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Dec. 31, 1946.
J. K. JOHNSON
[2,413,451
ULTRA-HIGH-FREQUENCY TUNING UNIT
Filed Oct. 3, 1942
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INVENTOR
‘JOHN KELLY JOHNSON ~
BYWe
if
ATT RNEY
.
‘
Patented Dec. 31, 1946
2,413,451
UNITED STATES PATENT OFFICE
2,413,451
ULTRA HIGHFREQUENCY TUNING UNIT
John Kelly Johnson, Deer-?eld, Ill.,_ assignor, by
mesne assignments, to Haz'eltine Research, Inc.,
Chicago, 111., a corporation of Illinois
Application October 3, 19112, Serial No. 460,645
6 Claims.
1
The present invention relates to ultra-high
frequency tuning, units for wave~signal apparatus
and, particularly, to suchunitsofthe type where
(01. 250-40)
2
of the disadvantages and limitations of the prior
art arrangements of this type.
It is a further object of the invention to‘ pro
vide a new and improved ultra-high-frequency
is effected primarily by variation of inductance
tuning unit having a light weight, ruggedness and
of a resonant-tuned circuit..
rigidity, and one readily adapted to be manufac
Inductively-tuned resonant circuits have an
tured within close tolerances in large quantities
average value of resonant impedance higher than
at low cost.
that which can be obtained by a condenser-tuned.
It is an additional object of the‘ invention to
circuit tunable over the same frequency band. 10 provide an ultra-high-frequency' tuning unit for
Whereas condenser-tuned circuits have mini
inductive tuning and adapted to have a high ratio‘
mum impedance at the low-frequency end of a
of inductive reactance to resistance, that is‘, a
tuning band and high impedance at the high
high Q, and one possessing electrical symmetry
frequency end, the inductively-tuned circuit con
and balance.
versely has minimum impedance at the high-fre
It is a further object of the invention to pro‘
quency end and maximum impedance at the low
vide an ultra-high-frequency tuning unit in whichv
frequencyv end of the band- This characteristic
its point of mechanical support provides a con
of inductively-tuned circuits has particular im
venient point at which to establish a radio-fre
in tuning over a relatively wide frequency band
portance, for example, in ultra-high-frequency
quency neutral point while preserving mechanical
oscillators where, the low impedance of a con 2-0 symmetry and electrical balance relative thereto.
denser-tuned‘ circuit frequently causes the oscil
In accordance with the invention, an ultra
lator to cease to function near the low-frequency
high-frequency tuning unit comprises a single
end‘ of its operatingef'requency band. The mini
mum impedance of‘ an inductively-tuned circuit,
on the other hand, is relatively high so that the
operation of the oscillator is generally depend
able over its entire frequency range.
Ultra-high-frequency tuning. units of the in
ductive tuning type generally are of quite low
ohmic resistance to enable them to have a desir
able high ratio of inductive reactance to resist
ance. They consequently involve the use ofelec
tri'cal conductors of relatively large cross section.
The length of the conductor is generally only a
matter of inches since it must havean inductance
of the order of a few microhenries. It. is gen’
turn loop member of conductive material having
a predetermined cross-sectional circumference,
the loop member having at least two spaced ter
minal points thereon. The tuning unit includes
a rotatable member comprising a ring of conduc
tive material‘ having a cross-sectional circumfer
ence at least as large as that of the loop member,
and means for supporting. the rotatable‘ member
within the stationary member for rotation about
a diameter thereof symmetrical with respect to
the terminal points comprising a shaft journalled
in the stationary member in electrical contact
therewith and mechanically and electrically con
nected to' the rotatable member. The average
erally'quite desirable that such tuning. units do
minimum spacing of’ the rotatable member from
the stationary member is appreciably less than
not involve the use of either-?exible pigtails or
the aforementioned circumference. The sta
sliding contacts- The prior art low-frequency
arrangements forv effecting inductive-tuning with 40 tionary member is‘ thereby adapted to be electrically connected to ground in the vicinity of
out sliding contacts or ?exible pigtails involve in
the shaft to provide a tuning unit electrically bal
ductors of relatively»large-inductance, few types of
anced to ground and the rotatable member, upon
which are well adapted for‘ultra-high-frequency
rotation, is‘ effective substantially to vary the
inductive tuning. One such prior art arrange
ment is‘the powderedeiron-core type of inductor. 45 inductanceof the stationary'member while main
taining the electrical balancetov ground;
The. latter, when used at‘ ultrahigh-frequencies,
For a better understanding of the present in-~
presents difficulty in obtaining a su?iciently high
vention,
together with other and further objects
ratio of inductive reactance to resistance (high
thereof, reference is had to the following descrip
Q), cannot generally be built witha. desirable de- P
tion taken in connection with the accompanying
greeof- ruggedness of construction,. and requires
drawing, and its scope will be pointed out in the
afairly massive core.
appended claims.
It is an objectof thepresent invention, there
Referring now to the drawing, Fig.’ 1- illustrates
fore, to provide. a new andlimprovediultra-high- ‘
an ultra-high-frequency tuning unit. embodying
frequency tuning unit. which avoidsone. or‘ more 55 the present invention; Fig. 2- is. an- exploded » view‘
‘2,413,451
3
' 4
showing the assembly of the tuning-unit rotor
ductlve reactance to resistance for the tuning
member and shaft; Fig. 2A is a cross-sectional
view taken along the line B-B of Fig. 2; Fig. 3
unit.
The tuning unit also includes means for sup
porting the members I0 and I3 for relative move—
ment to vary the spacing therebetween. In the
particular construction shown, the member I3 is
is a top view, partly in cross section, of the
tuning unit of Fig. 1; Fig. 4 is a circuit diagram
of an ultra-high-frequency oscillator which util
supported within the stationary member In for
rotation about a diameter thereof. This means
comprises a web l4, integrally formed with the
oscillator varies with rotation of the tuning-unit 10 rotatable member I3, and a shaft I5 journalled
izes the tuning unit of the present invention;
while Fig.6 isa graph representing the manner
in which the operating frequency of the Fig. 4
rotatable member.
in an aperture l6 provided in the stationary mem
ber Ill and base member I2. The shaft I5 is in
electrical contact with the stationary member l0
_
Referring now more particularly to Fig. 1 of
the drawing, there is illustrated an ultra-high
The tuning unit
and is mechanically and electrically connected to
the rotatable member I3 by rivets I1, I‘! which
comprises a stationary single-turn loop member
If! comprising a ring of conductive material hav
secure the shaft I5 to the web I4. The web I4
and ring 13 are preferably formed as an integral
frequency tuning unit embodying the present in
Vention in a preferred form.
member of aluminum by die casting.
ing a substantially circular cross section of rela
The preferred manner of assembling the shaft
tively large area and of predetermined cross
sectional circumference. There is secured to the 20 l5 and rotatable member I3 is best shown by the
exploded views of Figs. 2 and 2A, of which Fig.
member I!) stiffening side ?anges II, II, useful
primarily to give added rigidity to member I0, and
III,‘ the ?anges II, II and the base member, I2
2A is a cross-sectional view taken along the line
>B—B of Fig. 2. The shaft I5 is provided with a
slotted end E8 to receive the web It of the ro
tatable member and is inserted through a collar
are preferably formed of aluminum by die cast
ing. This material has the advantages that it
provides a tuning unit of very light weight, is
adapted to be easily die cast, and has high elec
which secure the shaft and rotatable member in
a base member l2 for securing the tuning unit
to a suitable support. The stationary member
I9 integrally formed with the rotatable member
l3. Suitably spaced holes 20 are provided in the
shaft I5 and web I4 to receive the rivets I1, I?
trical conductivity, thus enabling the attainment 30 assembled relation. It will be understood, of
course, that the assembly of the shaft IS with
of a high ratio of inductive reactance to resist
the rotatable member I3. is performed while the
ance, or high Q, for the tuning unit. Die cast
rotatable member I3 is positioned within the sta
ing is desirable from the standpoint that tuning
units embodying, the invention may be manu
factured within close tolerances in large quanti
ties at low cost.
tionary member I0.
35
‘
The diameter of the loop'member I9 is, of
course, governed by the desired operating, fre
quency range considered in connection with the
total capacitance of the circuit which will be con
nected across the open ends of the member ID.
The factors affecting choice of a suitable di
ameterv are well understood and such diameter
may readily be determined by one skilled in’ the
art.
.
.
'
The rotatable member I3 is adapted upon ro
tation to vary the inductance of the stationary
member ID. In order that an appreciable varia
tion of the inductance of the stationary member
IE may be effected within the range of rotation
of the rotatable member IS, the average minimum
spacing of the rotatable member is from the
stationary member l0 should be appreciably less
than the cross-sectional circumference of the
material of the stationary member I ii and prefer
ably is less than one-third of suchcircumference.
As previously stated, the highefrequency' current
penetrates the loop member Iii only to a very
'
The large cross section of the conductive ma
terial of the loop member Ill aids in attaining a
high ratio of inductive reactance to resistance,
and thus a high Q, for the tuning unit. Varia
tions of crosssection have only a second-order
small depth and hence flows almost entirely on
its surface. For this reason, the circumference
50 of the material of member If! is an important
effect on the inductance of the member It. In
the ultra-high-frequency range for which the
tuning unit of the present invention is particu
larly suited, for example tuning ranges of. the
thin strip-like material or of a material having a
order'of and above 100 megacycles, the high- .
frequency current penetrates the loop member
ID to only -a very small depth. For thisreason,
the member I0 may be constructed of hollow tub
factor in regard to the minimumaverage spacing
of members Iii and I3 and this is true whether
the member Ill be formed of awide and relatively
circular cross section, as shown. Speci?cally, if
the conductive material of the stationary member
III has a thickness of predetermined value in the
plane of the member H], or is of circular cross
section ‘with a predetermined cross-sectional
ing, if desired, without materially impairing the
diameter, the minimum spacing of the rotatable
Q-of the tuning unit. This in general, however, Gs) member I3 from the stationary member l? should
willincrease the cost of the unit since die casting
be not appreciably greater than the predeter
cannotbe used'with this construction. It has
mined value of thickness or cross-sectional diam
been found that the Q of the tuning unit is
eter. In general, the closer the minimum spac
slightly vimproved by‘ omission of the stiffening
ing of the rotatable member l3 from the station
?anges I I, I I and this may be done in a particular
ary member I I], the greater will be the variation
application where the loop member II] has sum
of inductance of the stationary member ID with
in the range of rotation of the rotatable mem
cient rigidity without the use of the flanges.
ber l3.
The tuning unit also includes a member I3 com
prising a ring‘ of conductive material preferably 70 While the shaft I5 need not be J'ournalled in
the stationary member I0 at a point diametrically
having a substantially circular cross section of
opposite the gapwbetween the ends of the latter,
relatively ‘large cross-sectional area equal at least
thisconstruction is desirable from the standpoint
to that of the stator member III. A largecross
that the rotatablev member I3 is supported sym
sectional area of the material of the rotatable
metrically about a diameter of the stationary
member I3 aids in attaining a high ratio of in-'
5
2,41 3,451
member Iii! and rotates- about such diameter, thus
providing a tuning unit which is mechanically and
. electrically balanced. Consequently, the tuning
unit is electrically balanced to ground when a
ground connection is made thereto through the
supporting member l2, as» by mechanically and
electrically connecting the member l2 to a
grounded metal panel of an ultra-high-frequency
tential, indicated as. +3, and the control elec-‘
trode of the vacuum tube is connected to ground
through a suitable radio-frequency choke 28 and
a grid-leak resistor 29. The cathode of the vac
uum tube is connected to ground through a radio
frequency choke 30 and the heater of the tube is
energized through the choke 30 and a radio-fre
apparatus.
The ends of the stationary tuning member in
quency choke 3| from a suitable source of ener
condensers to one or more electrodes of a vacuum
electrode and cathode thereof, the condensers 32
gizing potential indicated as +A. A condenser 32
10 is connected between the anode and cathode of
are, in most applications, coupled through small
tube 26 and a condenser 33 between the control
device included in an ultra-high-frequency ap
paratus. The tuning unit of the present inven
vtion has the advantage that these coupling con
densers may easily and readily be constructed on
the end portions of the stationary member ID.
This construction is highly desirable from the
standpoint that it provides a minimum of stray
circuit inductance, the latter being quite trouble
some and very undesirable at ultra-high-frequen
cies. This construction is best shown in Fig. 3,
which is a top view, partly in cross section, of the
tuning unit shown in Fig. 1. Each such coupling
condenser comprises an end portion of the ?ange
II as a plate thereof, a second condenser plate
2|. conveniently formed of thin-tinned copper
having a connecting lug 22 formed as an integral
part thereof, and a suitable sheet of dielectric 9,
which may be a sheet of mica, for separating the
condenser plate 2| from the end portion of the
?ange I I. The coupling condenser is maintained
in assembled relation by machine screws 23 which
are threaded into the flange H and are elec
trically insulated by insulating bushings 24 from
the condenser plate 2|. In the event that the
stiffening ?anges H are omitted from the tuning
unit, as may be desirable in certain applications
and 33 being shown in broken lines for the reason
that they may be comprised in whole or in part
of the inherent interelectrode capacitances exist
ing between the anode, cathode, and control elec
trode of this tube.
Considering now the operation, of the Fig. 4 os
cillator arrangement, oscillatory voltages de
veloped across the resonant circuit Ill-l3 and 25
produce corresponding potentials across the con
densers 32 and 33 which are coupled in series
across the resonant circuit, The voltage devel
oped across the condenser 33 and applied to the
s, control electrode and cathode of tube 26 is effec
tively a voltage fed back to the control electrode
from the output circuit of this tube and has such
phase
respect to the voltage developed in the
latter circuit that it is e?ective to produce sus
tained oscillations in the resonant circuit ||l—-|3
.25. A certain portion of the voltage developed
in the output circuit of tube 26 may also be fed
back through the inductor |B—|3 to the control
electrode of tube 26, thus being also effective to
produce sustained oscillations in the resonant
circuit, but it is believed that the major portion
of the feed-back voltage is that across the con
denser 33. It will be understood that the fre
quency of oscillation of the resonant circuit Ill-l 3
as hereinbefore suggested, the coupling con
denser is formed directly upon an end portion of 40
and
is determined by suitable adjustment of
the stationary member Ill. In this case, the con
the rotatable member | 3 with respect to the loop
denser plate 2| preferably is formed to have a
member it to provide the desired value of induct
contour corresponding to that end portion of the
stationary member ID over which the condenser
anee for operation at the desired operating fre
_ quency.
plate 2| is secured. A coupling condenser of in
creased capacity may be had by adding additional
A typical tuning characteristic of the tuning
dielectric sheets and condenser plates, as desired.
unit is represented by the graph of Fig. 5 and
Fig. 4 represents a circuit diagram of an ultra
it will be seen that the tuning unit has its lowest
high-frequency oscillator and illustrates one ap
inductance, and thus resonates at the highest fre
plication of the tuning unit of the present inven
quency,
when the plane of they rotatable member
tion. The inductor ||'|—|3 of this arrangement
l3
coincides
with that of the stationary member
schematically represents the stationary member
til and that it has its highest inductance, and
Ill of the tuning unit, the inductance of which is
thus resonates at the lowest operating frequency,
variable, as indicated by the arrow, by virtue of
when the plane of the rotatable member I 3 is nor
the effect of the rotatable member l3 on the sta
tionary member In. The inductor |0-—|3 is . mal to that of the stationary member It. It will
be apparent from this graph that the resonant
grounded at its center point, as by grounding the
supporting member l2 of the tuning unit, and is
tunable to a desired operating-frequency band
frequency of the tuning unit varies approximate
ly linearly with angular variations of its rotatable
by a ?xed condenser 25 shown in broken lines for
member l3 over a considerable range of variation
the reason that it may be comprised in whole or 60 of the latter.
in part of the inherent structure and circuit ca
As illustrative of a speci?c embodiment of the
pacitance existing between the end terminals of
the stationary loop member Ill. The oscillator
includes a vacuum tube 26 having its anode cou
pled through one of the coupling condensers
||—2| to one end of the inductor ||l—| 3 and
having a control electrode coupled through a sec
ond coupling condenser ||-2| to the other ter
minal 0f the inductor Ill-l3. It will be under
stood that the coupling condensers ||-2| are
constructed on individual end portions of the
invention, the following parameters are given for
an embodiment of the invention of the type
shown in Fig- 12
Stationary member | ii, of die~cast aluminum:
Diameter
of
circular
cross
sec
tion ___________________ __inches__
1/;
Spacing of ends of member |il_do-___
1/4
External
d iameter
of
member
in __________ __‘ ________ __inches__
2%
stationary loop member |0 of the tuning unit as
Rotatable member I3, of die-cast aluminum:
previously described. The anode of the vacuum
Diameter of circular cross sec
tube 26 is energized through a suitable radio-fre
tion ___________________ __inches__
17/4,
quency choke 21 from a source of energizing po 75
External diameter __________ _-do____
1%
2,413,451
7
Minimum air-gap spacing of members It!
and I3 _____ _'_ _____________ __inches__ ,
l '
11s
i?cations as fall within the true spirit and'scope _
Inductance of stationary member ID:
Minimum, with members l0 and 13 in
,
.same plane ______ _~_microhenries__ 0.045
r.
1
l
Maximum, with member I3 normal to
~
member l0 ______ _-_microhenries__
8
invention, and it is, therefore, aimed in the ap
pended claims to cover all such changes and mod
'
of the invention.
What is claimed is:
1. An ultra-high-frequency tuning unit com
prising, a stationary single-turn loop member
0. 0'75
including a ring of conductive material having
a substantially circular cross section of predeter
,
H—2i __________ __micromicrofarads__
50
Vacuum tube 26, Hytron ____________ __ Type 615 1O mined radial diameter, said loop member having
at least two spaced terminal points thereon, a ro
Inductance of chokes 21, 2B, 30 and.
tatable member comprising a ring of conductive
3| ____________________ __microhenry__
'1
Capacitance of couplin g condensers
material having a substantially circular cross sec
Resistor 29 ___________________ __ohms__ 30, 000
+3 ____________________ ._ _____ __volts__
tion at least substantially as large as that of said
450
Operating frequency range of Fig. 4 cs
15 loop member, and means for supporting said ro
cillator __megacycles__ Approximately 150-200
While the movable member l3 of the tuning
unit of the present invention is shown as rotatable
about a diameter of the stationary member 10,
it will be evident that the member l3 need not 20
be rotatable but may, if desired, ‘be moved axial
ly with respect to the member l0. In fact, the
member l3 may be the stationary member and the
member It the movable member, the latter then
having either rotatable or axial movement rela 25
tive to member i3. Further, the members 10 and
i3 may be interchanged without departing from
the invention, the inner member then becoming
the open-circuited single-turn loop and the outer
member the closed-circuit member which varies
the inductance of the inner loop member upon
relative movement of the members. Additional—
1y, while the members 10 and I3 are shown as
ring-shaped members, it will be apparent that
these members may have other con?gurations,
for example elliptical or square or may even have
con?gurations differentifrom each; other, al
though in general the greatest variation of in
ductance of the loop member is eifected when
the two members have the same con?guration.
Moreover, while the single-turn loop member
Iii is shown as being of the open-circuited type,
it will be understood that it may also, like the
member i3, be of the closed-circuit type compris
.ing a simple ring of conductive material. In 45
this event, the tuning unit of the invention is
suitable for operation at much higher frequencies
where waves of current or voltage are produced
around the periphery of the loop- member ID, the
loop member being coupled to associated appara- ,
tus in any suitable well-known manner.
'_ From ‘the above description of the invention, it
will be apparent that a tuning unit embodying
the invention has the advantages of very light
weight, great ruggedness and rigidity, and is one
readily adapted to be manufactured within close
tolerances in large quantities at low cost. The
tuning ,unit of the invention also has the advan
tags that it is adapted to have a high ratio of
tatable member within said stationary member
for rotation about a diameter thereof symmetri
cal with respect to said terminal points including
a shaft journalled in said stationary member in
electrical contact therewith and mechanically
and electrically connected to said rotatable mem
ber, the average minimum spacing of said rotat
able member from said stationary member being
not greater than one-half said radial diameter,
whereby said stationary member is adapted to be
electrically connected to ground in the vicinity of
said shaft to provide a tuning unit electrically
balanced to ground and said rotatable member,
upon rotation, is effective substantially to vary
the inductance of said stationary member while
maintaining said electrical balance to ground.
2. An ultra-high-frequency tuning unit com
prising, a stationary single-turn loop member in
cluding a ring of conductive material having a
thickness of predetermined value in the plane
of said member and a predetermined cross-sec
tional circumference, said loop member having
at least two spaced terminal points thereon, a
rotatable member including a ring of conductive
material having a cross-sectional circumference
at least as large as that of said loop member, and
means for supporting said rotatable member
within said stationary member for rotation
about a v‘diameter thereof symmetrical with
respect to said terminal points including a
shaft journalled in said stationary member in
electrical contact therewith and mechanically
and electrically connected to said rotatable mem
ber, the average minimum spacing of said ro
tatable member from said stationary member be
ing not greater than one-half said predetermined
value of thickness, whereby said stationary mem
her is adapted to be electrically connected to
ground in the vicinity of said shaft to provide a
tuning unit electrically balanced to ground and
said rotatable member, upon rotation, is effective
substantially to vary the inductance of said sta
tionary member while maintaining said electrical
balance to ground.
'
3. An ultra-high-frequency tuning unit com
inductive reactance to resistance, or high Q, and Gil
prising, a stationary single-turn loop member in
possesses electrical and mechanical symmetry
cluding a ring of conductive materialhaving a
and balance. A radio-frequency ground connec
circular cross section of predetermined diameter,
tion is readily made to the unit while preserving
said loop member having at least two terminal
mechanical symmetry and electrical balance to
points thereon, a rotatable member including a
ground. The tuning unit of the invention has
ring of conductive .material having a circular
the additional advantages that a wide variation
cross section of diameter not substantially less
of inductance of the unit may be effected in con
than said predetermined diameter, and means for
tinuous manner between minimum and maxi
supporting said rotatable member within said
mum values and without the need of any sliding
stationary member for rotation about a diameter
contacts or ?exible pigtails.
thereof symmetrical with respect to said terminal
While there has been described what is at pres
points including a shaft journalled in said sta
ent considered to be the preferred embodiment of
tionary member in electrical contact therewith
this invention, it will be obvious to those skilled
and mechanically and electrically connected to
in the art that various changes and modi?cations
said” rotatable‘member, the average minimum
may be made therein-without departing from the
2,413,451
spacing of said rotatable member from said sta
tionary member being appreciably less than one
half said predetermined diameter, whereby said
stationary member is adapted to be electrically
connected to ground in the vicinity of said shaft
to provide a tuning unit electrically balanced to
ground and said rotatable member, upon rota
tion, is effective substantially to vary the induc
tance of said stationary member while main
taining said electrical balance to ground.
10
having a cross-sectional circumference at least
as large as that of said loop member, and means
for supporting said rotatable member within said
stationary member for rotation about a diameter
thereof symmetrical with respect to said end por
tions including a shaft journalled in said station
ary member in electrical contact therewith and
mechanically and electrically connected to said
rotatable member, the average minimum spac
10 ing of said rotatable member from said station
4. An ultra-high-frequency tuning unit com‘
ary member being appreciably less than said cir
prising, a stationary single-turn loop member in~
cumference, whereby said stationary member is
eluding a ring of conductive material having a
adapted to be electrically connected to ground
predetermined cross-sectional circumference,
in the vicinity of said shaft and is adapted to
said loop member having at least two spaced ter
be coupled by said coupling condensers to a wave
minal points thereon, a rotatable member in
signal-translating circuitelectrically balanced to
cluding a ring of conductive material having a
ground to provide a tuning unit electrically bal
cross-sectional circumference at least as large as
anced to ground and said rotatable member, upon
that of said loop member, and means for so me
rotation, is effective substantially to vary the in
chanically supporting said members that said loop 20 ductance of said stationary member While main
member is effectively electrically symmetrical
with respect to said terminal points and said ro
tatable member rotates about a diameter of said
loop member to vary the spacing therebetween
while maintaining said electrical symmetry, said
supporting means including a shaft journalled in
said stationary member in electrical contact
therewith at a point equidistant from said ter
minal points and mechanically and electrically
connected to said rotatable member, the average
minimum spacing of said rotatable member from
said stationary member being appreciably less
than said circumference, whereby said stationary
taining said electrical balance to ground.
,
6. An ultra-high-frequency tuning unit com
prising, a stationary single-turn loop member
comprising a ring of conductive material having
a predetermined cross-sectional circumference,
said loop member having at least two spaced
terminal points thereon, a rotatable member com
prising a ring of conductive material having a
cross-sectional circumference at least as large as
that of said loop member, and means for support—
ing said rotatable member within said stationary
member for rotation about a diameter thereof
symmetrical with respect to said terminal points
comprising a shaft journalled in said stationary
member in electrical contact therewith and me
chanically and electrically connected to said ro
member is adapted to be electrically connected to
ground in the vicinity of said shaft to provide
a tuning unit electrically balanced to ground and
said rotatable member, upon rotation, is effective
tatable member, the average minimum spacing
substantially to vary the inductance of said sta
of said rotatable member from said stationary
tionary member while maintaining said electrical
member being appreciably less than said circum
balance to ground.
40 ference, whereby said stationary member is
5. An ultra-high-frequency tuning unit com
adapted to be electrically connected to ground
prising, an open-circuit single-turn loop member
in the vicinity of said shaft to provide a tuning
including a ring of conductive material having
unit electrically balanced to ground and said ro
a predetermined cross-sectional circumference,
tatable member, upon rotation, is effective sub
the end terminal portions of said loop member
stantially to vary the inductance of said station
being relatively closely spaced, a pair of coupling
ary member while maintaining said electrical
condensers each including an individual one of
balance to ground.
said end portions as a plate thereof, a rotatable
member including a ring of conductive material
JOHN KELLY JOHNSON.
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