JP4023369B2 - Dipole antenna - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a dipole antenna that is used to receive a UHF band television wave and realizes a wide band and high gain characteristic in a small size.
[0002]
[Prior art]
As a conventional dipole antenna, a folded dipole antenna having a half-wavelength has been widely used since it has wideband characteristics (see, for example, Non-Patent Document 1).
[0003]
FIG. 6 shows a conventional general folded dipole antenna 1 in which a conductor having a wavelength of about 1 wavelength having a radius r is folded back so as to keep the width W, and includes an impedance converter 2 for matching with a normally used cable impedance of 75 ohms. It is a thing.
[0004]
The folded dipole antenna 1 can be analyzed as a circuit combination of a ½ wavelength dipole antenna and a ¼ wavelength line whose tip is short-circuited, and has a broadband frequency characteristic with an output impedance of nearly 300 ohms. In order to convert the output impedance of 300 ohms to 75 ohms, the received radio wave from the antenna is efficiently sent to the antenna output terminal 3 via the impedance converter 2 which is usually a 4: 1 balun transformer.
[0005]
In addition, as a conventional example of downsizing a dipole antenna, there is one that includes a loading coil and a matching circuit at the center of the dipole antenna for use at a wide frequency range (see, for example, Patent Document 1). FIG. 7 shows a dipole antenna described in the patent document. In FIG. 7, a loading coil 5 is connected to the center of a dipole antenna made of an antenna element 4, and a matching circuit 6 comprising an inductor, a matching capacitor, and a variable capacitance diode is provided. Furthermore, in order to obtain the required frequency band characteristic of the antenna output impedance, the capacitance of the varicap diode is changed to match the cable impedance by controlling the varicap diode applied voltage. Although a DC circuit applied to the varicap diode is not shown in FIG. 7, a configuration in which the applied voltage is changed according to the frequency to be used is used.
[0006]
[Non-Patent Document 1]
Yasuto Mushiaki: “Antenna / Radio wave propagation”, Corona, 1971, PP. 43-45 Figure 4.7
[Patent Document 1]
JP-A-9-130132 (page 6, FIG. 1)
[0007]
[Problems to be solved by the invention]
However, in the configuration of the conventional folded dipole antenna, when the folded portion has the same diameter r, the output impedance of the simple dipole is four times that of the simple dipole, and it is 4: 4 to match the cable with 75 ohm characteristics used for transmission of television signals. 1 impedance converter was required. In addition, a circuit board and a holding structure are necessary to provide the required impedance converter at the center of the antenna.
[0008]
Furthermore, in recent years, there are problems in the ease of handling and the manufacturing price related to the strength of the folded portion when it is attached to a portable mobile radio device.
[0009]
In the case of the dipole antenna having no folded portion shown in Patent Document 1, in order to obtain broadband frequency characteristics, a complicated matching circuit including a loading coil, a capacitor, and an inductor is required. A circuit board and a holding structure are required to hold the antenna at the center.
[0010]
SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems. By realizing a dipole antenna with a simple structure, a dipole antenna that matches a cable impedance over a wide frequency range without requiring an impedance converter or a matching circuit. The purpose is to provide.
[0011]
[Means for Solving the Problems]
In order to solve the above-described conventional problems, a dipole antenna according to the present invention short-circuits both ends of a coaxial half-wavelength antenna element composed of an outer conductor and a center conductor, and further, the coaxial antenna element The outer conductor is divided into two at the center, and an antenna output terminal for taking out the received radio wave output from the center end of the outer conductor is provided, thereby realizing a wideband frequency dipole antenna.
[0012]
With this configuration, the antenna element can be matched to 75 ohms, which is the impedance of the connection cable, without bending the antenna element and without an impedance converter. Furthermore, it has a simple coaxial structure and has realized a wideband frequency characteristic dipole antenna without a loading coil or a complicated matching circuit.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0014]
(Embodiment 1)
FIG. 1 is a configuration diagram of a dipole antenna according to Embodiment 1 of the present invention.
[0015]
In FIG. 1, the same components as those in FIGS. 6 and 7 are denoted by the same reference numerals and description thereof is omitted.
[0016]
In FIG. 1, an antenna element 4 having a coaxial structure with a diameter r by an outer conductor 8 and a center conductor 7 has a short-circuit portion 10 that is electrically short-circuited at both ends of the coaxial. In this structure, the antenna output terminal 3 is used to extract the received radio wave output from the central side of the outer conductor 8. Further, the dielectric 9 has a role of supporting the central conductor 7, and the length L of the antenna element 4 is selected to be approximately ½ wavelength of the frequency used.
[0017]
As shown in FIG. 2A, the output impedance Za of the antenna element 4 having such a structure is the same as the antenna radiation impedance Zb having a simple dipole structure with an external conductor and a quarter-wavelength coaxial impedance short-circuited at the terminal. It is obtained by the characteristic that Zc, which is twice Zd, is connected in parallel.
[0018]
The antenna impedance Zb made of the outer conductor 8 having a diameter r is a characteristic that shows inductivity when the frequency L is about 70 ohms and the frequency is about 70 ohms at a frequency of 1/2 wavelength, and shows capacitance when the frequency is low. The coaxial impedance Zd of length L / 2, one end of which is short-circuited, is Zd = Zod · tan (kL / ε), the electrical execution length becomes open characteristics at a frequency of ¼ wavelength, and the frequency is high. When the frequency is low, inductivity is exhibited. Here, Zod is the characteristic impedance of the coaxial line, and k is 2π / λ. Further, Zc is Zc = 2Zd, assuming that coaxials whose ends are short-circuited are connected in series.
[0019]
FIG. 2B is a graph showing the change characteristics of the impedances Zb and Zc obtained in this way. The frequency FL is such that the coaxial length L is ½ wavelength. Za obtained from the parallel connection of Zb and Zc having the characteristics shown in FIG. 2 <b> not only cancels the capacitive and inductive characteristics at frequencies lower than the frequency FL, but also has an effect of approaching 75 ohms of the connection impedance. Even if the frequency is separated from the frequency FL, the antenna impedance has an effect of approaching 75 ohms.
[0020]
For example, since the frequency of the TV radio wave is broadcast at 470 MHz to 770 MHz, when the center frequency FL = 600 MHz, the antenna element length L can be calculated by the speed of light / frequency / 2, and L = 25 cm.
[0021]
FIG. 3 shows an example of the output characteristics of the dipole antenna having a diameter of 8 mm and a length of 25 cm according to the first embodiment of the present invention, and a simple dipole antenna characteristic example having the same size without a folded portion, centered on 75 ohms. Shown in Smith chart. How wide the frequency characteristic region near the center of the Smith chart indicates the good broadband frequency characteristic. In the example of the dipole antenna according to the present invention, the radio wave frequency of 470 MHz to 770 MHz is contained in the circle within VSWR = 2, indicating that the frequency characteristics are good.
[0022]
In the folded dipole antenna of the conventional example, the output impedance of the feeding unit 12 shown in FIG. 6 is close to 300 ohms of VSWR = 4 due to the effect of the folding element when the diameter of the antenna element on the feeding unit 12 side is the same as the folded element unit. In the dipole according to the present invention, the action of condensing the output impedance of the center conductor 7 corresponding to the folded portion to 75 ohms is used. This is because the electromagnetic field radiated from the folding element affects the side element part close to the power feeding part to increase the impedance, and the characteristic difference is that the central conductor 7 of the present invention does not contribute to the electromagnetic field radiation action. Is caused.
[0023]
Although the dielectric 9 changes in bandwidth and center frequency due to the difference in dielectric constant, necessary characteristics can be obtained by appropriately selecting the antenna element length L. In addition, when the central conductor can be mechanically held, it is not indispensable for the present embodiment, and the dielectric 9 is included in the embodiment of the present invention even if the dielectric 9 is partially or entirely missing.
[0024]
Further, in the present embodiment, the outer conductor 8 may be other than a circular pipe, and when a mesh-like braided wire made of a metal wire is used, the outer conductor 8 can be mechanically flexible.
[0025]
(Embodiment 2)
FIG. 4 is a configuration diagram of a dipole antenna according to the second embodiment of the present invention.
[0026]
4, the same components as those in FIGS. 1 and 7 are denoted by the same reference numerals, and the description thereof is omitted.
[0027]
4, the outer conductor 6 in FIG. 1 is replaced with two conductive plates 12 having a recess in the length direction of the width W and the length L, and a conductive wire 13 and an insulating dielectric 14 along the recess. Both ends of the covered wire 15 are connected to the end portion of the conductive plate 12 by the short-circuit connection portion 16. The two conductive plates 12 are fixed at positions symmetrical to each other by a support 17 and screws 18, and the antenna output terminal 3 is provided at the center end of the conductive plate 12 near the center of symmetry.
[0028]
Since the diameter of the conductive wire 13 of the covered wire 15 along the dent is sufficiently smaller than the width of the conductive plate and falls into the dent, the high-frequency current flowing in the conductive wire 13 is not radiated electromagnetically to the outside. This is the same as the coaxial in the first embodiment having the impedance Zd determined by the plate 12 and the covered wire 15. The outer conductor 8 and the conductive plate 12 have different external shapes. However, if the width W of the conductive plate 12 is made slightly larger than the coaxial diameter r, the same radio wave radiation characteristic can be obtained, and the ½ wavelength length L used. Thus, it plays the same role as an antenna element that radiates an electromagnetic field.
[0029]
According to such a configuration, the outer conductor 6 and the central conductor of the first embodiment are replaced with the conductive plate 12 and the covered wire 15, so that the antenna output impedance obtained by the conductive plate has the same effect as described in the first embodiment, A dipole antenna that can be used in a wide band frequency can be realized.
[0030]
In addition, no matching circuit is required, and there is no unnecessary protrusion in the device, so that reliability can be improved and handling can be facilitated.
[0031]
In this embodiment, an operating part may be provided on the support body to provide a folding function and a storage function, or a structure that can be separated from the device with a connector structure including a covered wire.
[0038]
【The invention's effect】
As described above, according to the dipole antenna of the present invention, the antenna element can be matched to 75 ohm which is the impedance of the connection cable without bending the antenna element and without an impedance converter. Furthermore, it is possible to realize a dipole antenna having a broadband frequency characteristic with a simple coaxial structure and without a loading coil or a complicated matching circuit.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a dipole antenna according to a first embodiment of the present invention. FIG. 2 is a characteristic explanatory diagram of a first embodiment of the present invention. FIG. 3 is an output impedance explanatory diagram of a first embodiment of the present invention. 4] Configuration diagram of a dipole antenna according to Embodiment 2 of the present invention [FIG. 5] Configuration diagram of a conventional dipole antenna with a folded structure [FIG. 6] Configuration diagram of a conventional dipole antenna using a matching circuit
DESCRIPTION OF SYMBOLS 1 Folded dipole antenna 2 Impedance converter 3 Antenna output terminal 4 Antenna element 5 Loading coil 6 Matching circuit 7 Center conductor 8 Outer conductor 9 Dielectric 10 Short-circuit part 12 Conductive plate 13 Conductive wire 14 Insulating dielectric 15 Covered wire 16 Short-circuit connection Part 17 Support 18 Screw

Claims (1)

Linearly arranged two conductive plates remembering recessed flat plates, Sowase the covered wire which conductive wire and made of an insulating dielectric into the recesses of the conductive plate, the ends of said covered wire on the outside of the conductive plate terminals A dipole antenna having a structure in which an antenna output terminal is provided at the inner end where the two conductive plates face each other .