JP2006074627A - Bar antenna - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bar antenna capable of miniaturizing particularly with low back, in addition to producing a satisfactory radiation characteristic and realizing a high gain. <P>SOLUTION: On a strip-shaped core 1 being formed of Ni-Cu-Zn ferrite in which CoO is added for 0.2-10 weight%, and having a loss factor μ" of 3×10<SP>-2</SP>or less in a UHF band (315 MHz band), there is wounded a coil having a length of λ/4 while being wounded, and a first trimmer capacitor 3 and a second trimmer capacitor 4 for impedance matching are attached externally. A low profile bar antenna 10 having the above structure can be incorporated into an apparatus of small size. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a bar antenna that is most suitable for mounting on a small device using the UHF band, and is compatible with both a small configuration and good radiation characteristics.

  The UHF band weak radio represented by the 315 MHz band does not require a license for a radio station, and is widely used mainly for data communication at a short distance. For example, it is used in fields such as a keyless entry system for automobiles, an electronic key system for houses, a television broadcast receiving system for mobile phones, and a wireless telemeter system. Devices in these fields have been downsized due to the development of integrated circuit technology, and accordingly, there is a high demand for downsizing of mounted antennas.

  Conventionally, a λ / 4 whip antenna or a loop antenna directly formed on a printed circuit board has been used as an antenna in equipment using weak electromagnetic waves. However, for installation of these antennas, for example, in the case of the 315 MHz band, an installation area of about 20 cm for the λ / 4 whip antenna and about 50 × 30 mm for the loop antenna is required, and this type of antenna is mounted on a small device. It is difficult.

  As a miniaturized antenna, a ceramic antenna shortened by utilizing the wavelength shortening effect of the dielectric (the wavelength is shortened in proportion to the -1/2 power of the relative dielectric constant), or a λ / 4 element in a spiral shape A helical antenna shortened by winding it around is used. However, these antennas have a problem that the gain is remarkably lowered when the antenna is downsized.

Many proposals have been made on a bar antenna having a configuration in which a coil is wound around a core (see, for example, Patent Documents 1 and 2), but none has been sufficiently reduced in size.
JP 2001-102832 A JP 2004-147098 A

  In the portable small-sized communication device as described above, a transmission / reception antenna is often incorporated in a casing or embedded in a circuit board in order to reduce the overall configuration. Therefore, an antenna mounted on such a small device is particularly required to have a low profile (thinner).

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a bar antenna that can achieve a good radiation characteristic and achieve a high gain, and that can be downsized, in particular, a low profile. To do.

The bar antenna according to the present invention includes a core made of ferrite having a loss coefficient μ ″ of 3 × 10 ⁻² or less in a radio wave band to be used, and wound around the core. A coil having a length of λ / 4 or less and a capacitor for matching impedance.

In the bar antenna of the present invention, the length in the state of being wound around the core made of ferrite having a loss coefficient μ ″ of 3 × 10 ⁻² or less, that is, the length considering the wavelength shortening effect is λ / 4 or less coil is wound and impedance matching is achieved with an external capacitor, so that even a small and low profile configuration can generate a large magnetomotive force by flowing a large resonance current. , Get good radiation characteristics.

  The bar antenna according to the present invention has the above-described configuration, and is characterized in that a spacer is provided between the core and the coil.

  In the bar antenna of the present invention, a spacer is provided between the core and the coil, and the coil is wound without being in close contact with the core. Therefore, the wavelength shortening effect is reduced, the number of turns of the coil is increased, and the magnetomotive force applied to the core is increased.

  The bar antenna according to the present invention has the above-described configuration, wherein the cross-sectional shape of the core is rectangular.

  In the bar antenna of the present invention, the cross-sectional shape of the core is rectangular. Therefore, the height can be further reduced.

  The bar antenna according to the present invention has the above-described configuration, and the core material is a Ni—Cu—Zn-based ferrite material to which CoO is added in an amount of 0.2 to 10 wt%.

In the bar antenna of the present invention, a Ni—Cu—Zn based ferrite material added with 0.2 to 10% by weight of CoO is used for the core. Therefore, a low loss of 3 × 10 ⁻² or less can be easily realized.

In the bar antenna of the present invention, a core made of ferrite having a loss coefficient μ ″ of 3 × 10 ⁻² or less, and a length wound in a spiral state around the core is λ / 4 or less. Since a certain coil and a capacitor for impedance matching are provided, it is possible to achieve downsizing, in particular, low profile while having good characteristics.

  In the bar antenna of the present invention, a spacer is provided between the core and the coil to reduce the wavelength shortening effect. Therefore, the number of turns of the coil can be increased, the magnetomotive force applied to the core can be increased, and the characteristic Improvements can be made.

  In the bar antenna of the present invention, since the core has a rectangular cross section, the profile can be further reduced.

In the bar antenna of the present invention, the Ni—Cu—Zn-based ferrite material to which CoO is added in an amount of 0.2 to 10% by weight is used as the core material, so that it is 3 × 10 ⁻² or less necessary for good characteristics. Low loss can be easily achieved.

  Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments thereof. FIG. 1 is a diagram showing an exemplary configuration of a bar antenna 10 according to the present invention.

In FIG. 1, 1 is a strip-shaped core, and the core 1 is Ni—Cu—Zn based ferrite (Fe ₂ O ₃ : 20 to 49 mol%, NiO: 0 to 50 mol%, CuO: 5 to 20 mol%, Zn: balance). Further, the loss coefficient μ ″ of the core 1 in the UHF band (315 MHz band) is 3 × 10 ⁻² or less. A coil 2 is spirally wound around the center of the core 1. The length in a state where the coil 2 is wound, that is, the length considering the wavelength shortening effect (hereinafter referred to as the extension length of the coil 2) is set to λ / 4 or less. A capacitor 3 is connected in series, and a second trimmer capacitor 4 is connected in parallel, and these first and second trimmer capacitors 3 and 4 are impedance matched with a coil 2 that functions as an L element, as will be described later. Is for doing.

The reason why the loss coefficient μ ″ of the core 1 is set to 3 × 10 ⁻² or less is as follows. When the loss is large, energy is consumed to warm the core 1 and is sufficiently utilized for radio wave radiation. As a result, the radiation factor of the core 1 is reduced to 3 × 10 ⁻² so that the input energy can be effectively used for radio wave radiation. It is as follows. The numerical value of this loss factor can be easily realized by using Ni—Cu—Zn based ferrite to which 0.2 to 10 wt% of CoO is added.

  In a bar antenna in which a coil is spirally wound around a soft ferrite core, the frequency characteristics of the impedance of the coil are measured. When the frequency is low, that is, when the wavelength is not a problem, the bar antenna always functions as an L element. When the wavelength increases and the wavelength becomes a problem, the coil becomes inductive when the extension length of the coil is λ / 4 or less, and becomes capacitive when it exceeds λ / 4. When the coil exhibits capacitance, a portion where the direction of current is reversed is generated, and the magnetomotive force applied to the core is reduced.

  Therefore, in the present invention, the extended length of the coil 2 is made inductive by setting it to λ / 4 or less, and impedance matching is performed by the external first and second trimmer capacitors 3 and 4. A large resonance current flows due to the resonance action between the coil 2 and the first and second trimmer capacitors 3 and 4, and a large magnetomotive force can be applied to the core 1 so that a magnetic field can be efficiently radiated to the external space. As a result, it is possible to realize a low profile of 3 mm or less and a shortened length of λ / 20 or less while maintaining the same characteristics as the conventional λ / 4 whip antenna.

  The bar antenna of the present invention is a magnetic field coupling type in which an electric field is not dissipated to the outside, and has a structure that does not allow a resonance current to flow through the ground plane. Therefore, it can be easily mounted on a small device for which it is difficult to secure a ground contact area, and can be mounted on a plastic casing. In addition, since it is a magnetic field coupling type, it is hardly affected by stray capacitance due to the proximity conductor, so that the performance degradation is small even if it is incorporated in a small device. In addition, mismatching is less likely to occur.

  FIG. 2 is a diagram showing a configuration of another example of the bar antenna 10 according to the present invention. In FIG. 2, the same parts as those in FIG.

  In the example shown in FIG. 2, the spacer 5 is provided on the surface of the core 1 in the region where the coil 2 is wound, and the coil 2 is wound with a gap without being in close contact with the core 1. The degree of the wavelength shortening effect is determined by the electric field behavior in the vicinity where the coil 2 is in close contact with the core 1. Therefore, in this example, the coil 2 is wound without being in close contact with the core 1 to reduce the wavelength shortening effect, and the number of turns of the coil 2 is increased to increase the magnetomotive force applied to the core 1. . Other configurations are the same as those in FIG.

  Hereinafter, specific examples of the bar antenna of the present invention in the 315 MHz band will be described. As the core 1, the coil 2, and the first and second trimmer capacitors 3 and 4, the following were used.

(Core 1)
Materials: Fe ₂ O ₃ : 45 mol%, NiO: 35 mol%, CuO: 15 mol%, ZnO: 5 mol%, CoO: 1 wt% Ni-Cu-Zn ferrite, whose characteristics are Magnetic constant μ ′ = 8, relative permittivity ε ′ = 8
Dimensions: width 12mm, length 50mm, thickness 2mm
(Coil 2)
Material: Polyurethane-coated copper wire (JIS (UEW type 2) equivalent material)
Dimensions: Diameter 0.5 mm, extension length λ / 4 or less (specifically, 80 to 95 mm when close to the core 1)
Winding state: 3 turns, 2 mm space between windings, 0.5 mm space with core 1 (first trimmer capacitor 3)
Capacity: 3pF
(Second trimmer capacitor 4)
Capacity: 40 pF

  Next, characteristic evaluation of the bar antenna 10 according to the present invention will be described. The bar antenna 10 having the numerical examples as described above was manufactured and its characteristics were measured. The impedance was adjusted by connecting to an impedance analyzer. This adjustment was adjusted to 50Ω by a method of adjusting the capacities of the first and second trimmer capacitors 3 and 4.

  An oscillator was connected to the manufactured bar antenna 10, a 315 MHz standard dipole antenna was used on the receiving side, and the received power was measured in an anechoic chamber with a distance of 3 m between both antennas. As a comparative example, an impedance matching circuit using two capacitors is added to a wire antenna (30 x 50 mm) formed with a copper foil pattern using a glass epoxy (FR-4) substrate, and a 50Ω coaxial line is connected. The received power was measured under the same conditions as the bar antenna 10 of the present invention described above, using the manufactured comparative antenna.

  When the input is 5 dBm, the measurement result of the received power is −60 dBm in the conventional comparative antenna, whereas the measurement result of the received power is −40 to −50 dBm in the bar antenna 10 of the present invention. When the bar antenna 10 according to the present invention is used as a transmission antenna, the power reaching the reception side with the same transmission power is increased by 10 dB or more compared to the case where a conventional wire antenna is used despite the small configuration. I was able to. Therefore, by using the bar antenna 10 of the present invention, an antenna having excellent radiation characteristics can be incorporated into a small transmitter that has been difficult in the past.

It is a figure which shows the structure of an example of the bar antenna which concerns on this invention. It is a figure which shows the structure of the other example of the bar antenna which concerns on this invention.

Explanation of symbols

1 Core 2 Coil 3 First Trimmer Capacitor 4 Second Trimmer Capacitor 5 Spacer 10 Bar Antenna

Claims (4)

A core made of ferrite having a loss factor μ ″ of 3 × 10 ⁻² or less in a radio wave band to be used, and a length in a state of being wound around the core being λ / 4 or less A bar antenna comprising a coil and a capacitor for matching impedance.   The bar antenna according to claim 1, wherein a spacer is provided between the core and the coil.   The bar antenna according to claim 1 or 2, wherein a cross-sectional shape of the core is rectangular.   4. The bar antenna according to claim 1, wherein the core material is a Ni—Cu—Zn-based ferrite material to which CoO is added in an amount of 0.2 to 10% by weight. 5.

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