JPH11345708A - Antenna for medium wave broadcasting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a medium wave broadcasting antenna capable of improving the magnetic permeability and quality coefficient of a magnetic material in the broadcasting frequency band of an AM radio or the like, obtaining high sensitivity even in a miniaturized state, and when it is constituted as size allowed to be loaded on a compact portable equipment, obtaining proper mechanical strength and high sensitivity. SOLUTION: The antenna consists of Mn-Zn ferrite of <=4 μm average crystal grain size using Fe2 O3 , MnO and ZnO as main components and is provided with rod-like or plate-like magnetic core setting up 1.7 MHz effective permeability to >=40 and a coil. The ferrite consists of 52 to 57 mol.% Fe2 O3 , 23 to 45 mol.% MnO and 5 to 20 mol.% ZnO as main components.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a medium-wave broadcasting antenna suitable for use as a magnetic antenna incorporated in an AM radio or the like and capable of realizing a compact antenna.

[0002]

2. Description of the Related Art Conventionally, AM radios for medium wave reception include:
Magnetic antennas using a ferrite material such as Ni-Zn ferrite or Mg-Cu-Zn ferrite as a magnetic core material have been used. In recent years, portable radios and portable cassette recorders with radios have been reduced in size. There is a demand for miniaturization of the body antenna. By the way, the relationship between the SN ratio indicating the sensitivity of this type of magnetic antenna and the received power is as follows.
Pr is set to received power (W), k is set to Boltzmann coefficient (= 1.38 × 10 ⁻²³ JK ⁻¹ ),
When Tsys ≒ Ts (temperature: K) is set and Δf is set as the bandwidth, it is expressed by the following equation (1). S (signal) / N (noise) = Pr / k · Tsys · Δf (1)

Subsequently, Pt is used as the power of the transmitter (power).
of transmitter: W), and Aet is the effective aperture of transmitting antenna.
enna: m ² ), Aer is set to the effective area of the receiving antenna (m ² ), and r is set to the distance (m), the received power Pr becomes the following equation (2). Can also be represented. Pr = (Pt · Aet · Aer) / r ² · λ ² (W) (2) where k is a radiation efficiency coefficient (radiation ef)
ficiency factor), it can be expressed by the following equation (3). Aer = (1.5 · λ ² · k) / 4π (3) From the above, it can be assumed that S / N∝k, and the radiation efficiency coefficient k is equal to the SN ratio which is the sensitivity of the antenna. You can see that they are related.

Considering the radiation efficiency coefficient k, Rr is the radiation resistance, RL is the copper loss of the coil, R
If f is the iron loss of the ferrite of the antenna constituent material, the following equation (4) is satisfied. k = Rr / (Rr + RL + Rf) (4) Here, if μer is the effective magnetic permeability, n is the number of turns of the conductor wound around the antenna, and A is the cross-sectional area of the ferrite core, the following equation (5) is established. I do. Rr = 31200 · μer ² · n ² · (A / λ ² ) ² (5)

From the above, k∝Rr∝μer ² ·
The relationship of A ² is derived, and it is understood from this relationship that the SN ratio is proportional to the square of the effective magnetic permeability μer. In general, R
Since r and RL show smaller values than Rf, Rf ∝
From the relationship of 1 / Q, it becomes clear that k can be considered to be roughly proportional to the quality factor Q of the magnetic material.

The effective magnetic permeability (μer) is given by the following equation (6).
It is represented by μer = μr · (1 + N · μr-N) ^-1 (6)

Here, μr is the relative magnetic permeability of the antenna material,
N represents a demagnetizing factor determined by the shape of the magnetic material.

Thus, in order to increase μer, N
, That is, making the shape of the magnetic material elongated is effective.

[0009]

However, Ni-Zn ferrite or Mg-Cu-Zn ferrite, which has been used as a conventional magnetic core material, has a low magnetic permeability μr in the frequency band of AM radio, so that the effective magnetic permeability is low. μer
And the quality factor Q is small, the iron loss R
f was large, and the radiation efficiency coefficient k of the antenna could not be made sufficiently large. For this reason, when the cross-sectional area A of the antenna is small, the SN ratio becomes small, causing a problem that the sensitivity of the antenna deteriorates. Therefore, in order to prevent this, the broadcasting frequency band of 0.5 to 1.7 MHz (500 k
(Hz to 1700 kHz), a magnetic core material having a high magnetic permeability μr and a high quality factor Q is desired, and a small and high-performance antenna which has a small occupation size and does not hinder mounting on a small radio or the like is desired.

Therefore, an object of the present invention is to provide a magnetic material having a magnetic permeability (μ) in a broadcasting frequency band such as AM radio.
r) and the quality factor (Q) of the magnetic material is high, so that good sensitivity can be obtained even if the size is reduced, and even if the size is small enough to be mounted on a small portable device, appropriate mechanical strength and good sensitivity are obtained. It is an object of the present invention to provide a medium wave broadcasting antenna having the following.

[0011]

According to the present invention, there is provided a Mn-Zn ferrite comprising Fe ₂ O ₃ , MnO and ZnO as main components and having an average crystal grain size of 4 μm or less. It is characterized by comprising a rod-shaped or plate-shaped magnetic core and an coil having an effective magnetic permeability of 7 MHz of 40 or more. The present invention further, Fe ₂ O ₃ 52~57mol
%, MnO 23-45 mol%, and ZnO 5-5
It is made of Mn-Zn ferrite having 20 mol% as a main component and having an average crystal grain size of 4 μm or less, and is provided with a rod-shaped or plate-shaped magnetic core and an coil having an effective magnetic permeability of 1.7 MHz of 40 or more. What you do.

According to the present invention, Mn--Zn having an appropriate composition
By selecting a ferrite material, the magnetic permeability can be increased. In addition, by reducing the average crystal grain size, the frequency of domain wall resonance can be shifted to a higher frequency side than the frequency band of AM radio, thereby reducing loss due to domain wall resonance in the broadcast frequency band of AM radio. Can be. Therefore, a Mn-Zn ferrite having a high quality factor of the magnetic material in the AM radio broadcast frequency band can be obtained. Further, by providing a coil on a magnetic core formed in a rod shape or a plate shape, it is possible to obtain an antenna having good sensitivity in a broadcast frequency band of AM radio even if it is small.

Next, in the above-described structure, it is preferable that the rod-shaped or plate-shaped magnetic core has a demagnetizing factor of 0.03 or less. Further, the frequency of the rod-shaped or plate-shaped core 5
In the band of 00 kHz to 1.7 MHz, it is preferable that the initial magnetic permeability is 700 or more and 1 / tan δ is 20 or more. Furthermore, SiO ₂ 0.01 to 0.0
3 wt%, CaO 0.01-0.1 wt%, and Ta
It preferably comprises at least one additive selected from the group consisting of ₂ O _{5 0.02~0.2wt%.}

Further, in the present invention, as a raw material of the Mn-Zn ferrite, Fn produced by a hydrothermal synthesis method is used.
It is preferable to use a raw material powder containing e ₂ O ₃ , MnO and ZnO as main components. By adding these elements,
The electric resistance of the ferrite can be increased, and the eddy current loss can be reduced. Further, when these additive elements are dissolved in the defects of the ferrite main component phase, lattice defects in the ferrite are reduced, and the magnetic properties are improved. In the present invention, the length of the rod-shaped or plate-shaped core is 30 to 80 m.
Preferably, m, thickness or width is in the range of 1 to 5 mm.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The Mn-Zn ferrite constituting the medium wave antenna according to the present invention contains Fe ₂ O ₃ , MnO, and ZnO as main components and has an average crystal grain size of 4 μm or less, preferably 3.5 μm or less. An antenna structure having a coil formed by processing Mn-Zn ferrite of this composition into a rod or plate, and applying a winding to the rod or plate. In the Mn-Zn ferrite, Fe ₂ O ₃ 5
2~57mol%, MnO23~45mol%, and is preferably in the composition in the range of ZnO 5 to 20 mol%, _{further, Fe 2 O 3 52~55mol%,} MnO
35-41 mol%, and ZnO 7-10 mol%
More preferably, the composition is Such Mn-Z
The n ferrite was obtained by mixing raw material powders containing Fe ₂ O ₃ , MnO and ZnO as main components, drying, calcining, granulating, and then molding using a predetermined mold. It is preferably manufactured through a step of firing the molded body.

The raw material powder mainly containing Fe ₂ O ₃ , MnO and ZnO used in the present invention is preferably produced by a hydrothermal synthesis method. Here, the hydrothermal method is
A type of solution method for synthesizing powder from the liquid phase, known as a method for producing powder with relatively small formation of coarse fixed particles, in which an alkaline suspension is subjected to a hydrothermal reaction at high temperature and pressure. Thus, powder particles are produced.
In this method, two or more types of compounds such as a solution and a solid oxide participate in the reaction, and a desired compound is synthesized. When the reaction occurs in this manner, one of the compounds exists as a solution. Alkali metal ions are frequently used.

In order to produce a raw powder of Mn-Zn ferrite by a hydrothermal synthesis method, for example, first, Mn, Zn, F
A metal compound aqueous solution containing a predetermined amount of e is prepared. As the compounds of these elements, various water-soluble compounds can be used, but chlorides, nitrates and the like are preferably used. Then, for example, NaOH, K
An aqueous solution of OH, NH ₃ OH or the like is contacted and mixed to form an alkaline suspension. Next, this alkali suspension is placed in a pressure vessel such as an autoclave, and is heated at 120 to 250 ° C.
To produce a ferrite precipitate. And after washing the obtained ferrite precipitate with water, it is dried,
Obtain the desired raw material powder.

Further, the Mn-Zn ferrite according to the present invention is preferably added with at least one additive (additive) selected from SiO ₂ , CaO and Ta ₂ O ₅ . These additives have an effect of suppressing grain growth of the main component of ferrite, increasing electric resistance, and reducing eddy current loss. In addition, since these additive elements form a solid solution in defects of the main phase of ferrite, lattice defects in ferrite are reduced, and an effect of improving magnetic properties is obtained. These additives may be contained as impurities in the raw material powder, but may be positively added in appropriate amounts.

SiO ₂ and CaO are particularly effective in segregating or precipitating at grain boundaries of ferrite to increase electric resistance. However, excessive addition of these additives causes abnormal grain growth, so that the amount of addition is limited. Effect of increase in the resistance of the ferrite is sufficiently exhibited, the range of the abnormal grain growth does not occur amount added, 0.01～0.03Wt% in the case of SiO _2, in the case of CaO .01 to 0.
1 wt%. Ta ₂ O ₅ has the effect of increasing the electrical resistance by segregating to the crystal grain size of the ferrite and making the crystal grains finer. However, an excessive addition of Ta ₂ O ₅ adversely reduces the electric resistance, so that the addition amount is limited. The effect of increasing the electrical resistance and making the crystal finer is fully exhibited,
The range of the amount of addition in which the decrease in electric resistance does not occur is from 0.02 to
0.2 wt%.

In the present invention, when these additives are added, they are first added in a step of mixing the raw material powders, and are temporarily calcined in a non-oxidizing atmosphere to add the additive elements to the crystals of the main component phase. After the solid solution is formed, it is preferable to granulate, shape, and perform main firing. Here, the non-oxidizing atmosphere means an atmosphere in which a reduction reaction occurs in the main component of ferrite in the ferrite powder at the time of heating, for example, an N ₂ atmosphere, a vacuum atmosphere,
r atmosphere. On the other hand, if the temperature at the time of pre-firing is too high, sintering proceeds, which leads to growth of crystal grains and causes deterioration of characteristics. On the other hand, if it is too low, the reduction reaction does not proceed, and the solid solution of the added element is hindered. Therefore, in order to sufficiently dissolve the additive element in ferrite and suppress grain growth, the temperature of the preliminary firing is preferably about 700 to 930 ° C.

Mn-Zn used for the antenna according to the present invention
According to ferrite, frequency is 500 ~ 1700kHz
In the above, the apparent magnetic permeability can be improved by the high magnetic permeability of 500 or more. Further, by reducing the average crystal grain size to 4 μm or less, preferably 3.5 μm or less, the frequency of magnetic resonance can be shifted to a high frequency side of 3 to 12 MHz. Then, 0.5 to 1.9M including the broadcast frequency band of AM radio is thereby obtained.
The quality factor Q of the magnetic material at Hz can be achieved at a high value of 10 or more, and a ferrite material having excellent characteristics as a core material of an AM radio antenna can be obtained. Therefore, the Mn-Zn ferrite having such high μ and Q values is formed into a rod shape or a plate shape, and this is used as a magnetic core to form a coil by winding around the core. An antenna having high sensitivity can be obtained.

Next, when the above-mentioned Mn-Zn ferrite is processed into a rod shape or a plate shape to form an antenna, the length is preferably 30 to 80 mm, and the thickness or width is preferably 1 to 5 mm. Alternatively, the width is more preferably in the range of 1 to 4 mm. Even when the above-described Mn-Zn ferrite is used, if the length is less than 30 mm, satisfactory receiving characteristics cannot be obtained as is apparent from the examples described later, and the length is longer than 80 mm. When formed, the performance as an antenna is improved, but the risk of breakage increases due to the mechanical brittleness inherent to the ferrite material, and it is disadvantageous in terms of mounting on a small portable device. Also, if the thickness or width is less than 1 mm, there is a risk of breakage as in the case of the length, and the problem is to make the antenna smaller than a conventional antenna. Since the high magnetic permeability of the Mn-Zn ferrite having the above-described composition related to the present invention can be used, the size of the Mn-Zn ferrite can be reduced. In view of the above, it is most preferable that the length be 40 to 60 mm and the thickness or width be 2 to 3 mm in order to achieve both mechanical strength and sensitivity at a high level as an antenna.

Next, in order to control the crystal grain size in the above-mentioned Mn-Zn ferrite, a method of controlling the temperature at which the green compact of the formed ferrite powder is fired and the time for maintaining the temperature at the maximum temperature may be used. This makes 0.9-1
Mn—Zn ferrite having an average crystal grain size of about 0 μm can be easily obtained.

[0024]

EXAMPLE First, a raw material powder containing Mn, Zn, and Fe was produced by a hydrothermal synthesis method. That is, a ferrite precipitate is obtained by subjecting an alkaline suspension containing predetermined amounts of Mn, Zn, and Fe to heat treatment, and then the precipitate is washed with water and dried to obtain 53.5 mol% of Fe ₂ O ₃ and 39. 5mo
A ferrite raw material powder having a composition of 1% and 7.0 mol% of ZnO was obtained. SiO ₂ was added to the obtained raw material powder.
0.02 wt%, CaO 0.02 wt%, Ta ₂ O ₅
After the slurry obtained by adding 0.04 wt% and mixing with a ball mill was dried, the slurry was calcined at 900 ° C. for 3 hours in an N ₂ atmosphere. After crushing and drying the powder after calcination,
A compact was produced by compacting into a rod. Then, the molded body was subjected to main firing in a vacuum to obtain a rod-shaped magnetic core made of Mn-Zn ferrite having a length of 50 mm, a width of 3 mm, and a thickness of 2 mm. The operation conditions at the time of the main firing were 1050 ° C. for 4 hours, and the temperature increasing rate was 180 ° C./hour. The average crystal grain size of the Mn-Zn ferrite obtained after the main firing was 1.0 μm.

As a comparative example, a rod-shaped magnetic core (length 34 mm, width 8 mm, thickness 2 mm) made of Mg-Cu-Zn ferrite was prepared. Its composition is 39.2 mol of Fe ₂ O ₃
%, MgO 37.6 mol%, ZnO 15.1 mol%, C
uO was 8.1 mol%, and the average crystal grain size was 3.7 μm.

The magnetic properties of the Mn-Zn ferrite core obtained above and the Mg-Cu-Zn ferrite core of the comparative example were measured. Here, in general, the magnetic permeability μr
Is expressed by the real part (μ ') and the imaginary part of the permeability μr.
There are two values of (μ ″), and the magnetic core material of the antenna preferably has a high real part (μ ′) and a low imaginary part (μ ″). The quality factor Q is expressed by (μ ′) / (μ ″), and the larger the value of Q is, the more preferable the core material of the antenna is. As shown in Table 1 below, the ferrite cores of the above Examples and Comparative Examples The results of measurement of the effective magnetic permeability μer and the quality factor Q. The measurement of the effective magnetic permeability is performed by measuring the inductance values of the air-core coil and the coil containing the ferrite core at a frequency of 500 kHz, 1 MHz (1000 kHz), and 1.
It was measured at 5 MHz (1500 kHz) and was determined from the rate of change.

"Table 1" Example core Comparative example core μer 500 kHz 54 201 MHz 54 20 1.5 MHz 54 20 Q 500 kHz 110 39 1 MHz 96 58 1.5 MHz 72 62

From these results, it can be seen that 500 to 1700 kHz (1.7 MHz) which is the main frequency band of AM radio broadcasting.
In the mid-wave band, the Mn-Zn ferrite core of the example has a μ value compared to the Mg-Cu-Zn ferrite core of the comparative example.
Both the value of er and the value of Q are high, and it is recognized that the antenna is excellent as a medium wave antenna.

FIG. 1 shows the rod-shaped ferrite cores of the above embodiment and comparative example at a frequency of 100 to 2000 kHz.
2 shows the frequency dependence of μer and Q in FIG. Here, Q (quality factor of the magnetic material) is generally used as a value indicating the performance of ferrite, and it is preferable that this value is large as an antenna material. The magnetic properties were measured under the same conditions as above. In the figure, ● represents an example,
Δ indicates a comparative example.

From the results shown in FIG. 1, in the range of 500 to 1700 kHz which is the main broadcast frequency band of AM radio (the range between the chain lines in FIG. 1), the M obtained in the embodiment is obtained.
The rod core of n-Zn ferrite had a higher Q value than the rod core of the comparative example, and it was recognized that the rod core had excellent characteristics as a ferrite core for an antenna.

FIG. 2 shows the frequency dependence of μ ″ for the bar-shaped ferrite cores of the example and the comparative example at a frequency of 100 to 5000 kHz. The measurement of the magnetic characteristics was performed under the same conditions as described above. In the figure, ● shows the measurement result of the example, and ○ shows the measurement result of the comparative example.

From the results shown in FIG. 2, the ferrite magnetic core of the comparative example has the maximum value of μ ″ indicating the loss at about 2000 kHz, whereas the ferrite magnetic core of the embodiment has a μ ″ of 5000 kHz.
Hz, and the peak of μ ″ is shifted to the high frequency side. This indicates that the frequency of the magnetic resonance of the ferrite core of the example having a small crystal grain size is lower than that of the ferrite core of the comparative example. This indicates that the frequency is shifted to a higher frequency side than the frequency.

Therefore, in FIG. 1, the quality factor Q (=
μ ′ / μ ″), the ferrite core of the example shows a significantly higher value than the comparative example. From these results, the average crystal grain size of the Mn—Zn ferrite is 2.4 μm.
By reducing the size to m, the frequency of magnetic resonance can be set to a high frequency range, and good characteristics with a quality factor Q of 10 or more in the range of 0.5 to 1.7 MHz, which is the broadcast frequency band of AM radio, can be obtained. It can be seen that it can be obtained.

Next, the frequency dependence of the relative loss coefficient tan δ / μ ′ was measured for the ferrite of the above example.
The relative loss factor tan δ / μ ′ is represented by a quality factor Q = (μ ′) /
(Μ ″) is the reciprocal of tan δ divided by μ ′,
If the material is the same, it is a numerical value that does not change even if the shape changes, and is a numerical value specific to the material. This relative loss coefficient tanδ
The smaller the value of / μ ', the smaller the material-specific loss. FIG. 3 shows the measurement results. In FIG.
The measurement results of the examples are indicated by ●, and those indicated by the measurement results Δ are 10% when a ferrite having the same composition as that of the example is manufactured.
A sample having an average crystal grain size of 0.9 μm obtained by main firing at a temperature increasing rate of 180 ° C. after holding at 00 ° C. for 4 hours and corresponds to another example. Further, in FIG.
The measurement results of other comparative examples (samples having the same composition as the example, but firing at 1200 ° C. at the time of main firing and having an average crystal grain size of 10 μm) are indicated by Δ.

FIG. 3 shows that the frequency is 0.5 to 1.5 MHz (5 MHz).
(1500 kHz to 1500 kHz), the ferrite of each example was compared with the ferrite of each comparative example in terms of the relative loss coefficient ta.
nδ / μ ′ is small. Therefore, the frequency 0.5
At 1.5 MHz, the ferrite of the embodiment has a small material-specific loss, and if the shape of the antenna is optimized,
It can be seen that an antenna with small loss can be created.

FIG. 4 shows the frequency characteristics of the S / N ratio when the ferrite core of the previous embodiment and the ferrite core of the comparative example are mounted on an AM radio. From the measurement results shown in FIG. 4, in the mid-wave band of 500 to 1700 kHz, the ferrite core of the example has a volume of about 1/2 that of the ferrite core of the comparative example, although the volume of the ferrite core is about ５〜.
It is clear that B also has an improved S / N ratio.

FIG. 5 shows that the thickness of the ferrite core of the embodiment is 2
7 shows an effective magnetic permeability curve when the length and width are fixed to mm. The measured frequency is 1 MHz, which is approximately the center value of AM radio. However, as can be seen from the frequency characteristics of the effective magnetic permeability shown in FIG.
Since it shows a substantially constant value up to Hz, the data in FIG. 5 corresponds to all frequencies used in AM radio. It can be seen that a ferrite bar antenna having various effective magnetic permeability can be manufactured by adjusting the length of the ferrite core in the range of 20 to 70 mm and the width in the range of 1 to 8 mm. In FIG. 5, curve a
1 is a conventional core (2x8x34mm size conventional core)
5, the curve a2 shows the same volume line as that of the conventional magnetic core. In FIG. 5, the area below the curve a1 is the sensitivity reduction area, and the antenna core has a length and width corresponding to this area. When a conventional magnetic core (2 × 8
This is a region where the sensitivity is lower than that of the conventional magnetic core having a size of × 34 mm, and a region above the curve a2 in FIG. 5 is a region where the volume is increased, and the antenna is manufactured with a length and width corresponding to this region. The conventional magnetic core (2 × 8 × 34mm
This is a region where the volume is larger than that of the conventional magnetic core and cannot be reduced in size. From the above, when manufacturing the antenna of the present invention, by manufacturing the antenna with the size of the region sandwiched between the curves a1 and a2, it is possible to obtain an antenna having a smaller volume and a higher sensitivity than the conventional magnetic core. It is clear that. In addition, when the effective magnetic permeability is set to 40 or more, the width and length of the antenna may be set in a region above the curve in which the numerical value 40 is entered. From the above, the antenna sensitivity is increased by setting the effective magnetic permeability to 40 or more, the antenna sensitivity is improved by devising the shape, and the length of the rod-shaped or plate-shaped core is 30 to 80 in consideration of the mechanical strength.
It is clear that the range of mm and the thickness and width of the magnetic core should be set in the range of 1 to 5 mm (in FIG. 5, the area surrounded by the curves a1 and a2 and the curve indicating the effective magnetic permeability 40). .

In FIG. 5, the curve a3 is a half volume line for the conventional magnetic core, and the curve a4 is a sensitivity line three times as large as the conventional magnetic core. If the antenna is manufactured by setting the length and width in the region, it is possible to obtain an antenna having a volume equal to or less than 1/2 with respect to the conventional magnetic core and having excellent sensitivity with respect to the conventional magnetic core. If the antenna is manufactured by setting the length and width in the region between the curve a2 and the curve a4, the antenna has three times or more the sensitivity and the volume is smaller than that of the conventional core. It turns out that it can obtain.

FIG. 6 shows that the thickness of the ferrite core of the embodiment is 2
It shows the value of the radiation efficiency when the length and width were fixed and fixed to mm. It can be seen that a ferrite bar antenna exhibiting various radiation efficiencies can be manufactured by adjusting the length of the ferrite core in the range of 20 to 70 mm and the width in the range of 1 to 8 mm.
In FIG. 6, a curve a6 represents a conventional magnetic core (2 × 8 × 34 m
Same radiation efficiency line as m-size conventional magnetic core), curve a7
Is the same volume line as that of the conventional core, curve a8 is a half volume line of the conventional core, and curve a9 is 3 of the conventional core.
In order to make the volume smaller than that of the conventional magnetic core, it is necessary to set the length and width in a region below the curve a7 in FIG. To obtain efficiency, curve a in FIG.
It is necessary to set the length and width in the area above 6
In order to reduce the volume to １ ／ or less of the volume of the conventional magnetic core, it is necessary to set the length and width in a region below the curve a8 in FIG. That is, when the ferrite core manufactured in the present embodiment is used, a target volume and a dimension of the ferrite core that gives sensitivity can be obtained. For example, it can be seen that the width of the flight core needs to be 3.3 mm or less and the length needs to be 41 mm or more in order to achieve 1/2 the volume and twice the radiation efficiency, that is, twice the sensitivity, of the conventional core.

FIG. 7 shows that the thickness of the ferrite core of the embodiment is 2
The value of the demagnetizing coefficient when the length and width are fixed to mm is shown. It can be seen that by adjusting the length of the ferrite core in the range of 20 to 70 mm and the width in the range of 1 to 8 mm, ferrite bar antennas exhibiting various demagnetizing coefficients can be manufactured. In FIG. 7, a curve a10 represents a conventional magnetic core (2 × 8 × 3).
The same demagnetizing factor line as that of the conventional magnetic core of 4 mm size, the curve a11 is the same volume line as that of the conventional core, the curve a12 is a 1/2 volume line with respect to the conventional core, and the curve a13 is the triple demagnetizing field of the conventional core. Since the coefficient lines are shown, in order to make the volume smaller than that of the conventional magnetic core, the curve a11 in FIG.
It is necessary to set the length and the width in the region below, and to obtain a higher demagnetizing factor than the conventional magnetic core, it is necessary to set the length and the width in the region above the curve a10 in FIG. There is a need to set the length and width in a region below the curve a12 in FIG. 7 in order to make the volume less than half the volume of the conventional core, and more than three times the conventional core. To obtain sensitivity, curve a in FIG.
You need to set the length and width in the area above 13
Further, in order to set the demagnetizing factor to 0.03 or more, the length and width of the Mn-Zn ferrite may be set so as to be on the curve of 0.03 shown in FIG. FIG. 7 also shows that the demagnetizing factor N, which is twice as sensitive in a half volume as the conventional magnetic core, is 0.024. If the shape of N is 0.024 and μer is 40 or more, about twice the sensitivity can be expected as compared with the conventional magnetic core. Ferrite permeability μ
r can be represented by μr = μer (1 + μrN−N),
Μr at this time is 700. Furthermore, since it is desirable for the ferrite material to be smaller than 9 × 10 ⁻⁵ which is the minimum value of tan δ / μ ′ in the comparative example shown in FIG. 3, the value of 1 / tan δ may be 20 or more. Required.

As described above, from the measurement results shown in FIGS. 5, 6 and 7, in order to obtain an effective magnetic permeability, radiation efficiency and demagnetizing coefficient exceeding the conventional core, the length of the rod-shaped or plate-shaped core is required. It is clear that it is preferable to set the thickness to 30 to 70 mm and the thickness or the width to 5 mm or less. However, in terms of the length, the longer the antenna, the easier it is to obtain the characteristics as an antenna.
If the length of the antenna is longer than 30 mm, there is a risk of breakage due to the inherent brittleness of the ferrite, and if the length is less than 30 mm, the sensitivity as an antenna is reduced.
The range of m is preferred. Further, the thickness and the width are preferably in the range of 5 mm or less in consideration of the conventional magnetic core, but the thickness and the width of less than 1 mm are liable to breakage, so that the range of 1 to 5 mm is preferable. Even within these ranges, a range of 40 to 60 mm in length and a thickness or width of 2 to 3 mm is more preferable for achieving both mechanical strength and sensitivity as an antenna.

FIG. 8 shows the real part (μ ′) of the magnetic permeability of the Mn—Zn ferrite constituting the magnetic core of the antenna of the embodiment.
And the dependence of the value of tan δ / μ on the crystal grain size. In the antenna, it is necessary to increase the effective magnetic permeability in order to improve the radiation resistance and reduce the copper loss. In addition, reduction of tan δ is necessary for reduction of iron loss. Therefore, it is necessary that the material properties have a high μ and a small value of tan δ. In other words, it can be said that the smaller the value of tan δ / μ, the better. From the crystal grain size dependence shown in FIG. 8, the value of tan δ / μ of the Mn—Zn ferrite used in the present invention tends to decrease due to the refinement of the crystal grain size. It can be seen that the value is about the same as that of Zn ferrite.

In order to obtain a better value of tan δ / μ than the conventional magnetic core from the crystal particle size dependency shown in FIG. 8, the crystal particle size must be 4 μm or less, which is 1/1 of the conventional magnetic core.
To reduce the average crystal grain size to 3.5 μm or less,
m was found to be necessary. Also, tan
The minimum value of δ / μ is at the crystal grain size of 2 μm, and the value of tan δ / μ tends to increase even if the particle size is further reduced. 5
The range is preferably from 3.5 to 3.5 μm.

[0044]

As described above, according to the present invention, F
eO, MnO and ZnO as main components or Fe ₂ O ₃ 5
It is composed of Mn-Zn ferrite having a main component of 2 to 57 mol%, 23 to 45 mol% of MnO, and 5 to 20 mol% of ZnO and having an average crystal grain size of 4 μm or less.
Since it is equipped with a rod-shaped or plate-shaped core and a coil having an effective magnetic permeability of 40 or more in Hz, the magnetic permeability is higher than that of an antenna made of a conventional material such as a conventional Ni-Zn ferrite or Ni-Cu-Zn ferrite. Further, by reducing the average crystal grain size to 4 μm or less, the antenna in which the domain wall resonance frequency is set to a higher frequency side than the medium wave range and the iron loss in the broadcast frequency band of AM radio is reduced. can get. The Mn-Zn ferrite constituting this antenna has a frequency of 0.1 including the broadcast frequency band of AM radio.
The quality factor Q of the magnetic material at 5 to 1.9 MHz is sufficiently high, and has an advantage that a good sensitivity can be obtained even if the size is reduced.

Next, if the demagnetizing factor of the magnetic core is 0.03, it is possible to provide an antenna having good sensitivity for medium wave broadcasting. If the Mn-Zn ferrite constituting the antenna of the present invention is manufactured from a raw material powder obtained by a hydrothermal synthesis method, the crystal grain size can be easily reduced, so that the value of tan δ is reduced. It is possible to reduce the iron loss by making the antenna smaller, thereby obtaining an antenna with excellent sensitivity with a good radiation efficiency coefficient. Therefore, according to the present invention, there is an effect that it is possible to provide a small-sized antenna having good sensitivity as a medium-wave broadcasting antenna for a small portable device.

[Brief description of the drawings]

FIG. 1 is a graph showing frequency dependence of permeability μer and quality factor Q of ferrite cores for antennas of an example and a comparative example.

FIG. 2 is a graph showing the frequency dependence of the magnetic permeability μ ″ of the ferrite cores for antennas of the example and the comparative example.

FIG. 3 is a graph showing the frequency dependence of the relative loss coefficient of the ferrite cores for antennas of the example and the comparative example.

FIG. 4 is a graph showing frequency characteristics of S / N ratios of the antennas of the example and the comparative example.

FIG. 5 is a graph showing the dependency of the length and width on the effective magnetic permeability when the thickness is fixed to 2 mm in the ferrite core for an antenna of the example.

FIG. 6 is a graph showing the dependence of the length and width on the radiation efficiency when the thickness is fixed at 2 mm in the ferrite core for an antenna of the example.

FIG. 7 is a graph showing the dependence of the length and width on the demagnetizing factor when the thickness is fixed to 2 mm in the ferrite core for an antenna of the example.

FIG. 8 is a graph showing the crystal grain size dependence of μ ′ and tan δ / μ in the ferrite core for an antenna of the example.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akihiro Makino 1-7 Yukitani Otsuka-cho, Ota-ku, Tokyo Alps Electric Co., Ltd. (72) Inventor Noboru Nakagawa 6-7-35 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation

Claims (7)

[Claims] 1. A rod-shaped or plate-shaped core made of Mn-Zn ferrite having Fe ₂ O ₃ , MnO, and ZnO as main components and having an average crystal grain size of 4 μm or less, and having an effective magnetic permeability of 1.7 MHz of 40 or more. And a coil. _2. 52 to 57 mol% of Fe ₂ O ₃ , MnO
23-45 mol%, and ZnO 5-20 mol
% As a main component and Mn—Zn having an average crystal grain size of 4 μm or less.
Made of ferrite, the effective permeability of 1.7MHz is 40
A medium wave broadcasting antenna comprising the above-described rod-shaped or plate-shaped magnetic core and a coil. 3. The medium wave broadcasting antenna according to claim 1, wherein the bar-shaped or plate-shaped magnetic core has a demagnetizing factor of 0.03 or less. 4. A frequency 50 of the rod-shaped or plate-shaped magnetic core.
In the band of 0 kHz to 1.7 MHz, the initial permeability is 7
The antenna for medium wave broadcasting according to claim 1 or 2, wherein 1 / tan δ is 20 or more. 5. SiO ₂ 0.01-0.03 wt%, C
aO 0.01 to 0.1 wt%, and Ta ₂ O ₅ 0.0
At least one selected from the group consisting of 2 to 0.2 wt%
4. A method according to claim 1, wherein said additive comprises a seed additive.
The medium wave broadcast antenna according to any one of the above. 6. A raw material powder of Fe ₂ O ₃ , MnO, and ZnO produced by a hydrothermal synthesis method as a main material of the Mn—Zn ferrite. The antenna for medium waves according to any one of claims 1 to 5. 7. The rod-shaped or plate-shaped magnetic core has a length of 30.
The medium wave antenna according to any one of claims 1 to 6, wherein the antenna has a thickness of 1 to 80 mm and a thickness or a width of 1 to 5 mm.