US3662139A

US3662139A - Cavity resonator having means for reducing leakage of r.f. energy at a covered access point

Info

Publication number: US3662139A
Application number: US16496A
Authority: US
Inventors: Kenneth E Love
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1970-03-04
Filing date: 1970-03-04
Publication date: 1972-05-09
Anticipated expiration: 1989-05-09

Abstract

A conveyorized multimode cavity resonator is disclosed as utilized for applying microwave energy to products to be treated. The cavity includes an access port in one of the walls thereof such access port being covered by a removable cover. A conductive vane is carried from the cover and extends into the cavity adjacent the marginal lip of the access opening for reducing leakage of microwave energy from the cavity through the space between the cover and the wall of the cavity when the cover is seated in place over the access opening.

Description

United States Patent 1 1 ,662,139 Love 1451 May 9, 1972 [541 CAVITY RESONATOR HAVING MEANS 3,182,164 5/1955 lronfield ..219/10.s5 FOR REDUCING LEAKAGE OF 3,249,731 5/1966 1011115611.... ..219 10.55 3,351,730 ll/l967 Pahlman... ..219/1o.55

ENERGY AT A COVERED ACCESS 2,958,754 11/1960 11111111 ..219/1o.55

POINT Primary Examiner.l. V. Truhe Assistant ExaminerHugh D. Jaeger Attorney-Stanley 2. Cole and Leon F. Herbert [57] ABSTRACT A conveyorized multimode cavity resonator is disclosed as utilized for applying microwave energy to products to be treated. The cavity includes an access port in one of the walls thereof such access port being covered by a removable cover. A conductive vane is carried from the cover and extends into the cavity adjacent the marginal lip of the access opening for reducing leakage of microwave energy from the cavity through the space between the cover and the wall of the cavity when the cover is seated in place over the access opening.

8 Claims, 4 Drawing Figures 1 CAVITY RESONATOR HAVING MEANS FOR REDUCING LEAKAGE OF R.F. ENERGY AT A COVERED ACCESS POINT DESCRIPTION OF THE PRIOR ART Heretofore, multimode cavity resonators have employed covered access ports. The marginal lip of the access port was provided with a three section quarter wave choke cooperating with an overlaying portion of the access cover to reduce the leakage of radio frequency from'the cavity through the space between the non-electrically contacting cover and the underlying marginal lip of the access port. It has been found that when the access port is of relatively large dimensions and when the cavity is operated at relatively high power levels, such as on the order of several kilowatts, that excessive leakage of microwave energy is obtained generally along the long sides of the rectangular access cover. It is desirable to reduce this leakage to an acceptable level which currently requires that the radio frequency leakage be less than milliwatts per square centimeter.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is'the provision of a cavity resonator having means for reducing RF energy around a covered access port.

One feature of the present invention is the provision of a conductive membercarried from the access port cover and projecting into the cavity adjacent the marginal lip of the access port for reducing leakage of microwave energy from the cavity through the space between the cover and the underlying lip portion of the access port.

Another feature of the present invention is the same as the preceding feature wherein the conductive member comprises a conductive vane extending generally parallel to the adjacent marginal lip of the access port.

Another feature of the present invention is the same as the preceding feature wherein the conductive vane extends into the cavity. from the cover by approximately an integral number of odd quarter wavelengths at the operating frequency of the cavity resonator.

Another feature of the present invention is the same as any one or more of the preceding features wherein the conductive member extending into the cavity is slanted downwardly in the region adjacent the lowest marginal edge of the access opening for draining condensate collected on the cover over the adjacent lip of the access opening.

Another feature of the present invention is the same as any,

one or more of the preceding features wherein the marginal lip of the access port is corrugated to form at least three radially successive choke sections concentrically disposed surrounding the lip of the access port to further reduce leakage of radio frequency energy around the covered access port.

Other features and advantages of the present invention will become apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a conveyorized microwave oven employing features of the present invention,

FIG. 2 is an enlarged sectional view of a portion of the structure of FIG. 1 taken along the line 2-2 in the direction of the arrows,

FIG. 3 is an enlarged sectional view of a portion of the structure of FIG. 1 taken along line 3-3 in the direction of the arrows, and

FIG. 4 is a plot of RF voltage versus distance radially outward from the lip of the access port showing the effect of the RF choke structure of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a conveyorized microwave oven 1 incorporating features of the present invention. The conveyorized microwave oven 1 includes an elongated box-shapedcavity resonator 2 excited with microwave energy in acertain operating range of frequencies, for example, in S-band via a pair of input waveguides 3 and 4 disposed at opposite corners of the cavity 2. In a typical example, at S- band, the cavity 2 has a length I of 4 feet, a width w of 2 feet, and a height h of 3 feet.

With these dimensions, a multitude of microwave modes are excited within the cavity .2 for treating materials with microwave energy, such materials being fed into the cavity 2 via a conveyor'belt 5 passing through aligned openings in opposite end walls of the cavity 2. End traps 6 and 7 are disposed at opposite ends of the cavity surrounding the conveyor belt 5 for preventing the escape of microwave energy outwardly in the cavity through the openings in the end wall for passage of the conveyor belt 5 therethrough. The conveyor belt,-after passing through the cavity 2, returns underneath the cavity. A conveyor drive wheel 8, driven from a motor, not shown, drives the conveyor belt over an idler wheel 9 disposed at the opposite end of the conveyor belt 5.

A relatively large rectangular access port 11 is disposed in one vertical sidewall 12 of the cavity 2. A rectangular metallic cover 13 covers the access port 11 to prevent escape of microwave energy through the access port 11. A rectangular viewport 14 is centrally disposed in the cover 11 and includes a glass pane 1'5 and a conductive screen inwardly disposed thereof to allow viewing of the interior of the oven 2 without escape of microwave energy through the viewport 14. In a typical example, the access cover 13 is rectangular having a length, as of 3.5 feet, and a width, as of 2 feet.

The cover 13is removably secured in place over the access port 11 by means of four clamping type toggle latches 16 disposed at the four corners of the cover 13. The cover 13 is removable from the port 11 by undoing latches l6 and swinging the cover upwardly and away from the resonator 2 on the pivotable supports thereof. The pivotable support structure for the cover 13 comprises 4 parallel arms 17 each of which is pivotably secured to the wall 12 of the cavity 2 at pivot hinges 18. Pivot hinges 18 are disposed at the four corners of a rectangle. Two parallel rods 19 are fixedly secured to the cover 13 via clamps 21 and the opposite ends of each of the rods 19 are pivotably secured to the respective ends of the two pairs of arms 17 such that as the cover 13 is swung up and away from the access port 11 the plane of the cover 13 always remains parallel to the plane of the side wall 12 of the cavity 2.

The relatively large access port 11 readily facilitates cleaning and maintenance of the interior surfaces of the cavity 2. In a typical example, the cover 13 and cavity 2 are made of 18-8 stainless steel. In use, the cavity 2 is excited with a relatively large amount of microwave power, as of 5 kilowatts average power at S-band, for treating the products passable therethrough on the conveyor 5.

Referring now to FIGS. 2-4, there is shown an embodiment of the present invention for reducing the escape of microwave energy from the cavity through the space between the access cover 13 and the underlying marginal edge of the access port 11, which underlies the cover 13 when the cover is seated in place over the access port 11. Wall 12 at the marginal edge of the access port 11 is corrugated at 23 to define with the overlying marginal lip of the cover 13 a succession of concentrically disposed radial quarter wavelength choke sections. The choke sections are provided for reducing the escape of microwave energy from the cavity through the space between the marginal edge of the access opening 11 and the non-electrically contacting cover 13. A relatively thin sheet of dielectric material 24, as of teflon 0.010 inches thick, is affixed to the cover 13 by means of a suitable adhesive and is disposed at the marginal edge of the cover 13 between the cover 13 and the underlying corrugated portion 23 of the sidewall 12 of the cavity 2. In a typical example, the wall 12 is made of 0.063 inch thick stainless steel, whereas the cover 13 is made of 0.050 inch thick stainless steel.

Each of the corrugations, of the corrugated portion 23 of the wall 12, has a radial dimension taking into efiect the dielectric loading produced by the insulative sheet 24 of a quarter of a wavelength at the operating frequency of the cavity 2. The depth of the corrugations d is on the order of oneeight of a wavelength and the resultant choke section comprises an inner section 25 of very low impedance radial transmission line concentrically surrounded by an intermediate quarterwave section 26 of relatively high impedance radial transmission line which in turn is surrounded by another section 27 of relatively low impedance radial transmission line to reflect a short circuit at the juncture of the inner choke section 25 with the lip 28 of the access port 11. The resultant RF voltage versus radial distance for the structure of FIG. 3 is shown in FIG. 4, such choke structure reflecting a short circuit to the cavity 2 at 28.

Although the three section radial choke structure 25-27 serves to substantially reduce the escape of microwave energy from the cavity 2 through the space between the cover 13 and the wall 12, excessive amounts of such microwave energy may escape, especially along the long sides of the rectangular cover 13 where the cover is not as perfectly seated against the corrugated portion 23 as it is along the short sides of the rectangular cover 13. More particularly, it is found that when the cavity 2 is operating at a power level on the order of kilowatts average, or more, that the radiation escaping around the cover 13, when in place, especially along the long sides thereof can be on the order of 20 milliwatts per square centimeter which is twice the maximum of milliwatts per square centimeter, as set forth by Federal regulations.

However, it has been found that the provision of

conductive vane members

31 and 32 conductively connected to the cover 13, as by spotwelding, and extending along the long sides of the cover 13 adjacent the marginal bottom and top lips 28 of the access port 11 substantially reduces the escape of microwave energy to on the order of l to 2 milliwatts persquare centimeter for an average power within the cavity of 5 kilowatts. The

conductive vane members

31 and 32 are typically made of 18-8 stainless steel having a thickness of 0.031 inches and extending substantially from one short side of the cover 13 to the opposite short side thereof. In a preferred embodiment, the

conductive vanes

31 and 32 are slanted downwardly at approximately a 45 angle and have a length from the root to the tip of approximately n number of quarter wavelengths at the operating frequency of the cavity (where n is an integer odd number and preferably 1) and are preferably spaced from the root of the vane to the marginal lip 28 by approximately a quarter of a wavelength to present a high impedance at the end of the

vanes

31 and 32 which project into the cavity 2 to provide essentially a short circuit at the marginal lip 28 for microwave energy within the cavity 2. By slanting the

vanes

31 and 32 downwardly, condensate tending to collect on the inside of the cover is drained from the cover over the lower lip portion of the access port, as shown in FIG. 3 to prevent an unwanted accumulation of condensate such as water vapor on the access cover or in the corrugated choke section 23.

The radial choke structure 23 has been shown as incorporated into the wall 12 of the cavity 2. This is not a require ment as the choke structure 23 may, alternatively, be incorporated into the overlaying portion of the cover 13.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a microwave apparatus, a cavity resonator means for excitation by microwave energy in a certain operating frequency range and having an access opening in a wall thereof, a removable cover for closing over the access opening such that there is an overlap between a portion of said cover and the lip of said access opening, insulating means between the overlapping portion of said cover and the lip of said access opening to prevent electrical contact between said cover and the lip of said access opening and to provide a transmission line microwave energy seal around the access opening, electrically conductive vane means carried from said cover and disposed adjacent the marginal lip of said access opening and projecting into said resonator when said cover is closed for perturbing the microwave fields within said cavity at the lip of said access opening to reduce leakage of microwave energy from said cavity through the space between said cover and said lip of said access opening when said cover is seated in place over said access opening.

2. The apparatus of claim 1 wherein said conductive vane means projects from said cover into said cavity resonator by approximately an integral number of odd quarter wavelengths in the operating frequency range of said cavity resonator.

3. The apparatus of claim 1 wherein said access opening is in a vertical wall of said cavity and said vane means is disposed adjacent the lowest marginal edge of said access opening and is slanted downwardly from said cover, when in place over said access opening, for draining condensate collected on said cover over the adjacent lip of said access opening.

4. The apparatus of claim 1 wherein said transmission line microwave energy seal includes a corrugated structure which surrounds the lip of the access opening.

5. The apparatus of claim 4 wherein said corrugated transmission line structure includes at least three radially successive corrugation sections concentrically disposed surrounding the lip of the access opening and each corrugation section being approximately a quarter wavelength long in the radial direction in the operating frequency range of the cavity.

6. The apparatus of claim 1 wherein the root portion of said vane means is disposed approximately a quarter wavelength, at the operating frequency of the cavity from the adjacent lip of the access opening.

7. The apparatus of claim 1 wherein said access cover is rectangular and said vane means is disposed along at least one of the long sides of said rectangular cover.

8. The apparatus of claim 1 wherein said transmission line microwave energy seal comprises quarter wavelength choke means surrounding the lip of said access opening, said vane means projects from said cover into said cavity resonator by approximately an integral number of odd quarter wavelengths in the operating frequency range of said cavity resonator, and said vane means extends along a direction substantially parallel to said lip of the access opening.