JP2005020228A - Antenna equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide antenna equipment which is suitable for mounting on a portable terminal and which has a comparatively simple structure and a wide band characteristic of large specific band width. <P>SOLUTION: At least one second antenna element 12 of about λ'/2, which is excited by capacity coupling, is arranged in series at an opening end part of a first antenna element 11 whose one end is connected to a feeding point and which resonates at about λ/4. It is desirable that a third antenna element 13 whose one end is capacity-coupled and which resonates at about λ"/2 is disposed at the other end of the second antenna element 12. Electric lengths of the antenna elements are adjusted to become λ>λ'>λ". <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an antenna device that requires wideband characteristics.
[0002]
[Prior art]
As a conventional method for increasing the bandwidth of a linear antenna, an antenna element having an electrical length of λ / 2 is placed close to the open end of the antenna element excited by an electrical length of λ / 4, and both antenna elements are capacitively coupled to increase the bandwidth. There are known methods for achieving this. For example, in “Retractable antenna device for mobile communication device” disclosed in Patent Document 1, an example of application as an antenna for a mobile terminal in a 1.9 GHz band is shown, and a value of VSWR (Voltage Standing Wave Ratio) is 3 In the following, a broadband characteristic with a specific bandwidth of about 37% is realized.
[0003]
[Patent Document 1]
JP-A-7-94923 [Non-patent Document 1]
Antenna Engineering Handbook, The Institute of Electronics, Information and Communication Engineers Hei 6 Ohmsha P121
[0004]
[Problems to be solved by the invention]
However, in the method as described in Patent Document 1, it is impossible to realize a wide band because of its configuration in a low frequency band of 1 GHz or less. For this reason, for example, an antenna for terrestrial digital broadcasting that requires a low frequency band such as a center frequency of 620 MHz and a specific bandwidth of about 48% and a broadband characteristic is not sufficient for wideband using the above technique. Expected to disappear.
[0005]
A log periodic antenna is known as an antenna having a wideband characteristic with a specific bandwidth of 50% or more (see Non-Patent Document 1). However, since the wavelength is generally long at a frequency of 400 to 2000 MHz used in mobile communication, the size of the antenna increases due to its special shape. For this reason, the log periodic antenna is problematic in that it is difficult to carry when applied to a mobile communication terminal or the like.
[0006]
The present invention has been made in such a background, and an object of the present invention is to provide an antenna device having a wide bandwidth characteristic with a relatively simple configuration and a large specific bandwidth suitable for mounting on a mobile terminal. is there.
[0007]
[Means for Solving the Problems]
The antenna device according to the present invention has a second antenna element of about λ ′ / 2 excited by capacitive coupling at the open end of the first antenna element that is connected at one end to the feeding point and resonates at about λ / 4. At least one antenna element is arranged in series, and the electrical length of each antenna element is adjusted to satisfy λ> λ ′.
[0008]
The first antenna element excites the second antenna element by capacitive coupling. Further, the other end of the second antenna element may be provided with a third antenna element having one end capacitively coupled and resonating at about λ ″ / 2.
[0009]
The electrical length of each antenna element is adjusted to satisfy λ> λ ′> λ ″. As a result, the resonance frequency is shifted in each antenna element, and as a result, a wide band impedance characteristic is obtained.
[0010]
The first antenna element may have a capacity loaded shape. Thereby, even when the impedance characteristic of the first antenna element is reduced and the size of the ground conductor such as the portable terminal is not sufficient, impedance matching can be achieved without adding a matching circuit.
[0011]
In order to make the first antenna element have a capacity loading type, for example, it has a meander shape, a helical shape or a zigzag shape, or is loaded with a dielectric.
[0012]
Capacitive coupling between the antenna elements can be constituted by chip parts between the antenna elements. Alternatively, capacitive coupling between the antenna elements can be realized by overlapping the end portions of the antenna elements substantially in parallel.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0014]
First, FIG. 1 shows a schematic configuration of a first embodiment of an antenna device according to the present invention. The antenna device includes a ground conductor 15, a first antenna element 11 having an electrical length of about λ / 4, one end connected to a feeding point 14 located at one end of the ground conductor 15, and about λ ′ / The second antenna element 12 having an electrical length of 2 and the third antenna element 13 also having an electrical length of λ ″ / 2. The antenna elements 11, 12, 13 are configured as shown in the figure. The second antenna element 12 is arranged so as to be electromagnetically coupled with a predetermined capacity 16 to the open end of the first antenna element 11 excited with an electrical length of about λ / 4. The antenna element 13 is arranged so as to be electromagnetically coupled with a predetermined capacitance 17 to the opposite end of the second antenna element 12. Each antenna element is made of a conductive linear member. , For example, flexible be formed by a wire or the like having a. Ground conductor 15 is, for example, a ground layer and the shielding box on the base of the mobile terminal.
[0015]
Here, the electrical length is adjusted so that the resonance frequency of the second antenna element 12 observed from the power feeding unit of the first antenna element 11 is higher than that of the first antenna element 11 (that is, λ> λ ′> λ ″). Similarly, the electrical length is adjusted so that the resonance frequency of the third antenna element 13 observed from the feeding portion of the first antenna element 11 is higher than that of the second antenna element 12. That is, the lowest resonance frequency is constituted by the first antenna element 11, the next highest resonance frequency is constituted by the second antenna element 12, and the further higher resonance frequency is constituted by the third antenna element 13.
[0016]
By this adjustment, at the lowest frequency, the self-impedance of the resonating first antenna element 11 is increased, and the electrical lengths of the second and third antenna elements 12 and 13 are gradually shortened. Self-impedance decreases. With this arrangement, wideband impedance characteristics can be realized by reducing interference between the antenna elements in the impedance characteristics.
[0017]
Generally, the characteristic impedance used for connecting the antenna and the high frequency circuit is 50Ω. The first antenna element 11 in the first embodiment has an electrical length of about λ / 4, and a large amount of current flows on the ground conductor 15 depending on the size of the ground conductor 15. For this reason, the impedance characteristics of the antenna vary greatly depending on the size of the ground conductor 15. When the size of a ground conductor such as a portable terminal is not sufficient, the impedance is close to the configuration of a dipole antenna, and thus the impedance at the resonance frequency is generally higher than 50Ω.
[0018]
Further, the first antenna element 11 is coupled to the second antenna element 12 having an electrical length of about λ ′ / 2 and the third antenna element 13 having an electrical length of about λ ″ / 2, which has a high impedance. Further, the impedance characteristics of the entire antenna as viewed from the antenna feeding point 14 are further improved, and a matching circuit can be added to improve impedance matching, but a loss occurs due to the matching circuit. There is a possibility that interference between elements changes in the frequency band, which may hinder widening of the band.
[0019]
FIG. 2 shows a schematic configuration of the second embodiment of the present invention for dealing with such a problem. This configuration is basically the same as that of the first embodiment shown in FIG. 1, and the same reference numerals are assigned to the same components as those shown in FIG. The second embodiment is structurally different from the first embodiment in that the first antenna element 21 having a different configuration is used instead of the first antenna element 11. This is a method of adjusting the size of the characteristic impedance in order to obtain broadband characteristics with a simple configuration change in the same configuration as that of the first embodiment. For this purpose, a so-called capacity loaded antenna element 21 is used as the first antenna element. Examples of the capacity loaded antenna element 21 include a helical type 11a, a meander type 11b, a zigzag type 11c, and a dielectric loaded type 11d, as shown in FIGS. In the antenna element of the dielectric loading type 11d, a dielectric 26 made of a predetermined material is loaded around the conductor 25. The dielectric 26 is, for example, a resin such as Teflon (registered trademark) or ceramic, and the insulating material covering the second and third antenna elements or all the antenna elements is at least one of shape, size, or material. The first antenna element has a significant capacity loading effect. Thereby, the first antenna element can be adjusted to low impedance characteristics. As a result, when the second antenna element and the third antenna element are combined, matching to the 50Ω system can be easily realized.
[0020]
Next, FIG. 3 shows a specific configuration example of the first embodiment. This configuration is basically the same as that of the first embodiment shown in FIG. 1, and the same reference numerals are assigned to the same components as those shown in FIG. The configuration of FIG. 3 uses a chip component (capacitor) 18 as a specific method of realizing the capacitors 16 and 17 in the first embodiment. Thereby, the magnitude | size of the capacitive coupling between each element can be adjusted with a simple structure. The specific capacity of the chip component can be appropriately determined by experiments or the like. With this method, the capacitance can be changed by simply changing the value of the chip component, so that optimization can be achieved without adjusting the electrical length of each antenna element during mass production. The configuration shown in FIG. 3 can also be employed in the second embodiment shown in FIG.
[0021]
Next, FIG. 4 shows another specific configuration example of the first embodiment. This configuration is also basically the same as that of the first embodiment shown in FIG. 1, and the same components as those shown in FIG. 1 are denoted by the same reference numerals. In the configuration of FIG. 4, as a specific method of realizing the capacitors 16 and 17 in the first embodiment, the end portions of the antenna elements are overlapped substantially in parallel so as to show the antenna portions 40 and 41 in an enlarged manner. The structure adopted is adopted. In this case, the size of the capacitor 19 is adjusted according to the length and interval of the overlap at the end of each antenna element. By this method, the adjustment range of the size of the capacitor 19 is expanded, and the applicable frequency range can be expanded. The space between the overlapping antenna element ends may be a space, but a dielectric may be loaded between them to adjust the capacitance coupling value. The configuration of FIG. 4 can also be adopted in the second embodiment shown in FIG.
[0022]
As an actual antenna device, the antenna elements 11, 12, and 13 are covered with an insulating material (for example, synthetic resin or fiber), and all the antenna elements are integrated. As a specific form, the antenna device for a portable terminal can be configured as a string-like strap. The shape of the strap-like strap may be a line or a loop.
[0023]
An example in which the characteristics of the prototype according to the embodiment of the present invention are confirmed by experiment is shown in FIGS. This prototype employs the dielectric loading configuration shown in FIG. Similar results are expected in other configurations.
[0024]
FIG. 5 is a graph in which the horizontal axis represents frequency (400 MHz to 800 MHz), the vertical axis represents VSWR, and VSWR values corresponding to the frequency are plotted. FIG. 6 is a Smith chart (the center point of the circle is 50Ω) in which impedance characteristics of the antenna device according to the frequency (400 MHz to 800 MHz) are plotted. From these graphs, since the frequency range where the value of VSWR is 3 or less is 430 = 770 MHz and the center frequency is 600 MHz, the impedance with a specific bandwidth of 50% or more (specifically, 56.7% = (770-430) / 600) It can be confirmed that the characteristics can be secured. In the graph of FIG. 5, three local minimum points are observed. These three frequencies are considered to be caused by λ, λ ′, λ ″ of the three antenna elements.
[0025]
The preferred embodiment of the present invention has been described above, but various modifications and changes can be made. For example, in the above description, only the case where the number of antenna elements connected in series is three is shown, but in principle, it may be two or four or more. In the case of two cases, the effect of widening the band is reduced as compared with the case of three cases, but still has a significant effect as compared with the prior art. Although the antenna element is composed of a linear member, the antenna element is not necessarily limited to a linear shape.
[0026]
【The invention's effect】
According to the present invention, it is possible to obtain an antenna device having a wide bandwidth characteristic with a relatively simple configuration and a large specific bandwidth (for example, 50% or more). This antenna device is particularly suitable for use as an antenna for terrestrial digital broadcasting.
[0027]
By using a capacitively loaded antenna element for the first antenna element, impedance matching can be achieved without using a matching circuit in a portable terminal or the like whose ground conductor is not sufficiently large.
[0028]
For example, if the antenna elements connected in series are used as a strap of a portable terminal, they are not disturbed even if externally attached to the terminal, and are suitable for mounting on a portable terminal or the like.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of a first embodiment of an antenna device according to the present invention.
FIG. 2 is a diagram showing a schematic configuration of an antenna device according to a second embodiment of the present invention.
FIG. 3 is a diagram showing a specific configuration example of an embodiment according to the present invention.
FIG. 4 is a diagram showing another specific configuration example of the embodiment according to the present invention.
FIG. 5 is a graph of frequency versus VSWR showing characteristics of a prototype according to an embodiment of the present invention confirmed by experiments.
FIG. 6 is a Smith chart showing characteristics of a prototype according to an embodiment of the present invention confirmed by experiments.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... 1st antenna element, 12 ... 2nd antenna element, 13 ... 3rd antenna element, 14 ... Feeding point, 15 ... Ground conductor, 18 ... Chip component, 21 ... Capacity loading type antenna element, 40, 41. Antenna part

Claims (7)

At least one second antenna element of about λ ′ / 2 excited by capacitive coupling is arranged in series at the open end of the first antenna element connected at one end to the feeding point and resonating at about λ / 4. In addition, the antenna device is characterized in that the electrical length of each antenna element is adjusted to satisfy λ> λ ′. A first antenna element having one end connected to the feed point and resonating at about λ / 4;
A second antenna element having one end capacitively coupled to the open end of the first antenna element and resonating at about λ ′ / 2;
A third antenna element having one end capacitively coupled and resonating at about λ ″ / 2, on the other end of the second antenna element;
An antenna device, wherein an electrical length of each antenna element is adjusted to satisfy λ> λ ′> λ ″. The antenna device according to claim 1, wherein the first antenna element has a capacity loading type shape. 4. The antenna device according to claim 3, wherein the first antenna element is adjusted to have a low impedance by having one of a meander shape, a helical shape, and a zigzag shape. 4. The antenna device according to claim 3, wherein the first antenna element is adjusted to have a low impedance by being loaded with a dielectric. 3. The antenna device according to claim 1, wherein capacitive coupling between the antenna elements is realized by connecting a chip component between the antenna elements. 3. The antenna device according to claim 1, wherein capacitive coupling between the antenna elements is realized by overlapping end portions of the antenna elements substantially in parallel.

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