US2814298A - Impedance matching pad for microwave heating and method of use - Google Patents

Description

Nov. 26, 1957' GENTO I 2,814,298

H. F. AR IMPEDANCE MATCHING PAD FOR MICROWAVE HEAT NG AND METHOD OF USE Filed Nov. 18, 1953 2,814,298 Patented Nov. 26, 1957 EDAN CE NEATCHING PAD FOR MICROWAVE IEATING AND METHOD OF USE Henry F. Argento, Glencoe,

Manufacturing Company, tion of Delaware Application November 18, 1953, Serial No. 392,970 Claims. (Cl. 128-404) 11]., assignor to Raytheon Waltham, Mass., a corpora- In heating a body, such as the human body, with microwaves, a large proportion of the energy is reflected from the surface of the body and lost instead of being absorbed by the body.

Objects of the present invention are to avoid this loss and to cause substantially all of the energy impinging on the body to be absorbed by the body. Other objects are to provide for this purpose an impedance matching pad which is light and easy to handle, which will conform to the contour of the body and which is durable in use.

According to the present invention the aforesaid reflection is avoided by inserting in the path of the microwaves an impedance matching pad whose dielectric constant is the geometric mean of the dielectric constants of the body and the air or other medium through which the microwaves are transmitted to the body and whose thickness is approximately equal to an odd number of quarter wave-lengths of the microwaves in the pad, the pad being located a distance from the body which is a multiple of one-half of the wave-length, including a zero multiple, that is in contact with the body without any substantial spacing between the pad and the body. In this Way two reflections are produced, one at each face of the pad. The two reflections being substantially equal in amplitude and of opposite phase, they substantially cancel each other.

When the microwaves are transmitted through air, as they usually are, the aforesaid geometric mean is equal to the square root of the dielectric constant of the body. While the aforesaid pad may be spaced from the body one-half wave-length or other multiple of a half wavelength, it is preferably placed in contact with the body, in which case it is preferably in the form of a flexible path which conforms to the contour of the body. The preferred embodiment comprises a rubber pad containing approximately 50% to 80% titanium dioxide.

For the purpose of illustration, a typical embodiment is shown in the accompanying drawing, in which the figure is a diagrammatic view of an emitter E for irradiating a body B through the aforesaid pad P, the emitter comprising the usual antenna A, reflector R and coaxial input terminal T for supplying current to the antenna.

While the composition of the matching impedance pad may vary widely, depending on the conditions of use and the wave-length, for diathermic treatment of the human body with a wave-length of 12.2 cm., the pad is preferably made of butyl rubber loaded with titanium dioxide to increase the dielectric constant of the pad. Inasmuch as the wave-length in the pad is equal to the Wave-length in air divided by the square root of the dielectric constant of the pad, increase in dielectric constant permits a decrease in the thickness of the pad. The following are a few examples for a wave-length of 12.2

cm. in the region where the thickness of the pad approximate one centimeter:

Titanium Example Dioxide, Density Dielectric Thickness percent Constant in Cm. by wt.

The composition of the pad is preferably made accord- The composition is preferably compounded as follows. The butyl rubber is first milled for two or three minutes on a rubber mill with rolls preheated to about 60 C. After this plasticizing operation the heating of the rolls is discontinued and the titanium dioxide is added gradually while containing the milling from ten to twenty minutes depending on the amount of titanium dioxide. Then While the milling progresses, the zinc oxide, stearic acid, Captax, Tuads and sulfur are admixed successively. After milling the finished composition for one minute, the rolls are cooled with water and the mixture is stripped from the rolls. After letting the material stand for at least twenty-four hours, it is cut into discs of the desired size, the discs are stacked to the required thickness and inserted into a mold lubricated with magnesium stearate. After applying pressure of about eight hundred pounds per square inch, the resulting laminated pad is cured at about C. for an hour, after which the finished pad may be removed from the mold.

Uneven heating of a body may be accomplished by varying either the thickness or the dielectric constant of the pad throughout its area, maximum heating occurring where the thickness is equal to an odd number of quarter wave-lengths and the dielectric constant is equal to the aforesaid geometric mean and the amount of heating decreasing in proportion to departure from these optimum conditions. For example, if a restricted area, such as the eye of a patient, is to be protected while heating the surrounding area, the portion of the pad covering the surrounding area may have said optimum conditions and the thickness and/or geometric mean of the portion of the pad corresponding to the restricted area may depart more or less from said optimum conditions depending upon the degree of shielding desired throughout the restricted area. Conversely if the microwaves from the emitter are not uniform in intensity throughout the area of the pad, the intensity may be made uniform by varying the thickness and/ or geometric mean throughout the area of the pad. For example, in a small emitter the intensity is usually maximum at the center and gradually decreases toward the periphery; and to make the distribution uniform the pad may have said optimum conditions at the periphery and gradually depart therefrom toward the center. By making the distribution uniform maximum heating may be obtained throughout the area without danger of burning at any location within the area.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. A therapeutic heating system comprising a microwave energy generating means, means for directing the microwave energy to a body of living tissue, an impedance matching pad placed in a first medium wherein the dielectric constant of said pad is the geometric mean of the dielectric constants of said body and medium and whose thickness is approximately equal to an odd number of quarter wave-lengths of the microwaves in the pad for matching the impedance between said pad and said body and distributing said energy over an area of said body, whereby when the pad is placed in the path of the microwaves, at a distance from the body which is a multiple of one-half the wave-length, loss by reflection is minimized and said microwave energy is free to pass therethrough.

2. A therapeutic heating system comprising a microwave energy generating means, means for directing the microwave energy to a body of living tissue, an impedance matching pad placed in a first medium wherein the dielectric constant of said pad is the square-root of that of the body and whose thickness is approximately equal to an odd number of quarter wave-lengths of the microwaves in the pad for matching the impedance between said pad and said body and distributing said energy over an area of said body, whereby when the pad is placed in the path of the microwaves, at a distance from the body which is a multiple of one-half the wave-length, loss by reflection is minimized and said microwave energy is free to pass therethrough.

3. A therapeutic heating system comprising a microwave energy generating means, means for directing the microwave energy to a body of living tissue, a rubber matching pad placed in a first medium wherein the dielectric constant of said pad is the geometric mean of the dielectric constants of said body and medium and whose thickness is approximately equal to an odd number of quarter wave-lengths of the microwaves in the pad for matching the impedance between said pad and said body and distributing said energy over an area of said body, said pad containing approximately 50% to 80% titanium dioxide, whereby when the pad is placed in the path of the microwaves, at a distance from the body which is a multiple of one-half the wave-length, loss by reflection is minimized and said microwave energy is free to pass therethrough.

4. A therapeutic heating system comprising a microwave energy generating means, means for directing the microwave energy to a body of living tissue, a rubber matching pad placed in a first medium wherein the dielectric constant of said pad is the square-root of that of the body and whose thickness is approximately equal to an odd number of quarter wave-lengths of the microwaves in the pad for matching the impedance between said pad and said body and distributing said energy over an area of said body, said pad containing approximately 50% to titanium dioxide, whereby when the pad is placed in the path of the microwaves, at a distance from the body which is a multiple of one-half the wavelength, loss by reflection is minimized and said microwave energy is free to pass therethrough.

5. A therapeutic heating system comprising a microwave energy generating means, means for directing the microwave energy to a body of living tissue, an impedance matching pad placed in a first medium wherein the dielectric constant of said pad has a value which approximates the geometric mean of the dielectric constants of said body and medium throughout at least a part of the area of the pad and whose thickness has a value which approximates an odd number of quarter wave-lengths of the microwaves in the pad throughout at least a part of the area of the pad for matching the impedance between said pad and said body and distributing said energy over an area of said body, at least one of said values varying throughout the area of the pad to control the distribution of the microwaves which penetrate the body and said microwave energy is free to passtherethrough.

References Cited in the file of this patent UNITED STATES PATENTS 1,990,649 Ilberg Feb. 12, 1935 2,161,292 Hahnemann June 6, 1939 2,220,269 Patzold et a1. Nov. 5, 1940 2,242,886 Hirschland et al May 20, 1941 2,304,540 Cassen Dec. 8, 1942 2,407,690 Southworth Sept. 17, 1.946

2,599,944 Salisbury June 10, 1952 FOREIGN PATENTS 155,953 Australia Aug. 21, 1952 445,495 Great Britain Apr. 14, 1936 579,974 Great Britain Aug. 22, 1946 894,421 Germany Oct. 26, 1953 OTHER REFERENCES Proceedings of the I. R. E., vol. 32, No. 4, April 1944, pp. 233-241 (Transmission-Line Analogies of Plane Electromagnetic-Wave Reflections by Arthur Bronwell).

Application of Microwaves in Physical Medicine, a paper read at the Convention of the Institute of Radio Engineers at New York, N. Y., March 4, 1952, 9 pp. (Copy in Division 55.)

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