JP4793210B2 - Folded dipole antenna - Google Patents

Description

  The present invention relates to a small broadband antenna composed of a metal wiring developed on a surface. This antenna is useful when a dipole antenna is simply configured at low cost, and is used, for example, when an in-vehicle antenna is disposed on a dielectric such as a car window.

  As a conventional technique for realizing a planar small antenna that matches in a wide band, for example, a technique described in Patent Document 1 below is known. This conventional loop antenna has a great feature in that it has a folded dipole as a part thereof to achieve a wide band.

Further, as a conventional technique for attempting to widen an antenna band using a variable capacitance element, for example, a technique described in Non-Patent Document 1 below is known. This antenna is proposed to solve the problem that the antenna's input impedance may become difficult to match the internal circuit (receiver circuit) due to changes in the surrounding environment of the antenna (eg, the approach of the human body). It is characterized by a π-type matching circuit using varactor diodes and an analog circuit that dynamically controls the matching characteristics of the matching circuit.
JP-A-2005-204194 Ida, Takada, and two others, "Automatic antenna matching device for mobile communication devices by adaptive control", 2004 IEICE General Conference, B-1-262.

However, when trying to increase the bandwidth of an antenna using a variable capacitance element, the bias control circuit for controlling the capacitance becomes complicated, which is clearly disadvantageous in terms of cost.
Although a method of assembling a variable capacitance element to the antenna itself is also conceivable, many problems still remain in terms of circuit complexity, cost, aesthetics of the antenna, or adverse effects of the bias line on the radiation pattern shape of the antenna. Such a method is not always desirable.

  On the other hand, in the case of the conventional loop antenna described in the above-mentioned Patent Document 1, further widening of the band is expected by some addition or modification. From the viewpoint of cost reduction, the widening means is as simple as possible. Is desirable.

  The present invention has been made to solve these problems, and an object of the present invention is to further increase the bandwidth while suppressing the complexity and cost increase of the antenna.

In order to solve the above problems, the following means are effective.
That is, according to the first means of the present invention, in a folded dipole antenna composed of metal wiring pattern-developed on the surface, one set of two feed points (α, β) on the left and right sides that are close to each other. A first folded dipole composed of an outermost circumferential loop Z (path α- efa -bcdchgh-β) having a power supply unit O and a capacitor located on the opposite side thereof; An unbranched line s disposed inside the outermost loop Z and having both ends (g, h) connected to two different points in the middle of the left half of the outermost loop Z, and the outermost loop Z disposed of on the inside, at both ends (e, f) are respectively connected to different two points in the middle of the right half of the outermost loop Z, and a free branch of the line t that does not have even line s and the intersection also contacts a, a feeding section O and capacity and line s and line t, the outermost Le Become since a portion of the flop Z (path f-a-b-c- d-h), the feed point (alpha, beta) metal forming one open curve of unbranched having a capacity in the middle and end points of The wiring p (path α-e-line tf-a-b-c-d-h-line s-g-β) constitutes a second folded dipole, and the capacitance is the outermost loop. On the middle point on the metal wiring forming Z, a pattern is formed in the gap of the metal wiring (path bc) forming the outermost peripheral loop Z, and between the line s and both ends (g, h) The line connecting the line t and the metal line q (path g-line s-h-g) as a simple closed curve, and the line connecting the line t and both ends (e, f) are other The metal wiring r (path e−line t−f−e), which is a simple closed curve, is formed, and the metal wiring p and the metal wiring q are mutually connected to the line s. The metal wiring p and the metal wiring r share the line t. The metal wiring q and the metal wiring r are arranged apart from each other, and the lines s and t are excluded from the metal wirings p, q, r. The remaining portion forms the outermost peripheral loop Z, and the portion pp of the metal wiring p located between the metal wiring q and the metal wiring r includes a power feeding portion O, and the metal is interposed via the power feeding portion O. The metal wiring connected to the wiring q and the metal wiring r and having a pattern developed on the surface is a single center line that divides a pair of feeding points (α, β) on the left and right. The first folded dipole has a length L (path bc) in the longitudinal direction of the first folded dipole, and the length W (path ab) of the first folded dipole in the direction orthogonal thereto 10 times or more and 50 times or less, and forms a second folded dipole. The distance D2 between the metal lines parallel to the longitudinal direction satisfies L / 200 ≦ D2 ≦ L / 50 with respect to the length L in the longitudinal direction of the first folded dipole, and is viewed from the feeding point (α, β). When the first impedance, the second frequency, and the third frequency from the lower side are the frequencies at which the impedance becomes the pure impedance, the increase in the second frequency and the third frequency with respect to the decrease in the capacitance The folded dipole antenna is characterized in that the frequency band to be used is expanded with a characteristic that becomes larger than an increase of one frequency.

However, the surface on which the metal wiring is developed can be a surface of a dielectric substrate, a film substrate, glass or the like, and these metal wirings may be further sealed with an arbitrary dielectric material.
Further, the above-described capacitor may be a plurality of capacitors connected in series or in parallel. Particularly in that case, the combined capacity thereof corresponds to the above-mentioned capacity.
The desirable numerical range of the capacitance is about 0.1 pF to 2 pF, more preferably about 0.1 pF to 1.0 pF, and more preferably about 0.2 pF. If this value is too large, it will be difficult to obtain the broadening effect of the present invention. If this value is too small, the matching characteristics will deteriorate locally in the center of the frequency band where the antenna should operate well. This is undesirable because it can result in a band. That is, it is not desirable because a maximum point occurs in the graph of the voltage standing wave ratio (VSWR) with respect to the resonance frequency of the antenna, and the maximum value shows a large value of 3 or more.

The open curve or simple closed curve referred to here may be interpreted as the center line of the metal wiring. Since this metal wiring is actually formed by a known metal conductor such as a metal wire, of course, it has a thickness and a volume if strictly speaking.
Strictly speaking, the above open curve is cut off at the above capacity, but here, the above open curve (metal wiring p) is considered ignoring the gap.

The gaps (slits) for forming the capacitance do not necessarily need to be formed symmetrically. In this case, what is important is the position of the effective center of gravity of the capacity or the combined capacity, and it is sufficient that the position is on the center line.

Further, the second means of the present invention is the above-mentioned first means, wherein a part ss of the line s located near the feeding part O and a part tt of the line t located near the feeding part O are separated by a distance D1. It is arranged to face each other in parallel.

According to a third means of the present invention, in the second means, L / 10 ≦ D1 ≦ L / 4 is satisfied with respect to the length L in the longitudinal direction of the first folded dipole. Similarly, the distance D1 is set.

By means of the present invention described above, effectively the problems described above, or can be reasonably resolved.

The effects obtained by the above-described means of the present invention are as follows.
That is, according to the first means of the present invention, the frequency band that can be used by the antenna can be made wider than before by the arrangement of the capacitance. In addition, this broadband antenna can be formed very simply and inexpensively by patterning metal wiring and capacitance on the surface of the dielectric substrate. In particular, the above capacitance can be formed by a gap between the metal wirings developed on those surfaces, so there is no risk of an increase in cost due to the introduction thereof. Further, this capacitance is, for example, 0.2 pF. Since the size may be very small, the above configuration is suitable for miniaturization of the antenna.

Further, by the present onset bright lever, since the current is strong portion on the metal wiring above the capacitor is arranged, which makes it possible to obtain a frequency broadband action based on the capacity most efficiently.

Further, according to this onset bright, it is possible to maximize the gain of the antenna. However, the wiring pattern of the folded dipole antenna is ultimately determined by comprehensively considering the shape of the installation location (eg, the shape of the window frame that supports the glass), area, and visual effects. It is more desirable to do.

Further, according to this onset bright, it is possible to narrow forming the occupied area of the antenna, for example, in the case of disposing the antennas, etc. on the windshield of the vehicle, it is possible to improve the mountability of the antenna. Further, according to the fifth means of the present invention, a so-called 8-shaped directivity can be obtained over a wide band.

Further, according to the second means of the present invention, the second folded dipole can be formed in a substantially T shape, and further, the input of the second folded dipole can be achieved by adjusting the distance D1. Impedance can be easily adjusted.

According to the third and first means of the present invention, the voltage standing wave ratio (VSWR) of the antenna is effectively reduced based on optimization of each tuning parameter (D1 or D2). Can do.

Hereinafter, the present invention will be described based on specific examples.
However, the embodiments of the present invention are not limited to the following examples.

FIG. 1 is a plan view of a folded dipole antenna A1 according to the first embodiment. Each metal wiring in the figure is composed of a plate-shaped strip conductor.
The first folded dipole is composed of a rectangular outermost loop Z having points a, b, c, and d as vertices. On the other hand, the second folded dipole is composed of a metal wiring p made of a strip conductor surrounding a central substantially T-shaped planar region P, and feed points α and β are formed at both end points of the metal wiring p. Has been. Hereinafter, these feeding points α and β are collectively referred to as a feeding unit O.

  Each end point of the line s arranged inside the outermost peripheral loop Z is connected to points h and g on the outermost peripheral loop Z, respectively. Further, each end point of the line t arranged inside the outermost peripheral loop Z is connected to points e and f on the outermost peripheral loop Z, respectively. The further part ss of the line s constituting a part of the metal wiring p and the further part tt of the line t constituting a part of the metal wiring p are arranged to face each other in parallel by being separated by a distance D1. Yes. The opposing portions ss and tt may be hereinafter referred to as stubs (ss and tt).

  In addition, the power feeding portion O is inserted in the center of the metal wiring pp connecting the points g and e. In other words, the metal wiring pp is located on the right side of the power feeding part O and connects the power feeding point α and the point e (pp right part), and is located on the left side of the power feeding part O and the power feeding point β. It is a general term for a metal wiring (pp left part) connecting g, and the pp right part and pp left part also constitute a part of the metal wiring p. Starting from the feeding point α, the outermost peripheral loop Z is formed of a metal wiring on a forward path that sequentially passes counterclockwise through points a, b, c, and d and finally returns to the feeding point β.

  The line s passes only on the rectangular inner region surrounded by the outermost loop Z, connects the two points g and h on the outermost loop Z, and is located on the outermost loop Z. Absent. Similarly, the line t passes through only the rectangular inner region surrounded by the outermost peripheral loop Z, and connects the two points e and f on the outermost peripheral loop Z. The line t and the line s have neither intersection nor contact point.

Hereinafter, these structures will be described using the two-dimensional region U.
This two-dimensional region U is a region surrounded by the line segment connecting the feeding points α and β and the outermost peripheral loop Z, and coincides with a rectangular inner region having points a, b, c and d as vertices. To do. The two-dimensional area U is composed of the sum area of the three two-dimensional areas P, Q, and R shown in FIG. 1 and the two lines s and t. However, none of P, Q, R, s, and t intersect each other. The metal wiring p that forms an open curve surrounding the two-dimensional region P and the metal wiring q that forms the circumference (simple closed curve) of the two-dimensional region Q share a part of the metal wiring s. Similarly, a metal wiring p that forms an open curve surrounding the two-dimensional region P and a metal wiring r that forms a circumference (simple closed curve) of the two-dimensional region R share a part of the metal wiring t.

  The metal wiring p surrounding the substantially T-shaped two-dimensional region P forms an open curve, and two end points of the open curve correspond to the power supply points α and β of the power supply unit O. That is, the two end points α and β of the metal wiring p forming an open curve surrounding the two-dimensional region P are close to each other at the power feeding unit O. A portion pp of the metal wiring p provided with the power supply unit O forms a point g on the metal wiring q that forms a circumference (simple closed curve) of the two-dimensional region Q and a circumference (simple closed curve) of the two-dimensional region R. A point e on the metal wiring r is connected via a power feeding unit O.

  A width D2 in the drawing indicates a distance between two substantially parallel lines constituting the second folded dipole. The left end portion of the T-shape is bent at a right angle with respect to the negative direction in the y-axis direction and the positive direction in the x-axis direction at a portion ahead of the point c. Similarly, the right end portion of the T-shape is bent at right angles to the negative direction in the y-axis direction and the negative direction in the x-axis direction at a portion ahead of the point b. In addition, the power feeding unit O is provided at the approximate center at the bottom of the T-shape.

A capacitor C1 is provided on the metal wiring p shared by the first folded dipole and the second folded dipole by the outermost peripheral loop Z. The capacitor C1 is patterned with a metal wiring having a slit ps having a width D3. The slit ps has two right-angled bent portions. According to this configuration, the necessary capacitance C1 can be formed in the same manner as the metal wiring of the antenna by a printing technique or the like. Therefore, a low-cost antenna having a broadband characteristic can be realized.
Further, since the central portion of the metal wiring p constituting the T-shaped upper portion shared by the first folded dipole and the second folded dipole is a portion where current is strong, by providing the capacitor C1 here, The allowable frequency bandwidth of the antenna A1 can be maximized.

Hereinafter, the appropriate range is disclosed about the length of each part of metal wiring.
With respect to the lengths L and W in the x and y directions of the folded dipole antenna A1, if L / 50 ≦ W ≦ L / 10, the antenna can be narrowed in the y-axis direction and wide. A stable 8-shaped directivity can be obtained over frequency.
The appropriate range of the stub width D1 (interval between the wire ss and the wire tt) is L / 10 to L / 4. At this time, the voltage standing wave ratio (VSWR) at the power feeding unit O is Can be small.
In addition, an appropriate range of the distance D2 between the two lines of the second folded dipole is L / 200 to L / 50. With this setting, the voltage standing wave ratio (VSWR) at the power feeding unit O is further reduced. can do.

Next, FIG. 2A shows impedance characteristics in the power feeding portion O of the folded dipole antenna A1 of FIG. This impedance characteristic is obtained by simulation assuming the following numerical values.
(Assumption condition 1)
W = 15mm
L = 284mm
τ = 5μm
D = 1mm
D1 = 40mm
D2 = 2mm
D3 = 60 μm
LL = 5mm
C1 = 0.2pF
Here, τ and D are the thickness and width of each metal wiring, and LL is the length of the slit ps in the x-axis direction.

In the graph of FIG. 2A, the solid line indicates the value of the real part of the impedance, and the dotted line is the imaginary part. The horizontal axis indicates the frequency normalized by the center frequency f ₀ (710 MHz). Further, the frequencies of the horizontal axis intercept where the imaginary part is 0 are set to f ₁ , f ₂ , and f ₃ from the lowest.
FIG. 2B shows the voltage standing wave ratio (VSWR) of the antenna A1. The frequency f was standardized with a frequency f ₀ (that is, the above center frequency: 710 MHz) having an impedance of 230Ω at the power supply unit O as 1. At this time, the frequency f is 0. 66f ₀ (470 MHz) to 1. The voltage standing wave ratio becomes 3 or less over the range of 34f ₀ (950 MHz). Therefore, for example, when an antenna is connected to an RF cable having a characteristic impedance of 50Ω, a balun of 50Ω to 230Ω may be inserted between the power feeding unit O and the RF cable.

On the other hand, as a result of performing the same simulation on the conventional loop antenna 10 of FIG. 1 of Patent Document 1 under the following two conditions, in each case, the frequency band where the voltage standing wave ratio is 3 or less is It was 470 MHz to 710 MHz.
(Assumption condition 2)
W = 50mm
L = 200mm
τ = 5μm
D = 1mm
D1 = 40mm
D2 = 5mm
(Assumption condition 3)
W = 20mm
L = 250mm
τ = 5μm
D = 1mm
D1 = 40mm
D2 = 2mm

  From this simulation result, it can be seen that the frequency band has been doubled as compared with the prior art due to the provision of the capacitor C1 on the metal wiring p and, at the same time, the elongated antenna. In addition, the frequency band that can be transmitted and received by the antenna is often shifted downward due to adverse effects such as the approach of the human body, so that the frequency band that can be transmitted and received is greatly expanded upward is a very desirable result. I can say that.

FIG. 3A shows the relationship between the value of the capacitance C1 of the folded dipole antenna A1 and the frequencies f ₁ , f ₂ , and f ₃ . As can be seen from this graph, the frequencies f ₂ and f ₃ increase as the capacitance value decreases, but at this time, the frequency f ₁ does not change much. In other words, it can be seen from this graph that the antenna frequency band becomes wider when the capacitance value is reduced.

FIG. 3B shows the relationship between the value of the capacitance C1 and the impedance of the folded dipole antenna A1. As can be seen from this graph, as the capacitance value is decreased, the impedances at f ₂ and f ₃ increase. At this time, the impedance at the frequency f ₁ does not change much. Here, an increase in impedance at f ₃ indicates that the frequency of use of the antenna is increased.

4A to 4C, f = 0... Of the folded dipole antenna A1. It shows each directivity on xy plane at _{66f 0, f = f 0 =} 710MHz, f = 1.34f 0. As can be seen from these graphs, in the frequency band where the voltage standing wave ratio is 3 or less, the directivity of the folded dipole antenna A1 is approximately 8 characters.

  FIG. 5 shows a plan view of the folded dipole antenna A2 of the second embodiment. The folded dipole antenna A2 is a slightly modified version of the folded dipole antenna A1 of the first embodiment, and is characterized in that a capacitor located on the opposite side of the power feeding unit O is constituted by a capacitor C2. is there. The capacitor C2 is formed by overlapping the metal wiring p constituting the upper portion of the T-shape of the second folded dipole in a double parallel manner inside the interval D1, and a slight gap between the parallel metal wirings. A simple gap forms a slit ps having a capacity C2. The width of the slit ps may be approximately the same as the width D3 of the first embodiment.

In addition, the structure of the capacitor C2 is formed by forming a thick portion at a position opposite to the power feeding portion O of the metal wiring p and forming a slit ps in the left-right direction (x-axis direction) at the center of the extremely thick portion. You may think that it was comprised.
Of course, the operation and effect substantially the same as those of the first embodiment can be obtained also by the formation form of the capacitor C2.

FIG. 6 shows a plan view of the capacitor C3 of the third embodiment. This capacitor C3 can be applied in place of the capacitor C1 in the folded dipole antenna A1 of the first embodiment. The structure of the capacitor C3 is obtained by patterning the capacitor in a so-called meander shape that is slightly complicated, and it may be considered that a plurality of capacitors C1 are connected in parallel. Therefore, the capacitance C3 is useful when it is desired to increase the capacitance value compared to the case where the capacitance C1 is applied.
In addition to the number of parallel connection stages, these capacitance values depend on the width of the slit ps, the length in the x-axis direction, the length in the y-axis direction, the thickness of the metal pattern (the length in the z-axis direction), and the like. Can also be optimized.

[Other variations]
The embodiment of the present invention is not limited to the above-described embodiment, and other modifications as exemplified below may be made. Even with such modifications and applications, the effects of the present invention can be obtained based on the functions of the present invention.

(Modification 1)
For example, in each of the above embodiments, at least a part of the slit ps forming the capacitor is formed in the left-right direction (x-axis direction), but the slit ps may be formed only in the vertical direction (y-axis direction). FIG. 7A shows a plan view of the capacitor C4 of the first modification. The capacitor C4 is formed by thickening a portion located on the opposite side of the power supply portion O of the metal wiring p and forming a slit ps straight in the vertical direction. According to such a capacitance formation mode, the capacitance structure on the metal wiring p can be simplified. It is also possible to make the desired folded dipole antenna completely symmetrical.

(Modification 2)
FIG. 7B shows a plan view of the capacitor C5 of the second modification. In the second modification, the center line of the antenna is removed, and the capacitors C5a and C5b are arranged symmetrically with respect to the center line. That is, the capacitor C5 of the second modification example is a combined capacitor of the capacitor C5a and the capacitor C5b connected in series. For example, in this way, individual capacitors do not necessarily have to be arranged on the center line of the antenna. Further, the shapes of the capacitors C5a and C5b are the simplest, and according to such a structure, it is very easy to form a capacitor to be disposed on the metal wiring p on the opposite side of the power feeding portion O. Can do.
In addition, the structure of the serial connection of the capacitor C5 according to the second modification is useful, for example, when it is desired to make the capacitance value smaller than when the capacitor C1 is applied.

(Modification 3)
In the first embodiment, the metal wiring pattern is formed based on the straight line (line segment) and the right angle, but the antenna wiring pattern is not necessarily formed based on the straight line (line segment) or the right angle. Absent. For example, the folded dipole antenna of the present invention may be formed by a wiring pattern based on a curve as disclosed in the above-mentioned Patent Document 1.

(Modification 4)
In addition, various modifications can be given to the antenna of the present invention. For example, referring to various examples and modifications disclosed in Patent Document 1 mentioned above, The embodiment may be partially imitated or applied.

  The folded dipole antenna of the present invention has a configuration in which, for example, the superiority of the folded dipole antenna is particularly remarkable when disposed on a transparent insulator such as a window glass. The present invention is not limited to this, and can be used in any form of use for transmission / reception of radio waves of any frequency that can be used for communication.

Plan view of folded dipole antenna A1 according to the first embodiment. The graph which shows the impedance of antenna A1 of Example 1. Graph showing the voltage standing wave ratio (VSWR) of the antenna A1 Graph showing the relationship between the capacitance value of antenna A1 and the frequency Graph showing the relationship between the capacitance value of the antenna A1 and the impedance A graph showing the directivity of the antenna A1 (f = 0.66f ₀ ) A graph showing the directivity of the antenna A1 (f = f ₀ ) A graph showing the directivity of the antenna A1 (f = 1.34f ₀ ) Plan view of folded dipole antenna A2 of the second embodiment Plan view of capacitor C3 of Example 3 Plan view of the capacitor C4 of Modification 1 Plan view of the capacitor C5 of Modification 2

A1: Folded dipole antenna C1: Capacitance O: Feeding part p: Wire (forms an open curve)
q: Wire (forms a simple closed curve)
r: Wire (forms a simple closed curve)
s: shared part of p and q t: shared part of p and r pp: part of p (near the power feeding part O)
ss: a part of s (opposite to tt)
tt: a part of t (opposite part with ss)
ps: slit provided in the wire p D1: distance between ss and tt D2: distance between two parallel lines of the second folded dipole D3: width of the slit ps

Claims (3)

In the folded dipole antenna composed of the metal wiring pattern developed on the surface,
Outermost peripheral loop Z (path α- efa -b ) having one feeding part O composed of a pair of feeding points (α, β) on the left and right sides close to each other and a capacitor located on the opposite side A first folded dipole consisting of -c-dh-g-β) ;
An unbranched line s disposed inside the outermost loop Z and having both ends (g, h) connected to two different points in the middle of the left half of the outermost loop Z;
Arranged inside the outermost loop Z, both ends (e, f) are respectively connected to two different points in the middle of the right half of the outermost loop Z, and have no intersection or contact with the line s. An unbranched line t and
The line s, the line t, the power feeding unit O, the capacitance, and a part of the outermost peripheral loop Z (path fa-bbcdh), the power feeding point (α, β) Is a metal wiring p (path α-e-line tf-a-bcdh-line sg-) that forms one unbranched open curve having the above-mentioned capacitance in the middle. β) constitutes the second folded dipole,
The capacitor is patterned with a gap of the metal wiring (path bc) forming the outermost peripheral loop Z on the middle point on the metal wiring forming the outermost peripheral loop Z ,
The line s and the line connecting both ends (g, h) constitute a metal wiring q (path g-line s-h-g) that is a single simple closed curve,
The line t and the line connecting both ends (e, f) constitute a metal wiring r (path e-line t-f-e) that is another simple closed curve,
The metal wiring p and the metal wiring q share the line s with each other,
The metal wiring p and the metal wiring r share the line t with each other,
The metal wiring q and the metal wiring r are arranged apart from each other,
The remaining part excluding the line s and the line t from the metal wiring p, q, r forms the outermost peripheral loop Z,
A portion pp of the metal wiring p located between the metal wiring q and the metal wiring r includes the power feeding unit O, and the metal wiring q and the metal wiring r are interposed via the power feeding unit O. Connected,
The metal wiring developed in a pattern on the surface is developed symmetrically with respect to one center line that divides the pair of feeding points (α, β) on the left and right two points left and right,
The length L (path bc) in the longitudinal direction of the first folded dipole is not less than 10 times and 50 times the length W (path ab) of the first folded dipole in the direction orthogonal thereto. And
The distance D2 between the metal lines parallel to the longitudinal direction forming the second folded dipole is set to the longitudinal length L of the first folded dipole.
L / 200 ≦ D2 ≦ L / 50 is satisfied,
When the impedance viewed from the feeding point (α, β) is a pure impedance, and when the first frequency, the second frequency, and the third frequency are set from the lower side, the second is reduced with respect to the decrease in the capacitance. A folded dipole antenna characterized in that the increase in frequency and third frequency is greater than the increase in first frequency, and the frequency band used is expanded . A part ss of the line s located near the power feeding part O and a part tt of the line t located near the power feeding part O are:
2. The folded dipole antenna according to claim 1, wherein the folded dipole antennas are arranged to face each other in parallel by being separated by a distance D <b> 1 . The folded dipole antenna according to claim 2 , wherein the distance D1 satisfies L / 10 ≦ D1 ≦ L / 4 with respect to a length L in a longitudinal direction of the first folded dipole.

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