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Патент USA US3068387

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Dec. 11, 1962
_
C. K. BIRDSALI.. ETAL
3,068,377
ELECTRON DISCHARGE DEVICE
Filed March 7. 1955
„QQN
IN VEN TORS
United States Patent Oñfice
1
3,068,377
Patented Dec. 11, 1962
2
24.
3,062,377
ELECTRON DISCHARGE DEVICE
Charles K. Birtlsall, Los Angeies, and George R. Brewer,
Palos Verdes Estates, Calif., assignors to Hughes Air
craft Company, Culver City, Calif., a corporation of
Delaware
Filed Mar. 7 1955, Ser. No. 492,664
1 Claim. (Cl. 315-36)
Cathode 24 is maintained at a potential consider
ably below ground by a source of potential 34 having
its positive terminal grounded. A voltage of the order
of 1000 volts with respect to ground is representative of
the voltage normally impressed upon cathode 24 by
source 34. Focusing electrode 28, which is connected
to the negative terminal of battery 32, may have a
frusto-conical internal surface of revolution disposed at
an angle of 671/2 degrees from its axis of symmetry.
This invention relates to microwave tubes and more
Anode 30 is maintained at a potential a few hundred
particularly to a high-gain traveling-wave tube amplifier.
volts positive with respect to the potential of cathode
v As microwave tubes find application in new fields, it
isfrequently desirable to obtain as much gain as pos
24 by a connection to a tap 33 on potential source 34.
sible from such tubes. A certain known but unavoid
able input or electromagnetic excitation loss in a travel
ing-wave tube amplifier reduces gain. The loss takes
place when the total input energy divides into three for
ward waves.
.Only one of the waves exists as a grow
ing Awave and hence vthe energy divided between the two
other waves becomes a total loss.
20
It is therefore anv object of the invention to provide
a relatively high-gain traveling-wave tube.
, It is another object of the invention -to provide means
whereby the radio-frequency input loss in a traveling
wave tube- ampliiiermay be reduced.
.
In accordance with the present invention, a relatively»
A solenoid 54 is axially positioned symmetrically
about the envelope 20. An appropriate direct current
is maintained in solenoid 54 by means of a potential
source, such as a battery 56, so as to produce an axial
magnetic field of the .order of 100() gauss within the en
velope 2li to constrain the electron stream produced
by gun 22.
.
Proceeding along from the electron gun 22 in the
direction of electron ilow, there are positioned succes
sively about the path of the electron stream, a matching
input ferrule 58 connected by an input antenna lead 60.
to an input helix 62, a principal helix 63 which is,rin
turn, connected by an output antenna-lead 64 to a match-l
ing output ferrule 66, and a collector electrode 68 which
short input helix anda relatively long principal helix
is positioned at the end of the path so as -to collect the
stream electrons. Electromagnetic wave energy is cou
are maintained at different average direct-current po
pled from input helix 62 to principal helix 63. by means
tentials in a traveling-wave tube. The potential of the
input helix is maintained at a value which will provide 30 of a coupling helix 65 which is disposed about the adja
maximum gain at the output end of the principal helix
cent ends of input and principal helices 62 and 63.
but generally at a lower potential than that of the prin
Coupling helix 65, however, has a pitch angle negative.y
with respect to but approximately equal in magnitudeî
cipal helix in order to minimize the radio-frequency
to that of input and principal helices 62 and 63 to pro-,
input loss of the tube for the production of maximum
gain.> The principal helix is then maintained at a po-- 35 vide the desired coupling and may be spaced about the4
tential for optimum gain. Means are also provided to
outside of envelope 20 as shown. What is meant by ay
negative pitch angle is that the coupling helix has op
couple electromagnetic wave energy from the input helix
to the principal helix.
posite screw sense to that of input and principal helices
The novel features which are believed to be character
istic of the invention, both as to its organization and 40
method of operation, together with further objects and
advantages thereof, will be better understood from the
following description considered in connection with the
accompanying drawing in which several embodiments
62 and 63.
All of the helices, which serve as the slow-wave cir-~
cuit for the traveling-wave tube 10, preferably are made
of a material such as tungsten or molybdenum, the prin
cipal requirement being that they retain their form,
especially with respect to the ratio of their pitches toV
of the invention are illustrated by way of example. It 45 their diameters. In accordance with the present inven-l
tion, input and coupling helices 62 and 65 are main
is to be expressly understood, however, that the draw
tained at a suitable fixed potential which may be ground.,
ing-is for the purpose of illustration and description
Coupling helix 65 is connected directly to ground
only, and is not intended as a definition of the limits
whereas input helix 62 is connected to ground through
of the invention.
' FIG. 1 is a sectional view of an embodiment of the 50 input antenna-lead 60 and input ferrule 58. Alterna
tively, coupling helix 65 may be maintained at the
traveling-wave tube amplifier of the invention shown
with associated circuitry; and
Y FIG. 2 is a broken section of a traveling-wave tube
in which an alternative embodiment of the invention
same potential as that of principal helix 63, or at a cer-A
tain potential between those of input helix 62 and prin
cipal helix 63. Principal helix 63 is maintained at a
55 potential somewhat positive with respect to ground by4
is illustrated.
t’ Referring to the drawing, in FIG. l an embodiment
a connection from output ferrule 66 to the positive ter
of the microwave amplifier of the invention is shown
minal of a potential source 69, the negative terminal
of which is grounded.
comprising a traveling-wave tube 10 including an input
matching cavity 12 having a coaxial input cable 14
As previously mentioned, input and principal helices
connected thereto and an output matching cavity 16 60 62 and 63 are connected to ferrules 58 and 66 by leads
connected to a coaxial output cable 18. An envelope
60 and 64, respectively. Leads 60 and 64 are located
20, which provides the evacuated chamber of traveling
parallel to the electric fields excited within matching
wave tube 10, consists of a long cylindrical structure
cavities 12 and 16. Matching cavity 12 has the con
which has an enlarged portion at its left extremity as
figuration of a rectangular toroid with a concentric col
65 lar 70 disposed about and spaced from matching ferrule
illustrated in FIG. 1.
Within the enlarged portion at the left extremity of
5S. An opening 72 is provided in the end plate of
the envelope 20, there is located an electron gun 22 for
cavity 12 facing the left end of input helix 62. Cavity
developing an electron stream. Gun 22 comprises a
16 is similarly constructed, having a corresponding con
cathode 24 with a heater 26, a focusing electrode 2S
centric collar 74 arranged about and spaced from match
and an accelerating anode 30. Heater 26 is connected 70 ing (ferrule 66 and an opening 76 facing the right end
across a source of potential, such as battery 32, the
negative side of heater 26 being connected to cathode
of principal helix 63.
_
The center conductor 78 of coaxial input cable 14 ex
Secam?
3
tends through an aperture in the annular wall of cavity
12 and is connected to concentric collar 70 while the outer
conductor of cable 14 is bonded to the periphery of the
aperture. Likewise, the center conductor 80 of coaxial
output cable 18 extends through an aperture in the annu
lai’ wall of cavity 16 and is connected to concentric collar
74 while the outer conductor of a cable 18 is bonded to
the periphery of the aperture, in the same manner as be
fore, or vice-versa. Cavities 12 and 16 are fabricated
¿il
second and P is the pitch angle of the helix, [P] being the
absolute magnitude of the pitch angle. The pitch angles
of helices 122 and 124 should therefore be approximately
equal in absolute magnitude but may be opposite in sign.
A helix is not, of course, the only type of slow-wave
structure which may be used with the tube of the present
invention.
Numerous other types, such as a disc-loaded
with an inner surface composed of highly conductive ma
waveguide, are illustrated in chapter IV in “Traveling
Wave Tubes” by J. R. Pierce, D. Van Nostrand and Co.,
New York, 1950. An input section and a principal sec
terial and are broadly resonant so as not to limit the
tion of a slow-wave structure there illustrated may be in
frequency bandwidth of operation.
sulated from each other, e.g. with a dielectric ring. In
that case means for maintaining the sections at different
The configuration
shown and described for the cavities 12 and 16 in the
drawing, provides suitable impedance matching from in
direct-current potentials and means for coupling electro
put and principal helices 62 and 63 to coaxial cables 14 15 magnetic energy from one section to the other may be
and 18, respectively, over a range of frequencies such as,
provided in accordance with the present invention.
for example, from 2000 to 4000 megacycles per second.
An electromagnetic wave coupled from external cir
The stream electrons are intercepted by collector 68 at
cuitry into a conductive helix will cause alternating-cur
the opposite extremity of envelope 20 with respect to elec
rent velocity modulation to exist on the beam of electrons
tron gun 22. A potential of the order of 200 volts posi
directed therealong. Part of the input signal energy thus
tive with respect to ground may be applied to collector 68
is used to excite space charge waves on the electron
in order to prevent secondary electrons, which' may be
stream. These waves interacting with the stream cause
produced by the stream electrons impinging on its- surface,
the total tìeld to’ be represented by three forward propa
from reaching principal helix 63 or ferrule 66. This po
gating wavesä Approximately one-third of th'e‘input sig
tential isîapplied by means of a connection from collector 25 nal energy will be found in each of these wavesv while only
68 to the positive terminal of a source 84', the negative
that in the growing wave is extracted as useful output
terminal of whichis grounded.
energy.V An apparent “loss” in energy at the inp'ut of the
In the operation of the' tube 10, an input signal to be
tube is thus realized.
Y
amplified i's applied through coaxial input cable 14 to in
As the three excited waves travel along the helix, they
put cavity 12. The input wave in flowing along the ex 30 interact in such away that their total “voltage” varies
posed portion of conductor 78 within cavity 12 excites an
along the helixv and in addition the character ofthe totall
electromagnetic field within that cavity. This ñeld in
wave as evidenced by the phase relationships between the'
duces a corresponding current in antenna-lead 60 connect
voltage and current changes. If the helix is severed into'
i'ng‘input'ferrule 58 to input helix 62 to launch a traveling
two portions at some point while preservingv radio-fre'
wave' along the'input helix 62. Interaction between the 35 quency continuity along the two portions of the helix, by
electron stream and the traveling-wave ultimately results
adjusting this inter-wave interaction a wave of any d‘e
in a net transfer of energy from the stream to the wave
sired character may be derived from the first portion to
causing it to “grow” or be amplified. The wave suffers
feed the second portion of the severed helix. The charac
a relatively small coupling loss in the transition from
ter o-f the wave may be modiñed to approximate the grow
input helix 62 to principal helix 63. However, the com 40 ing wave on the second helix portion in which case 'a
bined coupling losses will be less than the excitation loss
larger proportion of the signal energy may go into the
of a single helix of the same length if an appropriate length
growing wave. This results in greater signal gain. The
and operating potential of input helix 62 are selected.
inter-wave interaction on the input helix can be further
AtV the right end of input helix 62, wave energy is
coupled from input helix 62 to principal helix 63 by
coupling helix 65 thereby insuring the continuity of the
adjusted for optimum performance by adjusting the direct
current voltage of the iirst helix.
ì
cipal helix 63, the amplified electromagnetic wave, in
flowing along output antenna-lead 64 connecting output
In order to better understand the operation of the
traveling-wave tube of the invention, it is desirable to re
view the gain equation of a traveling-wave tube. This
equation is as follows:
helix 63 to output ferrule 66, excites an electric field in
cavity 16. This electric field induces a corresponding out
put signal on center conductor 80 of coaxial cable 18.
where A` is the total loss factor of the tube; B='54'..6x1v
wave traveling along on the helices.
At the end of prin
G=A+BCN
(2)
Wave energy may be coupled from the isolated input
where x1 is the real part of an incremental propagation'
helix 62 to the principal helix 63 in a number of ways.
For example, in FIG. 2 a broken section of an evacuated
constant 61 of the growing wave; C is a gain parameter of
envelope 120 is illustrated housing an input helix 122
wavelengths, i.e., the length of the tube divided by the’
electronic wavelength ke'=u0/f, where uo is the direct
current electron velocity, and f is frequency.
Of the three waves propagated along the helix the’
60 growing wave is the principal contributor to the useful>
which is disposed about a principal helix 124. The en
vlelopeV 120 is contracted at its _right end whereby it is
disposed contiguously about both helices 122 and 124,
input’ helix 122 having a larger diameter than that of
principal helix 124. Input helix 122 may be disposed
contiguous to a portion of principal helix 124 to provide
optimum electromagnetic coupling. Both of the helices
the tube; and N is the length of the tube 1n electronic
output power of the circuit or helix at the output end of
the traveling-wave tube.
v
`
n
.
.
The loss factor A of the traveling-wave tube 10' 1s given
122l and 124 may then be appropriately tapered at their
mutual coupling ends. It is to be noted that input helix 65
122 has a negative pitch with respect to principal helix
where A1 is the apparent loss suffered by the input signal
124;
in modulating the electron stream, A2 is the loss suffered
The principal requirement in coupling wave energy in
by the output wave because not all of the energy stored
this manner is that all helices must propagate waves of
in the electron stream can be coupled out to the helix
frequencies within the operating frequency band at sub 70 and thus to the load, and, A3 is the coupling loss between
stantially the same velocity. The propagation velocity
helices.
v1', is given approximately by
When designing the amplifier of the invention, helices
62, 63 and 65 may'have approximately the same absolute
(1)
pitch. Helices 62 and 63 may have equal diameters.
where cis the velocity of light or 3 X1010 centimeters per 75 The actual pitch and diameter of helicesV 62 and 63 will
3,068,377
5
6
gun including a cathode maintained at a predetermined
reference potential for producing an electron stream,
means for directing said stream along a predetermined
path, a collector electrode disposed opposite said electron
gun to intercept the stream electrons, an input helix
be determined by the operating frequency range oir band
fof the traveling-wave tube 10. The length of principal
helix 63 will generally be determined by the radio-fre
quency power output or gain required of the traveling
wave tubelt). Principal helix 63 will be maintained at
a potential to produce `a maximum x1 in order to obtain
disposed about said path adjacent said electron gun for
propagating an electromagnetic wave at a predetermined
maximum gain from the tube 16.
A2 is unaffected by changes in the length or potential
velocity, said predetermined velocity being small in com
parison to the velocity `of light, a principal helix electro
optimum length iand direct-current potential of input 10 magnetically coupled to said input helix and disposed be
tween said input helix and said collector electrode for
helix 62, it is necessary to plot the length of input helix
propagating said electromagnetic wave, means for main
62 as a `function of
taining said principal helix at a predetermined direct~
of input helix 62.
However', in order to determine an
current potential producing maximum amplification of
If a family of curves are thus plotted for ditïerent values
said electromagnetic wave, and means for maintaining
said input helix at a predetermined direct-current potential
minimizing the coupling loss to Said input helix and be
tween both of said helices with regard to the growing
wave portion of said electromagnetic wave.
of the direct-current potential of input helix 62, then
minimum loss, A40, may be plotted as `a @function of tube
length. In this manner, an optimum length for the helix
62 may be found where A40 is itself a minimum. By
using the optimum length, loss A4 may again be plotted
as a function of the direct-current potential of the input
helix 62. in this manner, an optimum direct-current
potential may ybe ascertained making signal gain a
maximum.
It has been determined experimentally that when two
helices having an electromagnetic coupling, such as the 25
helices 62 `and 63, having coupling helix 65, are insulated
References Cited in the tile of this patent
UNITED STATES PATENTS
2,588,831
2,660,689
2,694,159
Hansell _____________ ._ Mar. l1, 1952
Touraton et al _________ __ Nov. 24, 1953
Pierce ________________ __ Nov. 9, 1954
1,053,556
France ______________ __
FOREIGN PATENTS
from each other in a traveling-wave tube, a substantial
improvement in the gain of the tube may be obtained.
The invention is thus borne out both in theory and in
practice.
What is claimed is:
A traveling-wave tube ampliñer comprising yan electron
30
ept. 30, 1953
OTHER REFERENCES
Article by P. D. Lacy, pages 132 to 135, Electronics
for November 1954.
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