DESCRIPTION
COBRA ANTENNA
TECHNICAL FIELD
5 [0001]
The present invention relates to a cobra antenna that
can be used as an antenna for a wide frequency band ranging
from an FM band to a UHF band and that can be implemented with
a simple structure.
10
BACKGROUND ART
[0002]
Various forms of antennas have conventionally been used
as an antenna for receiving various broadcast waves such as
15 television broadcast or FM broadcast. For example, a dipole
antenna or a Yagi-Uda antenna is often used for receiving
television broadcast and FM broadcast.
On the other hand, the various broadcast waves have
increasingly being received in a room, in a car or during travel
20 on foot. The antenna used in such cases needs to be easily
handled, for example, for assembly or installation.
[0003]
Such an easily-assembled or easily-handled antenna is
typified by a dipole antenna that is implemented by the antenna
25 elements that are simply structured. A cobra antenna is known
as an embodiment of the dipole antenna. The cobra antenna
is used with some turns of a coaxial wire around a ferrite
core (for example, Non-patent Document 1).
[0004]
30 Fig. 5 is a view for showing an exemplary cobra antenna
that has been produced by modifying a dipole antenna . As shown
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in Fig. 5, a cobra antenna 100 includes a central conductor
(core wire) 300 and a ferrite core 400. On the assumption
that the radio wave to be received has a wavelength of X, the
central conductor 300 is A./4 in length and is connected, as
5 an upper element, on a feeding point 200. The ferrite core
400 is provided under and A./4 away from the feeding point 200.
A coaxial cable (coaxial wire) 500 is wound around the ferrite
core 400. Although the coaxial cable 500 is wound 3 times
in Fig. 5, the number of turning (the number of winding) does
10 not necessarily need to be three times. The number may be
once or twice.
[0005]
When the coaxial wire is wound around the ferrite core
400 three times or more, the impedance tends to drastically
15 decrease regardless of the size of the ferrite over about the
frequency of 100 MHz . For example, it has been reported that,
when the number of winding is once, the impedance of the antenna
tends to increase even though the frequency exceeds 100 MHz;
however, when the number of winding is three times, the
20 impedance drastically decreases.
[0006]
In the cobra antenna shown in Fig. 5, a choke coil is
formed by a ferrite core 300 and the coaxial cable 500 wound
around the ferrite core. The choke coil separates a feeder
25 part below the ferrite core 400 so that a A./4 dipole antenna
can easily be formed. An egg-shaped glass or the like is
attached to the upper core wire 300 of the dipole antenna for
insulation so that the antenna can be hung from a tree branch
or a wooden frame. This can facilitate the installation of
30 an antenna. A cobra antenna structured in such a manner can
also be applied to an antenna of a car-mounted mobile device.
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CITATION LIST
NON-PATENT DOCUMENT
[0007]
5 Non-patent Document 1: Chapter 1 ANTENA NO KISO, p. 84 in "WIRE
ANTENNA" edited by CQ ham radio HENSHU BU
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
10 [0008]
However, when the cobra antenna shown in Fig. 5 is used
as an antenna for a wide frequency band ranging from an FM
band to a UHF band, an interference of radio waves sometimes
occurs depending on the length of the coaxial cable 500 from
15 the ferrite core 400 to a receiver. In other words, there
is a problem about radio wave interference in that the
high-frequency current received by the upper part of the
coaxial cable 500 leaks into the lower part of the coaxial
cable 500. The upper part extends from the ferrite core 400
20 to the feeding point 200. The lower part extends from the
ferrite core 400 and is connected to the receiver. The leakage
of the high-frequency current is considered to occur due to
the impedance mismatch between the upper side and the lower
side across the ferrite core 400. There is a disadvantage
25 in that the leakage causes the gain characteristics as an
antenna to become bad.
[0009]
The occurrence of the leakage of the high-frequency
current depends on the length of the coaxial cable 500 from
30 the ferrite core 400 to the point connected to the receiver.
Thus the occurrence becomes a strict limitation when the length
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of the part of the coaxial cable 500 is determined. In other
words, in a conventional cobra antenna 100, the length of the
coaxial cable 500 from the ferrite core 400 to the receiver
cannot freely be determined. It is considered that the
5 interference due to the high-frequency current occurs because
the cobra antenna 100 uses the outer sheath of the coaxial
cable 500 as an antenna. Thus, there is a problem in that
the required performance cannot be obtained when the cobra
antenna 100 is connected to a connector of the receiver without
10 modification.
[0010]
The present invention has been made in light of the
foregoing problems, and an object of the present invention
is to provide a small cobra antenna that can be used as an
15 antenna for a wide frequency band ranging from an FM band to
a UHF band, and has a high performance as an antenna. The
cobra antenna also minimizes the limitation on the length of
the coaxial wire.
20 SOLUTIONS TO PROBLEMS
[0011]
To solve the above-mentioned problems and achieve the
object of the present invention, the cobra antenna of the
present invention includes a junction constituting the feeding
25 point. An antenna element is electrically connected to one
terminal of the junction. The antenna element has a length
corresponding to the frequency of the broadcast wave to be
received. A coaxial wire is connected to the other terminal
of the junction. A ferrite core is positioned at a location
30 a length identical to the length of the antenna element away
from the other terminal of the j unction connected to the coaxial
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wire. The coaxial wire is wound around the ferrite core about
once to three times. A high frequency interrupting part is
provided at the front side of a connecter of a receiver connected
to the other terminal of the coaxial wire. The high frequency
5 interrupting part is for interrupting the high-frequency
current from the coaxial wire.
[0012]
Note that the high frequency interruptingpart is a second
ferrite core that has high impedance against a high-frequency
10 wave. The above-mentioned coaxial wire passes through the
inside of, or is wound around, the second ferrite core . Further,
on the assumption that the frequency to be received has a
wavelength of X, the antenna element is X/4 in length and the
length from the junction of the coaxial wire to the ferrite
15 core is A./4.
[0013]
The cobra antenna of the present invention can prevent
the high-frequency wave picked up by the coaxial wire from
entering the receiver by including, in front of the connector
2 0 of the receiver, the second ferrite core that has high impedance
against a high-frequency wave.
EFFECTS OF THE INVENTION
[0014]
25 According to the present invention, the length of the
part of the coaxial wire except the antenna wire can freely
be determined. This reduces the limitation on the placement
of the antenna. Thus, the cobra antenna according to the
present invention can fully exert the performance as an antenna
30 regardless of the equipment to be connected to the antenna,
and regardless of the length of the coaxial wire of the antenna.
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BRIEF DESCRIPTION OF DRAWINGS
[0015]
Fig. 1 is a schematic view showing the comparison between
5 an exemplary embodiment of the cobra antenna of the present
invention (B) and a conventional cobra antenna (A).
Fig. 2 is a schematic view showing the comparison between
the frequencies-gain characteristics of the cobra antenna of
the exemplary embodiment of the present invention (B) and the
10 frequencies-gain characteristics of the conventional cobra
antenna (A).
Fig. 3 is a view showing an example where the cobra antenna
of the exemplary embodiment of the present invention is
attached as a car-mounted antenna.
15 Fig. 4 is a view showing the route used for the field
test for a car on which the cobra antenna of the exemplary
embodiment of the present invention is mounted as a car-mounted
antenna.
Fig. 5 is a view for describing a conventional cobra
2 0 antenna.
MODE FOR CARRYING OUT THE INVENTION
[0016]
An exemplary embodiment of the present invention
25 (hereinafter, sometimes referred to as the present example)
will be described below based on Figs. 1 to 4, and described
in the following order.
1. Description of the basic structure and the basic
principle of a cobra antenna
30 2. The structure and the characteristics of the cobra
antenna of the exemplary embodiment of the present invention
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3. The field test performed using the cobra antenna of
the exemplary embodiment of the present invention
[0017]

Fig. 1A shows the same cobra antenna as the conventional
cobra antenna described in Fig. 5. Fig. IB shows the cobra
antenna of the present example. First, the commonalities
between Figs. 1A and IB will be described.
10 Each of cobra antennas 10 shown in Figs . lAand IB includes
an antenna element 2, a junction 3, a coaxial wire 5, and a
ferrite core 4. The length of the antenna element 2 is A./4
on the assumption that the radio wave to be received has a
wavelength of X. The junction 3 is afeedingpoint. The length
15 of the coaxial wire from the junction 3 to the ferrite core
4 is A./4, which is the same as the length of the antenna element
2.
[0018]
An end of the coaxial wire 5 is connected to the antenna
20 element 2 through the junction 3. Further, the coaxial wire
5 is wound around the ferrite core 4 about once to three times.
The other end of the coaxial wire 5 is connected to a connector
6 of a receiver 8. In this case, a connector that has a low
loss of the high-frequency signal is preferably selected as
25 the connector 6. At the antenna element 2, an outer sheath
(protective coating) 5a and a shield wire (external conductor)
5b of the coaxial wire 5 are removed.
[0019]
At the junction 3, the outer sheath 5a and the shield
30 wire 5b of the coaxial wire 5 are removed, and a core material
2c (inductor) is exposed. A core wire 5d of the coaxial wire
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5 is connected to a core wire of the antenna element 2 by means
of, for example, soldering. The junction 3 is molded and formed
on a substrate 7. The junction 3 is a feeding point Fp of
the cobra antenna 10.
5 [0020]
With this configuration, the coaxial wire 5 from the
junction 3 (the feeding point) to the ferrite core 4 (A./4 in
length) and the antenna element 2 (X/4 in length) form a dipole
antenna of A./2.
10 [0021]

As described above, the commonalities between the cobra
antennas shown in Figs . lAand lBhavebeendescribed. However,
15 the cobra antenna of the present example shown in Fig. IB differs
from the conventional cobra antenna shown in Fig. 1A in that
the cobra antenna of the present invention is provided with
a second ferrite core 4a in front of the connector 6 of the
receiver 8.
2 0 [0022]
Hereinafter, the conventional cobra antenna shown in
Fig. 1A will be referred to as a cobra antenna (one-core product)
and the cobra antenna of the present invention will be referred
to as a cobra antenna (two-core product).
25 In the conventional cobra antenna (one-core product) ,
as already described, a high-frequency coupling occurs between
the coaxial wire 5 from the ferrite core 4 to the junction
3 and the coaxial wire 5 from the ferrite core 4 to the connector
6. This degrades the performance of the antenna. Because
30 the degrading depends on the length to the coaxial wire 5 from
the ferrite core 4 to the connector 6, the length of the part
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becomes a limitation when this type of cobra antenna is used
as a car-mounted antenna.
[0023]
In the cobra antenna (two-core product) of the present
5 example shown in Fig. IB, the second ferrite core 4a is provided
at a position near the receiver 8. Because the ferrite core
4a has high impedance against a high-frequency wave, the
high-frequency current leaking from the antenna is not
propagated to the receiver side.
10 [0024]
Fig. 2A and Table 1 are the graphs showing the peak gains
of the vertical polarization (V) and of the horizontal
polarization (H) of the conventional cobra antenna (one-core
product) shown in Fig. 1A. The horizontal axis of Fig. 2A
15 denotes the frequencies (MHz) and the vertical axis denotes
the peak gains (dBd).
The frequencies to be measured are set at FM/VHF bands
(70 MHz to 220 MHz) . The vertical polarization (V) is denoted
by a dash line. The horizontal polarization (H) is denoted
20 by a solid line.
[0025]
Table 1 shows the value of the peak gain of the vertical
polarization (V) and the value of the peak gainof the horizontal
polarization (H) at each measurement point in the graph shown
25 in Fig. 2A. Note that, in Table 1, only the measured values
of the frequencies from 7 6 MHz to 107 MHz are shown from among
the frequencies shown in the horizontal axis of Fig. 2A.
[0026]
As shown in Fig. 2A and Table 1, the peak gain of the
30 vertical polarization (V) becomes -11.50 dBd at 86 MHz and
-10.85 dBd at 95 MHz. The peak gain of the horizontal
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polarization (H) becomes -16.70 dBd at 86 MHz and -14.85 dBd
at 95 MHz. In other words, it is found that the conventional
cobra antenna (one-core product) also can receive both of the
vertical polarization and the horizontal polarization in the
5 FM/VHF bands.
[0027]
[Table 1]
Vertical polarization
Freq[MHz] 76 | 78.5 [ 81 | 83.5 [ 86 | 95 [ 101 | 107
Peak[dBd] -12.04 -12.60 -12.81 -12.14 -11.50 -10.85 -11.87 -12.96
Horizontal polarization
FreqfMHz] 76 | 78.5 | 81 | 83.5 I 86 I 95 I 101 I 107
Peak[dBd] -18.76 -18.80 -18.61 -17.72 -16.70 -14.85 -15.14 -15.50
[0028]
10 On the other hand, the frequency gain characteristics
of the cobra antenna (two-core product) of the present example
are shown in Fig. 2B and Table 2. As is obvious from Fig.
2B and Table 2, both of the vertical polarization (V) and the
horizontal polarization (H) reach maximum values near 95 MHz .
15 The vertical polarization (V) is -8.25 dBd and the horizontal
polarization (H) is -13.65 dBd. In comparison with the
conventional type (one-core product) shown in Fig. 2A and Table
1, the peak gains at 95 MHz become higher. The frequency-gain
characteristics are obviously improved. In other words, it
20 is found that the performance of the cobra antenna (two-core
product) of the present example is superior to that of the
conventional cobra antenna (one-core product) .
[0029]
[Table 2]
Vertical p o l a r i z a t i on
Freq[MHz] 76 [ 78.5 I 81 | 83.5 | 86 I 95 | 101 [ 107
PeakfdBd] -12.40 ~12.80 " ^ 2 . 8 1 -11.92 -10.70 -8.25 -8.87 -10.83
25
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Horizontal p o l a r i z a t i on
Freq[MHz] 76 | 78.5 | 81 | 83.5 | 86 | 95 | 101 | 107
~Peak[dBd] -20.31 -20.20 -19.96 -18.71 -17.30 -13.65 -13.67 -14.76
[0030]
Figs. 2A and 2B show the minimum values at about 130
MHz. It is indicated that setting the resonance frequency
at 100 MHz causes the Q factor of the antenna to become high
5 at about 130 MHz and causes antiresonance (mismatch) so that
the frequency cannot be received. Note that the resonance
frequency that has been set at 100 MHz resonates with a high
frequency. Specifically, the odd multiples of the resonance
frequency or, namely, even the triple, or quintuple of the
10 basic resonance wavelength can be received. As for the cobra
antenna (two-core product) of the present example, the
resonance can occur even when the frequency is set at 200 MHz.
[0031]

Fig. 3 is a view showing an example where the cobra antenna
(two-core product) of the present invention is mounted on the
car belonging to the inventor to perform a field test for the
cobra antenna (two-core product) . Needless to say, the
20 conventional cobra antenna (one-core product) has also been
mounted on the car to perform the same measurement for
comparison.
[0032]
As shown in Fig. 3, the antenna element 2 from the j unction
25 3 of the cobra antenna 10 to the tip is horizontally attached
at the windshield from the rearview mirror. The coaxial wire
5 from the junction 3 to the ferrite core 4 is longitudinally
attached at the left side. This forms the cobra antenna 10
as a V-shaped antenna having the junction 3 as a center (starting
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point). The junction 3 is a feeding point.
[0033]
In consideration of the fact that an FM band of 90 MHz
has a wavelength X of 3.33 m, in each of the cobra antenna
5 (two-core product) of the present example and the conventional
cobra antenna (one-core product), the antenna element 2 is
set as 0.83 m equal to A./4 in length, the coaxial wire 5 from
the junction 3 to the ferrite core 4 is similarly set as 0.83
m equal to X/4 in length, and then the antenna is set as X/2
10 (1.66 m) in length.
[0034]
The coaxial wire 5 from the ferrite core 4 to the connector
6 of the receiver 8 is horizontally routed on the dashboard
of the car. Note that, in the cobra antenna (two-core product)
15 10 of the present example, the second ferrite core 4a is inserted
into the front (proximity) of the connector 6 of the receiver
8.
The coaxial wire 5 can only pass through the hole of
the second ferrite core 4a. However, the coaxial wire 5 can
2 0 also be wound around the ferrite core 4a about once to three
times and be connected to the connector 6. As described above,
in the cobra antenna (two-core product) 10 of the present
example, the ferrite core 4a is positioned in front of the
connector 6. Accordingly, the receiver 8 side has high
25 impedance against the high-frequency current picked up by the
coaxial wire 5. The coaxial wire 5 connects the ferrite core
4 to the connector 6. Thus, even though the coaxial wire 5
from the first ferrite core 4 to the connector 6 picks up the
leaked high-frequency current, the leaked high-frequency
30 current does not adversely affect the receiver 8 side.
[0035]
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As shown in Fig. 3, the cobra antenna (two-core product)
of the present example and the conventional cobra antenna
(one-core product) have separately been mounted on the car
to perform a field test.
5 Fig. 4 is a view showing the course for the test of each
reception performance of the cobra antennas that have actually
been mounted on the inventor's car by the inventor. The type
of the car was Toyota Carolla (registered trademark). The
equipment used as the receiver 8 was a personal navigation
10 device (PND) manufactured by SANYO Electric Co., Ltd. (GORILLA
NV-SD750FT) (GORILLAis a registered trademark) . Thereceived
frequency was 81.9 MHz from VICS Yokohama and the output was
5 kW.
[0036]
15 As for the sample of the cobra antenna 10, the distance
from the junction 3 to the tip of the antenna element 2 was
83 cm and the distance from the junction 3 to the ferrite core
4 was also 83 cm. Further, in the test, the second ferrite
core 4a was provided about 5 cm away from a plug to be inserted
20 into the connector 6 of the receiver 8. However, the distance
can be determined as needed.
[0037]
As shown in Fig. 4, in the field test, the conventional
cobra antenna (one-core product) was first mounted on the car
25 and the car run on Nakahara-Kaido way shown in the drawing
to append the VICS updated every five minutes in the running
section. Next, the cobra antenna (two-core product) of the
present example was mounted on the car and the car run on the
same course to append the VICS every five minutes in the running
30 section in the same manner.
[0038]
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The test results are the following.
the conventional cobra antenna (one-core product) : 6/11
times, 54% reception rate
the cobra antenna (two-core product) of the present
5 example: 12/14 times, 78% reception rate
As is obvious from the results, it can be confirmed that
the cobra antenna (two-core product) of the present invention
can almost certainly update the data every five minutes in
comparison with the conventional type (one-core product).
10 [0039]
As described above, the cobra antenna (two-core product)
as the exemplary embodiment of the present invention has been
described in comparison with the conventional cobra antenna
(one-core product) . In the above-mentioned description, the
15 antenna using a coaxial wire (wire rod) has been described.
However, an antenna constituted of a substrate, a film, and
a metal wire can be used for the antenna element part to exert
the same effect. Further, needless to say, the present
invention can be used for the equipment in a room except a
20 car although the present example has been described as an
example that has been mounted on the car.
REFERENCE SIGNS LIST
[0040]
25 10, 100 Cobra antenna
2, 300 Antenna element
3 Junction
4, 4a, 400 Ferrite core
5, 500 Coaxial wire
30 5a Protective coating
5b Shield wire
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5c Core material
5d Core wire
Fp, 200 Feeding point
6 Connector
5 7 Substrate
8 Receiver

CLAIMS
1. A cobra antenna comprising:
a junction constituting a feeding point;
5 an antenna element electrically connected to one
terminal of the junction and having a length corresponding
to a frequency of a broadcast wave to be received;
a coaxial wire with one end electrically connected to
the other terminal of the junction;
10 a first ferrite core provided at a position a length
identical to a length of the antenna element away from the
other terminal of the junction to which the one end of the
coaxial wire is connected, the coaxial wire being wound around
the first ferrite core; and
15 a high frequency interrupting part, provided at a front
of a connector of a receiver to which the other end of the
coaxial wire is connected, for interrupting a high-frequency
current from the coaxial wire.
20 2 . The cobra antenna according to claim 1, wherein the high
frequency interrupting part has high impedance against a
high-frequency wave and the high frequency interrupting part
is a second ferrite core through which the coaxial wire passes
or around which the coaxial wire is wound.
25
3. The cobra antenna according to claim 1 or 2, wherein
the length of the antenna element and a length of the coaxial
wire from the junction to the first ferrite core are X,/4 when
a wavelength of a frequency to be received is assumed to be
30 X.
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4. The cobra antenna according to any of claims 1 to 3,
wherein the antenna element connected to the one terminal of
the junction is formed by a core part including a core wire
except an outer sheath and a shield wire of the coaxial wire,
5 and the core wire of the antenna element is electrically
connected to a core wire 'of the coaxial wire at the junction.