US1823327A - Shielding for radio-frequency amplifiers - Google Patents

Description

Sept. 15, 1931. w. A. M DONALD ET AL 1,823,327

SHIELDING FOR RADIO FREQUENCY AMPLIFIERS Filed Dec. 24, 1927 2 Sheets-Sheet l Fred. E. fo/lnsfan Sept. 15, 1931;

W. A. M DONALD ET AL SHIELDING FUR RADIO FREQUENCY AMPLIFIERS Filed Dec. 24, 1927 2 Sheets-Sheet 2 v INyENT OR5 Wly/l'amlllacflonald BY fled. Z: J 07717.52 on 2W, 7 r ATTORNEYS Patented Sept. .1931

UNITED STATES PATENT OFFICE WILLIAK A. MACDONALD, 01 LITTLE NECK, AND FRED E. JOHNSTON, OF FLUSHING, NEW YORK, ASSIGNORS TO HAZELTINE CORPORATION, JERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWARE SHIELDING FOR RADIO-FREQUENCY AMPLIFIERS Application filed December 24, 1927. Serial li o. 242,844.

This invention has to do with vacuum tube circuits and relates more especially to highfrequency amplifiers in radio receiving sets.

In order to utilize multistage tuned radiofrequency amplification to the best advantage it is trostatically segregate the several tuned stages from each other either by wide separation, which is disadvantageous, or by means of metal shielding.

Where shielding has been employed heretofore it has been the ractice .to enclose all the apparatus of each uned radio-frequency i stage within an individual metal compartthe exacting workmanship required. In order to screen" effectively closely adjacent high-frequency stages from each other and prevent all undesirable magnetic and electrostatic interaction between them, it is essential that all seams in the shielding be tightly closed and preferably soldered, that such openings as are necessary to provide access to the compartments be tightly closed, and that holes for the wiring to pass through shall be as small as possible. Shielding, such as that referred to, in the form of copper or aluminum compartments when properly constructed is entirely satisfactory but ex ensive.

he present trend is toward production in largevolume of radio receiving sets which are both highly eflicient and inexpensive, and the successful accomplishmentof such an undertaking is dependent upon the manufacturers ability to eliminate every non-essential item in so far as it is possible to do so without detriment to the essential qualities of the product. In line with this endeavor,

the object of the present invention is to provide metal shielding which is less expensive than that heretofore employed by equally, or at least, sufliciently, effective.

In accordance with the present invention necessary to magnetically and elecit has been found'that very elfective interstage shielding may be obtained by the provision of individual shields for the radiofrequency coupling transformers in the form of copper or aluminum tubes of slightly larger physical dimensions than the transformers. These shielding tubes encircle, or enclose the transformers except at the ends, thereby restricting their respective magnetic and electrostatic fields and effectively prevent interaction between the transformers.

By virtue of the excellent shielding properties of the tubularmetal shields, the coup ling transformers of a multistage radio receiver may be located closely adjacent each other and, if desired, with their magnetic axes parallel, without fear of interaction.

In "some cases, however, where the radiofrequ'ency amplification is extraordinarily high, experience has indicated that it is sometimes desirable to provide supplemental shielding of the compartment type, segregating the several radio-frequency stages of the receiver from each other. Such v supplemental shielding may be made of the less expensive metals, as for example, iron, or evenbrass or zinc-which metals have heretofore been found impracticable for this purpose because of their high specificresistances and the consequent introduction of excessive-losses in the coupling transformers. Such additional shielding has, however, little or no effect upon the electricalv characteristics of the coupling transformers where the tubular shields as herein described are also employed. This is primarily be cause of the fact that the major portion of the magnetic field of each stage is confined bythe tubular shield surrounding the'transformer, leaving only the magnetic field resulting from the wires connecting the transformers with their associated apparatus, and the electro-static fields of said apparatus. The magnitudes of these remaining fields are relatively very small; hence it hasbeen found unnecessary in many instances to exercise great care in the construction of such supplemental shielding. That is to say, the

soldered at the points of contact, and the seals do not have to be made especially tight. For the same reason other ex ensive operations may be partially or mm 1 eliminated.

In addition to the saving w ich may be realized on the cost of the shielding itself, the invention is adapted to bring about further manufacturing economies and mechanical and electrical advantages resulting from simplified construction. This applies particularly to the arrangement herein described for mounting the radio-frequency transformers and for bonding the shields to each other.

In the accompanying drawings,- Figs. 1 and 2am plan and side views,re-

, spectively, of a unit comprising three radiofrequency transformers together with their respective tubular shields;

Fig. 3 is a cross-sectional View taken along line 3-3 of Fig. 1

of a high-frequency vacuum tube amplifier of the tuned multistage type, and especially amplifiers for use-in radio receivers. Highfrequency amplifiers are, of course, used for other purposes than thatof radio reception,

and the invention is not to be construed as limited to radio receivers, although that now appears to be its most important application. 7

In Figs. 1,2 and 3 there is shown an assemblage of three radio- frequency coupling transformers 1, 2 and 3, respectively. These transformers are of the usual cylindrical form comprising a core of dielectric material upon which the primary and secondary coils are wound. Each of the transformers is partly enclosed within a metal shielding tube 4, 5 and 6, respectively. These tubes 7 may be of circular, square or polygonal cross-section and are usually made of copper or aluminum on account of the desirable v of the. transformers which they encircle and electrical qualities of these metalsparticularly their high conductivity. The shielding tubes are made with an inside diameter somewhat greater than the outside diameter their length is preferablyv such that they overlap, or extend beyond the transformer windings at both ends. In keeping with the object of the invention the metal tubes are preferably made as small as the controlling conditions will permit. Experience has indicated that the dimensions of the shielding tubes are not highly critical. If, however the clearance between the transformer .win ings and the inside surface of the surrounding tube is too smallthe distributed capacity of the coil will-be measurably increased and is apt to be excessive.

Some typical examples of transformer and shielding tube dimensions which have been found to be entirely satisfactory will now be given. In'the first example, the tubular transformer core'is 1 4 inches outside diameter and 2 ,4; inches long. The tubular shields for that particular transformer are case between the windings and the inside.

of the shielding tube is approximately inch. The .tubular shield in that case overlaps the ends of the windings about inchat each end. In another case, the tubular "transformer core is 2 4 inches outside diameter and 2 inches long. The tubular shield has an inside diameter of 3 1 inches and is 2% inches long. The primary winding in the latter case comprises 13 turns of No. 36 double silk covered copper wire and the secondary winding 70 turns of No. 28 enameled Wire. As in the previous case the radial clearance betweenthe Winding and the inside of the tubular shield is approximately -inch and the overlap at the ends is about inch in each instance. The foregoing dimensions are given purely as examples and are not to be regarded as in any way limiting.

In the structure of Figs. 1, 2 and 3, the three tubular shields 4, 5 and 6 are illustrated as being riveted together along their lines of contact, and may conveniently be I fastened in like manner to a supporting base. This form of construction is not only a manufacturing convenience, but results in mechanical and electrical advantages. For example, after the tubular shields have been fastened together mechanically the entire coil assembly, including the shields, is a structurally rigid unit which can easily be handled as such in the factory. F urthermore, the structure is sufficiently strong to support additional apparatus on the top thereof, if required, as when the structure is placed in a radio set with the axes of the coils'horizonta'l, as shown in the figures. As to an important electrical advantage, it

'will be evident that when the adjacent shields are secured together with good electrical connection, the electrical path from one end to the other through the shields as a whole'will be of extremely high conduc tivity since, as already pointed out, the tubular shields are preferatly constructed of copper. Thus when one or more of the windings of each coil is electrically connected directly to its respective shield, as is now customary in the art, the entire tubular shield structure will serve as a common return or ground lead of such low electrical resistance as to introduce ractically no undesirable interstage coup ings due to a high impedance ground.

For the purpose of supporting the transformers there is provided a strip 7 of suitable insulatin material. This strip is fastened to the's ielding tubes by means of a number of small brass cli )S 8 one end of each of which is welded, sol ered or riveted to one of the tubular shields. The clips 8 project through holes provided in the strip 7 and are bent down, as shown in Fig. 1, thereby securing the strips 7 in place. The

transformersl, 2 and 3 are in turn fastened to the strip of insulating material 7 7 by means of metal clips 9 similar to those indicated by reference numeral 8. The clips 9 are each riveted at one end to a transformer core and pass through openings provided in the supporting strip 7 and are bent over, as shown in Fig. 1. The clips 9 may be made to serve as soldering terminals for the transformer windings thereby effecting an additional economy. his arrangement for supporting the transformers and centering them within the tubular shields provides a very inexpensive form of construction and great y expedites the assembling operation.

Figs. 4 and 5 illustrate an' alternative form of shielding which is slightly cheaper'than that previously described, and frequently more advantageous both electrically and mechanically. By reference to Fig. 4 it will be seen that the tubular shields are made of two identical half-portions 10 and 11, respecf tively, each formed with three semi-circular sections. The two half-portions are preferably welded or soldered together along their: lines and surfaces of contact, which in thiscase comprises a flat portion or web 14,,

whereby there are obtained the same mechanical and electrical advantages previously described in connection with the structure of Figs. 1, 2 and 3. Furthermore, the use of this form of shield usually results in somewhat improved electrical characteristics because a permanent and highly conductive connection between the semi-circular sections may readily be obtained. The transformers may be attached to the shields of Figs. 4 and 5 in the same manner as illustrated in'Figs. 1, 2 and 3.

Where the radio-ire uency amplification is unusually high and it comes necessary to provide additional shielding, an arrangement such as that illustrated by Figs. 6 and 7 may be utilized to advantage. The latter figures illustrate a metallic housing 12 having three compartments in each of which is enclosed all the high-frequency apparatus of an amplifier stage. Within each of the compartments there is contained a tubular shield 13- encircling the radio-frequency coupling transformer. in the manner illustrated in Figs. 4 and 5. The Fig. 4 type of shield is frequently advantageous in connection with a radio-frequency amplifier such as that illustrated in Figs. 6 and 7 because the flat portion or web 14 serves to link the shields together and may be made of any length desired; thus resulting in the desired spacing between the individual coils. The top view of the radio-frequency amplifier of Fig. 6 illustrates the use of such spacing. The housing 12 is of the usual form of rectangular compartment shield except that it may be made of iron or some other type of inexpensive metal, and need not be con structed so prescisely as the ordinary copper shielding heretofore in use. Shielding of this type made of iron would not be at all satisfactory in the absence of the hi ductive inner shielding herein escribed. That is true even though care be exercised to insure all the seams being made tight. \Vithout the highly conductive shields, the losses which would ordinarily be introduced by iron shields would be altogether excessive; but when the highly conductive shields are used in combination with theslightly conductive iron shields, as illustrated in Figs. 6 and 7, it is found that the losses are negligible, while the screening or shielding efiect is all that could be desired.

It appears that the open-ended tubular metal shields are fully effective to prevent magnetic and electrostatic coupling between the transformers, tionis unusually high there is likely to be some undesirable interstage magnetic and electrostatic coupling between the leads and "connected components which extend beyond ,the tubular" shields. Apparently it is only coupling of the latter character, (i. e. between I ment shieldin is required to prev'gfont, and since thisis o comparatively small magnitude the iron shielding does not need t'obe very precise; nor is it of any practicalconsequence that the iron introduces some small hysteresis losses.

No details of construction of the housing 12 have been sho'wn'because such construc tion is within the ordinary skill of sheet metal workers.

It should be clear from Figs. 6 and 7 that the coupling transformers 1, 2 and 3, tuned by variable condensers 15, successive vacuum tubes arranged in cascade in the usual manner; or, alternatively, one of them may be utilized to couple the input circuit of a vacuum tube with an antenna circuit, all ofwhich may be readily understood without the necessity of resorting to further.

illustrations.

but where the amplificaleadsie etc.) that the iron compart-i 16 and 17, couple hly coned fixed tubular shield functioning toreduceundesirable coupling between said coils.

radio-frequency ampl of radio-frequency amplification,

- am nd,

fixed-tubular metal shield open a both ends encircling said transformer and conductivity,

than the outside each encirc mg one of Although the present invention has been described as applied to a transformercoupled radio-frequency amplifier, it may be app led with equal effectiveness in we s different from that herein specifical y described. For example, the tubular metal shields may, under certain conditions, be usefully employed for shielding radio-frequenc inductance coils as distinguished from trans ormers, per se.

We claim: 1. In an amplifier, a plurality 'ofstages of radio-frequency amplification, each .sta c including-a coupling'coiharid an open-en ed fixed tubular metallic shield encircling said coil and effective to restrict the magnetic field thereof: l 1 e 4 2. In an amplifier, aEIuraIityof stages of including a coupling coil, and an open-endofmetal of high conductivity surrounding said coil, said shields magnetic a plurality" of; stages each stage transformer, and a 3. In an amplifier,

ing a coupling extending a substahtial distance beyon'dthe windin s of 7 said transformer at both ends, said shields functioning to "reduce undesirable magnetic coupling between said coils. "-1 In anamplifier, a plurality of high-freuency coupling transformers of the conventional. cylindrical type, each of said transformers being. provided with an 0 enended fixed tubular metal shield of igh said shields encircling their respectively associated transformers and be ing of substantiall larger inside diameter diameter of their associated transformers, said shields substantiall overlapping the windings of their assoc ated transformers at both ends.

'5. In an amplifier, a lurality of stages of radio-frequency amp ification, each of said stages including a radio-frequency coupling transformer and other a paratus, an open-ended tubular metallic shield of high conductivit for .each'of said transformers,

eachpfsand-tubular shields encircling its assoclated transformer and extending at former, and supplemental shieldin of which is enclosed the high-frequency apparatus of one of said stages.

6. In a radio-frequencfy-amplifier, a plurality of coupling trans ormers and a inrality of o en-ended tubular metal shie ds said transfoers and functioning to restrict. the ma thereof so as to reduce undesirah e couplin' between said transformers,

stages inclu "ng catiom, each stage I shields.

pling means including a field h is fhi h a t h ving ds gs 1e 0 g can no ivitfaf 0 non 1d coils, and guppla said shields be.

parallel side-by-side role together-along their lines v shields open at both'ends, each shield encircling one of said transformers and. ex-

. tendin at both ends a substantial distance beyon the transformer windings, whereby undesirable magnetic cou ling between said transformers is considerably reduced.

.8. In an amplifier, a lurality of stages of radio-frequenc amp ification, each of radio=frequency coupling means including a coil, a metallic shield of high conductivity 1 surrounding each of said coils andextending beyond each end thereof, said shields being mechanically and electrically connected along their adjacent surfaces, and supplemental shielding of low conductivity comprising a plurality,

of compartments in each 0 which one of said stages is enclosed.

9. In an amplifier, a lurality of stages of radio-frequency amp cation, each of saidstages comprising a vacuum tube and radio-frequency coupling means including a coil, said coil bein surrounded by an openended highly con uctive metallic shield ex tending at each 'en beyond said coil, and supplemental shiel ng of low conductivity comprising a plural ty of compartments in each of which one of said stages is enclosed.

10. In an amplifier, 'a plurality of stages of radio-frequenc amplification, each of said stages inclu ing radiofrequenply coupling means including a coil, said co' being surrounded by .an o n-ended metallic shield extending at eac end beyond said coil, and supplemental shielding means en- T closing a p 11. In an amplifier, a plurality of stages of radio-frequency amp ification, each of said stages comprising radio-frequency coupling means mcludi a 1 coil, a metallic shield of high conductivity for each of said coils, each of said shields having 0 n ends and enclosing its associated coil an extending at each end beyond said coil, and supplemental shieldi means of low conductivity relative to sai first-named shields comprising a pluralitg of compartments in each of which one 0 said stages is enclosed, whereby maintained in proportion to the ratio 0 tendency towards undesired couplin 12. In an amplifier, a lurality 0 stages of radio-frequency amp cation, each of said stages comprising radio-frequency coucoil, a metallic and enclosing each of sa the efiectiveness of shieldinfthis e urality of said first-mentioned mental shielding means of relatively low conductivity enclosing a plurality of said first-mentioned shields, whereby the undesired transfer of energy between stages is reduced in proportion to the ratio of unrestricted coupling.

13. In an amplifier, a plurality of stages of radio-frequency amplification, each of said stages including a radio-frequency coupling transformer, a plurality of shielding structureseach comprisinga plurality of semitubular' portions, a web member between said portions, said shielding structures being electrically and mechanically united to form 15 open-ended tubular enclosures around each of said radio-frequency coupling transformers. whereby the magnetic field of each of said transformers is restricted;

14. The combination, with a multistage radio frequency amplifier, each stage including a coupling coil, of ashielding structure for said coils comprising a plurality of tubular metallic shields of high conductivity, said shields being disposed in parallel contiguous relation and securely fastened together along I their lines of contact, whereby said shielding structure is mechanically strengthened and the radio-frequency conductivity between the extremities thereof is increased.

15. A radio-frequency shielding structure formed from two sheets of metal into complementary halves, each half comprising a row of semi-tubes, said halves being mechanically and electrically bonded along their lines of contact, eachtube being adapted to enclose one or more radio-frequency coils.

16. A shielding structure according to claim 15 wherein the halves are formed with a web member between the semi-tubes, whereby the required separation between adjacent tubes is obtained while maintaining high intershield conductivity. 7 In testimony whereof we afiix our signatures.

5 WILLIAM A. MACDONALD.

FRED E. J OHNSTON.

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