embodiment of the present invention shown in FIGS. 2 and 3 above, a two-feeding point small loop antenna with two feeding points was described, but a multiple feeding point small loop antenna as shown in FIGS. 8 and 10 is also applicable. If an antenna is used, an amplifier, a load, a transmitter, etc. can be attached to each feeding point, and more functions can be provided to one small loop antenna than in the above-mentioned embodiments. Therefore, it is effective in making a small loop antenna multi-functional.

Figure 8 shows an embodiment of the present invention in which power supply terminals A, B, ..., F are provided inside the loop, and Figure 9 shows power supply terminals A, B, ..., F provided outside the loop. This is an embodiment of the present invention.
The multi-feeding point small loop antenna shown in FIG. 8 has the terminal inside, so it has the advantage that the entire antenna including the connection circuit can be configured compactly. Also, the 9th
Since the embodiment shown in the figure has a power feeding terminal on the outside, it is suitable for connecting an external electrical network to the antenna, and such a connection may be convenient depending on the antenna device.

As described above, the multi-feeding point small antenna according to the present invention can achieve effects such as reducing the number of switches etc. by connecting one small loop antenna to a plurality of external circuits. It also contributes to miniaturization of the shape.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of a conventional small loop antenna.
FIG. 2 is a block diagram of one embodiment of the present invention, FIG. 3 is a block diagram of another embodiment of the present invention, FIG. 4 is a diagram showing a specific example of a variable capacitance element used in the present invention, and FIG. The figure is a diagram showing an example of the circuit of an amplifier used in the present invention.
7 is a configuration diagram of another example of a conventional small loop antenna, FIG. 8 is a configuration diagram of yet another embodiment of the present invention, and FIG. FIG. 9 is a configuration diagram of still another embodiment of the present invention. 11...Loop-shaped conductor, 12...Variable capacitance element, 13...Amplifier, 14...Terminal, A, B,
C,...,H...Power supply terminal, SW ₁ ...Switch.

Claims (1)

[Claims] 1. A device comprising: a loop-shaped conductor with a portion cut off; and a capacitive element connected to the cut-off portion of the conductor, the conductor having a plurality of power supply terminals. A small loop antenna with multiple feeding points. 2. The multi-feed point small loop antenna according to claim 1, wherein the loop-shaped conductor has its loop area, peripheral length, and equivalent radius adjusted to fall within the range of 0.5f ₀ /f _n 4.0. However, f ₀ is the resonant frequency of the antenna, and f _n is the resonant frequency at which the input admittance is minimum. 3. The capacitive element is a variable capacitive element that can change the resonance frequency over a wide band.
A small loop antenna with multiple feeding points as described in .