U.S. patent number 3,716,745 [Application Number 05/165,263] was granted by the patent office on 1973-02-13 for double octave broadband traveling wave tube.
This patent grant is currently assigned to Litton Systems, Inc.. Invention is credited to Robert Matthews Phillips.
United States Patent |
3,716,745 |
Phillips |
February 13, 1973 |
DOUBLE OCTAVE BROADBAND TRAVELING WAVE TUBE
Abstract
An O-type traveling wave tube of the invention includes a slow
wave structure having a broad band characteristic over a
predetermined range of frequencies and in some cases is capable of
broad band operation over a double octave of frequencies. In this
the slow wave structure comprises a combination of a substantially
nondispersive delay line, such as a helix, with a substantially
dispersive delay line, such as a ring loop section. The ring loop
sections complement in gain the gain drop off characteristic of the
helix at the edges of the frequency band at which the gain from the
helix circuit tapers off. In this the overall gain of the traveling
wave tube is enhanced and extended over a frequency spectrum
greater than that which is available solely with the helix
construction.
Inventors: |
Phillips; Robert Matthews
(Redwood City, CA) |
Assignee: |
Litton Systems, Inc. (San
Carlos, CA)
|
Family
ID: |
22598156 |
Appl.
No.: |
05/165,263 |
Filed: |
July 22, 1971 |
Current U.S.
Class: |
315/3.6; 315/3.5;
330/43 |
Current CPC
Class: |
H01J
23/27 (20130101); H01J 25/38 (20130101) |
Current International
Class: |
H01J
25/38 (20060101); H01J 23/27 (20060101); H01J
23/16 (20060101); H01J 25/00 (20060101); H01j
025/34 () |
Field of
Search: |
;315/3.6,3.5
;330/43 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Assistant Examiner: Chatmon, Jr.; Saxfield
Claims
What I claim is:
1. An O-type traveling wave tube having a uniform level of gain
over at least a one octave frequency range which comprises:
a cathode for providing a source of electrons,
a collector electrode spaced from said cathode for collecting
electrons,
an accelerator electrode spaced in between said cathode and
collector for accelerating electrons from said cathode for travel
through an interaction region defined in the space between said
accelerator electrode and said collector,
and slow wave structure means located within said interaction
region, said slow wave structure including an input for receiving
signals to be amplified and an output for passing signals amplified
as a result of interaction in said interaction region to a load;
and said slow wave structure including:
first and second substantially nondispersive delay lines spaced
apart in said interaction region, said first and second delay lines
possessing an overall gain characteristic which is of a
substantially uniform level over a first predetermined range of
frequencies and which decreases to lesser levels of gain at other
frequencies above and below said first range;
said input being coupled to the end of said first non-dispersive
delay line most proximate said cathode and said output being
coupled to an end of said second nondispersive delay line most
proximate said collector;
a first substantially dispersive delay line section spaced from and
located in the space between said first and second delay lines,
said dispersive delay line section having a second predetermined
gain characteristic effective to provide gain substantially only in
an increment of frequencies adjacent and outside said first
predetermined range for complementing the gain characteristic of
said first and second delay lines in an increment of frequencies
adjacent and outside said first predetermined range without
providing substantially any gain within said predetermined
frequency range to thereby extend said uniform level of gain over
an additional increment of frequencies adjacent and outside said
first predetermined range;
support means for supporting all said delay lines in said
interaction region;
and microwave loss material located on said support means at those
locations thereon corresponding to the locations of the opposed
ends of said nondispersive and dispersive delay lines for absorbing
microwave energy propagating to said opposed ends.
2. The invention as defined in claim 1 wherein each of said first
and second delay lines comprises further: elongated helixes of
electrically conductive material.
3. The invention as defined in claim 1 wherein said substantially
dispersive delay line comprises an equivalent contrawound helix
assembly.
4. The invention as defined in claim 3 wherein said equivalent
contrawound helix assembly comprises further a ring loop line.
5. The invention as defined in claim 2 wherein said substantially
dispersive delay line comprises a ring loop assembly.
6. A broad band double octave O-type traveling wave tube of the
type having a metal envelope containing a cathode and accelerator
means for generating an electron beam of a predetermined velocity
for travel into an interaction region and a slow wave structure
located within said interaction region; said slow wave structure
comprising a first section of helix having an end most proximate
said cathode connnected to a microwave input coupling means and an
end most remote from said cathode connected electrically in common
with said envelope; a second section of helix located in said
interaction region spaced from said first section of helix; said
second helix having its end most remote from said cathode connected
to a microwave output coupling means and having its end most
proximate said cathode connected electrically in common with said
envelope; a ring-loop type slow wave circuit of a predetermined
length; said ring-loop structure comprising a plurality of spaced
rings of metallic material in which adjacent rings are coupled
together electrically by short metal loops; said ring-loop section
being located within said interaction region in between said first
and second helix sections and having at least one end thereof
connected electrically in common with said envelope; said first and
second helix sections capable of providing a relatively flat gain
characteristic over a predetermined range of frequencies,
designated .omega..sub.1 to .omega..sub.2, and having a decreasing
gain characteristic in an adjacent upper range of frequencies,
.omega..sub.2 to .omega..sub.u, and in a lower range of
frequencies, .omega..sub.1 to .omega. , where .omega..sub.u >
.omega..sub.2 and .omega. < .omega..sub.1, and said ring-loop
structure including a first plurality of rings spaced apart a first
predetermined distance and a second plurality of rings spaced apart
a second predetermined distance different from said first
predetermined distance and having a gain characteristic effective
substantially only in the region of frequencies .omega..sub.2 to
.omega..sub.u and .omega..sub.1 to .omega. and which gain
characteristic complements that of said helix structure
substantially only within the region of frequencies of
.omega..sub.2 to .omega..sub.u and .omega..sub.1 to .omega. to
extend said relatively flat gain characteristic for said traveling
wave tube over the larger frequency range of .omega. to
.omega..sub.u.
7. An O-type traveling wave tube of the type containing a source of
electrons; means for forming said electrons into an electron beam
for travel into an interaction region; a collector electrode for
collecting electrons traveling from said interaction region, input
coupling means, output coupling means, a slow wave structure
located within said interaction region for receiving signals,
.omega., to be amplified at one end and providing amplified signals
at an output end; and said tube having a gain characteristic of a
predetermined uniform level over a predetermined range of
frequencies, between .omega..sub.a and .omega..sub.b, where
.omega..sub.a is a predetermined first frequency and .omega..sub.b
is a predetermined second frequency in said range of frequencies;
the improvement thereto wherein said slow wave structure comprises:
first substantially nondispersive slow wave structure means
substantially nondispersive over the frequency range of
.omega..sub.b to .omega..sub.i, where .omega..sub.i is a third
predetermined frequency within the range of .omega..sub.a to
.omega..sub.b, and having a substantially uniform flat gain
characteristic over said frequency range .omega..sub.b to
.omega..sub.i and a gain drop off at frequencies between
.omega..sub.i and .omega..sub.a ; said first nondispersive slow
wave structure comprising two spaced helix sections and said input
means coupled to the input end of said first helix section and said
output means coupled to the output end of said second helix
section; and second substantially dispersive slow wave structure
means disposed between said two spaced helix sections; said
dispersive slow wave structure means having a gain characteristic
limited to a narrow range of frequencies substantially less than
the difference .omega..sub.b -.omega..sub.a and said gain
characteristic of said second slow wave structure being
substantially complementary with said gain characteristic of said
first slow wave structure means in the frequency range between
.omega..sub.i and .omega..sub.a to provide in combination with said
nondispersive slow wave structure a uniform gain characteristic
over the frequency range of .omega..sub.a to .omega..sub.b.
8. The invention as defined in claim 7 wherein .omega..sub.b >
.omega..sub.i > .omega..sub.a .
9. The invention as defined in claim 7 wherein .omega..sub.b <
.omega..sub.i < .omega..sub.a.
10. The invention as defined in claim 8 wherein said second slow
wave structure comprises a ring loop line.
11. An O-type traveling wave tube of the type which includes: a
source of electrons, means for forming electrons from said source
into an electron beam for travel through an interaction region, a
collector electrode for collecting electrons traveling from said
interaction region, and a slow wave structure in said interaction
region for receiving signals to be amplified by interaction with
said electron beam in said interaction region, said tube having a
gain characteristic of a predetermined flat level over the range of
frequencies .omega. to .omega..sub.u, where .omega. is the lower
frequency of the range and .omega..sub.u is the upper frequency of
the range, the improvement wherein said slow wave structure
comprises:
first nondispersive slow wave structure means substantially
nondispersive over the frequency range of .omega..sub.x to
.omega..sub.y and having a substantially flat gain characteristic
over the frequency range .omega..sub.x to .omega..sub.y and a gain
drop off at frequencies greater than .omega..sub.y and lesser than
.omega..sub.x, where .omega. < .omega..sub.x < .omega..sub.y
< .omega..sub.u ; said first nondispersive slow wave structure
including:
a first helix section, and
a second helix section,
said second helix section being spaced from said first helix
section; and
second dispersive slow wave structure means having a relative
peaked gain characteristic and effective to provide gain
substantially only within the range of frequencies between .omega.
and .omega..sub.x and between .omega..sub.u and .omega..sub.y, said
gain characteristic of said second slow wave structure means being
substantially complementary with said gain characteristic of said
first slow wave structure means in the range of frequencies between
.omega..sub.u and .omega..sub.y and in the range of frequencies
between .omega. and .omega..sub.x ; said second dispersive slow
wave structure means being located in between and spaced from said
first and second helix sections; and
microwave energy input coupling means coupled to one end of said
first helix section and microwave energy output coupling means
coupled to one end of said second helix section.
12. The invention as defined in claim 11 wherein said second
dispersive slow wave structure comprises a ring-loop line.
13. The invention as defined in claim 11 wherein said second slow
wave structure comprises a ring-loop line.
14. The invention as defined in claim 11 wherein said second slow
wave structure means comprises a member of the contra-wound helix
family of circuits.
15. The invention as defined in claim 14 wherein said member of
said contrawound helix family of circuits comprises further a ring
loop line.
16. An O-type traveling wave tube which comprises:
a cathode for providing a source of electrons;
a collector electrode spaced from said cathode for collecting
electrons;
an accelerator electrode spaced in between said cathode and
collector for accelerating electrons from said cathode for travel
through an interaction region defined in the space between said
accelerator electrode and said collector; and
slow wave structure means located within said interaction region,
said slow wave structure including an input at one end for
receiving signals to be amplified and an output at its other end
for passing signals amplified as a result of interaction in said
interaction region to a load, said slow wave structure
including:
first and second substantially nondispersive delay lines spaced
apart in said interaction region, each of which comprises an
elongated helix of electrically conductive material, said first and
second delay lines possessing an overall gain characteristic which
is of a substantially uniform level over a first predetermined
range of frequencies and which decreases to lesser levels of gain
at other frequencies above and below said first range;
a first substantially dispersive delay line section spaced from and
located in the space between said first and second delay lines,
said substantially dispersive delay line comprising a ring-loop
assembly having a first portion with a relatively wide spacing
between rings for providing gain at high frequencies, and a second
portion with a relatively small spacing between rings for providing
gain at low frequencies, said dispersive delay line section having
a gain characteristic which complements the gain characteristic of
said first and second delay lines in an increment of frequencies
adjacent and outside said first range to extend said uniform level
of gain for an additional increment of frequencies adjacent and
outside said first predetermined range;
support means for supporting all said delay lines in said
interaction region; and
microwave loss material located on said support means at those
locations thereon corresponding to that of the opposed ends of each
said delay lines for absorbing microwave energy propagating to said
ends.
17. The invention as defined in claim 16 wherein the pitch of each
of said first and second of said ring loop structures are tapered
to provide a tailored gain characteristic.
18. An O-type traveling wave tube of the type which includes:
a source of electrons;
means for forming electrons from said source into an electron beam
for traveling through an interaction region;
a collector electrode for collecting electrons traveling from said
interaction region; a slow wave structure in said interaction
region for receiving signals to be amplified by interaction with
said electron beam in said interaction region;
input coupling means coupled to one end of said slow wave structure
and output coupling means connected to the other end of said slow
wave structure;
said tube having a gain characteristic of a predetermined flat
level over the range of frequencies .omega. to .omega..sub.u, where
.omega. is the lower frequency of the range and .omega..sub.u is
the upper frequency of the range, the improvement wherein said slow
wave structure comprises:
first nondispersive slow wave structure means substantially
nondispersive over the frequency range of .omega..sub.x to
.omega..sub.y and having a substantially flat gain characteristic
over the frequency range .omega..sub.x to .omega..sub.y and a gain
drop off at frequencies greater than .omega..sub.y and lesser than
.omega..sub.x, where .omega. < .omega..sub.x < .omega..sub.y
< .omega..sub.u, and comprising a first helix section and a
second helix section, with said second helix section being spaced
from said first helix section; and
second dispersive slow wave structure means having a relative
peaked gain characteristic, said gain characteristic of said second
slow wave structure means being substantially complementary with
said gain characteristic of said first slow wave structure means in
the range of frequencies between .omega..sub.u and .omega..sub.y
and in the range of frequencies between .omega. and .omega..sub.x,
said second dispersive slow wave structure being located in between
and spaced from said first and second helix sections and comprising
a ring-loop line having a first section operative to provide gain
only over a limited increment of frequencies in the high range of
frequencies and a second section operative to provide gain only
over a limited increment of frequencies in the lower range of
frequencies.
Description
FIELD OF THE INVENTION
This invention relates to a broad band traveling wave amplifier
tube and, more particularly, to an O-type traveling wave tube that
has a gain characteristic which is essentially flat over a broad
band of frequencies.
BACKGROUND OF THE INVENTION
The O-type traveling wave tube is a microwave vacuum tube amplifier
which utilizes in operation electronic "interaction" between
traveling electrons and a microwave signal propagating along a slow
wave structure, whereby, by means of such interaction, kinetic
energy is transferred from the electrons to the microwave signal
increasing the amplitude of the signal and lowering the velocity of
the electrons.
In a conventional traveling wave tube the electrons are formed into
an electron beam containing electrons of a predetermined average
velocity for travel into the "interaction region." Typically, this
beam is formed with a source of electrons, the cathode, a focusing
electrode for shaping the electron beam, and an accelerator
electrode maintained at a high positive potential relative to the
cathode for accelerating the electrons released at the cathode to a
predetermined velocity at which the electrons enter the interaction
region. In some instances a pervious grid electrode may be included
in such a structure to be used to turn the beam on and off.
The slow wave structure is located within the interaction region
and in present broad band tubes comprises an electrical conductor
wound in the shape of an elongated helix. This helix includes an
input terminal at the end of the helix most proximate the cathode
and an output terminal at the end most remote from the cathode. A
collector electrode is located at the remote end of the helix to
collect electrons passing through the helix and a focusing solenoid
or magnet system is provided surrounding the envelope or container
housing the recited elements for maintaining the electron beam
focusing in the interaction region. The electron beam is directed
into the interaction region through the center of the helix. A
microwave signal coupled to the helix input propagates along the
wire at a predetermined velocity substantially equal to the
velocity of light. However, the actual or effective lateral
movement of the signal across the tube is only a fraction of that
velocity because of its circuitous spiral path around the loops of
the helix; hence, the signal is "slowed." The pitch and diameter of
the helix is designed so that the lateral propagation of the signal
between input and output is slightly less than the velocity at
which the electrons in the electron beam are traveling. And the
helix diameter is also designed to permit the electromagnetic
fields of the signal to extend into the electron beam to permit
interaction. As a result of electromagnetic field interaction
between the microwave signal and the electron beam over this
interaction region, the electrons are slowed and thus give up
energy to the microwave signal, which thereupon is evidenced by a
growing amplitude of signal up to the output end of the helix, and
the electron beam is finally collected at the collector.
In a more sophisticated traveling wave tube the helix is "severed;"
that is, the helix is broken up into two or more sections. This is
accomplished by severing the helix at some point along its length
and electrically grounding each of the thus formed "ends," and by
depositing sufficient microwave loss material at the "sever"
location to fully absorb the microwave signal at the end of the
first helix section in addition to absorbing any reflected signals
traveling from the second helix section toward the input end. This
type of arrangement enjoys use in that it isolates the input and
output of the tube.
As a result of interaction between the propagating microwave signal
on the helix with the electron beam, the electrons are not only
slowed down but they become "bunched;" that is, some electrons in
the electron beam are slowed down more than other electrons and, as
a result, the faster moving electrons catch up with the slower
moving electrons so as to form bunches, regions along the electron
beam having a high density of electrons, and, in like manner, voids
or regions having low density of electrons. This "bunching" or
series of nodes or antinodes of electron density corresponds on a
sinusoidal basis with the frequency of the applied microwave signal
and increases proportionately with the degree of amplification of
that signal; thus, although the microwave signal applied to the
input is terminated at the end of the first helix section by the
sever, the "bunched" electron beam continues its travel into the
second helix section. In entering the second helix section, the
varying density electron beam induces the corresponding
electromagnetic signal on the helix of the second section. This
induced microwave signal thereupon proceeds to travel along the
turns of the second helix section and by continued interaction with
the same electron beam the signal grows and hence is amplified.
Thus, in the severed circuit arrangement the output circuit from
the traveling wave tube is electrically isolated from the source of
microwave energy applied to the input terminal, and the sole
coupling of energy between the input and output ends of the slow
wave circuit is due to the electron beam. These structures and
principles are well described in greater detail in the
literature.
Mention was made of a microwave signal without regard to its
frequency or to the bandwidth of frequencies over which a given
traveling wave tube effectively operates with uniform output. Broad
band operation requires that signals of a predetermined level and
of a predetermined frequency, .omega..sub.1, applied to the input
should be amplified and appear at the tube output with essentially
the same level as any other frequency, .omega..sub.n, of the same
level within that range or band of frequencies. As a practical
matter, the gain over a frequency range must be within about (6)
decibles to be considered a "flat" or uniform gain over that
frequency range. Obviously, in order to obtain broad bandwidth, one
must use a broad band slow wave structure within the traveling wave
tube. Frequently, this broad band characteristic of the slow wave
structure is described in other terms, i.e., as "nondispersive:" a
slow wave structure in which the velocity of propagation of the
signal along the structure is essentially independent of the
frequency of the signal. Conversely, very narrow band slow wave
structures are termed "dispersive:" the velocity of propagation of
the input signal along the structure is highly dependent upon the
signal frequency.
As is evident from the described mode of operation, for interaction
to occur the propagation velocity of signals to be amplified along
the slow wave structure must be slightly less than the velocity of
the electrons in the electron beam. Thus, if the signal propagation
velocity is substantially greater than the velocity of the electron
beam, no interaction and, hence, no amplification of the signal
occurs. Likewise, if the velocity of propagation of the signal is
much less than the velocity of the electrons, there is no
interaction and hence no amplification of the signal. Ideally, it
is thus desired for the slow wave structure to permit all signals
regardless of their frequency to travel along the slow wave
structure at a given identical velocity. Unfortunately, this ideal
is not possible. Hence the range of operation of any given
traveling wave tube is limited to a certain bandwidth. The more
dispersive the slow wave structure, the more narrow is the range of
frequencies which will be amplified by that traveling wave tube
structure. Conversely, as a general rule, the more nondispersive
the slow wave structure, the wider the range of frequencies which
can be amplified with uniform gain by the traveling wave tube.
The conductive wire helix is the most nondispersive slow wave
structure known, so much so that it is used as a standard and
ofttimes spoken of as nondispersive. Typically, the helix is
capable of effective operation over but an octave of
frequencies.
Basically, any O-type traveling wave tube constructed to have
uniform gain over at least one octave of frequencies incorporates a
substantially nondispersive structure, continuous or severed. The
most commonly used of these is the helix. Even so, given a helix of
predetermined diameter and pitch, a limiting factor at the lower
frequencies is dispersiveness. This is due primarily to the nature
of behavior of the electromagnetic fields at the low frequencies
where the lines of force extend over physical distances large
compared to the helix pitch. These lines or fields "jump" from turn
to turn of the helix and signal propagation is not confined to
travel spirally along the turns of the helix. At the low
frequencies the velocity of propagation of the signals is faster
than that which occurs when the signal travels solely along the
turns of the helix. Because of this, the signal travels at a
velocity exceeding the velocity of electrons in the electron beam
and there is decreased interaction.
A second limiting characteristic to the helix occurs in operation
at the upper edges of the frequency spectrum. While the helix, per
se, is not truly "dispersive" at these frequencies in the sense
defined previously, the interaction efficiency goes down again due
to inherent limitations in the physical nature of the
electromagnetic fields. For interaction between the propagating
signal in the helix and the electrons in the electron beam, the
fields must extend into the central area of the helix. At very high
frequencies, however, the wavelengths of these fields become small
in respect to the helix diameter and they do not fully extend into
the electron beam. As the interaction efficiency thus goes down,
signal amplification is reduced. As is commonly recognized, the
most broad band or nondispersive slow wave structure is limited at
the low frequency range by a dispersive characteristic and at the
high frequency end of the range due to less efficient interaction
with a uniform frequency gain characteristic between these two
ends.
Heretofore, structures have been proposed for purposes of
broadbanding microwave tubes. One prominent approach has been to
use a series of dispersive circuit structures and to tune each to a
different frequency so as to provide "stagger tuning." This
approach proves difficult because of the difficulties of matching
together the numerous sections of slow wave structure necessary to
obtain a smooth overall bandwidth characteristic. Moreover,
inasmuch as successive sections require loss material for severing
and isolation, excessive losses or attenuation do not permit
sufficient gain.
Other schemes for broadbanding microwave tubes usually involve
essentially narrow band tubes for operation at a single or very
narrow range of frequencies wherein it is desired to "slightly"
extend the narrow bandwidth of operation of the tube. In traveling
wave tubes this involves a highly dispersive slow wave structure
for maximum amplification at a single frequency so as to maximize
efficiency of the tube at that frequency and obtain highest gain.
Such structures are broadbanded somewhat by modifications to the
slow wave structure to make them slightly less dispersive. By way
of example, slow wave structures have been produced consisting of a
dispersive series of vanes which are coupled together on either
side by elongated helixes. Hence, the broadband characteristic of
the helix serves to broadband somewhat the very narrow band
characteristic of the main portion of the slow wave structure, that
being the highly dispersive vane assembly.
OBJECTS OF THE INVENTION
Accordingly, it is an object of my invention to provide a new broad
band traveling wave tube.
It is a further object of my invention to provide a traveling wave
tube construction capable of providing essentially uniform gain
over a double octave of frequencies without loss in efficiency.
It is a still further object of my invention to provide a broad
band O-type traveling wave tube in which the bandwidth
characteristic of a basic helix slow wave structure is enhanced at
both the upper frequencies and the lower frequencies.
And it is a still further object of the invention to provide a new
slow wave structure for an O-type traveling wave tube.
SUMMARY OF THE INVENTION
Briefly, in accordance with my invention, a traveling wave tube is
provided in which the helix slow wave structure is severed and
consists of two helix portions spaced apart a predetermined
distance within the electron interaction region. A dispersive slow
wave structure is located within the interaction region and between
the two helix sections to enhance the gain characteristic of the
helix at one or more ranges of frequencies.
In accordance with a further object of my invention, the dispersive
slow wave structure includes a first portion having its gain
characteristic complementary over a predetermined range with the
gain characteristics of the helix structure at the high end of the
frequency band, and a second portion having its gain characteristic
complementary with that of the helix structure at the lower end of
the band.
Further, in accordance with the invention, said dispersive slow
wave structure comprises a delay line of the contrawound helix
variety, such as the ring loop or ring bar.
Those characteristics of my invention which I believe to be novel
together with the objects and advantages of my invention and the
relationship and cooperation of the elements comprising the
invention, in addition to obvious substitutions and equivalents for
those elements, become more apparent from a consideration of the
following detailed description of the embodiments of my invention
taken together with the illustrations thereof in the drawing.
In the drawing:
FIG. 1 illustrates symbolically an O-type traveling wave tube
embodying the slow wave structure of the invention;
FIG. 2 illustrates in greater detail mechanically the dispersive
delay line;
FIG. 3 illustrates ideally the gain frequency characteristics of
each portion of the slow wave structure together with the gain
characteristic for the entire traveling wave tube; and
FIG. 4 illustrates the frequency versus normalized phase velocity
characteristic of each portion of the slow wave structure.
DETAILED DESCRIPTION OF THE INVENTION
The dashed line 1 in FIG. 1 represents the envelope of the
traveling wave tube. As is conventional, envelope 1 comprises a
vacuum-tight nonmagnetic stainless steel material which confines
internally a region in vacuum. And, as is customary, the envelope
is grounded. Cathode 3 provides a source of electrons. A filament
or heater 4 is provided for heating cathode 3 to enhance emission
of electrons. Both cathode 3 and heater 4 include electrical
conductors which pass through a vacuum-tight terminal in the
envelope to permit electrical connections to be made.
An accelerator electrode 5 is spaced from cathode 3 and
electrically grounded to the tube envelope. Accelerator electrode 5
contains a central passage 7 for permitting passage of electrons
emitted from the cathode 3.
A collector anode 9 is provided at the right hand end of the tube.
The collector electrode is electrically grounded in the illustrated
embodiment. Typically, the region within envelope 1 between the
accelerator electrode and the collector electrode may be referred
to as the electronic "interaction" region, as hereinafter becomes
more apparent. A "slow wave structure" or delay line is provided in
the interaction region. The slow wave structure of the invention as
schematically illustrated in FIG. 1 includes a first helix portion
or section 11 and a second helix portion or section 13 separated
and spaced from the first helix portion. The input end 10 of helix
11 is connected to an RF input terminal 9. The right hand end 12 of
helix 11 is electrically connected to the tube envelope and is
therefore at an electrical ground potential. The left hand or input
end of helix 13 is also connected to the tube envelope 1 and is
therefore the electrical ground potential. The right hand or output
end 15 of helix 13 is connected to the RF output terminal 16. These
helix sections form characteristically as hereinafter explained in
detail a nondispersive structure. The slow wave structure includes
further a dispersive section of slow wave structure such as is
available with the contrawound helix family of circuits and
particularly a ring loop section located in the interaction region
in between helix section 11 and 13 and is spaced therefrom. The
ring loop section used here by way of illustration includes a first
section 17 and a second section 19, which sections are illustrated
ad being electrically and mechanically connected together at their
respective right and left hand ends although they need not be. The
input end 18 of section 17 is connected to the tube envelope 18 and
the right hand end 20 of section 19 is electrically connected to
the tube envelope.
As is conventional, these slow wave structures are physically
supported in their respective locations within the tube envelope 1
by a series of three elongated ceramic support rods, which are
simply illustrated in FIG. 1 by a single dashed line 23. The
support rods are coated with a microwave loss or dissipative
material, suitably carbon, in predetermined amounts and at
predetermined locations thereon. Thus, as schematically
illustrated, a tapered density carbon loss material appears on the
support rods at adjacent the right hand end of helix 11, the left
hand end of ring loop section 17, the right hand end of ring loop
section 19, and the left hand end of helix 13, and are suitably
labeled 25, 27, 29 and 31, respectively. The loss materials are
conventionally provided to "match" the lines electromagnetically
and prevent passage of microwave energy traveling in either
direction past those points. Hence the carbon loss prevents
coupling between sections of the slow wave structure and prevents
electromagnetic energy from being reflected back from the tube
output terminal to the source, which avoids undesired
self-oscillation.
A source of voltage, V, represents the high voltage power supply.
This power supply has its negative polarity terminal connected to
the lead to cathode 3 and its positive polarity terminal connected
to electrical ground. A source of filament current is applied to
the filament leads. As is apparent such connection of the voltage
source places accelerator electrode 5, the helix sections, the tube
envelope 1, and anode 9 at a high positive voltage relative to
cathode 3.
A magnetic field longitudinal of the slow wave structure and
typically formed with permanent magnet rings or a solenoid is
illustrated by the symbol B and the arrow.
FIG. 2 better illustrates the dispersive ring loop slow wave
structure, schematically illustrated in FIG. 1 as elements 17 and
19, in mechanical perspective. For purposes of continuity and
perspective, the bracketed sections of the ring loop are labeled
17' and 19' and the portions of the helix sections 11' and 13' are
illustrated. It is apparent from the illustration that the ring
loop slow wave structure is made up of a series of ring members or
rings 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53 and 55. These
rings are spaced apart a predetermined distance and are
mechanically and electrically joined together by loop sections or
loops 34, 36, 38, 40, 42, 44, 46, 48, 50, 52 and 54. In the
preferred construction, alternate ones of these coupling loops join
together adjacent rings on the upper side of the slow wave
structure and the remaining ones are joined together on the bottom
side.
Ring loop 17' has a highly dispersive velocity-frequency
characteristic effective only at the higher range of frequencies of
the helix, while ring loop section 19' is highly dispersive and
effective only at the lower range of frequencies of the helix, as
hereinafter discussed in greater detail.
For purposes of illustration, each of the sections of the ring loop
is made up of six rings with section 17 comprising rings 33, 35,
37, 39, 41 and 43, whereas section 19' comprises rings 45, 47, 49,
51, 53 and 55. Coupling loop 44 joins together the two sections.
Preferably, the rings of either section together with the coupling
loops are stamped out of a single strip of molybdenum material and
then are bent at the coupling loop portions to form the series of
joined rings.
Basically, the narrow range of frequencies at which a ring loop
line is effective is determined primarily by the ring to ring
spacing. Thus, section 19' is operative at the lower frequency
range and the rings are spaced more closely together than the ring
to ring spacing found in ring section 17' effective in a narrow
range of higher frequencies.
As hereinafter explained, it is possible to tailor or adjust the
electromagnetic characteristics of each section within the narrow
frequency band by varying slightly the distance between individual
rings of that section so as to have a tapered gain characteristic
at that range of frequencies.
In a practical embodiment of my invention, I use a series of 40
rings spaced apart by an approximate distance of .100 inches for
fulfilling the function of section 17' and using a series of 60
rings spaced apart by an approximate distance of .050 inches for
operation of section 19'. It is apparent and within the scope of my
invention, however, for different numbers of rings to be employed
in alternative embodiments. Moreover, as is also apparent, the
number of rings in each separate section need not necessarily be
equal to the number of rings in the other section. It is not
necessary to couple together sections 17 and 19, moreover, although
I prefer to do so. Should it be desired to separate these two
sections of dispersive slow wave structure, one need only to remove
coupling ring 44, by way of example, to ground the top end of each
of rings 41 and 44, and apply dissipative loss to the support rods
to terminate the wave.
FIG. 3 illustrates ideally, by way of example, normalized
dispersion characteristics of the various portions of the slow wave
structure used in the traveling wave tube of FIG. 1. The dispersion
curve is one of the conventional graphical tools used by tube
designers for determining the operational characteristic of a
traveling wave tube. Thus it is apparent that the helix
characteristic is relatively flat between the frequencies .omega.A
to .omega.D. In terms of operation of the tube the phase velocity
of propagation of a microwave signal applied to the helix is over
that range of frequencies independent of frequency, and therefore
for electron beam interaction in which the electron beam is set to
a predetermined single velocity, interaction between the microwave
signal and the electron beam can be obtained over the entire range
of frequencies .omega.A-.omega.B. However, between the range of
frequencies .omega.E to .omega.A in the lower frequency ranges the
diagram illustrates that the phase velocity is somewhat dependent
upon and a function of frequency of the signal and, in particular,
in this lower range of frequencies the microwave signal of that
frequency travels at a greater phase velocity. Physically this is
explained because the electric fields at lower frequencies extend
over greater physical distances than is true at the high
frequencies, the low frequency signal has the ability to extend
across, couple, or "jump" from turn to turn of the helix rather
than ideally following only the spiral path of the conductor
forming the turns of the helix. Hence, the low frequency signal for
a given helix tends to travel therealong at a faster velocity than
those microwave signals in the range of .omega.A to .omega.B or
midband signals. This, of course, precludes or reduces electronic
"interaction" with an electron beam which, by design and as a
general rule, is set to contain electrons which travel at a single
velocity of the electrons must be slightly greater than the
velocity of the signal in order for interaction to occur.
Ring loop section 19' has the dispersion characteristic in which
the phase velocity of the applied signal along that section is
highly dependent upon and a function of the signal frequency, as is
indicated by the left hand curve in the figure. Likewise, ring loop
section 17' has a dispersion characteristic which is highly
variable and in which the phase velocity of an applied signal is
highly a function of the frequency of the signal as appears in the
right hand curve in the figure. Both dispersion characteristics are
representative of "high dispersiveness;" that is, they are
effective only over a frequency range of 10 to 25 percent of the
mid-band frequency. Thus, the dispersion characteristic for each of
these loop sections is only effective over a predetermined narrow
range of frequencies so that at the high end frequencies around
.omega.D only ring loop section 17' is effective, whereas around
the lower end of frequencies .omega.C only the ring loop section
19' is effective.
FIG. 4 illustrates the "gain" characteristic of the entire slow
wave structure and is represented as curve 60. By this graphical
representation, the individual contributions to "gain" or
amplification of the traveling wave tube can be evaluated. Thus
dispersion curves of FIG. 3 provide an understanding of whether or
not interaction will occur and to what degree for a given electron
beam velocity. The gain curve shows the result of such interaction
in terms of amplification. The gain of the helix labeled as curve
61 is relatively "flat" (.+-. 3 db) between frequencies .omega.A to
.omega.B. And either above .omega.B or below .omega.A the gain
decreases.
As previously explained, the reduction in gain at the lower range
of frequencies for the "nondispersive" helix type of slow wave
structure is due to loss of interaction between the low frequency
signal and the electron beam because the helix in that frequency
range is "dispersive," as is apparent from the curve of FIG. 3.
However, the departure of the helix gain from an ideal flat gain
characteristic at the upper range of frequencies, those above
.omega.B, results because the signals at the high frequencies are
of such short wavelength that the fields do not extend fully from
the helix into the helix center where those signals must interact
with the electron beam. Thus, in this way, a loss of interaction
efficiency occurs and is responsible for the reduced and decreasing
gain characteristic of the helix at the upper frequencies.
Curve 65 illustrates the gain characteristic for ring loop section
17 and curve 63 illustrates the gain characteristic for ring loop
section 19. As is apparent, each of the gain characteristics of the
ring loop sections is highly "peked" and dependent upon frequency.
Moreover, it is apparent that at the upper range of frequencies
ring loop section 19 has no gain and does not contribute any gain,
whereas at the lower range of frequencies ring loop section 17 has
no gain and does not contribute to the tube gain. The center
frequency (highest gain) of ring loop section 19' is at the
frequency .omega.C, which corresponds to a point on curve 61 of the
helix where the gain of the helix section is reduced approximately
20 db. Combining the ordinate of curves 61 and 63 in this region a
combined gain characteristic is obtained represented by the dashed
line 67 portion of curve 60. As is apparent, the gain
characteristic is raised to the level corresponding with the gain
of the helix in mid-frequency band and is relatively flat over a
predetermined additional range of lower frequencies from .omega.A
down to .omega.E (the 3 db point).
In this way, the bandwidth of the amplifier is increased over that
bandwidth available solely with a "nondispersive" helix over an
additional range of frequencies .omega.E to .omega.A.
In like manner, by design the peak of the gain characteristic of
ring loop section 17 is set at the frequency .omega.D which
corresponds to a point on the gain curve 61 of the helix where the
gain is substantially reduced. A resultant or combined gain
characteristic is obtained by adding together the ordinator of the
graph which is represented by the dashed line portion 69 of curve
60. As is apparent, the combination of gain from the helix and ring
loop 17 raises the overall gain of the helix at the high frequency
end and is relatively flat up to the frequency .omega.F where gain
commences to decline. Thus, given a flat gain characteristic for
the helix effective over the frequency range of .omega.A to
.omega.B a resultant flat gain characteristic is obtained for the
amplifier effective over a significantly larger frequency range
.omega.C to .omega.D by the addition of the highly dispersive
ring-loop sections 17 and 19.
The operation of the traveling wave tube, schematically illustrated
in FIG. 1 and containing the slow wave structure of the invention,
is in great part conventional, well known, and described in the
literature. Basically, electrons emitted by cathode 3 are attracted
by the accelerator electrode at a high positive potential relative
to the cathode and are accelerated up to a predetermined design
velocity. The electrons are formed into a beam, pass through
passage 7 and enter the interaction region. Obviously, the
velocities which the electrons attain up through passage 7 is
dependent upon the power supply voltage, V, as well as the distance
between cathode and electrode 5. Typically with a supply voltage 10
kilovolts these electrons are traveling at velocities of 20 percent
of the velocity of light upon entering the interaction region.
These electrons travel ideally in a straight path through the helix
section 11, ring loop sections 17 and 19, and helix section 13, and
then to collector electrode 9 where the electrons return through
electrical ground back to the positive polarity terminal of the
power supply. As is well known, however, electrons in a beam
because of their similar electrical charge repel one another which
could cause the beam to spread. In addition, if any electrons enter
the interaction region with a transverse velocity component they
would collide with the slow wave structure. The magnetic field B,
illustrated in the figure, axial with the beam path and created by
a solenoid or magnet system outside the tube envelope 1 serves to
focus those electrons to maintain them in a compact beam. The
longitudinal or axial magnetic field forces any electrons traveling
in a path away from the straight path defined are caused to spiral
around the beam path and in this way prevents electrons from
reaching the slow wave structure.
A microwave signal of a given frequency, .omega., assumed to be in
the band of frequencies between .omega.A and .omega.B, is applied
to the RF input terminal and proceeds to travel or propagate up
lead 10 into and along the turns of the helix section 11, spiraling
around therealong to the end of helix 11. Since electromagnetic
energy, such as this signal, travels, essentially, at the speed of
light, a known constant, c, along the turns of the helix, the
actual longitudinal travel of that signal is less than the speed of
light because of the circuitous path around the spirals of the
helix taken by the signal. Essentially, this longitudinal travel is
slowed down by a factor in direct proportion to the diameter of the
helix and the pitch or distance between turns of the helix,
normally about 20 percent the speed of light and approximately 10
percent less than the electron velocity of the electron beam.
Hence, the designation "slow wave structure." In propagating along
the slow wave structure, such as helix 11, the microwave signal
possesses fields which extend from the structure into the region of
the electron beam and creates periodically alternating fields which
act to accelerate or decelerate relative to the electrons traveling
through the helix. Under the influence of the decelerating field
the electrons in the electron beam are slowed down. Conversely,
under the influence of the accelerating fields the electrons in the
electron beam are speeded up. These two groups of accelerating and
decelerating electrons form together into regions along the
electron beam path where the electrons are said to be "bunched."
The microwave signal applied to the slow wave structure changes the
electron beam into a beam comprising spaced bunches of electrons.
These bunches tend to form at the locations of nodes along the slow
wave structure; that is, the position between the accelerating and
the decelerating fields, or zero field.
The phenomenon of interaction occurs in that the electron beam
velocity by design is made to be on the average slightly greater
than the velocity of longitudinal travel of the microwave signal
along the slow wave structure. Typically, as was stated the
electron beam velocity is from 1 to 10 percent greater than that of
the electromagnetic signal on the slow wave structure. The
traveling bunches of electrons hence are attempting to travel at a
greater speed than that of the microwave signal and the
decelerating fields generated thereby so that the electron bunches
are increasingly under the influence, during travel, of a
decelerating electromagnetic field. The microwave field, hence,
tends to slow down the bunch of electrons. And in slowing down the
electrons some kinetic energy is given up by the electrons which
is, in turn, transferred to the microwave signal. Hence, by
continued "interaction" over the length of a slow wave circuit, the
average velocity of the electrons in the beam diminishes, bunching
becomes more pronounced, and the average amplitude of the microwave
signal applied to the slow wave structure input tends to increase.
A more detailed and scientific explanation of these phenomena is
available in the literature.
In itself, the foregoing phenomenon describes basically the
operation of an O-type traveling wave tube. Generally, however, it
is desirable to isolate the input signal source applied to the
microwave tube from the output or RF load end for reasons,
including the prevention of internal reflections in the tube from
causing self-oscillation and in preventing any mismatch in the load
from affecting the operation of the input signal source. To do so
the input and output circuits are "severed," typically by the use
of microwave attenuation material, combined with or without
physical separation of the parts of the helix. This is described in
the further description in the mode of operation of the traveling
wave tube of FIG. 1.
At a position near the output end of helix 11 the input signal,
.omega., has been amplified to a high level and the underlying
electron beam is correspondingly "bunched." The amplified signal
travels to attenuator 25 where it is absorbed and dissipated. The
attenuator thus prevents the microwave signal .omega. continuing
along the slow wave structure. However, the electron beam has been
bunched in proportional intensity and these bunches continue to
travel in a course toward helix 13, omitting for the present the
description of any interaction in ring loop sections 17 and 19. In
entering helix 13, the bunches of electrons "induce" within the
first turn of the helix the microwave signal of frequency, .omega.,
corresponding to the input signal which originally bunched the
electrons beam, somewhat analogously to the manner in which the
grooves of a phonograph record reproduces the signals which
originally were used to cut those grooves. Subsequently, this
induced microwave signal, .omega., travels along the helix section
13 and continues electronic interaction with the electron beam,
further bunching the beam, essentially in the same manner as was
discussed in connection with the helix operation of helix 11, and
the microwave signal, .omega., continues to grow in amplitude until
the output of the helix is reached. The signal travels via lead 15
to the RF output terminal where it is coupled to an electrical
load, not illustrated.
The electrons in the beam which have given up much of their energy
at this point continue to travel to collector 9 with which they
collide, dissipate most of their remaining energy in heat at the
collector, return to ground and back to the original D. C. power
supply. Attenuator 31 coupled to the front end of helix section 13
dissipates any reflected microwave energy, i.e., any microwave
energy which might travel from the RF output terminal into the
helix in a reverse direction toward the front end of the helix.
Such reflections could cause oscillations and are thus desirably
eliminated in this manner.
The electron bunches formed during interaction contain the
information representative of the original microwave input signal
applied to helix 11, much as a fingerprint is representative of the
fingertip that made the impression, and it is this information that
is induced upon helix section 13 as the bunched electrons enter the
second helix section.
As was previously assumed, the microwave signal, f, applied to the
RF input terminal was in the mid-range of frequencies for which the
embodiment of FIG. 1 was designed to operate. Reference to FIGS. 3
and 4 show that the microwave frequency signals within this range
are uninfluenced by ring loops 17 and 19 which are effective only
at the higher or lower frequencies, respectively. Thus any signal
induced on structures 17 and 19 during passage of the electron beam
bunches is uneventful in that it is quickly dissipated without
interaction.
During passage through ring loop sections 17 and 19, there is no
change either to the bunching of the beam or to the slow wave
structure, as is apparent from FIG. 3 and from FIG. 4. Ring loop
section 17, represented by curves 65 and 17', are ineffective or
lacking in gain, as may be variously termed, to signals in the
mid-band range of frequencies. In like manner, as the electron beam
passes through the second ring loop section 19, and it too is
ineffective to change the characteristic of the bunching, there is
no interaction inasmuch as the characteristics of the ring loop
section 19, as illustrated in curve 19' of FIG. 3 and 63 of FIG. 4,
have no influence upon signals in the mid-band range and may be
considered neutral or inert.
Considering now the operation of the tube with a high frequency
band edge signal, .omega.D, applied to input lead 10, the .omega.D
signal interacts with and causes bunching of the electron beam in
helix section 11 in the same manner as discussed with respect to
the mid-band section .omega.. As the bunched electron beam proceeds
into ring loop section 17 it induces a corresponding microwave
signal on the ring loop. It is within this frequency region, as
represented graphically by FIGS. 3 and 4, that ring loop section 17
is effective. Accordingly, the signal induced on ring loop section
17 thereupon interacts further with the electron beam and is
amplified. Conversely, the amplified signal causes greater bunching
of the electron beam. Much in the same manner as the original
microwave input signal is dissipated in a terminating attenuator
25, previously discussed, the signal generated upon and amplified
in ring loop section 17 proceeds along the loop through ring loop
section 19 to a similar terminating attenuator 29, where it is
dissipated. The increasingly bunched electron beam travels through
section 19 where it undergoes no further change inasmuch as ring
loop section 19 is ineffective as evidenced by the gain curve 63 of
ring loop section 19, and the bunched electron beam proceeds to
enter the second helix section 13 where, in the normal manner
previously described, a signal is induced upon the initial turns of
the helix and through interaction that signal is amplified and
taken at the output.
The converse situation occurs with respect to a microwave signal in
the low frequency range, such as .omega.C, applied to the RF input
terminal. In this operation the ring loop section 17 is ineffective
and the second ring loop section 19 is effective to cause further
bunching and amplification of he electron beam. As before, the
amplified signal is dissipated in attenuator 29 at the end of ring
loop section 19 and the increasingly bunched electron beam proceeds
into the final helix section 13 where it induces a corresponding
signal upon the initial turns of this helix, which thereupon
interacts with the electron beam, is amplified progressively along
the helix section 13 and is taken from the RF output terminal.
As is evident from FIG. 4, the gain of the helix sections, taken
together, is substantially flat over a predetermined frequency band
.omega.A to .omega.B and drops off the frequencies above .omega.B
and similarly drops off at frequencies below .omega.A.
Considering first the characteristics of ring loop section 19' as
represented by curve 63 in FIG. 4, the ring loop section has a
predetermined and peaked gain characteristic which by design
centers around a frequency .omega.C and drops off at a
predetermined rate at frequencies either above or below .omega.C.
The center frequency of gain for the ring loop section 19' is
designed so that it occurs where the gain of the helixes alone is
down about 20 db relative to the gain characteristic of the helix
section. By adding together the relative gain illustrated in curves
61 and 63 in this region, the curve representing the summation of
such gain is derived and is indicated by the dashed lines 67. By
this combination of the relatively nondispersive helix and the
dispersive ring loop section 19' the bandwidth of the traveling
wave tube is seen to have been extended relatively flat from
frequency .omega.A to a lower frequency .omega.C.
In like manner, the design characteristic of ring loop section 17'
is highly dispersive and is peaked at a frequency .omega.D within
the region above frequency .omega.B at which the gain
characteristic of the helix section is decreasing. At frequencies
either above or below .omega.D the gain characteristic of ring loop
section 17' decreases. The center frequency .omega.D, for which the
ring loop is designed to have peak gain, is selected so as to fall
at a point along the gain characteristic of the helix section as
illustrated in curve 61, so that the gain of the helixes alone is
again down about 20 db. By combining the contributions of signal
gain from each of the helix section and ring loop section 17', a
gain curve represented by the dashed lines 69 is obtained and is
seen to be of the same highly level over a predetermined range of
frequencies as the helix gain at the mid-band frequencies. The gain
for the tube is seen to have increased from the high frequency
.omega.B to a higher frequency .omega.F.
In this way, the tube so constructed has a broad band or flat gain
characteristic, however termed, over a greater frequency range of
.omega.E to .omega.F. This is substantially greater than the
bandwidth .omega.A to .omega.B available solely with a
nondispersive helix-type structure.
As was previously noted, the helix alone as a slow wave structure
has uniformly, because of its substantially nondispersive
characteristics, been chosen as the slow wave structure for broad
band operation. With the addition of dispersive slow wave
structures at the band edges of the helix it is seen that a new
structure is obtained that has a broad band characteristic greater
than that possible with a series of highly dispersive slow wave
structures or with the helix alone.
It is further apparent that the broad banding is accomplish without
any reduction in the tube efficiency in that no attenuation is used
and that the contribution of the dispersive lines is either a
contribution to signal gain or none at all.
In the foregoing description of the preferred embodiment of the
invention, a traveling wave tube structure was described which
provided the maximum possible gain enhancement at both the higher
and lower frequency ranges of an ordinary helix by the inclusion of
two individual highly dispersive ring loop lines operative and
effective at the higher and lower frequency band edges
respectively. It is apparent that further modifications of this
invention can provide improved bandwidth characteristics in a
traveling wave tube by simply enhancing the gain at either the
lower range of frequencies or, alternatively, at only the high
range of frequencies. Such embodiments, however, do not provide the
same extent of broadbandedness as is found in the preferred
embodiment, but do provide a substantial increase in bandwidth over
those traveling wave tubes which use a helix slow wave structure
without more.
Thus, a traveling wave tube which has a broad band gain
characteristic but in which the gain is enhanced solely at the low
frequency band edge in an embodiment of my invention requires a
modification to the traveling wave tube, schematically presented in
FIG. 1, to omit ring loop section 17. In that embodiment the end of
ring loop section 19 shown connected to ring loop section 17 is
instead connected to the tube envelope 1 where it is electrically
grounded. In addition, a microwave attenuative material on support
rod 23, illustrated as 29 in FIG. 1, would instead be located
proximate the left end of ring loop section 19.
In like manner, a broad band traveling wave tube of the invention
is provided in another embodiment in which the gain of the
traveling wave tube is enhanced solely at the higher or upper band
edges. That embodiment requires a modification to the embodiment
illustrated schematically in FIG. 1 in which ring loop section 19
is omitted and in which the right end of ring loop section 17,
illustrated connected to ring loop section 19, is connected instead
to the tube envelope 1 where it is placed at an electrical ground
potential. Additionally, the microwave loss material 27 located on
support rods 23 adjacent the left end of ring loop section 19 is
located in such an embodiment at the left hand edge of ring loop
section 17.
In a practical construction of one embodiment of the invention in
which solely the upper frequencies were enhanced with the inclusion
of a single ring loop section, the helix had a gain characteristic
which was flat essentially over the frequency range of 1.6 GHz to
4.0 GHz. A ring loop section having 40 rings and of the same
diameter as the helix was constructed in which the rings were
spaced apart by a uniform pitch of approximately .100 inches. In
this way the gain of the traveling wave tube was enhanced over an
additional range of frequencies of 4.0 GHz to 5.4 GHz. It is noted
however that in such construction that the gain was slightly peaked
at the high frequency end by a factor of 3 db which suggests that
the ring loop provided slightly larger than the desired gain.
However, as is apparent, this is easily corrected by tapering or
adjusting the ring to ring spacing. In this way the dispersion
characteristics of this highly dispersive line, and accordingly the
gain characteristic, is changed. For example, by adjusting the ring
to ring spacing so that it varies from .08 inches to .10 inches
with at least one half of the rings at a constant .10 inches, the
peaking is found to be smoothed out. Thus, by adjusting the
amplitude of the gain characteristic of the dispersive ring loop
the overall gain of the traveling wave tube can be maintained at
the desired level and is accordingly enhanced.
The foregoing embodiments are intended to be illustrative of the
invention and not as a limitation to my invention.
As is apparent, many alternative embodiments and substitutions for
the element become apparent to one skilled in the art from this
specification, all of which come within the spirit of my invention.
Accordingly, it is expressly understood that my invention is to be
broadly construed within the breadth and scope of the appended
claims.
* * * * *