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Och 29, 1946-
\ 2,410,109
- Filed Feb. 15.4 1,943
Patented Oct. 29, 1946
VARIABLE '(31;‘:'1I(')1,‘:);ES6NATOR .1
Bell Telephone Laboratories, Incorporated, New
York, N. Y., a corporation of New “York
Application February 13, 1943,-~Serial No.’ 475,"?50
13 Claims.
(c1. 1.78_44)'
This invention relates to cavity type electrical
resonators sometimes termed space resonators
and particularly to such» resonators‘ with dimen
sions variable for the-purpose of changing the
resonant frequency.
tion- may be had from the following description
onator incorporating features of. the invention
A principal object of» the invention is to pro
‘i 1 ~
Fig. 1 is a general view of a typical cavity rese.
and the accompanying ?gures, of which: i
and showing a method of mounting an electron
vide'such a resonator adjustable over a relatively
wide range of frequency.
A more complete understanding of the inven
tube for the purposeof passing-an electron stream
through the resonator.
Another object is to provide such a wide range
Figs. 2 and 3 show, in section, a conventional
movable plunger type of cavity resonator with
the plungers in the positions for minimum and
tor optimum conditions for coupling between the
frequencies respectively.
high frequency ?eld of the resonator, and ‘an
Figs. 4 and 5 show; in section, a cavity reso
electron stream passing therethrough.
In ultra-high frequency devices electrical cir 15. nator typical'of the invention with the plu'ngers
in the positions for minimum and maximum fre
cuits oftthe cavity or space resonator type pre
quencies respectively.
sentadvantages on account of their low electri
an alter
cal losses and the fact thatrthey completely en
‘y g
close and thereby shield the high frequency ?eld, -' r native plunger shape.
thus preventing unwanted radiation.‘ A disade 20
ed by ‘the walls of’the rectangular tubular mem:
vantage of some forms of cavity structure is‘that
her 6 and the plungers 2 and 3, ‘all of conduct
they are non-adjustable, and. being usable only
ing material. The plungers make sliding con
at a single frequency must be replaced when it
ta‘ct'wi'th themner surface‘ of‘ thetubularmeme
is desired to operate at a di-iierent frequency. A
small amount of adjustment is often provided 25 be!‘ ‘through the ?exible ?ngers i1 and are move
able toward and away‘ from" each other along the
such as‘ by varyingthe amount a metallic tun
‘member by any suitable means
ing member projects into theinterior space of
through the rods ‘4‘ and 5 connected to them.
the resonator or by changing the volume of the
They openings‘ 1 and18 are to permit the insertion
resonator by jileiiihgi a portion of; its boundary
of means‘ for passing an electron beam through
or‘ sh'éllf: Such adjustments are for tuning-‘pure 30 the
cavity space to interact with a high frequency
poses and provide relatively little change in the
cavity resonator that is simple of adjustment.
Another object is to retain in such a resona
electric ?eld‘ therein, and thereby effect a trahse
fer of high frequency energy between the electric
nantji" ency is sometimes had by movinga
?eld and the electron stream. In practice" the
sidewall of the resonator likeja piston in a cyl
openings may permit thev insertion of an elec
iride'r'." 'O'neofr more walls may be moved so that
tron tube through the space of the resonator,
the’. reson'ator‘dimension in the‘directionv-oi wall
‘as in the manner shown-in Fig‘. 1 where" I75
in ment ','is changed thereby changing _ the res
a three "gap electron gun generally
n frequency. ,
similar to the two gap electron gun‘ shownni'n
A "onsiderable range of; adjustment of the res
Figs. 1 and 5' of the" copending application No‘.
on‘a'iit' frequency is possible with the usual mov 40 386,794, ?led April 4, 1941,‘ by A, E. Anderson and
ablewall type-of resonator just{described;~ I-Iow-.
A.‘-L. Samuel." In this‘ copending application ‘an
ever, it'if's“ sometimes desirable tomake available
arrangement-of the electron gun’ with'a' cavity
a’ 'still' greater'lrange of adgiustnrientv such as may
is shown in Fig. 1 an'd‘thje' "circuit is
berequired -'in[,apparatus designed tooperate over
shown in Fig. 5. These showings are‘typicalionly
a'wide range of frequency and be adjustable con 45 as the present invention is applicable to diil‘erent
. tinuoiisly over that range.
types‘v of cavity resonators which may be‘ ‘a'sso‘;
QSuCh’a greater range of adjustment is obtained
ciated with different 1-types of electron tubes and
accordingto this invention by the use of mov
circuits.~ Furthermoreresonators accordingi'to
able 7w ls so shaped that?as they are moved in
the invention may‘ be used in circuits i-ndepend-é
vire'ction two dimensions of the resona 50 ently of electron tubes as‘ variable circuit ele
resonant" frequency. A‘ greater change in reso
fo" arechanged. in»; this manner the vratio be
tween'tne maximum and minimum volumes of
the cavity is made large and a corresponding large
natiobetweien- the “maximum and minimum fre
duencies is obtained.
ments. ‘ The member '6: of-Fig; 1 is split along
the line IS, in the‘ plane‘ of the axis of the" elec‘-'
tron tube and'the centers of the openings» 7 and
8 to permit separating the two split parts of
When the
.55 member 6 for insertion of the tube.
tral portions are apart to clear the electron stream
tube is in place the two parts of member 6 are
clamped together by a suitable means for main
While not essential it is desirable that two mov
able plungers be employed so that the cavity
ducting material and connect with electrodes in 5 space is always symmetrical with respect to the
position of the electron stream.
the tube. They effectively form part of the shell
The volume variation and corresponding reso
of the resonator, being clamped to member 6, and
taining good electrical contact along the line It.
The large discs I3 and I4 are of electrically con
nant frequency variation between the plunger po
close the space around the tube in the openings
1 and 8.
sitions of Figs. 2 and 3 may of course be in
The purpose of the clamping of the Various 10 creased-by simply moving the flat surface plungers
farther apart in Fig. 2. This expedient, how
shell members and of the ?exible ?ngers I1 on the
ever, is not the equivalent of increasing the range
movable members 2 and 3 is to maintain low
by shaping the plungers according to the inven
impedance high frequency paths throughout the
tion as illustrated in Figs. 4 and 5 (also in Figs.
1 and 6). In the case of simply moving the
plungers farther apart there is a practical limit
boundary of the resonator and so enclose the
space and the high frequency field within as
completely as possible during operation and
throughout adjustment of the movable members.
Some openings in the shell are unavoidable such
as those for injection of the electron stream, the
to the minimum frequency attainable because
after a certain amount of separation of the
plungers the frequency changes relatively slowly
introduction of leads for power supply or high 20 as the separation is increased. In the use of the
shaped plungers advantages accrue because the
maximum frequency (minimum cavity volume) is
frequency coupling and those due to mechanical
imperfections. The term substantially closed will
therefore be used in describing the cavity reso
nator to indicate that the shell is completely
closed except for such necessary openings.
determined to a great extent by the size of the
recess in each plunger which permits the use of
25 a relatively large total plunger’ area and corre
spondingly large transverse dimensions of the
It will be noted- that the surfaces of the
plungers 2 and 3 facing each other are not flat
tubular member of the cavity shell which then
in turn requires relatively small separation of the
plungers for the minimum frequency (maximum
tially ?at portions along the edges parallelrto
cavity volume). Thus the shaped plunger type
.the line of the openings 1 and 8 may come close 30
of’ resonator has characteristics at both ends of
together, practically in contact, over a substan
the frequency range which make it advantageous
or simply curved but are shaped so that substan
tial area when the plungers are moved to the
where a large range of adjustment is required.
What is claimed is:
positions nearest each other while the curved
portions extending along the center parallel to
1. A substantially closed electrically resonant
the line of the openings 1 and 8 clear the space 35 cavity of which the resonant frequency is de
required by the electron tube and with the por
tions of the tubular member near the openings
1 and 8 enclose a relatively small space around
the position of the electron stream. This is the
minimum size of the cavity resonator determin
ing the maximum resonant frequency. It will be
seen that the volume of space in this minimum
size resonator can be made relatively small so
that the ratio between that and the maximum 45
like, back and forth in opposite directions to vary
the dimensions and thereby the resonant fre
quency of the cavity, the resonant frequency be
ing determined by the position of the movable
shell portion and the contour of the inner sur
are the maximum distance apart) can be made
to the other extreme position at least two of the
three orthogonal'principal dimensions of the cav—
ity are e?ectively changed to cooperate in chang
pendent upon its dimensions and which has a
rigid portion of the cavity shell movable, plunger
face of the shell portion being'such that when
that portion is moved from one extreme position
size resonator (that obtained when the plungers
large thereby'providing a relatively large range
of frequency adjustment. A comparison between
ing the resonant frequency thereof. .
the use of conventional ?at surface plungers and 50 2. A space resonator, the resonant frequency
the shaped surface plungers in accordance. with
of which is dependent upon two of its principal
the invention is illustrated by Figs. 2 and 3 and
Figs. 4 and 5. These ?gures show sections along
dimensions which are at right angles, electrically
conducting walls substantially enclosing the
the axis of the tubular member and perpendicu
space within the resonator, rigid portions of
lar to the line of the openings ‘1 and 8.’ Figs. 2 55 the walls being movable over a range of adjust
and 3 show respectively the minimum and maxi
ment in the direction of one of the said two di
mum frequency positions of the ?at surface
mensions to alter the resonant frequency and the.
plungers 9 and“) and Figs. 4 and 5 show respec
volume of the enclosed space, and the internal
tively the corresponding minimum and maximum
the ‘ shaped
faces of the movable wall portions being so
60 shaped that within the range, of adjustment
plungers. It is-readily apparent that the ratio
of volumes and therefore the range of frequency
variation is greater in the case of the shaped
' the resonant frequency and’ the ‘volume ‘of the
enclosed space are altered by changes in both of
the two said dimensions which are effective in
surface plungers of Figs. 4 and 5 which are like
those illustrated in Fig. l.
_ I
The shaped plungers need not be of the exact
determining the‘ resonant frequency.
3. A substantially'closed electrically resonant
cavity of which the resonant frequency is de
curved shape shown in Figs. 1, 4 and 5. For ex
ample an alternative shape is shown in Fig. 6.
pendent upon two of its three orthogonal prin
cipal dimensions and having aportion of the
The important feature is that substantially ?at
enclosing wall movable along'the direction of one
portions along the edges vparallel to the electron‘ 70 of the said two dimensions between a'position‘
stream come close together in the minimum vol
of maximum cavity volume and "a position of
minimum cavity volume, ‘the movable portion'
ume position in a manner to close off part of the
cavity space and effectively reduce ‘a cavity die
mension other than the dimension in the direc
tion ofmovement of the plungers while the cen
of the wall being so shaped that when it is
moved from the maximum volume position to,
75 the minimum volume position 1both- of the said
dimensions effective in determining thelresonan
variable. frequency substantially; closed
spa'ceoresonator o'ifa type'iin which‘ the ‘resonant
frequencyv ‘varies- inversely as the volume; of the
frequency ‘are reduced:
. -
A.’ "A substantiallyrclosed “electrically resonant
cavity of <which theresoiiant frequency. is de1
enclosed space fandis' ‘dependent upon two wo‘fits
pendent vupon its dimensions having a ‘rigid- pore 5 orthogonal. principal dimensions, ‘comprising
electrically conducting walls substantially ' en
tion-of the: cavity wall movable: back and- forth
to ; vary. the volume "of the‘ cavity mawmeh ‘the
closing ‘the resonator space; ati1east one wall por
tion being 'ir'iiova'b'l'e ‘over a" range 'of adjustment
tdVaiiy-th ‘ 'eson'ant frequency andthe volume
contourof theinner surface ‘of the movable wall
portion is such that when-it ‘is moved? from the“
position of'iimaximum cavity volumei‘t’oithe po-‘
sition of‘ minimum. cavity volume 'at' least one
dimension in aydirection-other than the direc':
tion of motion of the movable wall portion is
effectively; reduced toffalter the resonant fre
5. A substantially closed electrically resonant
‘ '
space, the volume increasing as‘
equen'cy decreases ‘and ‘decreasing as the
éy increases‘; and Ttheiinter-nal face of -‘at
ewaiiiporitioncteihzgiso shaped ‘that with
iii‘lth'e ' range of. adjustment ithe 1 resonant lire
15 quency and the volume of the enclosed space are
varied by effective changes in both of the two
said orthogonal dimensions upon which the res
cavity of which the resonant frequency is de
pendent upon it dimensions having a rigid por
tion of the cavity wallmovable back and forth
onant frequency is dependent.
10. A substantially closed electrically resonant
along a line of direction to vary the volume of 20 cavity which has a rigid portion of the cavity
the cavity and its resonant frequency in which
shell movable, plunger like, to vary directly, a_
the inner surface of the movable wall portion is
?rst dimension of the cavity along the ?rst of
such that when it is moved from the maximum
three orthogonal axes, the contour of the cavity
volume position to the minimumvolume posi
shell being such that the distance between the
tion the cavity dimension along the said line of 25 interior surface of the said movableshell portion
direction is reduced and also another cavity di
and the interior surface of an opposite shell por
mension in a direction perpendlcular'to the ?rst
said line of direction and upon which the res
tion in the direction of the said ?rst dimension
' is less at points removed from the center of the
onant frequency of the cavity is dependent is
cavity along a second orthogonal axis than at
effectively reduced as a frequency determining 30 points in the central portion of the cavity,
whereby, at the extreme where the said ?rst di
mension has its smallest value there are portions
of the cavity space removed from the center
6. An electrically resonant cavity arranged to
be excited by an electron stream projected there
through along a certain path, at least one wall of
along the said second orthogonal axis, and be
the cavity extending in directions generally par 35 tween the said movable shell portion and the said
allel to the path of the electron stream being
opposite shell portion, which are made narrow in
movable toward and away from the path in di
the direction of the said ?rst dimension and are
rections generally perpendicular to the path to
thereby effectively shielded electrically from the
vary the volume of the cavity space and having a
contour in a section perpendicular to the path
such that when it is moved from the position of
mimmum cavity volume all of the cavity dimen
sions perpendicular to the path are effectively
7. In combination with an electron tube a sub
stantially closed electrically resonant cavity hav
wider space between the same said shell portions
in the said central portion of the cavity and
eliminated as a factor determining the effective
dimensions of the resonant cavity, while at the
extreme where the said ?rst dimension has its
greatest value'the said portions of the cavity
" space removed from the center, and narrow in
mg a portion of the enclosing wall movable be
the ?rst said extreme, are widened'in the direc
tion of said ?rst vdimension and are thereby ef
tween a position of maximum cavity volume and
fectively coupled electrically to the also widened
a position of minimum cavity volume, the mov
said wider space in the center of the cavity and
‘ able portion of the wall being so shaped that 50 made an effective part of the resonant cavity
when it is moved from the maximum volume po
whereby at the second said extreme as compared
sition to the minimum volume position two of, the
with the ?rst said extreme the cavity size is ef
three orthogonal principal dimensions of the cav~
fectively greater in the direction of the said sec
ity are effectively reduced, one of the said two di
ond axis as well as in the direction of the said
7 mensions being in the direction of motion of the 55 ?rst axis.
movable wall portion and the other being in a
11. A resonant cavity according’ to claim 10 in
direction at right angles thereto.
combination with means for producing electrical
electrically resonant cavity comprising
two piston like side walls in contact with and
oscillations therein with a mode of oscillation
such that vthe main components of the electric
slidably movable within a tubular shaped mem 60 vector'inside the cavity are parallel to the third
ber which together with the tubular shaped
of the said three orthogonal axes.
member enclose the cavity space and are mov
. 12. A resonant cavity according to claim'lO in
. able to vary the volume and resonant frequency
which the frequency of resonance varies inverse
gorthe cavity, the inner surfaces of the movable
ly as the dimension along the said second or
walls facing each other and having contours “ thogonal axis whereby the shielding of the said
tours such that if moved to the position of mini
space removed from the center, along that axis
mum cavity volume and maximum resonant'fre
has the effect of increasing the frequency of
quency central portions of the surfaces are spaced
apart substantially and enclose a space deter
13. A substantially closed electrically resonant
mining the resonant frequency ' of the cavity 70 cavity the resonant frequency of which is vari
while other substantial portions of the surfaces
able inversely with the volume of the enclosed
are sufficiently close together that the space be
space and having its volume adjustable between
tween is negligible, having substantially no effect
positions of maximum and minimum by the
in determining the volume and resonant fre
movement of at least one of two opposite por
quency of the cavity.
75 tions of the enclosing wall to vary the separation
of the two interior surfaces of the saidwall. por
tions, the interior surface ' least one ‘of the
areas. and‘ is ine?ective in Jaetermining the vol
ume and resonant frequency of the cavity and
said wall portions being otherthan planar and
such‘ also that‘in the maximum volume adjust
so shaped that over the entire range ofuvolume
ment .the same said other parts of area’ are sep
adjustment a part of its area is more distant
from an opposite area of the interior surface of
arated from the interior surface of the said other
the other said wall portion than are other‘ parts
of its area and such that in the minimum vol
ume adjustment the said other parts of its area
wall portion whereby the intervening space is in
timatelyjoinedywith the space between the said
Incredistantly apartsurface areas to make up
the totalvolume of enclosed space, is an appre
are close to the interior surfaceof the ,saidyother 10 ciablytlarge proportion of the total enclosed
space and is thereby effective in determining the
wall portion whereby ‘the, intervening space ,is
electrically shielded from and thereby is effec-,
tively cut off from the cavity space remaining be
tween the said more distantly apart surface
resonant frequency of the cavity.
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