EP2528164A1

EP2528164A1 - Vehicle-use antenna device

Info

Publication number: EP2528164A1
Application number: EP11734613A
Authority: EP
Inventors: Hiroyuki Kobayashi; Shuichi Tajima; Hisashi Fujisaki; Shoichi Negami; Yoichi Iso; Tomoyuki Fujieda; Masami Suzuki; Takeshi Ito
Original assignee: Furukawa Electric Co Ltd; Pioneer Solutions Corp
Current assignee: Pioneer Corp
Priority date: 2010-01-21
Filing date: 2011-01-18
Publication date: 2012-11-28
Also published as: EP2528164A4; JP2011151623A; WO2011090007A1; JP5547500B2

Abstract

A vehicle-mount antenna device is provided in which stable antenna characteristics can be obtained with an influence of common mode noise being reduced. A vehicle -mount antenna device 200 is configured with an antenna unit 210 and a connector-equipped coaxial cable 120 being connected to each other. The antenna unit 210 includes a feeding element 111 and a parasitic element 112, with the feeding element 111 being connected to a + input of an amplifier circuit 113 and the parasitic element 112 being connected to a ground plane 115 of the amplifier circuit 113. An output from the amplifier circuit 113 is connected to a center conductor 121 of the coaxial cable 120 via a filter 216, and the ground plane 115 is connected to an outer conductor 122 of the coaxial cable also via the filter 216.

Description

Technical Field

The present invention relates to a vehicle-mount antenna device mounted on a vehicle and, in particular, to a vehicle-mount antenna device in which an antenna element and a receiver are arranged so as to be positioned away from each other and are connected via a cable.

Background Art

In recent years, needs for a wideband UHF antenna for use in a digital terrestrial television and others have been increased also in a vehicle-mount antenna device. In the vehicle-mount antenna device, a decrease in size is strongly demanded, and the placeable location is also restricted. For this reason, in general, while an antenna element is placed on a front windshield or a rear windshield suitable for reception of radio waves (for example, Patent Document 1), a receiver is placed at another position away from the antenna element. A connection between the antenna element and the receiver is made with a coaxial cable.
Since the coaxial cable connecting the antenna element and the receiver is routed at an inconspicuous place such as a bottom of a vehicle body, the coaxial cable is routed in a state of being near or in contact with a metal part of the vehicle body (hereinafter referred to as a vehicle body metal part). While many of UHF antennas of a balanced type, the coaxial cable and an amplifier circuit for amplifying a reception signal are unbalanced circuits, and therefore a mismatch occurs therebetween, thereby causing a common mode current in the coaxial cable. With this common mode current, shielding characteristics of the coaxial cable are significantly degraded, and noise from the vehicle body metal part tends to flow into the coaxial cable. Conventionally, there is a problem such that common mode noise is mixed from the vehicle body metal part to an outer conductor of the coaxial cable to degrade antenna characteristics. There is also a problem in which this influence of common mode noise onto the antenna characteristics significantly varies depending on the routing position of the coaxial cable or the like.
An example of a conventional vehicle-mount antenna device is shown in FIG. 7. Description is made by using FIG. 7(a), FIG. 7(b), and FIG. 8. FIG. 8 is an equivalent circuit diagram when a conventional vehicle-mount antenna device 900 is arranged so as to be near a vehicle body metal part 10. FIG. 7(a) and FIG. 7(b) both are schematic structural diagrams of an antenna unit 910 configuring the conventional vehicle-mount antenna device 900, FIG. 7(a) being a front view and FIG. 7(b) being a rear view. Also, an equivalent circuit when the antenna unit 910 is placed at a position away from a receiver and a coaxial cable connecting therebetween is routed near the vehicle body metal part is shown in FIG. 8.
The antenna unit 910 is, by way of example, a wideband vehicle-mount antenna for use in an UHF frequency band. The antenna unit 910 has a feeding element 911 and a parasitic element 912, the feeding element 911 being connected to an amplifier circuit 913 and the parasitic element 912 being connected to a ground plane 915 provided on a back surface of a circuit board 914. Furthermore, the amplifier circuit 913 is connected to a center conductor 921 of a coaxial cable 920, and the ground plane 915 is connected to an outer conductor 922 of the coaxial cable 920.
FIG. 8 shows how noise mixed from the vehicle body metal part 10 to the coaxial cable 920 propagates inside the vehicle-mount antenna device 900 when the coaxial cable 920 is routed near the vehicle body metal part 10 and is connected to a receiver 930. Since the coaxial cable 920 and the antenna unit 910 are arranged near the vehicle body metal part 10, capacitances (parasitic capacitances) are formed between these cable and unit and the vehicle body metal part 10. That is, capacitances C1 and C2 are formed between the vehicle body metal part 10 and the outer conductor 922 of the coaxial cable 920, and capacitances C3 and C4 are formed between the vehicle body metal part 10 and the feeding element 911 and between the vehicle body metal part 10 and the parasitic element 912, respectively.
The capacitance formed between the vehicle body metal part 10 and the outer conductor 922 of the coaxial cable 920 is divided into the capacitances C1 and C2 in FIG. 8, and a capacitance at a position where noise is mixed into the outer conductor 922 is taken as C1, and a capacitance formed at another position is taken as C2. When a current I1 as common mode noise is mixed to the outer conductor 922 via the capacitance C1, currents when this current I1 flows inside the vehicle-mount antenna device 900 via the outer conductor 922 are shown as I2 to I5.
Part (a current I2) of the current I1 as common mode noise returns from the outer conductor 922 to the vehicle body metal part 10 via the capacitance C2, and the remaining current I3 flows into the circuit board 914. Between a + side input and a - side input of an amplifier circuit 913, an input impedance Z_A (50 Ω) is present. For this reason, as for the current I3, part thereof (the current I4) flows into the parasitic element 912 via the ground plane 915 of the circuit board 914 on which the amplifier circuit 913 is mounted, and the remaining current I5 flows into the feeding element 911 via the input impedance Z_A.
The currents I4 and I5 return to the vehicle body metal part 10 via the capacitances C3 and C4 between the parasitic element 912 and the vehicle body metal part 10 and between the feeding element 911 and the vehicle body metal part 10. Here, a voltage drop occurring due to the input impedance Z_A and the current I5 becomes a voltage input to the amplifier circuit 913, is amplified by the amplifier circuit 913 and received as normal mode noise by the receiver 930, thereby degrading the antenna characteristics.
Thus, to reduce normal mode noise received by the receiver 930, the current I5 passing through the input impedance Z_A is preferably reduced. To reduce the current I5, methods can be thought, including a method (a first method) of reducing the current I3 passing through the outer conductor 922 of the coaxial cable 920 and a method (a second method) of increasing the current 14 flowing to a parasitic element 912 side to reduce the current I5.
In the first method described above, measures have been conventionally taken in which the coaxial cable 920 is brought near the vehicle body metal part 10 to reduce the current I3. With this, the capacitance C2 between the outer conductor 922 and the vehicle body metal part 10 is increased to decrease an impedance from the outer conductor 922 to the vehicle body metal part 10. As a result, while the current I2 is increased, I3 (=I1-I2) is decreased, and the current I5 is also decreased.
Also, in the second method described above, the current I4 flowing to the parasitic element 912 side without changing the current I3 to reduce the current I5 (=I3-I4). When an impedance between the feeding element 911 and the vehicle body metal part 10 is taken as Z_F and an impedance between the parasitic element 912 and the vehicle body metal part 10 is taken as Z_G, the current I5 has a relation with the current 13 as follows.
I5=I3-Z_G/ (Z_A+Z_F+Z_G)
From the equation above, the current I5 can be reduced by relatively decreasing the impedance Z_G with respect to (Z_A+Z_F). Measures to decrease Z_G have been conventionally known such as increasing the parasitic element 912 or increasing the capacitance C3 by bringing the parasitic element near the vehicle body metal part 10. There is another method of setting Z_G at 0 by directly connecting the parasitic element 912 to the vehicle body metal part 10.

Prior Art Document

Patent Document

Patent Document 1: Japanese Patent Application Laid-open No. 2008-153738

Summary of the Invention

Problem to be Solved by the Invention

However, in the first method described above, the capacitance C2 significantly varies depending on how the coaxial cable 920 is routed and, as a result, noise characteristics also fluctuate, and therefore stable antenna characteristics cannot be obtained. Moreover, also in the second method described above, there are problems, such as those in which it is difficult to decrease the size of the antenna unit and the shape, mount position, and mount conditions of the antenna unit are greatly restricted.
The present invention was made to solve the problems described above, and has an object of providing a vehicle-mount antenna device in which stable antenna characteristics can be obtained with an influence of common mode noise being reduced.

Means for Solving the Problem

A first aspect of the vehicle-mount antenna device of the present invention includes an antenna unit having a feeding element and a parasitic element, a circuit board with a back surface including a ground plane, an amplifier circuit mounted on the circuit board and amplifying a reception signal received by the antenna unit, and a coaxial cable electrically connecting the amplifier circuit and a receiver, and the device further includes a filter for reducing common mode noise mixed in an outer conductor of the coaxial cable.
In another aspect of the vehicle-mount antenna device of the present invention, the filter is connected between the amplifier circuit and the coaxial cable, the feeding element is connected to the amplifier circuit, and the parasitic element is connected to the ground plane.
In still another aspect of the vehicle-mount antenna device of the present invention, the filter is arranged between the amplifier circuit and the antenna unit, the feeding element is connected to the amplifier circuit via the filter, and the parasitic element is connected to the ground plane via the filter.
In still another aspect of the vehicle-mount antenna device of the present invention, the feeding element and the parasitic element are arranged substantially parallel to a connecting direction in which the coaxial cable is connected to the amplifier circuit.
In still another aspect of the vehicle-mount antenna device of the present invention, the filter is a common mode choke coil blocking common mode noise mixed in the outer conductor of the coaxial cable.
In still another aspect of the vehicle-mount antenna device of the present invention, the filter is a choke coil for increasing an impedance on a side of the circuit board when viewed from the outer conductor of the coaxial cable.
In still another aspect of the vehicle-mount antenna device of the present invention, the antenna unit has a wideband reception characteristic in a UHF frequency band.

Effects of the Invention

It is possible to provide a vehicle-mount antenna device in which stable antenna characteristics can be obtained with an influence of common mode noise being reduced.

Brief Description of the Drawings

FIG. 1 is a structural diagram showing a schematic structure of a vehicle-mount antenna device according to a second embodiment of the present invention.
FIG. 2 is an equivalent circuit diagram when the vehicle-mount antenna device of the second embodiment is arranged so as to be near a vehicle body metal part.
FIG. 3 is a structural diagram showing a schematic structure of a vehicle-mount antenna device according to a first embodiment of the present invention.
FIG. 4 is an equivalent circuit diagram when the vehicle-mount antenna device of the first embodiment is arranged so as to be near a vehicle body metal part.
FIG. 5 is an explanatory diagram describing the arrangement of the vehicle-mount antenna device in driving experiments.
FIG. 6 shows graphs representing experiment results of a reception power difference to a CNR difference measured in the driving experiments.
FIG. 7 shows a plan view and a rear view of an example of an antenna unit of a conventional vehicle-mount antenna device.
FIG. 8 is an equivalent circuit diagram when the conventional vehicle-mount antenna device is arranged so as to be near a vehicle body metal part.

Description of Embodiments

Vehicle-mount antenna devices in preferred embodiments of the present invention are described in detail with reference to the drawings. For simplification of the drawings and description, components having the same function are provided with the same reference numeral.

(First Embodiment)

A vehicle-mount antenna device according to a first embodiment of the present invention is described by using FIG. 3 and FIG. 4. FIG. 3 is a structural diagram showing a schematic structure of a vehicle-mount antenna device 200 and a receiving system 240 of the present embodiment, and FIG. 4 is an equivalent circuit diagram when the vehicle-mount antenna device 200 of the present embodiment is arranged so as to be near a vehicle body metal part 10.
The vehicle-mount antenna device 200 of the present embodiment is configured of an antenna unit 210 and a connector-equipped coaxial cable 120 electrically connecting the antenna unit 210 and a receiver 130. As shown in FIG. 3, the antenna unit 210 includes a feeding element 111, a parasitic element 112, and a circuit board 114. The circuit board 114 has a front surface where an amplifier circuit 113 is implemented and a back surface where an electrode pattern as a ground plane 115 of the amplifier circuit 113 is arranged, and a filter is arranged on a coaxial cable 120 side of the circuit board 114. The feeding element 111 is connected to a + input of the amplifier circuit 113, and the parasitic element 112 is connected to the ground plane 115 of the amplifier circuit 113. An output from the amplifier circuit 113 is connected to a center conductor 121 of the coaxial cable 120 via a filter 216, and the ground plane 115 is connected to an outer conductor 122 of the coaxial cable also via the filter 216. A signal from the vehicle-mount antenna device 200 is transmitted to the receiver 130 via the coaxial cable 120. The present embodiment is characterized in that the filter 216 is connected between the amplifier circuit 113 and the coaxial cable 120.
While the antenna unit 210 is placed on a front windshield, a rear windshield, or the like of a vehicle suitable for receiving radio wave, the receiver 130 is placed at another position away from antenna elements. For this reason, the coaxial cable 120 connecting both together is routed through a relatively long distance. The coaxial cable 120 has a center conductor 111 and an outer conductor 112, and is routed at an inconspicuous place such as a bottom of the vehicle. For this reason, at least part of the coaxial cable 120 is near the vehicle body metal part 10, and common mode noise tends to be mixed from the vehicle body metal part 10 to the outer conductor 112.
In FIG. 4, parasitic capacitances (hereinafter simply referred to as capacitances) formed by arranging the antenna unit 210 and the coaxial cable 120 near the vehicle body metal part 10 are shown as capacitances C1 to C4. Between the vehicle body metal part 10 and the outer conductor 122 of the coaxial cable 120, the capacitance C1 is formed at a position where noise is mixed and the capacitance C2 is formed at another position. Also, the capacitances C3 and C4 are formed between the vehicle body metal part 10 and the feeding element 111 and between the vehicle body metal part 10 and the parasitic element 112, respectively.
As for common mode noise, a current of noise mixed in the outer conductor 122 via the capacitance C1 is taken as I1, and currents when this current I1 flows inside the vehicle-mount antenna device 200 via the outer conductor 122 are taken as I2 to I5. As for the current I1, the current I2, which is part of the current I1, returns from the outer conductor 122 to the vehicle body metal part 10 via the capacitance C2 and the remaining current I3 flows into the circuit board 114 via the filter 216. Between a + side input and a - side input of the amplifier circuit 113 mounted on the circuit board 114, an input impedance Z_A (50 Ω) is present. For this reason, as for the current I3, the current I4, which is part of the current I3, flows into the parasitic element 112 via the ground plane 115 of the circuit board 114, and the remaining current I5 flows into the feeding element 111 via the input impedance Z_A.
The currents I4 and I5 return to the vehicle body metal part 10 via the capacitances C3 and C4 between the parasitic element 112 and the vehicle body metal part 10 and between the feeding element 111 and the vehicle body metal part 10. Here, when a voltage drop occurring due to the input impedance Z_A and the current I5 flows into the amplifier circuit 113 as a voltage input, the voltage input is amplified by the amplifier circuit 113 and received as normal mode noise by the receiver 130, thereby degrading the antenna characteristics. In the vehicle-mount antenna device 200 of the present embodiment, in order to reduce such normal mode noise, the filter 216 is connected between the amplifier circuit 113 and the coaxial cable 120.
To reduce the current I5, which can be a source of generating normal mode noise, the current I3 is preferably reduced. To reduce I3, the impedance of a line (a conducting line) where I3 flows is preferably increased. However, if the impedance of the outer conductor 122 is increased, transmission of a signal from the antenna unit 210 toward the receiver 130 is also prevented, and therefore it is required to increase the impedance of the conducting line of the current I3 without influencing signal transmission. Thus, in the present embodiment, the filter 216 is connected between the coaxial cable 120 and the amplifier circuit 113.
The filter 216 desirably has a high impedance with respect to the current I3 derived from common mode noise and does not influence a normal mode signal from the antenna unit 210 toward the receiver 130. In the present embodiment, a common mode choke filter is connected. With this, the filter 216 can cut off a common mode at a connecting point between the circuit board 114 and the coaxial cable 120 without influencing a normal mode signal, and common mode noise via the coaxial cable 120 can be inhibited from flowing into the amplifier circuit 113.
With the filter 216 being connected between the amplifier circuit 113 and the coaxial cable 120, the impedance on the amplifier circuit 114 side when viewed from the outer conductor 122 of the coaxial cable 120 is increased by an impedance Z_CMC of the filter 216, thereby reducing the current I3 flowing through the ground plane 115 of the circuit board 114. With this, as with the conventional anti-noise measure (the first method), an effect of reducing the current 13 can be obtained without being influenced by the routing position of the coaxial cable 120. Also, since the impedance with respect to the current I4 is not changed, the conventional anti-noise measure (the second method) can also be performed together.
Furthermore, while a common mode current from the coaxial cable 120 toward the amplifier circuit 113 is inhibited by the filter 216, a common mode current from the ground plane 115 of the amplifier circuit 113 to the antenna elements 111 and 112 is not inhibited. For this reason, the ground plane 115 can be designed so as to be operated as part of the parasitic antenna element 112, thereby efficiently making effective use of space. As a result, the antenna size can be designed so as to be maximum in a limited mount space, thereby making it possible to obtain excellent antenna characteristics.
In the vehicle-mount antenna device 200 of the present embodiment, common mode noise can be reduced by the filter 216 irrespectively of balanced or unbalanced type of the antenna elements 111 and 112. Also, since the high impedance characteristics with respect to the common mode does not depend on the wavelength, it is possible to easily reduce the size of the filter 216 and widen the bandwidth of the antenna elements 111 and 112. Furthermore, the shape and mount position of the antenna unit 210 are not restricted.
As described above, according to the vehicle-mount antenna device 200 of the present embodiment, with the filter 216 being connected between the amplifier circuit 113 and the coaxial cable 120, the common mode noise current I3 flowing to the amplifier circuit 113 can be reduced without being influenced by the routing position of the coaxial cable 120 and the placing position of an antenna unit 110. As a result, normal mode noise from the amplifier circuit 113 to the receiver 130 can be reduced. Furthermore, since the antenna size can be designed so as to be maximum in a limited mount space, antenna gain can be increased, thereby obtaining excellent antenna characteristics. In the vehicle-mount antenna device 200 of the present embodiment, stable antenna characteristics can be obtained without an influence of the routing position of the coaxial cable 120 or the like. Still further, the shape and mount position of the antenna unit 210 are not restricted.

(Second Embodiment)

A vehicle-mount antenna device according to a second embodiment of the present invention is described by using FIG. 1 and FIG. 2. FIG. 1 is a structural diagram showing a schematic structure of a vehicle-mount antenna device 100 of the present embodiment, and FIG. 2 is an equivalent circuit diagram when the vehicle-mount antenna device 100 of the present embodiment is arranged so as to be near the vehicle body metal part 10.
While the filter 216 is arranged between the coaxial cable 120 and the amplifier circuit 113 in the vehicle-mount antenna device 200 of the first embodiment, a filter 116 is connected between the amplifier circuit 113 and the feeding element 111 and the parasitic element 112 in the present embodiment. The filter 116 can reduce the current IS and inhibit a flow of common mode noise to the amplifier circuit 113.
To reduce the current I5, which can be a source of generating normal mode noise, the impedance of a line (a conducting line) where I5 flows is preferably increased. However, if the impedance of the feeding element 111 is increased, desired antenna characteristics are degraded, and therefore it is required to increase the impedance of the conducting line of the current I5 without influencing the antenna characteristics. Thus, in the present embodiment, the filter 116 is connected between the amplifier circuit 113 and the feeding element 111 and the parasitic element 112. As the filter 116, a balun can be used.
In an antenna targeted for a UHF band or longer wavelength, if a metal is near, operation cannot be made in a completely balanced state. Also in the vehicle-mount antenna device 100 of the present embodiment, since the vehicle body metal part 10 is near thereto, and operation can hardly be made in a completely balanced state. If a forced balun is used on condition that operation cannot be made in a complete balanced state, an unbalanced operation of the antenna elements may be inhibited, and excellent antenna characteristics may not be able to be obtained.
Thus, in the present embodiment, a choke balun (a choke coil) is used as the filter 116. With a choke balun as the filter 116 being connected between the antenna elements 111 and 112 and the amplifier circuit 113, the impedance with respect to the currents I4 and I5 is increased, thereby decreasing the currents 14 and I5. With that, the impedance on the amplifier circuit 113 side when viewed from the outer conductor 122 is increased, and the current I3 can also be decreased. With this, as with the conventional anti-noise measure (the first method), an effect of reducing the current I3 can be obtained without being influenced by the routing position of the coaxial cable 120. As a result, normal mode noise transmitted from the amplifier circuit 113 to the receiver 130 can be reduced, and excellent antenna characteristics can be obtained.
A relation between the current I3 and the current I5 is represented by the following equation. $I 5 = I 3 \cdot (Z_B + Z_G) / (Z_A + 2 \cdot Z_B + Z_F + Z_G)$

Here, Z_A is an input impedance between a + side input and a - side input of the amplifier circuit 113, Z_Bis an impedance of the filter 116, Z_F is an impedance between the feeding element 111 and the vehicle body metal part 10, and Z_G is an impedance between the parasitic element 112 and the vehicle body metal part 10.
To increase the noise reduction effect by the filter 116, the impedance Z_B of the filter 116 is preferably increased so as to increase the impedance on a circuit board 114 side when viewed from the outer conductor 122. As a result, I3 can be reduced. However, if Z_B is sufficiently large, it can be seen from the equation above that the influences of the impedances Z_F and Z_G are relatively decreased. From this, the conventional anti-noise measure (the second method) of decreasing the impedance Z_G to increase the current I4 cannot be usedtogether, and therefore the effect is limited compared with the first embodiment. Also, unlike the first embodiment, the ground plane 115 cannot be designed as part of the parasitic element, the antenna size is also smaller than that of the first embodiment.
As described above, according to the vehicle-mount antenna device 100 of the present embodiment, with the filter 116 being connected between the circuit board 114 and the antenna elements 111 and 112, the current 15 flowing through a line side where the amplifier circuit 113 is connected can be reduced without being influenced by the routing position of the coaxial cable 120 and the placing position of the antenna unit 110. As a result, normal mode noise flowing from the amplifier circuit 113 to the receiver 130 can be reduced, and excellent antenna characteristics can be obtained. In the vehicle-mount antenna device 100 of the present embodiment, stable antenna characteristics can be obtained without an influence of the routing position of the coaxial cable 120 or the like. Still further, the shape and mount position of the antenna unit 110 are not restricted.
To inspect the common mode noise reduction effect by the vehicle-mount antenna device 200 of the present embodiment, driving experiments were performed with the vehicle-mount antenna device 200 mounted on a vehicle. In the following, inspection results from the driving experiments are described. In the driving experiments, as shown in FIG. 5, the antenna unit 210 is mounted on a dashboard 11, the receiver 130 is placed below a rear seat, and a connection therebetween is made with the coaxial cable 120. Here, a reception signal of the antenna unit 210 is a Tokyo Tower DTV broadcast wave of 27 ch.
The driving experiments were performed in the case as shown in FIG. 5(a) where the coaxial cable 120 is in contact with the vehicle body metal part 10 for routing (the case is hereinafter referred to as a routing condition A) and the case where the coaxial cable 120 floats from the vehicle body metal part 10 by a length of 20 cm near the dashboard 11 (this portion is referred to as a cable sag portion 12) and the others are in contact with the vehicle body metal part 10 for routing (the case is hereinafter referred to as a routing condition B).
In the case of the routing condition B, the cable sag portion 12 of the coaxial cable 120 is in a state of receiving radio waves, like the antenna elements 111 and 112. As a result, in the case of the routing condition B, reception power is increased more than the case of the routing condition A. In the dashboard 11, various electric wirings are routed, and the electromagnetic noise level nearby is high. From this, noise tends to be received in the case of the routing condition B.
Also, for comparison, driving experiments were performed with a vehicle-mount antenna device without having the filter 216 (hereinafter referred to as an antenna device of a comparative example) mounted on a vehicle under the same conditions of those of the vehicle-mount antenna device 200 of the present embodiment. In each of the driving experiments, a GPS for obtaining location information of the vehicle is mounted.
In the driving experiments, the vehicle having the vehicle-mount antenna device 200 mounted thereon and the vehicle having the antenna device of the comparative example are caused to run on the same course according to the conditions described above, and reception power and CNR (Carrier vs. Noise Ratio) of each antenna are measured. Then, for each of the vehicle-mount antenna device 200 and the antenna device of the comparative example, a difference in the reception power and a difference in CNR between the case of the routing condition A and the case of the routing condition B at the same point (judged with GPS information) are calculated (which are referred to as a reception power difference and a CNR difference,
respectively).
The reception power difference and the CNR difference at a same point i are calculated by the following equations.
Reception power difference (point i)=reception power (routing condition B, point i)-reception power (routing condition A, point i)
CNR difference (point i)=CNR (routing condition B, point i)-CNR (routing condition A, point i)
The reception power difference and the CNR difference in the equations above are calculated for each of the case in which the vehicle-mount antenna device 200 is mounted and the antenna device of the comparative example is mounted.
Experiment results with the reception power difference being on the horizontal axis and the CNR difference being on the vertical axis are shown in FIG. 6. The reception power difference and the CNR difference are thought to represent the magnitude of influence of changes of the reception power and the CNR by the cable sag portion 12 in the case of the routing condition B. In the drawing, a straight line C with a gradient 1 passing through an origin represents positions plotted when a change in reception power and a change in CNR are the same. That is, a plot deviation amount from the straight line C represents a difference in noise inflow between the cable routing conditions A and B. That is, when plots are distributed on the straight line C, this represents that no influence of change in noise due to the cable sag portion 12 is present.
FIG. 6(a) shows experiment results when the antenna device of the comparative example without having the filter 216, and FIG. 6 (b) shows experiment results when the vehicle-mount antenna device 200 of the present embodiment is mounted. From FIG. 6 (a) when the device does not have the filter 216, pieces of experiment data are distributed by approximately 5 dB to a right side from the straight line C. That is, this means that the CNR is degraded by approximately 5 dB due to the cable sag part 12 when the same reception power is obtained. This shows that an inflow of noise due to the routing condition such as a sag of the cable cannot be prevented when the device does not have the filter 216.
By contrast, in the vehicle-mount antenna device 200 of the present embodiment having the filter 216, plots are distributed on the periphery of the straight line C. This shows that with the use of the filter 216, an inflow of common mode noise received by the cable at the cable sag portion 12 to the amplifier circuit is reduced. That is, in the vehicle-mount antenna device 200 of the present embodiment in which the filter 216 is arranged between the amplifier circuit 113 and the coaxial cable 120, the influence of common mode noise flowing from the vehicle body metal part 10 can be reduced. With this, stable antenna characteristics without being influenced by the routing condition of the coaxial cable 120 or the like can be obtained.
Note that the description in the present embodiments shows an example of the vehicle-mount antenna device according to the present invention and are not meant to be restrictive. Detailed structure, detailed operation, and others of the vehicle-mount antenna device in the present embodiments can be changed as appropriate within a range not deviating from,the gist of the present invention.

Reference Numerals

10:: Vehicle body metal part

11:: Dashboard
12:: Cable sag portion
100, 200:: Vehicle-mount antenna device
110, 210:: Antenna unit
111:: Feeding element
112:: Parasitic element
113:: Amplifier circuit
114:: Circuit board
115:: Ground plane
116, 216:: Filter
120:: Connector-equipped coaxial cable
121:: Center conductor
122:: Outer conductor
130:: Receiver
140, 240:: Receiving system

Claims

A vehicle-mount antenna device comprising: an antenna unit having a feeding element and a parasitic element; a circuit board with a back surface including a ground plane; an amplifier circuit mounted on the circuit board and amplifying a reception signal received by the antenna unit; and a coaxial cable electrically connecting the amplifier circuit and a receiver,
the device further comprising a filter for reducing common mode noise mixed in an outer conductor of the coaxial cable.
The vehicle-mount antenna device according to claim 1, wherein
the filter is connected between the amplifier circuit and the coaxial cable, the feeding element is connected to the amplifier circuit, and the parasitic element is connected to the ground plane.
The vehicle-mount antenna device according to claim 1, wherein
the filter is arranged between the amplifier circuit and the antenna unit, the feeding element is connected to the amplifier circuit via the filter, and the parasitic element is connected to the ground plane via the filter.
The vehicle-mount antenna device according to any one of claims 1 to 3, wherein
the feeding element and the parasitic element are arranged substantially parallel to a connecting direction in which the coaxial cable is connected to the amplifier circuit.
The vehicle-mount antenna device according to any one of claims 1 to 4, wherein
the filter is a common mode choke coil blocking common mode noise mixed in the outer conductor of the coaxial cable.
The vehicle-mount antenna device according to any one of claims 1 to 5, wherein
the filter is a choke coil for increasing an impedance on a side of the circuit board when viewed from the outer conductor of the coaxial cable.
The vehicle-mount antenna device according to any one of claims 1 to 6, wherein
the antenna unit has a wideband reception characteristic in a UHF frequency band.