US3491320A - Tuning coil with movable magnetic core - Google Patents

Description

Jan. 20,1970 TAMAKI oH sm EITAL 3,491,320

TUNING COIL WITH MOVABLE MAGNETIC CORE Criginal Filed March 29, 1967 3 Sheets-Sheet l F/g. PR/OR ART 2 PR/OR ART l6 f (X'lOOK /s) |4 1'0 2'0 5 0 L (mm) INVENTORS TAMAKI OHASHI TAKESHI ISHlNO IsAo YOKOYAMA ATTY6.

Jan. 20,1976 TAMAKI OHASHI ETAL 3,491,320

TUNING COIL WITH MOVABLE MAGNETIC CORE Original Filed March 29, 1967 3 Sheets-Sheet 2 R 3 PR/OR ART 6 v 4- IO (0 2'0 30 L (mm) 4 5 PRIOR ART PRIOR ART 2 Mill/fir INVENTORS TAMAKI OHASH TAKESHI ISl-HNO IsAo YOKOVAMA 5J3 W0%) W7 ATTYS,

Jam. 20,1970 TAMAKI oHAsm L 3,491,320

TUNING con; wxwa MOVABLE mimic CORE Original Filed March 29, 1967 a Sheets-Sheet s INVENTORS TAMAK/ OHASHI TAKESHI ISHINO I SAO YOKOYAMA Arrvs.

United States Patent TUNING COIL WITH MOVABLE MAGNETIC CORE Tamaki Ohashi, Tokyo, and Takeshi lshino and Isao Yokoyama, Akita-ken, Japan, assignors to TDK Electronics Co., Ltd., Tokyo, Japan, a corporation of Japan Continuation of application Ser. No. 626,742, Mar. 29,

1967. This application Jan. 31, 1969, Ser. No. 798,254 Int. Cl. H01f 21/10, 21/06 US. Cl. 336-87 1 Claim ABSTRACT OF THE DISCLOSURE A variable inductance coil including a solenoid type coil, an adjusting core made of magnetic material and mounted for axial movement relative to the coil, and a fixed magnetic sleeve disposed around the coil to enclose the same. The fixed magnetic sleeve extends longitudinally beyond the coil at the end where the adjusting core is located so that the fluctuation of the electric characteristics of the adjusting core is minimized.

This application is a continuation of application Ser. No. 626,742, filed Mar. 29, 1967, now abandoned.

This invention relates to a variable inductance (,u tuning) coil to be used in a method of a tuning system, for example, in a car radio set.

A dust core has been used exclusively for a conventional variable inductance coil. As the advantages of such a dust core, it can be enumerated that the temperature coefiicient is small, and that applicable frequency band is wide. On the other hand, its permeability is so low that the long stroke, as the variable inductance, is essential to cover a certain tuning frequency range, the gap between the sleeve core and adjusting core must be small and the close tolerance of the cores must be required.

Instead of the above described dust cores, there have been used ferrite core in Japan since some time ago to eliminate the defects of such dust cores. However, inherently ferrite is a material so high in the permeability that there have been many faults due to unknown factors and have been much troublesome especially in the tracking process of the radio set production.

As illustrated in FIG. 1, in a conventional ferrite variable inductance coil, a coil 2 is wound on a bobbin 1, a fixed magnetic sleeve core 3 (which shall be known as a sleeve core hereinafter) shorter than the coil 2 is applied outside the said coil and a shield case 4 is further applied outside the said sleeve core so that an adjusting core (moving core) 5 for tuning may be moved in the directions indicated by the arrows x and x for tuning.

Usually, the tuner for the car radio set has an antenna coil, tuning coil and oscillating coil or further a high frequency amplifying c-oil. As in adjusting them the permeability of the adjusting core is fluctuated, the position of the adjusting core is also fluctuated in case of obtaining the same inductance. Consequently, in mass-produced sets incorporating such a kind of cores, the frequency f-stroke L curves as shown, for example, in FIG. 2 of these tuners do not agree with each other. Besides, the curve of each coil in one tuner does not become parallel. Therefore, there is a great defect that, in case they are incorporated into radios, tracking will become diflicult. For example, in case, in FIG. 1, the core 5 is of a diameter of '5 mm. and a length of 30 mm. and the sleeve core 3 is of an outside diameter of 15 mm., an inside diameter of 12 mm. and a length of 14 mm, the average tuning frequency by the stroke L of the tuning core 5 and five tuning cores will be :as shown in FIG. 2. FIG. 3 shows the fluctuation of the tuning frequency by a range R. In FIG. 3, the curve 10 represents the fluctuation of the tuning frequency of the same as is shown in FIG. 2 and the curve 11 represents the case according to the present invention. As evident from FIG. 2 and the curve 10 in FIG. 3, even if the dimension allowance is $0.05 mm., the fluctuation between respective samples will be of a considerably large value as shown by the curve 10. Thus tracking has been diflicult.

As a countermeasure against it, as shown in FIG. 4, notches 6 and 6' have been made in a part of the adjusting core 5 or, as shown in FIG- 5, the winding pitch of the coil 2 has been varied. However, in either manner, the operation has been complicated, the production cost has been high and yet no sufficient effect has been obtained.

In the present invention, in order to reduce such fluctuation, the interaction between the sleeve core and the adjusting core have been investigated on the permeability of the cores and the relative positioning. In the present invention, as shown in FIG. 6, the sleeve core 3 is made longer by k than the coil 2 and FIG. 7 shows the fluctuationR-stroke L as parameter k, which is shown in FIG. 6.

In FIG. 7, the curves A, B and C represent fluctuations in case dust cores having a value of ,u =20 were used for the sleeve core and adjusting core. It is found that, when the value of k of the sleeve core was varied to 0 mm. (represented by the curve A), 5 mm. (represented by the curve B) and 10 mm. (represented by the curve C), the reduction of the fluctuation was shown as the value of k was varied toward k: 10 mm., and the deviation was 4 kc./ sec. in the value of k=10 mm. On the other hand, when only the sleeve core was made from the material of :450, the fluctuation reduced gradually as in 0, 2, 4, 6, 8 and 10 mm. in FIG. 7, and was of a practically satisfactory value at k: 10 mm.

From the above, if ,u, of the material of the sleeve core is made as large as possible, m of the adjusting core is made small and the length of the sleeve core is made longer than the coil, the fluctuation of tuning frequency will be minimized. Therefore, in the variable inductance coil represented by the curve 10 in FIG. 3, in case the adjusting core was made from dust core and the sleeve core (of :50) was made from ferrite 3 mm. longer than the coil, the fluctuation R was as in the curve 11 in FIG. 3.

As described above, according to the present invention, by means of making the permeability ,u, of the material of the sleeve core more than 50 and making the sleeve core slightly longer than the coil, the fluctuation of the inductance of the variable inductance coil can be made small and tracking can be made easy.

While there has been described in connection with the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the invention.

We claim as our invention.

1. A variable inductance coil comprising a single solenoid type coil wound on a fixed hollow bobbin, said single coil being wound with uniform spacing over its entire length, an adjustable core made of magnetic material and mounted for axial movement within one end of said hollow bobbin relative to said coil, a. fixed magnetic sleeve disposed around said coil and made of ferrite having a permeability of more than 50 and greater than the permeability of said core, said sleeve extending beyond said coil by a distance sufficientto minimize fluctuations in the tuning frequency of said coil, and a shield casing disposed around said fixed magnetic sleeve.

References Cited UNITED STATES PATENTS Weis 336134 Harvey 33687 Berg 336132 XR OTHER REFERENCES Bozorth: Ferromagnetism, D. Van Nostrand Company Inc., March 1951, pp. 246 and 870-871.

THOMAS J. KOZMA, Primary Examiner US. Cl. X.R.

Conradet a1 336136XR 10 336432 136

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