High frequency therapeutic radiator

Nov. 27, 1962 _F pOtzl 3,065,752

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AGEN’

Nov. 27, 1962

F. POTZL 3,065,752

HIGH FREQUENCY THERAPEUTIC RADIATOR

Filed Oct. 24, 1960

Sheets-Sheet 2

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INVENTOR

FRITZ POTZL

BY

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United States Patent Office

3,065,752

Patented Nov. 27, 1962

3,065,752

HIGH FREQUENCY THERAPEUTIC RADIATOR Fritz Potzl, Hamburg, Germany, assignor to North Amercan Philips Company, Inc., New York, N.Y., a corporation of Delaware

Filed Oct. 24, 1960, Ser. No. 64,644

Claims priority, application Germany Nov. 14, 1959 5 Claims. (CI. 128--422)

This invention relates to a high-frequencj' radiator utilized in the practice of diathermy, and other therapy, such, as, for example, thermotherapy, hyperthermy, etc., to irradiate small surfaces of the body, i.e. for localized heat treatment.

The invention proposes to excite a cavity resonator into oscillation by means of a microwave generator and to obtain therefrom a radiation for therapeutic purposes by interrupting the walls of the cavity by a slot of a particular shape, as will be described in greater detail hereinafter, which is transverse to the surface currents in the resonator. The alternating field transverse to the slots, in turn, causes the ambiance to oscillate, i.e. by means of the slots the desired energy is irradiated externally. By a suitable shape of the cavity resonator and of the irradiating slots, a satisfactory and uniform short-range field distribution may be obtained.

It is also possible, according to the invention, to cause the magnetic field which corresponds to the surface currents extending along the inner side of the resonator walls to penetrate into the ambiance. The alternating field would produce there an irradiating wave. Structurally, this could be realized by removing parts of the resonator wall with the exception of a few strips, which extend in the direction of the initial surface currents.

The invention has. for its object to provide a high-frequency radiator with a cavity resonator in which throughout the frequency band concerned, for example from 2400 to 2450 mc./s., the total input energy is irradiated in accordance with the different absorption properties of the body parts, and which correspond, to different loads for the radiator.

The novel high-frequency radiator and its associated cavity resonator of the invention features, inter alia, a cavity resonator which is excited via a coupling element and is provided with a diaphragm at one terminal surface. The diaphragm is provided with sectional restrictions in the y-direction and in the x-direction, for the capacitative and inductive load, which form an H-shaped slot having a size such that at the operational frequency the diaphragm is in resonance. The position of the coupling element is also provided to be in resonance at the operational frequency for the short-circuit condition of the cavity resonator as is determined by the nature of the coupling.

The high-frequency radiator, of which the size may be smaller than the size corresponding to the wavelength employed will be capable of irradiating very effectively comparatively small body surfaces over a larger frequency range, if suitably proportioned.

The above mentioned and other features and objects of this invention will become apparent by reference to the following description taken in conjunction with the accompanying drawing, in which:

FIG. la is a perspective view of one embodiment of the novel high frequency radiator of the invention;

FIG. lb is a correlated perspective view of a co-ordinate system helpful in explaining the operation of the embodiment of FIG. 1;

FIG. 2 is a plan view of the diaphragm utilized in the novel high frequency radiator illustrated in FIG. 1;

2.

FIG. 3 is a schematic diagram of the equivalent circuit of the diaphragm illustrated in FIG. 2;

FIG. 4 is a sectional view of another embodiment of the novel high frequency radiator of the invention;

FIG. 5 is a plan view of the embodiment of the radiator illustrated in FIG. 4; and

FIGS. 6 to 9 are plan views of other diaphragms having modified-shaped apertures utilized in the high frequency radiator of the invention.

Referring to FIG. la, a cavity resonator.3 is connected to a suitable source of energy, not shown, such as a microwave generator or the like via an energy supply cable, not shown, and receptacle or plug 1, and is provided with a radiating element 2 which, together, act as a highfrequency radiator as will become apparent hereinafter. Structurally, the cavity resonator 3 serves as a housing or mounting .for the cable plug 1, the radiating element 2, shown for example as a diaphragm with an H-shaped slot, and in the embodiment illustrated in FIGS. 4 and 5 for the dielectric cover plate 4 of the diaphragm.

To provide adequate passage of energy from the resonator 3, which means a suitable transmission of energy therefrom, the resonance frequency of the total equipment is selected to be in the proximity of the operational. frequency and within the bandwidth of the radiator. An adequate wide band is obtained by a corresponding formation of the coupling and of the radiating elements. If the resonator 3 is to be of minimum size it is advisable to use a square cavity resonator, for example, a portion of a rectangular cavity resonator of the kind shown in FIG. la. At the same resonance frequency such a resonator is smaller than, for example, a cylindrical cavity resonator.

Among the field distributions (modes obtainable in a square space, the H₁₀i-mode (or with the interchange of the sides b and a; cf. FIG. la, the En₀-mode) yields the minimum geometric dimensions at the operational frequency. A resonator oscillating at this mode is characterized by an. electromagnetic field distribution having only a Ϋ component of the electric field, which is constant in the. y direction and which varies sinusoidally in the x and z directions over one-half period, being zero at the walls.

The excitation of oscillations in a cavity resonator by means of a coaxial cable connection may be carried out, as is known, with.the aid of a loop or of a pin. Whereas the additional capacities of a loop, are usually so low that the inductive coupling predominates, a pin is capable, in accordance with its. length and its thickness, of providing either a capacitive or inductive coupling at will. The equivalent electrical circuit is a series circuit, The longitudinal currents define an inductance. The electric field lines from the pin to the surrounding resonator walls, particularly to the opposite wall, define a capacity. In order to maintain the resonance of the cavity resonator, in the case of coupling by a pin, the walls are, in general to be displaced. With a predominating capacitive coupling, a constant resonance frequency requires a smaller space. In the case of inductive coupling, the walls are to.be displaced to the outer side parallel to the axis of the pin in order to maintain the conditions of the peripheral values for the field distribution.

For a wide band adaptation it is advantageous to construct the coupling element between the coaxial conductor and the cavity resonator so that at the resonance frequency of the cavity it is also in resonance, which means that the inductive share and the capacitive share of the coupling are the same and the coupling pin 5. constitutes a real resistance (radiation resistance). Thus the resonance frequency of the cavity resonator 3 is no longer shifted by a reactive load. The load of the cavity resonator, in the case of resonance, with the real loss resist

3,065,702 ance of the coupling series circuit increases the bandwidth of the total equipment. A further widening of the resonance curve is obtained in that with deviations of the frequency from the resonance point with the series circuit a reactive component opposite the cavity resonator resonance causes a circuit load. When oscillating in the Ηιοι-mode and when coupled with an orientated coupling pin (prolonged coaxial inner conductor) in the y-direction (FIGS, la and lb) the cavity resonator is to be regarded, with respect to the end of the cable, in the equivalent diagram by way of an impedance as a parallel circuit, which is illustrated in FIG. 3. Herein Lh designates the cavity resonator inductance, which is defined by the current distribution on the walls; Ch is the cavity resonator capacity determined by the electrical field in the resonator; R_H is the loss resistance of the resonator, determined by the current losses in the walls; L_K designates the coupling inductance determined by the current distribution along the coupling pin and C_K is the coupling capacity, i.e. the pin cavity relative to the resonator walls, R_K is the coupling loss resistance, which is given by the current losses in the pin and, as the case may be, by the dielectric losses in the envelope of the pin.

So far the cavity resonator is considered as a final load of a microwave generator, not shown, and no radiating elements are provided. If one of the two walls parallel to the x—y plane (FIG. lb) is completely lacking, the microwave energy is transferred from the coaxial supply cable to a unilaterally short-circuited, rectangular cavity resonator, from which it leaves partly at the open end in the form of radiation. The cavity resonator is traversed in this case by a travelling wave, which is excited by the coupling pin 5. The variation of the field lines may also be determined. Whereas the pulsatory alternating field remains locally stationary in the cavity resonator, the field distribution propagates through the cavity resonator with a given phase velocity.

In order to obtain, for the input energy at the transition between the coaxial conductor and the cavity resonator, a transfer over a wide band substantially free of reflection, the position of the resonator short-circuit is to be observed apart from the shape of the pin.

It is advisable to embed the pin 5 in a cylindrical block 6 of a dielectric which is poor in losses and which extends from the coupling side 3' to the opposite resonator wall 3 (FIG. 4). The position of the short-circuit wall 2' thus becomes less critical and for a wider frequency band the transition remains poor in reflection. The optimum distance d between the short-circuit wall 2' and the axis of the pin 5 depends, with a given frequency, upon the impedance of the coupling pin 5. If the latter is in resonance, a value between 0.26 and 0.32α (Meinke Taschenbuch, page 316) has been found empirically, wherein a designates the width of the cavity resonator (FIG. la). This value diminishes, when the pin 5 is enveloped by a dielectric cylinder 6. In order to hold the coupling in resonance, the coupling pin 5 is to be shortened, otherwise the inductive coupling would predominate. For a predominantly inductive coupling the coupling pin 5 is to be chosen larger.

The accurate or optimum position of the short-circuit wall 2' is most effectively determined by a single prior experiment or trial for a given connection between the coaxial conductor and the cavity resonator and for a given diameter of the coaxial conductor, given sectional area of the cavity resonator and given frequency, since the field stray at the transitional area can not readily be calculated in advance.

Since an open cavity resonator in a high-frequency radiator for diathermic or therapeutic purposes would, as a rule, irradiate too large a body surface, the end of the open cavity resonator is covered, in accordance with the invention, by a diaphragm 2 so that the penetrating radiation is concentrated adequately in the short-range field. Such a diaphragm may, in this case, be a conductive sur face, for example of sheet iron with a suitable interruption 10 (one or more apertures, slots or the like), of which the dimension in the direction of the travelling waves (z-direction) is small with respect to λθ/y. Various forms of radiating apertures are shown in FIGS. 6 to 9 and these comprise a central slot portion 11 and side portions 12—12. Whereas in the case of an open cavity resonator the conductor end is unloaded, with the exception of a radiation resistance, a diaphragm constitutes a finite, in general a complex impedance. A shortening of the electric field lines, preferably a restriction of the sectional area in the y-direction then corresponds to a capacitative load. When the conductor current part (wall currents) is increased with respect to the shifting currents flowing through the apertured surface, an inductive load is caused by the diaphragm, which corresponds preferably to a restriction in sectional area in the x-direction. As is the case with the coupling of the coaxial cable, the adequate energy radiation requires such a construction of the diaphragm that, at the operational frequency, the capacitative and inductive effects just compensate each other, so that the diaphragm itself is in resonance. The cavity resonator is then loaded by the diaphragm 2 with a real resistance (radiation resistance and minimum wall losses in the diaphragm sheet). In the case of reactive load by the diaphragm the geometric dimensions of the cavity may, if desired, be varied to maintain the resonance.

With an operational frequency of, for example, 2400 mc./s., the required small irradiated surface of, for example, 30 x 30 mm.² is obtained by a very heavy capacitive load in the center of the diaphragm (narrow slot 11 in the x-direction) and long current paths at the sides 12—12 of the diaphragm sheet that form inductances at the two ends of the slot (FIGS. 7 and 8).

FIGS. 6 to 9 show such H-diaphragms, which are equivalents of a parallel combination of a series and a resonant circuit; cf. FIGS. 2 and 3, R_H and R_K are defined in this case as radiation resistances by the emitted energy. As stated above arrangements, which may be represented, as impedances, by such an equivalent diagram, give approximately a real resistance with a corresponding proportioning of the separate circuits over a wider frequency range.

An H-slot irradiates a wave of which the electric field lines are polarized in the y-direction. The emitted energy attains its maximum value, when the slot itself is in resonance. In accordance with the Babinet principle the recess in the resonator wall produces the same electric field in the free space as an aerial of the shape of the slot, which would lie in the plane of the initial diaphragm plate when turned through 90° with respect to the position of the slot and when excited in a similar manner. Then the H-slot concerned may be considered, at the operational frequency, approximately as a capacity loaded Herz dipole.

As is shown in FIGS. 4 and 5, the high-frequency radiator is connected via a coaxial plug 1 of a coaxial cable to the diathermy apparatus comprising the microwave generator. The inner conductor 5' may be prolonged by a screw-threaded rod, which operates as a coupling pin 5. In order to obtain a wider band with this arrangement, this rod is screwed, as stated above, in a cylindrical block 6. The resonator is made of sheet iron. The resonant cavity is short-circuited in a sectional area on the side 2', whereas the other side is covered by a diaphragm 2 with a H-slot 10. For the electrical prolongation of the minimum distance between the diaphragm and the surface to be irradiated, a Plexiglas block 4 is applied to the diaphragm 2, -which block urges at the same time the diaphragm sheet against the flange surfaces 20 of the resonant cavity. The length of the coupling pin 5, measured away from the resonator wall, the distance between the axis of the coupling pin and the resonator short-circuit 2' and the dielectric envelope 6 of the coupling pin may be

3,065,752 slightly varied in accordance with the various values influencing the adaptation to a wide range, without detracting from the advantageous properties of the novel highfrequency radiator. The Plexiglas block 4 may be replaced, if desired, by a cover plate of Plexiglas of a thickness of a few millimeters. In order to concentrate the radiation beam in the vicinity field to a sectional area of, for example 30 x 30 mm.², a diaphragm having an Idshaped slot as shown in FIG. 6 is particularly advantageous, but under given conditions, for example different concentration requirements, the other shapes of diaphragms shown may be effectively employed.