U.S. patent number 3,662,139 [Application Number 05/016,496] was granted by the patent office on 1972-05-09 for cavity resonator having means for reducing leakage of r.f. energy at a covered access point.
This patent grant is currently assigned to Varian Associates. Invention is credited to Kenneth E. Love.
United States Patent |
3,662,139 |
Love |
May 9, 1972 |
CAVITY RESONATOR HAVING MEANS FOR REDUCING LEAKAGE OF R.F. ENERGY
AT A COVERED ACCESS POINT
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.
Inventors: |
Love; Kenneth E. (Belmont,
CA) |
Assignee: |
Varian Associates (Palo Alto,
CA)
|
Family
ID: |
21777426 |
Appl.
No.: |
05/016,496 |
Filed: |
March 4, 1970 |
Current U.S.
Class: |
219/741;
219/699 |
Current CPC
Class: |
H05B
6/78 (20130101); H05B 6/76 (20130101) |
Current International
Class: |
H05B
6/76 (20060101); H05b 009/06 () |
Field of
Search: |
;219/10.55 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Jaeger; Hugh D.
Claims
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.
Description
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 10
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 member carried 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-shaped
cavity resonator 2 excited with microwave energy in a certain
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 l 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 15 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 13 is 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 16 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 effect 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 one-eight 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 5 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 10 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 1 to 2 milliwatts per square 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.degree. 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 requirement 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.
* * * * *