US3648205A

US3648205A - Device for varying the inductance of a helical inductor

Info

Publication number: US3648205A
Application number: US79858A
Authority: US
Inventors: Raymond S Bunch; Richard B Sanger
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1970-10-12
Filing date: 1970-10-12
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07

Abstract

An insulating member has an external helical thread that threads inside the turns of a helical inductor. A first part of the thread provides a first spacing between turns of the inductor, and a second part of the thread provides a second, greater spacing between turns of the inductor. A nonmagnetic, conductive element is positioned inside the insulating member in the vicinity of the second part of the thread. With the first part of the thread engaging the inductor, a high value of inductance is provided. When the insulating member is rotated so that the second part of the thread engages the inductor, the increased spacing of the inductor turns and the nonmagnetic, conductive element combine to decrease the inductance of the inductor by an amount dependent upon the amount that the second part of the thread and the element are introduced into the inductor.

Description

United States Patent Bunch et al.

[4 Mar. 7,1972

[54] DEVICE FOR VARYING THE INDUCTANCE OF A HELICAL INDUCTOR [72] Inventors: Raymond S. Bunch; Richard B. Sanger, both of Lynchburg, Va.

I 73] Assignee: General Electric Company [22] Filed: Oct. 12, 1970 [21] Appl. No.: 79,858

[52] 1.1.5. Cl. ..336/20, 336/75 [51] Int. Cl ..l-l01f2l/02 [58] Field of Search ..336/20, 75, 77, 136

[56] References Cited UNITED STATES PATENTS 2,458,071 1/1949 Gordon ..336/20 2,438,359 3/1948 Clapp ..336/75 3,517,363 6/1970 Curtis et a1. ...336/136 2,251,631 8/1941 Mayer ..336/75 Primary Examiner-Thomas J. Kozma Attorney-James J. Williams, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman I [5 7] ABSTRACT An insulating member has an external helical thread that threads inside the turns of a helical inductor. A first part of the thread provides a first spacing between turns of the inductor, and a second part of the thread provides a second, greater spacing between turns of the inductor. A nonmagnetic, conductive element is positioned inside the insulating member in the vicinity of the second part of the thread. With the first part of the thread engaging the inductor, a high value of inductance is provided. When the insulating member is rotated so that the second part of the thread engages the inductor, the increased spacing of the inductor turns and the nonmagnetic, conductive element combine to decrease the inductance of the inductor by an amount dependent upon the amount that the second part of the thread and the element are introduced into the inductor.

1 Claim, 4 Drawing Figures PATENTEDMAR 7 I972 RAYMOND 3. Emma, RICHARD 8. SANGER,

BY fgfr zmmsm DEVICE FOR VARYING TI'IE INDUCTANCE OF A HELICAL INDUCTOR BACKGROUND OF THE INVENTION Our invention relates to a device for varying the inductance of a helical inductor,'and particularly to such a device that is relatively simple in construction and efficient in operation.

In many applications, particularly in radio transmitters, it is frequently desirable to vary an inductor rather than a capacitor in order to change circuit parameters or to change the resonant frequency of a tuned circuit. The use of a variable inductor may often be desirable in cases where the fixed, unavoidable capacities are too great to permit addition of tuning capacity. In the case of some transmitters, the vacuum tube grid-cathode capacity was so large that any added capacity would have required an inductance that was to small to be practical. In such applications, the inductor to be varied may utilize solid copper wire wound in a self-supporting helix that has the desired inductance, power, voltage, and mechanical characteristics. Magnetic tuning slugs of powdered iron or ferrite materials have been provided to vary the inductance of such inductors. However, we have found that such slugs introduce serious radiofrequency power losses, especially at the very high radio frequencies, such as the 150 to 175 megaI-Iertz band of interest to us.

Accordingly, a general object of our invention is to provide a new and improved device for varying the inductance of a helical inductor.

Another object of our invention is to provide an improved inductor tuning device that is supported by the inductor without the need for additional mechanical supporting structure.

Another object of our invention is to provide an improved inductor tuning device that is capable of changing the magnitude of inductance over a fairly wide range.

Another object of our invention is to provide a relatively simple but effective device for varying the inductance of a helical inductor.

SUMMARY OF THE INVENTION Briefly, these and other objects are achieved in accordance with our invention by an insulated cylindrical device that has a helical thread which fits inside the turns of a helical inductor. Part of the thread turns are spaced relatively close together, and part of the thread turns are spaced relatively far apart. When the inductor turns are engaged by the close thread turns, the inductor turns are fairly close together and the inductor provides a relatively high inductance. When the inductor turns are engaged by the far apart thread turns, the inductor turns are fairly far apart and the inductor provides a relatively low inductance. A nonmagnetic, conductive element, preferably in the form of a copper sleeve, may be concentrically positioned along the longitudinal axis of the cylindrical device in the vicinity of the far apart thread turns so as to reduce the inductance of the inductor even more. Thus, we provide a device having a relatively simple but effective structure for varying an inductor.

BRIEF DESCRIPTION OF THE DRAWING The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the claims. The structure and operation of our invention, together with further objects and advantages, may be better understood from the following description given in connection with the accompanying drawing, in which:

FIG. I shows an end view of a device in accordance with our invention for varying the inductance of a helical inductor;

FIG. 2 shows a side view of the device of FIG. 1;

FIG. 3 shows a cross-sectional view taken along the line 3 3 ofFIG. l; and

FIG. 4 shows a perspective view of a helical inductor provided with a device in accordance with our invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIGS. 1, 2 and 3, we have shown a preferred embodiment of a device 10 constructed in accordance with our invention for varying the inductance of a spiral or helical inductor. Our device 10 is generally cylindrical in shape, and is made of an insulating material such as a thermosetting plastic. The external portion of the device 10 is provided with a continuous helical thread 12 which has a constant diameter and which, for ease of manufacture, extends along the full length of the device 10. The thread 12 is generally divided into two parts 12a, 12c. The first part 12a extends from one end to the approximate midpoint, and has thread turns which are spaced relatively close together, and which are shaped so as to engage the internal surface of the wire forming the inductive helix which is to be tuned. At approximately the midpoint of the device 10, a transition 12b is provided between the first part 12a and thesecond part 12c The second part 12c extends from the transition 12b to the other end, and has thread turns which are spaced relatively far apart and which are also shaped to engage the internal surface of the wire forming the inductive helix. Thus, the pitch of the second part is greater than the pitch of the first part 12a. As is known, a nonmagnetic, conductive sleeve or cylinder 14, preferably made of copper or brass and silver plated to keep losses low, may be concentrically positioned inside the device to extend along the longitudinal axis between the midpoint and the other end. At this other end, a recess 16 and a slot 17 (or hole) may be provided for receiving a screwdriver blade (or tool) so that the device 10 can be rotated. We have found that present molding techniques for thermosetting plastics permit the sleeve 14 to be positioned in a mold and the entire device, along with thread 12 and the recess 16 and slot 17, formed in a single molding operation.

FIG. 4 shows a perspective view of a spiral or helical inductor 20 with a device 10 in accordance with our invention. The device 10 and the inductor 20 are constructed so that the device 10 can be threaded inside the inductor 20 with the first part 12a of the thread 12 normally engaging the turns of the inductor 20. In this connection, it will be seen that the shape of the threads 12 of the device 10 corresponds to the shape of the wire or material forming the turns of the inductor 20 so that a threading match or engagement can be provided. This ensures that the device 10 is self-supporting within the turns of the inductor 20. With the device 10 in this position, the inductor 20 has its maximum or largest value of inductance. If the inductance is to be varied, the device 10 is turned or rotated (in a clockwise direction as viewed in FIG. 4) so that the second part 120 of the thread 12 moves into the inductor 20. As this takes place, the greater spacing of the thread turns forming the second part 12c causes the turns of the inductor 20 to spread apart in an axial direction, and thus reduces the inductance of the inductor 20. In addition, the copper sleeve 14 adds an eddy current flow so that the inductance is further reduced. The reduction of inductance can be varied to a fine degree, since a relatively small change in inductance is provided by a half or full turn of the device 10. However, the relatively large number of turns of the thread 12 permit the inductance to be varied over a relatively wide range.

In one embodiment of our device 10 which was constructed, the device had a length of approximately 1 inch and an external diameter of approximately one-half inch. The device fitted into a wound inductor which had approximately three turns and which was connected with a capacitor to provide a resonant frequency of approximately megaHertz. When the device was threaded into the inductor so that the turns were spaced farther apart and the copper sleeve 14 was within the inductor, the inductance was reduced so that the resonant frequency was increased to approximately megaHertz.

It will thus be seen that our invention provides a new and improved device which, although relatively simple, is effective in varying the inductance of a helical inductor. While we have shown only one embodiment, persons skilled in the art will appreciate that modifications can be made. For example, the physical dimensions and pitch of the thread parts having respective fixed pitches, a thread of constantly varying pitch may be provided. Such a thread would provide a finer control of inductance variation. The thickness and length of the copper sleeve may be varied. A magnetic core may be positioned in the device in the vicinity of the relatively closespaced threads (either with or without the nonmagnetic conductive sleeve) to provide an added inductance variation. Therefore, while our invention has been described with reference to a particular embodiment, it is to be understood that modifications may be made without departing from the spirit of the invention or from the scope of the claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An improved device for va ying the inductance of a cylindrically shaped wound coil having a plurality of turns of selected diameter and length, said device comprising:

a. an elongated cylindrical member of insulating material;

b. said member having first external threads which extend from one end to an intermediate point thereof for engaging the internal part of turns of a wound coil;

.c. said member having second external threads which extend from said first threads and said intermediate point to the other end thereof form engaging the internal part of turns of a wound coil;

d. said first threads having a first selected pitch and said second threads having a second selected pitch that is greater than said first pitch and that is joined thereto by a smooth transition so as to space the turns of a coil by said second threads and thereby change the inductance of a wound inductor;

e. and a nonmagnetic, conductive element symmetrically positioned about the longitudinal axis of said cylindrical member and within said cylindrical member in the vicinity of said second threads, for providing an added effect on the coil inductance.

R UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,648,205 Dated March 7, 19 72 t Raymond S. Bunch, Richard B. Sanger It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 3 after "thread" insert can be varied.

Instead of a transition between two thread 4 Column 4, line 6 cancel "form" and insert for Signed and sealed this 2 +th day of April 1973.

(SEAL) Attest:

EDWARD M. FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM (10-59) USCOMM-DC scan-P69 U.5. GOVERNMENT PRINTING OFFICE: [9G9 D365334 Patent No. 3,648,205 Dated March 7, 19 72 I t r( Raymond S. Bunch, Richard B. Sanger It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 3 after "thread" insert can be varied.

Instead of a transition between two thread Column 4, line 6 cancel "form" and insert for Signed and sealed this 2mm day of April 1973.,

v (SEAL) Attest:

EDWARD M. FLETCHER, J'R. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IO50 (10-69) sco -pc 5 37e.p59

w u.s, GOVERNMENT PRINTING OFFICE: I969 o-sss-asa

Claims

1. An improved device for varying the inductance of a cylindrically shaped wound coil having a plurality of turns of selected diameter and length, said device comprising: a. an elongated cylindrical member of insulating material; b. said member having first external threads which extend from one end to an intermediate point thereof for engaging the internal part of turns of a wound coil; c. said member having second external threads which extend from said first threads and said intermediate point to the other end thereof form engaging the internal part of turns of a wound coil; d. said first threads having a first selected pitch and said second threads having a second selected pitch that is greater than said first pitch and that is joined thereto by a smooth transition so as to space the turns of a coil by said second threads and thereby change the inductance of a wound inductor; e. and a nonmagnetic, conductive element symmetrically positioned about the longitudinal axis of said cylindrical member and within said cylindrical member in the vicinity of said second threads, for providing an added effect on the coil inductance.