US2090991A - Radio frequency coil with subdivided iron core - Google Patents

Description

1937. o. WOH LFAR TH ET AL 2,090,991

RADIO FREQUENCY con. WITH SUBDIVIDED IRON coma Filed May 7, 1935 ATTORN EY Patented Aug. 24, 1937 PATENT OFFICE RADIO FREQUENCY COIL WITH SUBDIVIDED IRON CORE Otto Wohlfarth, Berlin, and Alfred Agricola, Berlin-Tempelhof, Germany, assignors to Allgemeine Elektricitatz Gesellschaft, Berlin, Germany Application May 7, 1935, Serial No. 20,160 In Germany May 7, 1934 4 Claims.

The present invention relates to radio frequency inductance coils and more particularly to such coils which are provided with powdered magnetic material cores.

5 Radio frequency coils are known in the prior art which are furnished with subdivided ironcores, more particularly iron-cores made of compressed or molded iron powder. Coils of this kind, if the iron powder has been chosen properl ly and also the insulation thereof, when compared with air-cored coils, are of smaller dimensions for the same loss. Another merit of coils of this kind consists in that they have less leakage or stray. than air-cored coils.

Now, it has been found that with the applica- I tion of well known ways and means in making the iron-cores, i. e., extensive subdivision of the iron-core (grain size less than the ironlosses may be diminished to such a point where 2 their share in the total losses becomes so small that the losses occasioned in the coil and which are a'fun'ction of the frequency, especially capacitive losses and eddy-current losses, exercise a decisive influence upon the quality of the iron- 5 cored coils. So far as the quality of the radio frequency coil, especially its use in tuning circuits covering a comparatively extensive frequency range or band is concerned, for instance, the range included in broadcast waves, it is im- 30 portant that the frequency-dependent portion of the coil losses should be as low as possible in order that the ratio RmL will be stable and constant as far as. feasible for the frequency range here concerned (where R dielectric loss 35 angle, L inductance, and w=frequency, 21r).

As has been borne out by experiments of which this invention is the outcome, these losses are essentially affected by the shaping. and the selection of the material for the coils. Iron-cored 40 coils as known in the prior art have mostly been of the kind with a cylindrical and a disk winding. Both these types of winding, when employed in connection with iron-cored coils, involve the demerit that turns which have a1ter-. 45 nating voltage are directly adjacent the iron core. Now, according to this invention the winding is built up in such a fashion that the turns carry-- ing alternating voltage are so'much farther removed from the iron core, the higher such alternating voltage applied to them, the winding being at the same time so chosen that it will inherently be of low capacitance. The losses by virtue of capacitive leakance will then be found n to be particularly low, and a coil thus designed exhibits substantially lower losses than the coils so far used in combination with iron-cores.

In the drawing, Fig. 1 illustrates one form of the invention wherein a cross wound coil is provided with a core; Fig. '2 illustrates diagram- 5 matically the formation of the wire used to wind the coil; Fig. 3 is a curved sheet illustrating the characteristics of the coils; Fig. 4 is a modification of the coil structure shown in Fig. 1 and Figs. 5 and 6 illustrate diagrammatically certain 10 preferred embodiments of the invention.

The coil of the invention may be designed in the form of a cross-wound coil the end of which located towards the iron-core is connected with a fixed potential (grounded). The form of the coil in this scheme is preferably so chosen that its width b will be severaltimes the length l as shown in Fig. 1.

The cross-winding offers over the coil known in the art a chance for using the coil unsupported, that is to say, without the use of a bobbin orcoil body. This insures the advantage that the inside crosssectional area of the coil wind ing is completely filled by the iron-core s (Fig. 1) with the result that the field of force set up in the interior of the coil is homogeneous, and this means a further reduction of thelosses.

According to another, object and idea of this invention, with a view to reducing the frequency v dependent coil losses still further, recourse is had to a multiple subdivision of the strands used for the winding, in other words, the wired the coil consists of a multiple-stranded litz. The latter, for instance, could be composed and built up in such a way that several say, -three strands orlitzes each of which consists of three or more inter-stranded constituent or individual conductors, are in turn twisted or stranded together as shown in-Fig. 2, where al, a2, a3, designate thre'e'strands each of which is composed of three 40 individual conductors, and these three strands are again twisted or stranded together. By such multiple stranding the losses due to skin ellect are substantially diminished.

- In a form of coil as stated it is also possible to choose the diameter of the constituent conductors .of the strand extremely small so that both the eddy-current losses in the strand proper as well as the capacitive losses of-the. coil will be reduced. At the same time, the increase in the ohmic resistance of the coil due to the reduction in the crOss-sectionalarea of the wire, and which is independent of the frequency, has a favorable effect. As can be seen from Fig. 3, the simultaneous increase in the ohmic resistance-which nection to give the iron core the shape of a bodyv plays a large part in the aggregate coil losses especially where low frequencies are dealt with,

leads-to a further fiatteningbf the loss curve.

The curve e which stands for a coil made of comparatively stout wires and multiple-stranded litz, changes over to curve 1 when using a coil' made of slender wires and multiple subdivision of the strand, for the same number of conducthroughout the frequency band becomes considerably more constant.

Very favorable results were obtained with strands of 3 6 0.05 mm., in other words, strands in which the constituent wires or conductors consist of copper wire with a cross-section of only 0.05 mm., six of which were twisted together so that there resulted three strands which were, in turn, stranded together.

For the purpose of a further reduction of the frequency-dependent losses the strand, according to the invention is lined with one or more coats or layers of low-loss insulation material, such as low-loss paper, say, Japanese paper. In this manner spacing between the constituent spires of the coil is insured so that the field set up around the strand axis and which decreases rapidly with the distance will cut across the adjacent strands only to a limited extent.

The reduction of the losses attainable by choosing a coil form as hereinbefore disclosed is still further decreasable by shaping the iron body or core in a suitable way. In the iron-cored coils known to the earlier art, the said iron body mostly presents the form of an H or of a ring. These forms inhere quite a number of disadvantages, inter alia, that the winding and thus live portions of the coils come to be placed at close proximity to the iron core, and this is conducive to additional coil losses. In order to reduce losses of this kind, according to another basic idea of the invention the said iron-core is so formed and disposed in reference to the coil that the live parts of the coil are spaced apart from the iron core. It is advantageous in this conof rotation.

Exemplified embodiments are shown in Figs. 4 to 6. Fig. 4 shows an iron-cored coil which consists of winding to which has the shape of a cross winding, and the iron core comprising These parts SI and s2 are cup? shaped bodies presenting rotational symmetry parts sl .and s2.

and whose bottom surfaces interengage. The supply-lead zl on the iron body is grounded, while the opposite lead 22 is on voltage.

, As can be seen the edges of the cups areso bent away from the winding that those parts of the winding which are at a higher potential are farther removed from the iron body than the portions being at a lower potential. u

The fact that the iron body is cup shaped insures an adequately closed magnetic path. But in order that the entire magnetic path may be improved, the cups sl and s2, as shown in Fig. 5 may at the outermost edge 1" be again bent to-- wards one another. In this manner also the stray is diminished. If desired, as shown in Fig.

' shells are placed flatly against each other, then the bottom of the same, and, if desired, also the edges of the same, could be made elliptical instead of presenting'rotational symmetry so that by turning the two cups sl and $2 in reference to each other it is possible to alter the inductance of thecoils, a fact, that may be of advantage in matching or adapting coils.

We claim:

1. A radio frequency coil provided with a core composed of compressed finely divided magnetic material, said core comprising a pair of adjacent cooperating cup shaped members arranged with respect to each other so as to form an enclosure for the coil, one of said members being provided with a central abutment upon which the coil is wound, said abutment being placed on the side of the cup shaped member which is adjacent the other cup shaped member.

2. A'radio frequency coil provided with a core composed of compressed finely divided magnetic material, said core comprising a pair of adjacent elliptical cup shaped members mounted with respect to oneanother so as to form a substantial enclosure for the .coil, one of said members being provided with a central abutment on the inside portion thereof, the coil being supported upon said abutment, said two members being displaceable relative to one another to thereby provide means for varying the inductance value of the coil.

3. In an inductance device, a coil, a magnetic material core for the coil, said core comprising two' shell halves, one of said shell halves being provided with a pin, said coil being positioned around the pin, the other shell half having formed therein a recess for receiving the pin of the other shell ha1f,'said shell halves being constructed so that a substantial portion of the outer peripheries thereof are a greater distance respect to one another so as to form a substan-* tially closed magnetic path, one of said members being provided with a central abutment, said coil being. cross wound around said abutment, the radial diameter of the coil being several times its length, leads for connecting the coil to external circuits, one of said leads being on the outer periphery of the coil, the other thereof being connected to the coil'at a point which is near the abutment, said last named lead being grounded when the core is connected to an external circuit,

the other lead being connected to a point of comparatively high voltage whereby-the turns of the coil carrying the greater voltage are furthest away from the abutment.

O'I'IO WOHLFARTH. ALFRED AGRICOLA.

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