US2622238A - Resonant tank circuit for diathermy apparatus or the like - Google Patents

Description

J. A. BOLTSON Dec. 16, 1952 RESONANT TANK CIRCUIT FOR DIATHERMY APPARATUS OR THE LIKE Filed April 7, 1949 INVENTOR.

JACOB A. BOLTSON Patented Dec. 16, 1952 RESONAN T TANK CIRCUIT'FORDIATHERM'Y APPARATUS'OR THE LIKE Jacob A. Boltson, Flushing, N. Y. HannahBoltson, administratrix of said Jacob A. Boltson, deceased, assignor to Hannah. Boltson, doing business as The Boltson Company, Flushing,

Application April 7, 1949, Serial No. 86,034

8 Claims, 1

Thepresent invention is related to the art concerning resonanttank circuits, and particularly those adapted for use in short-wave diathermy apparatus requiring high stability of output frequency.

At, the present. state of the art, it is Well known to. use high frequency short-Wave oscillations for physiotherapy purposes, to produce and/or surface .heating of the body or members thereof for therapeutic benefit. Diathcrmy apparatus for these purposes essentially comprises a short-wave oscillation generator in combinationfwith means for usefully applying the generated short wave power to the subject to be treated. During normal use of such apparatus extremely wide variations in the conditions of electrical loading are encountered. This produces great difliculty in maintaining substantially constant, frequency within the requirements imposed by the Federal Communications Commission. Further difficulty has been encountered in minimizing spurious frequencies, such as harmonics,,from the generated output.

According to the present invention these difficult'ies. have been substantially minimized and almost entirely overcome by the use of a novel form of resonant tank circuit which provides a large degree of stability despite the wide variation in loading of the oscillator and which is inherently self-shielding to minimize production and radiation of spurious frequencies.

In its preferred form, the tank circuit of the present invention comprises a flat conductor ar ranged generally in the form of a spiral and completely enclosed in a metallic enclosure which provides a capacitive effect combining with the inductance of the conductor to form the resonant circuit.

The present invention thereby provides. an extremely simple and highly useful, self-shielded,

resonant circuit element, especially adapted for use in the frequency range between 5 and 1G0 meg-acyclesper second. In this frequency rang-3,.

conventional coil and conductor elements for producing resonance present difficulties because of the factors of stray and distributed capacitance which become troublesome as the Wave lengthbecomes commensurate with the physical dimensionsof the circuit elements. On the other hand, sections of conventional transmission lines cannot be utilized because, at the frequencies ini volved', their physical length becomes excessive; for example, at 30 megacycles a quarter-wave line section becomes 2 meters long, which is too large for practical use.

. By the present invention, a compact, useful internal,

Ill)

tank. circuit is provided of convenient size and with good efficiency-whichovercomes these disadvantages and usefully employs the natural capacitance of the device.

The present invention provides a completely enclosed condenser formed of a conductive plate suspended within and spaced closely to a conductive housing, the plate and housing forming the two condenser elements. the path of the current flow in the device is lengthenedto form the necessary inductive effect. This. is done preferablyby making the internal condenser element or plate in the form of a flat spiral, as by cutting a generally spiral slot therein to create a coil providing sufficient inductsince, the inductance being a direct functionof the length of conductive path over which the current is forced to flow.

Other objects and advantages of the present invention. willbecome apparent from. the follow ing detailed description, taken in conjunction with the appended drawing in which:

Figure 1 shows an exploded perspective crosssectional view of a preferred form of tank circuit according to the invention, and

Figure 2 is a plan view of coil ll of the tank circuit of Figure 1, the sectionv of Figure 1 being taken along line I-| of Figure 2.

Referring'to the drawing, the present tank circuit comprises a fiat generally spiral conductive coil ll. spiral coil l I may be formed of concentric circular arcuate portions [2, l3, Hi, each formed as a nearly complete circle. these arcuate portions are joined by radially extending connecting portions IE, IS. Thus, at the center the coil II has a substantially circular disc portion I! joined to the innermost arcuate portion I2 by the radial converting portion l8. Arcuate portion I2 is connected to the succeeding arcuate portion l3 by the radial connecting portion 15. Similarly, arcuate portion I3 is con-- nected to next outermost arcuate portion M by the radially extending connecting portion IS. The coil 1 l is preferably formed of relatively wide strips of conductive material arranged in the same plane and only slightly spaced from one another. In one example which has been constructed, the width of each of the arcuate portions l2, l3, M was one inch, while the gap l9 spacing-the consecutive arcuate portions was 2; inch. Gap 20 between center disc ll and the innermost arcuate portion 12 may have the same width as the remainder of the gap l9, or maybe substantially larger, such as of the order of of an inch in the example given.

To provide resonance.

For convenience of fabrication, this,

Consecutive ones of Coil I I is mounted symmetrically between two parallel circular plates 22 and 23 which are relatively closely spaced to coil II (in the embodiment indicated above this spacing was {a inch). Connecting the two plates 22, 23 is a cylindrical element 24 which is joined at either end to plates 22 and 23 to form a completely enclosed housing of pillbox configuration containing and. surrounding the coil I I. It will thus be seen that coil II presents a larger area in close juxtaposition to each of these plates 22 and 23 and therefore has a larger inherent capacitance. It is this capacitance in combination with the inherent inductance of the coil which provides the resonance of the tank circuit. Coil II is supported within the enclosure 22, 23, 24 by a plurality of insulating studs 26.

The inner disc portion I1 is conductively connected to the lower plate 23 by a preferably rigid conductor 21 which passes through an aperture 28 at the center of plate 23, and is then curved back to form a loop 29 conductively connected to plate 23 as to 3D. This conductor 21, therefore, acts both as a further support for the coil II, and as output connection. The outermost arcuate portion or turn I4 of coil I I is provided with an upward extension 32 which passes through a suitable aperture 33 in upper plate 22 to form a terminal 34.

In customary use the terminal 34 serves as the high potential terminal of the tank circuit, and the housing 22, 23, 24 serves as the low potential or ground terminal. Output is derived from the tank circuit by inductive coupling, by means of coupling loop 35 inductively coupled to the loop 29. It will be appreciated that this coupling is extremely low, since the coupling occurs adjacent the low potential point 30 of the tank circuit. Especially when used in diathermy equipment, this loose coupling permits the attaining of proper frequency stability since the reaction of the highly variable load upon the frequency of oscillation is thereby minimized.

The use of the extremely large capacitative effect between the coil I I and the top and bottom plates 22, 23 is highly important in the present invention, since this capacitance forms a very low impedance for harmonics or higher spurious frequency, whereby such harmonics or spurious frequencies are greatly minimized. In addition, the self-enclosing and self-shielding nature of the device and the low potential output coupling greatly minimize any radiation of such harmonics or spurious frequencies. Also, the design of the device is such as to produce extremely low thermal drift, because of the large heat-radiating area made available. An embodiment of this tank circuit provided a drift in frequency of only 12 kilocycles at an operating frequency of 27.12 megacycles, after one hour of operation under the worst conditions. therefore combines the desirable features of nonradiation of harmonics and high stability which are especially important in short-wave diathermy apparatus.

While the coil II has been illustrated as formed of circular arcs and having circular arcuate gaps, it will be understood that this form need not be strictly adhered to, but is preferred solely for ease of fabrication when the coil II is machined from a single sheet of conductive material. However, coil II need not be restricted in its manufacture of this method of fabrication. Where other methods of fabrications are used, the gap I9 may be made as a spiral gap and the This tank circuit conductive portion of the coil II may similarly be of spiral configuration without departing from the essential spirit of the invention.

In designing the coil II, it is found that in general the dimensions of the coil (that is, its diameter, number of turns, width of turns, and spacing of turns) may be empirically determined to provide a close approximation to the desired resonant frequency; thereafter, a fine adjustment to attain exactly the desired frequency of resonance is provided by loop 29. It will be appreciated that the inductance of loop 29 is added to that of coil I I, and is thereby reflected into and affects the resonance frequency of the tank circuit. By making loop 29 larger, for example, its inductance is increased and correspondingly increases the inductance of the tank circuit, reducing its resonant frequency. Conversely a decrease in the area of loop 29 serves to increase the resonant frequency of the tank circuit. In this way a minor adjustment can be made of the resonant frequency of the tank circuit after fabrication of the coil II.

In the form of the invention described above, the dielectric material between the coil II and the housing 22, 23, 24 is air. Where desired, other forms of dielectric may be used. Thus, where voltage arc-over may be an important factor, the housing may be evacuated by using insulating vacuum seals at the aperture 33, 28. Where increased capacitance may be desired, mica or other high dielectric constant material may be placed between coil I I and plate 22 and/or plate 23, or else the entire housing may be filled with a desirable dielectric material.

While the tank circuit described above is particularly suitable and adapted for use with short wave diathermy apparatus, especially in the 27.12 megacycle frequency range, it will be apparent that its utility is not restricted merely to such apparatus, but it is useful wherever such tank circuits are desired. Its relatively massive con struction, containing as it does a large area and amount of conductive material, renders it especially adaptable for uses in the high frequency inductive heating field, where relatively large amounts of power are handled.

In addition to serving as a tank circuit, it is also highly useful as a radio frequency choke coil of the resonant type. When so used, lead 29 would not be connected to plate 23 by loop 29 as shown, but would be connected directly to any desired circuit as a second terminal.

The present apparatus is also useful as a direct-current shorting and high frequency isolating element. For this purpose, for example, terminal 34 may be connected to upper plate 22 and the input terminal then provided by lead 21 (loop 29 being, of course omitted). Alternatively, terminal 34 may be the input terminal, and lead 21 may be directly connected to plate 23, without loop 29 being formed therein. Thus either terminal 34 or 21 may be connected to the housing 22, 23, 24, or either or both may be isolated from the housing and connected to exter nal circuit elements.

In place of the coupling loop 29, conductor 21 may be led straight through aperture 28, and coupled directly to an external circuit. Alternatively, two conductors may be led from coil II through aperture 29, one being conductor 2'! (not here coupled to plate 23) and th other being a similar conductor connected to coil II at some point intermediate center disc I! and outer terminal 34, according to the desired degree of coupling to the external circuit.

In some instances, it is desirable to provide two tank circuits, as in a tuned-grid oscillator, or in a system whereby two tank circuits are paralleled, only ne being coupled to the load whereby the other stabilizes the frequency being generated. Such a double tank circuit is readily provided in the present invention merely by stacking two of the above described circuits one upon the other. In effect, the housing then becomes a single container divided into two parts by a.-

septum extending thereacross, with a spiral element symmetrically arranged in ach part, and the dual circuit can be easily constructed in this manner.

The present invention therefore provides a simple but efficient resonant circuit usefully employing deliberately augmented stray capacitance to resonant with an inductance element, whereby the resultant structure has convenient size in th megacycle region, accompanied by selfshielding, reduction of harmonics, and high frequency stability.

It will be understood that the details of the device described above are susceptible of variation in its specific features, and it is merely an illustrative embodiment of the invention, which is not limited thereto but is as defined in the appended claims.

I claim as my invention:

1. A resonant circuit adapted for use in diathermy apparatus or the like, comprising a generally spiral plane coil, each of whose turns has a radial width of a larger order of magnitude than the radial separation between successive turns, and means providing substantial capacitance to resonate with said coil, comprising a pair of conductive flat plate walls disposed respectively on either side of said coil in parallel adjacent relation thereto, and conductive means connecting said walls and enclosing said coil whereby said circuit is rendered non-radiating and harmonic resonances are suppressed.

2. Apparatus as in claim 1 wherein said flat plate walls are closer to said plane coil than th radial width of said coil turns.

3. Apparatus as in claim 1, wherein one of said walls is formed with an aperture adjacent to the outermost end of said coil, said coil having a terminal extension connected to said outermost end and passing insulatedly through said aperture to the exterior of said housing.

4. Apparatus as in claim 1, wherein one of said walls is formed with an aperture centrally thereof, and opposite the center of said coil, said coil having a terminal conductor connected to its center and passing insulatedly through said aperture.

5. Apparatus as in claim 4, wherein said terminal conductor is bent in the form of a magnetic loop and has its exterior end connected to said one wall to form a coupling for said circuit.

6. A resonant circuit for diathermy equipment or the like, comprising a generally spiral plane coil formed of a plurality of concentric arcuate portions, each only slightly less than a complete circle, each of said arcuate portions having a radial dimension of an order of magni tude larger than the spacing between successive arcuate portions, each arcuate portion having one end connected with one end of its immediately surrounding arcuate portion by a radial connecting portion to form a generally spiral coil having circular arcuate portions, means providing substantial capacitance for resonating with said coil, comprising a flat conductive plate parallel to said coil and spaced therefrom by a distance less than said arcuate portion radial dimension, and a pair of terminals connected respectively to the outermost end and to the innermost end of said generally spiral coil.

7. A resonant circuit comprising a generally spiral plane coil, means providing substantial capacitance for resonating with said coil comprising a pair of flat conductive plates disposed adjacent to and parallel to said coil on either side thereof and a cylindrical conductive wall joining said conductive plates and surrounding said coil, whereby said wall and plates form a conductive housing surrounding said coil, one of said plates having an aperture substantially centrally thereof and one of said plates having an aperture adjacent the outer edge thereof, and a pair of coil terminals connected respectively adjacent the inner and outer ends of said coil and extending insulatedly through respective one of said apertures for connecting said resonant circuit to other circuit elements.

6. A resonant circuit comprising a generally spiral plane coil and means providing substantial capacitance to said coil to resonate therewith, comprising a flat conductive plate disposed parallel and adjacent to said coil to form a lumped capacitance therewith, said plate having an aperture centrally thereof, and an output coupling comprising a conductor connected at one end to the inner end of said spiral coil and passing insulatedly through said central aperture.

JACOB A. BOLTSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,837,678 Ryder Dec. 22, 1931 2,027,861 Fyler Jan. 14, 1936 2,163,775 Conklin June 27, 1939 2,367,576 Harvey et a1 Jan. 16, 1945 2,421,137 Wheeler May 27, 1947 FOREIGN PATENTS Number Country Date 223,706 Great Britain Oct. 30, 1924 297,850 Germany May 26, 1917

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