JPH0445312Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to an antenna circuit suitably implemented in, for example, a vehicle-mounted radio receiver.

Conventional technology In car radio receivers, frequency modulation (hereinafter abbreviated as FM) is performed using a single antenna.
Radio signal, amplitude modulation (hereinafter abbreviated as AM)
It would be convenient to be able to receive radio signals and signals used in car telephones. Furthermore, since the antenna is expanded and contracted by a motor or the like, it is not possible to attach a signal cable to the lower end of the antenna, making it difficult to shorten the signal cable. Therefore, the cable capacity of the signal cable increases, and the impedance due to the cable capacity increases. Especially for radio signals in relatively low frequency bands such as AM radio signals, the influence of cable capacity becomes large. Therefore, especially in vehicle antennas, it is necessary to send signals in a wide frequency band to a radio receiver without increasing loss in the signal cable.

of configurations associated with typical prior art antennas.
The equivalent circuit in the AM radio signal frequency band is
It is shown in FIG. The antenna 1 can be represented by a configuration of an antenna effective capacitance Ce and an antenna reactive capacitance Ca, and the AM radio signal received by this antenna 1 is represented by an AC power source Vi. Further, the cable 2 is shown including a line 1 between the terminals A1 and B1, and the line 1 between the terminals A1 and B1 is grounded via the cable capacitance Cb. The signal at terminal B1 is input to the radio receiver. The voltage Vb at this terminal B1 is expressed by the following equation.

Vb=Ce/Ce+Ca+Cb・Vi...(1) As shown in the first equation, if the cable capacitance Cb is large, the gain of the relatively low frequency AM radio signal received by antenna 1 will decrease, and the receiving sensitivity and This results in a decrease in the signal-to-noise ratio (S/N ratio).

In order to prevent such a decrease in receiving sensitivity and S/N ratio, between the antenna 1 and the cable 2,
That is, antennas with an amplifier interposed at the terminal A1 position to improve receiving sensitivity and S/N ratio have also been put into practical use. In such an antenna,
Since active elements are used, costs increase and problems arise, such as securing circuit characteristics that suppress distortion of the output signal when a strong electric field is input. In addition, new problems arise, such as loss associated with impedance conversion in the amplifier and insufficient impedance matching.

Problems to be Solved by the Invention An object of the invention is to provide an antenna circuit that can solve the above technical problems and improve reception sensitivity and S/N ratio over a wide frequency band.

Means for Solving the Problems The present invention provides an antenna and a first antenna in a first frequency band.
An antenna circuit provided between a radio signal and an antenna input circuit of a radio receiver that receives a second radio signal in a second frequency band that is a higher frequency band than the first frequency band, the antenna circuit comprising: a signal cable; a first cable connected between the signal cable and the antenna and converting the impedance in the first frequency band from high impedance to low impedance;
an impedance conversion circuit; a first filter circuit connected between the signal cable and the antenna and passing a signal in a second frequency band; a first filter circuit connected between the signal cable and the antenna input circuit; a second impedance conversion circuit that converts impedance in a frequency band from low impedance to high impedance; and a second filter circuit that is connected between the signal cable and the antenna input circuit and passes the signal in the second frequency band. An antenna circuit characterized by comprising:

Effect According to the present invention, between the antenna and the signal cable, there is a first filter circuit that passes the first radio signal in the first frequency band, and a first filter circuit that changes the impedance in the second frequency band from high impedance to low impedance. Means for adjusting the impedance constituted by the first impedance conversion circuit is provided. Further, between the signal cable and the antenna input circuit of the radio receiver, there is a second filter circuit for passing a second signal in a second frequency band, and a first filter circuit for passing a second signal in a second frequency band.
Means for adjusting impedance is provided, which includes a second impedance conversion circuit that converts impedance in a frequency band from low impedance to high impedance.

The second radio signal is sent from the antenna to the radio via the first filter circuit, and the impedance of the first radio signal is converted by the first impedance conversion circuit to reduce loss due to cable capacitance in the signal cable. is transmitted to the radio receiver. The second radio signal is sent to the antenna input circuit of the radio receiver via the second filter circuit, and the first radio signal is converted by the second impedance conversion circuit to an impedance matching the antenna input circuit of the radio receiver. and sent to the antenna input circuit of the radio receiver. Therefore, radio signals over a wide frequency band can be transmitted to a radio receiver without increasing losses in the antenna and signal cable.

Embodiment FIG. 2 is an equivalent circuit diagram of the antenna circuit 6 in the AM radio frequency band for explaining the principle of the present invention. The antenna 4 is represented by a configuration of an antenna effective capacitance Ce interposed between the ground level and an antenna reactive capacitance Ca interposed in series,
The AM radio signal which is received by this antenna 4 and is the first radio signal is represented by the AC power source Vi. Further, the signal cable 5 is shown including a line 2 between terminals B2 and C2, and this line 2 is grounded via a cable capacitance Cb. A transformer 7 for converting impedance is interposed between the antenna 4 and the signal cable 5. The signal at terminal C2 is input to the antenna input circuit of the radio receiver. The voltage Vc at the terminal C2 is expressed by the following equation, where n is the turns ratio between the input side and the output side of the transformer 7.

Vc=Ce/Ce+Ca+Cb/n ²・Vi...(2) As can be seen from the second equation, if the effect related to the cable capacity Cb by adding the transformer 7 is explained using the conventional technology, 1 /n can be ² . Therefore, the impedance caused by the cable capacitance Cb seen from the terminal A2 can be converted to 1/n ² by the transformer 7, and the loss in the signal cable 5 can be reduced.

FIG. 1 shows an antenna circuit 1 according to an embodiment of the present invention.
FIG. 1 is a diagram showing a specific configuration of FIG. The antenna circuit 11 includes an antenna 12, a signal cable 16, an impedance adjustment circuit 13 interposed between the antenna 12 and the signal cable 16, and an impedance adjustment circuit interposed between the signal cable 16 and the radio receiver 26. The circuit 17 is configured to include a circuit 17. The antenna 12 is attached to, for example, the body of an automobile, and is expanded and contracted by a motor 28.

Such an output from the antenna 12 is given to the impedance adjustment circuit 13 via the duplexer 27. The impedance adjustment circuit 13 has a low impedance with respect to the FM radio signal frequency band, and includes an FM radio signal wave circuit 14, which is a first filter circuit, and a transformer 22, for example, and is a first impedance conversion circuit. The impedance conversion circuit 15 is connected in parallel.

The FM radio signal, which is the second radio signal received by the antenna 12, is transmitted to the FM radio signal wave circuit 1.
4 to the signal cable 16.

The FM radio signal wave circuit 14 includes, for example, a coil 20 and a capacitor 21, and functions as a high-pass filter with low impedance for the FM frequency band.

A radio signal from the signal cable 16 is given to an impedance adjustment circuit 17. The impedance adjustment circuit 17 waves an FM radio signal,
It is composed of an FM radio signal wave circuit 15, which is a second filter circuit, and an impedance conversion circuit 19, which has an impedance conversion effect on AM radio signals and is a second impedance conversion circuit. The FM radio signal wave circuit 18 and the impedance conversion circuit 19 are connected in parallel, and the FM radio signal from the signal cable 16 is led out to the antenna input circuit 26 of the radio receiver via the FM radio signal wave circuit 18. . The FM radio signal wave circuit 18 includes, for example, a coil 23 and a capacitor 2.
4, and serves as a high-pass filter that waves relatively high-frequency signals such as FM radio signals. Also, impedance conversion circuit 1
9 is the first impedance conversion circuit 22 described above.
Similarly, it is configured to include, for example, a transformer 25 and the like. Therefore, FM radio signal wave circuit 1
The inductance of the coils 20, 23 and the capacitance of the capacitors 21, 24 in 4, 18 are:
Each is selected to have a resonant frequency in the FM radio signal frequency band.

Even the configuration shown in the second figure mentioned above is actually the same as that shown in the first figure.
This is affected by the capacity in the FM radio signal wave circuit 14. An equivalent circuit diagram showing the principle when such a capacitance component Cf is considered is shown in FIG. In addition,
For simplicity, the antenna effective capacitance Ce and the antenna reactive capacitance Ca are expressed as a capacitance C _A. Further, the transformer 7 corresponds to the transformer 15 in FIG. 1, and the antenna 12 corresponds to the antenna 4. It has a self-inductance L ₁ on the input side, a self-inductance L ₂ on the output side, and a mutual inductance M between the output side and the input side. Therefore, the AC power source Vi caused by the radio signal received by the antenna 4,
There is a relationship between the voltage level Vc applied to the radio receiver and the relationship expressed by the following equation.

Vc={ω ⁴ C _A Cf (L ₁ L ₂ −M ² )−ω ² C _A M}Vi/ω ⁴ (C _A Cf
+C _A Cb+CbCf) (L ₁ L ₂ −M ² ) −ω ² {L ₁ (C _A +Cf) + L ₂
(Cb+Cf)-2MCf}+1...(3) Here, ω represents the angular frequency of the received radio signal.

At this time, when the denominator of the third equation is zero,
This is the extreme value of Vc. Here, the mutual inductance M
When calculated assuming that is expressed as k√ ₁ · ₂ (k is the coupling coefficient of transformer 7), the extreme value of Vo is shown by the following equation.

_{Where ,} ^​ _{​ ​ ​ ​ ​ ​ ​} } ...(6) Z=1 ...(7) Thus, the voltage level Vc has extreme values for two different values of the frequency f, as shown in the fourth equation. If the frequencies corresponding to the extreme values of this voltage level Vc are f1, f2 (f1<f2), then the frequency f
The relationship between the voltage level Vc and the voltage level Vc is shown in FIG.
As can be seen from the fourth to sixth equations, as the coupling coefficient k becomes smaller, the frequency f2 becomes lower. Therefore, by increasing the coupling coefficient k of the transformer 7,
AM radio signal frequency band frequency f1 and frequency f2
If it falls between , flat reception characteristics can be obtained in the AM radio signal frequency band. As the transformer 7 capable of increasing the coupling coefficient k, a transformer 7 having a winding method such as a so-called Sanderch winding or a bifilar winding may be used, for example.

FIG. 5 is an equivalent circuit diagram of the antenna circuit 11 of FIG. 1 in the AM radio signal frequency band. The antenna 12 can be represented by a capacitance C _A that includes an antenna effective capacitance having a capacitance in series with the radio signal and an antenna reactive capacitance occurring between the radio signal and ground level. Also antenna 1
The radio signal received at 2 can be represented by the AC power source Vi.

The AM radio signal received by antenna 12 is
Since the FM radio signal wave circuit 14 has high impedance, the impedance conversion circuit 1
5 is input. In the impedance conversion circuit 15, the number of turns on the output side is at a ratio of 1 to the number of turns n on the input side of the transformer 22. Due to this
The voltage of the AM radio signal is reduced by 1/n by the transformer 22. In the signal cable 16, there is a cable capacitance between the radio signal and the ground level.
Contains Cb.

For high frequency signals, such as FM radio signals, the signal cable 16 is low impedance. However, the impedance due to the cable capacitance Cb of the signal cable 16 becomes large for relatively low frequency signals such as AM radio signals. In this embodiment, the voltage of the AM radio signal is reduced by the impedance conversion circuit 15, and the loss related to the cable capacitance Cb can be reduced.

A signal in a relatively low frequency band such as an AM radio signal from the signal cable 16 has a high impedance in the FM radio signal wave circuit 18, so it is applied to the impedance conversion circuit 19.
In the transformer 25 of the second impedance conversion circuit 19, a ratio m of the number of turns on the output side to the number of turns 1 on the input side is set, and the voltage of the AM radio signal input to this transformer 25 is amplified and Output to the antenna input circuit of the receiver.

The relationship between AC power supply Vi and output voltage V ₀ is expressed by the following equation.

V ₀ = Vi・m/n・C _A /C _A +Cb/n ² ...(8) Capacity of the antenna circuit 11 as seen from the radio receiver
_CTA is expressed by the following formula.

C _TA = C _A・n ² + Cb/m ₂ ...(9) For example, this capacitance C _TA is specified as 80 pF due to impedance matching with a radio receiver, etc., and the capacitance C _A and cable capacitance Cb are
2 and the length of the signal cable 16. Therefore, the turns ratios n and m of the transformers 22 and 25 are selected so as to satisfy the above formula 9.

Furthermore, the equivalent circuit of the antenna circuit 11 seen from the radio receiver is expressed by the configuration of L ₀ as the inductance of the transformers 22 and 25, L ₀ /2 as the inductance connected in parallel, and capacitance C _TA . It can be done.
If the resonant frequency of such a circuit configuration is fp, the inductance L ₀ is expressed as follows.

L ₀ = 2/(2πfp) ²・C _TA ...(10) Set the resonance frequency fp to AM, for example around 250kHz.
It is desirable to select a frequency outside the radio signal frequency band and flatten the frequency characteristics outside the AM radio signal frequency band. The inductance L ₀ of the transformers 22, 25 for this purpose is determined by Equation 10.

In this manner, in the antenna circuit 11, for example, when an AM radio signal and an FM radio signal are received by one antenna 12, the loss of the AM radio signal in the signal cable 16 can be reduced. For example, antenna effective capacity
When Ce is 15 pF, antenna reactive capacitance Ca is 5 pF, cable capacitance Cb is 120 pF, and turns ratio n, m is "4", when calculated using the first and second equations,
A gain improvement of about 9 dB is achieved.

In the embodiment described above, if the turns ratios n and m of the transformers 22 and 25 are too large, the loss will increase, and if the turns ratios are too small, the effect will be reduced. According to the experiments of the present inventors, specifically, the turns ratio n,
Good results can be obtained by selecting a value of 10 or less for m.

FIG. 6 is a diagram showing the configuration of an antenna circuit 31 according to another embodiment of the present invention. Note that the same reference numerals are used for parts corresponding to the antenna circuit 11 described above. In the antenna circuit 31, in the impedance adjustment circuit 13a, the impedance conversion circuit 15a includes coils 32, 33 and a transformer 22, and the impedance adjustment circuit 17
In a, the impedance conversion circuit 19a
is configured to include coils 34 and 35 and a transformer 25. In order to reduce loss due to stray capacitance of the transformers 22, 25, the transformers 22, 2
Coils 32 to 35 are added to the input end and output end of 5, respectively. As a result, the transformer 22,
Loss due to storage capacity of 25 is prevented,
Receiving sensitivity and S/N ratio can be further improved.

As described above, in the embodiment described above, it is possible to reduce the loss particularly in the AM radio signal frequency band caused by the cable capacity, and to significantly improve the reception sensitivity and S/N ratio in the radio receiver.
Therefore, if you want to receive signals in a wide frequency band with one antenna, for example, use an in-vehicle antenna.
It is particularly useful when receiving both FM and AM radio signals.

Further, the antenna reactive capacity usually changes more than the antenna effective capacity depending on the type of antenna.
If the present invention is applied to an antenna with a large antenna reactive capacity, the effect will be particularly remarkable. Further, the polarity of the transformers 22, 25 may be either positive phase or negative phase, but according to experiments, a greater effect can be obtained if positive phase transformers 22, 25 are used.

In this embodiment, an antenna for receiving FM radio signals and AM radio signals has been described, but the present invention is applied to an antenna that simultaneously receives radio signals and other signals such as those used in car telephones. It is also possible to do so.

Effects of the Invention As explained above, according to the present invention, when receiving a radio signal with an antenna, it is possible to reduce the loss of the received signal due to the capacitive impedance of the signal cable. Therefore, reception sensitivity and S/N ratio can be significantly improved over a wide frequency band.

[Brief explanation of drawings]

FIG. 1 shows an antenna circuit 11 according to an embodiment of the present invention.
2 is an equivalent circuit diagram for explaining the principle of the present invention, and FIG. 3 is an equivalent circuit diagram for explaining the principle when considering the capacitance Cf in the configuration of FIG. FIG. 4 is a graph showing the relationship between the receiving frequency f and the output voltage level Vc in the equivalent circuit shown in FIG. 3, and FIG.
Equivalent circuit diagram in the AM radio signal frequency band,
FIG. 6 shows an antenna circuit 31 of another embodiment of the present invention.
FIG. 7 is a diagram showing the configuration of the antenna 1 of the prior art.
and cable 2 in the AM radio signal frequency band. 11, 31... Antenna circuit, 12... Antenna, 13, 13a, 17, 17a... Impedance adjustment circuit, 16... Signal cable, 14, 1
8...FM radio signal wave circuit, 15, 15a,
19, 19a... Impedance conversion circuit, 2
2,25...Transformer.

Claims (1)

[Claims for Utility Model Registration] An antenna and an antenna input of a radio receiver that receives a first radio signal in a first frequency band and a second radio signal in a second frequency band that is a higher frequency band than the first frequency band. an antenna circuit provided between the signal cable and the antenna, the first antenna circuit being connected between the signal cable and the antenna and converting impedance in a first frequency band from high impedance to low impedance.
an impedance conversion circuit; a first filter circuit connected between the signal cable and the antenna and passing a signal in a second frequency band; a first filter circuit connected between the signal cable and the antenna input circuit; a second impedance conversion circuit that converts impedance in a frequency band from low impedance to high impedance; and a second filter circuit that is connected between the signal cable and the antenna input circuit and passes the signal in the second frequency band. An antenna circuit characterized by comprising: