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United States Patent im 



[75] Inventor: Henry G. Kosmahl, Olmsted Falls, 


[73] Assignee: The United States of America as 

represented by the United States 
National Aeronautics and Space 
Administration, Washington, D.C. 

[21] Appl.No.: 130,058 

[22] Filed: Dec. 8, 1987 

[51] Intel.* HOI J 25/34 

[52] U.S. Cl 315/3.5; 315/3; 


[58] Field of Search 315/3.5, 5, 5.12, 3; 


[56] References Cited 


2,924,738 2/1960 Chodorow 315/3.5 

3,443,146 5/1969 Buck 315/3.5 

4,439,746 3/1984 Epsztein 331/82 


Patent Number: 



Date of Patent: 

Dec. 26, 1989 

Primary Examiner — Robert L. Griffin 

Assistant Examiner — T. Salindong 

Attorney , Agent, or Firm — James A. Mackin; Gene E. 

Shook; John R. Manning 


It is an object of the invention to provide a miniature 
traveling wave tube which will have most of the advan- 
tages of solid state circuitry but with higher efficiency 
and without being highly sensitive to temperature and 
various types of electromagnetic radiation and sub- 
atomic particles as are solid state devices. 

The traveling wave tube which is about 2.5 cm in length 
includes a slow wave circuit (SWS) comprising aper- 
tured fins with a top cover which is insulated from the 
fins by strips or rungs of electrically insulating, dielec- 
tric material. 

Another object of the invention is to construct a SWS 
of extremely small size by employing various grooving 
or etching methods and by providing insulating strips or 
rungs by various deposition and masking techniques. 

12 Claims, 3 Drawing Sheets 




Dec. 26, 1989 

Sheet 2 of 3 4,890,036 






This invention was made under NASA contract 
NAS3-24565 and has been assigned to the Administra- 
tor of the National Aeronautics and Space Administra- 
tion. 1Q 


This invention relates to traveling wave tubes (TWT) 
and is directed more particularly to a miniature TWT. 

Because of the burgeoning use of satellites for com- 15 
munications, transmitting tubes operating in the 5 to 60 
GHz range are required in order that a greater number 
of messages may be carried in a particular radio fre- 
quency signal. Further, present and future needs will 
utilize phased array antennas or radar systems made up 2Q 
of a large number of small individual transmitters of 
relatively low power of from 0. 1 to 10 watts for each 
transmitter. Each array may consist of, for example, 100 
transmitters. Such arrays are electronically steerable 
and each transmitter may radiate RF directly into open 25 

Many phased array antennas utilize solid state devices 
to provide a large number of individual transmitting 
units which are electrically steerable. Unfortunately, 
solid state devices are sensitive to temperature and ex- 30 
traneous radiation such as gamma rays, x-rays, neutrons, 
and protons. Furthermore, solid state devices have a 
lower power output, a narrower frequency range and a 
lower efficiency than TWTs utilizing an electron beam. 

For passage of an electron beam, some TWTs of the 35 
prior art provided notches in the top edges of the propa- 
gating plates or fins to avoid the impossibility of me- 
chanically assembling such structures with accurately 
aligned apertures for the beam. However, for TWTs 
operating above, for example, 5 GHz, the spacing when 40 
a top cover is added is entirely insufficient for the beam. 

Accordingly, it is an object of the invention to pro- 
vide a traveling wave tube having many of the advan- 
tages of solid state devices such as miniature size and 
suitability to non-mechanical construction. 45 

It is another object of the invention to provide a 
traveling wave tube having 1 to 5 watts power output, 

3 kV or less electron accelerating voltage and operating 
in the 5 to 60 GHz range. 

Still another object of the invention is to provide a 50 
miniature TWT with apertured propagating fins, 
wherein the apertures are accurately aligned and are at 
a point of high impedance to a slow wave. 

An additional object of the invention is to provide a 
traveling wave tube which may not require severs. 55 


U.S. Pat. No. 4,439,746 to Epsztein discloses a micro- 
wave oscillator in which a wave guide cavity is pro- 
vided with upstanding, apertured vanes and having 60 
coupling slots provided in the cavity between succes- 
sive pairs of vanes. 

U.S. Pat. No. 3,443,146 to Buck discloses a traveling 
wave tube wherein the delay structure discloses aper- 
tured propagating members extending from at least one 65 
wall of the tube, the propagating members being con- 
nected to one another at alternate positions by bars to 
form the equivalent of a helix slow wave structure. 


U.S. Pat. No. 2,924,738 to Chodorow discloses a slow 
wave structure with interdigital apertured fins. The fins 
have alternating negative and positive potentials. 

Many traveling wave tubes are known which include 
helically wound or coupled cavity slow wave struc- 


In accordance with the invention, there is provided a 
miniature, low-voltage TWT including a slow wave 
structure comprising successive apertured fins extend- 
ing from a bottom base member. The top cover plate is 
disposed on the fins opposite the base member with an 
electrically insulating layer disposed between the fins 
and the top cover. The electrically insulating layer is a 
dielectric material having a dielectric constant of 10 or 

Techniques such as vapor deposition, sputtering, or 
ion beam implantation in a pattern which will be in 
register with the top edges of the apertured fins. Strips 
of dielectric material may be mechanically disposed 
between the fins and the top cover plate. 

Preferably a cold cathode is utilized to provide an 
electron beam which interacts with an injected RF 
signal and passes through the apertures in the fins. At 
the end of the tube opposite the cathode, a multi-stage 
depressed collector may be incorporated to capture the 
spent electrons at high efficiency. 


FIG. 1 is a schematic top view of a travelling wave 
tube embodying the invention. 

FIG. 2 is an oblique view schematic drawing of the 
amplifying SWS and its top cover. 

FIG. 3 is a cross sectional view of the TWT of FIG. 

I taken along the line 3—3. 

FIG. 4 is a bottom view of a top cover plate for the 
SWS showing rung insulating layer members each rung 
having a gap. 

For purposes of clarity, the drawings are not to scale 
and dimensions are not in proportion to actual sizes. 


Referring now to FIG. 1, there is shown a traveling 
wave tube 10 constructed of an electrically insulating 
material such as glass. Disposed inside the TWT 10 is a 
slow wave circuit 11 comprised of a plurality of thin, 
apertured fins 12 which all extend in a common direc- 
tion from a base member 13. The SWS may be copper 
or silicon. If silicon is employed, a plating of an electri- 
cally conductive material such as gold or copper must 
be applied to the silicon. 

With relationship to an electron beam 14 emitted by a 
cathode 15, the fin 12 at the extreme right of the SWS 

II may be considered as a first upstream fin while the 
fin 12 at the extreme left of the SWS may be identified 
as the last downstream fin. Thus, an input coupling slot 
16 in the base member 13 is located immediately down- 
stream of the first fin 12 while an output coupling slot 17 
is positioned upstream of the last fin 12. The cathode 15 
is preferably a cold cathode but a miniature thermionic 
cathode may be used. 

The coupling slots 16 and 17 are preferably the same 
width as the fins 12 in order to provide the correct 
matching to impedance matching transformers horns 
which will be attached to the slots 16 and 17 as will be 
described presently. If the coupling slots 16 and 17 are 



as just described, that is, the same width as fins 12, there 
will be no structure to support the first and last fins 12. 
Accordingly, flanges 18 and 19 are provided along the 
length of the base plate to provide the necessary support 
for the first and last fms 12. 5 

At the end of TWT 10 opposite the cathode 15 there 
is provided a multistage depressed collector comprised 
of collector plates, 20, 21, and 22. A high voltage, on the 
order of 1-3 kV, is applied between a terminal 23 and 
ground 24 across a voltage divider 25. Collector elec- 10 
trode 20 is connected to the ground point and collector 
electrode 22 is connected to the high negative voltage 
terminal 23 while the collector electrode 21 is con- 
nected to an intermediate point on the voltage divider 
25. The voltage divider 25 is exemplary only, as a high 15 
efficiency traveling wave tube would utilize well 
known components other than resistors to provide an 
intermediate voltage for collector plate 21. 

The length of the SWS is indicated by the double 
ended arrow 26 while the width of the fin 12 is indicated 20 
by the double ended arrow 27. Opposing arrows 28 
specify the spacing between fms 12 while double ended 
arrows 29 indicate the distance from the front surface of 
a particular fin to the front surface of the next down- 
stream fm 12 (the period of the SWS). The thickness of 25 
the fms 12 is shown by the opposing arrows 30. 

Referring now to FIG. 2, there is shown an oblique 
schematic view of the SWS 11 of FIG. 1 wherein parts 
identical to parts in FIG. 1 are identified by like numer- 
als. As shown, the slow wave circuit 11 is provided 30 
with a top cover plate 31 having longitudinal strips of 
electrically insulating material 32 and 33 attached to its 
lower surface. The insulating strips 32 and 33 run 
lengthwise along the edges of the top cover plate 31 to 
establish a separation between the strips. The purpose of 35 
the separation is to avoid any buildup of charge on 
strips 32 and 33 and to provide a space between the top 
of the fms 12 and the cover plate 31 for the slow wave 
to propagate along the electron beam while the circuit 
wave travels a much longer path up and down the fins 40 
12, the bottom base walls and then around the top of 
each of the fins 12 into the next fm. An accumulation of 
charge on strips 31 and 32 could cause arcing at various 
points on the SWS 11. 

The fins 12 are provided with respective aligned 45 
apertures 34 through which electron beam 14 passes. In 
order to match the impedance of the input coupling slot 
16 and the output coupling slot 17 to wave guides, re- 
spective horns or matching transformers 35 and 36 are 
attached to the base member 13. 50 

Referring now to FIG. 3, parts corresponding to 
those of FIGS. 1 and 2 are identified by corresponding 
numerals. A double ended arrow 37 indicates the height 
of the fm 12 while opposed arrows 38 show the diame- 
ter of the aperture 34. The distance of the center of 55 
aperture 34 from the top edge of fm 12 is indicated by 
opposing arrows 39 while opposing arrows 40 show the 
thickness of the insulating strips 32 and 33. 

FIG. 4 is a bottom view of a top cover plate 31 for an 
alternate embodiment of the invention in which the 60 
electrically insulating layer attached to the cover plate 
31 is in the form of a row of rungs 41 and a row of rungs 
42, each row extending inwardly from respective oppo- 
site edges of the cover plate 31. The rungs 41 are di- 
rectly opposite respective rungs 42. 65 

The rungs extend inwardly only to the extent that 
they not interact with the electron beam to become 
charged as discussed previously with respect to the 


separation of the electrically insulating strips 32 and 33 
of FIG. 2. The rungs 41 and 42 are precisely located so 
they will be in register with respective ones of said fms 
12 when the top cover plate 31 is disposed against the 
top edges of the fms 12. 

The miniature TWT 10 of FIG. 1 utilizes a voltage of 
only 1 to about 3 kV to accelerate the electrons of the 
beam 14. The current of the electron beam is also quite 
low being in range from about 1 to 10 mA. 

A TWT embodying the invention for operation in the 
5 to 60 GHz range and at 1 to 5 watts of power will 
have the approximate dimensions as set forth in Table I 








TWT length, 30 GHz 

about 2.5 cm 


Fin width 

Xq/ 4 or less 


Distance between fins 

(0.2- 1.0 mm)- 
(0. 1-0.7 m) 


Spacing of fins (period) 

0.2- 1.0 mm 


Fin thickness 

0. 1-1.0 mm 


Fin height 

1.0-10 mm 


Aperture diameter 

0.25-1.0 mm 


Distance from top of fin 

0. 1-0.3 times 

to aperture center 

fm height 


Thickness of electrically 

0. 1-1.0 times 

insulating layer 

fm spacing 

where \o is the free space wavelength. 

The center of the aperture 34 is about 0.7 to 0.9 times 
the dimension 37 (fm height) from the base 13. This 
position is also defined as 0.1 to 0.3 times the dimension 
37 from the top edge of the fm 12. 

To construct a SWS in accordance with the invention 
an elongated block of material such as copper or silicon 
is cut to the length of the required SWS. Next a passage- 
way is drilled through the block from end to end. The 
location of the passageway is dictated by opposing ar- 
rows 39 of FIG. 3 as specified in TABLE I above. One 
of the long surfaces of the block is then subjected to 
transverse electron discharge grooving, transverse ion 
beam etching, or reactive sputtering to remove mate- 
rial. The removal of material is continued until the fms 
12 formed by the grooving process are of the desired 

The second fin from each end of the SWS are elimi- 
nated during the grooving process to provide space for 
the respective input and output coupling ports, 16 and 
17, respectively. The coupling ports 16 and 17 are 
formed by one of the same processes used for the groov- 
ing and are sized for impedance matching with the 
coupling transformers or horns 35 and 36 of FIG. 2. 
These transformers or horns are attached by soldering 
or the like to the coupling openings 16 and 17 in the base 
plate 13. If the SWS is made from silicon, plating with 
an electrically conductive material will be required as 
mentioned previously. 

A top cover the same length as the SWS and of the 
same material is provided to be disposed against the top 
edges of the fms 12 opposite the base plate. A thin layer 
of electrically insulating material is disposed as strips 32 
and 33 on the bottom surface of the top cover plate to 
separate it from the fms 12. The strips 32 and 33 may be 
Mylar, mica, quartz, boron nitride, aluminum oxide, 
polytetrafluoroethylene, or the like either attached to 
the top cover by a suitable adhesive or lain lengthwise 
on the fms before the cover is put in place. The electri- 
cally insulating strips 32 and 33 may also be formed by 



the electro-deposition, ion beam implantation, vapor 
deposition, or sputtering of materials such as diamond, 
aluminum oxide, boron nitride, quartz, or polytetrafluo- 

Another embodiment of top cover plate 31 shown in 5 
FIG. 4 utilizes inwardly extending rungs 41 and 42 
which, as explained previously, are in register with the 
fins 12 of the SWS 11. To form the rungs 41 and 42, the 
bottom surface of the top cover plate 31 is masked by 
various well known techniques with a slit provided at 10 
the desired position of each rung member. The electri- 
cally insulating dielectric material is then deposited by 
one or more of the various means discussed previously. 
The masking material is then removed and the top 
cover is cleaned and placed on the fins 12. The top 15 
cover may be retained in place by various clips or 
straps, by adhesives or by soldering. 

It will be understood that various changes and modi- 
fications may be made to the above-described invention 
without departing from its spirit and scope as set forth 20 
in the claims appended hereto. 

I claim: 

1. A miniature traveling wave tube (TWT) having a 

slow wave circuit (SWS) utilizing an electron beam, the 
SWS comprising: 25 

an elongated base member having an input port and 
an output port, said ports being at opposite ends of 
said base member; 

a plurality of parallel rectangular fins extending in the 
same direction from said base member, each of said 30 
fins having an aperture, the apertures ail being 
aligned on an axis parallel to said base member for 
passage of the electron beam through the apera- 
tures, the center of each aperture being positioned 
a measured distance from a top surface of said base 35 
member, said measured distance being between 0.7 
and 0.9 times the distance from said top surface of 
the base member to a top edge of each respective 
fin, the alignment of the apertures being established 
by a drilled passageway in a block of material from 40 
which the base and fins are formed; 

an elongated top cover disposed adjacent to top 
edges of said fins and opposite said base member; 

an electrically insulating layer selected from the 
group of dielectric materials consisting of diamond, 45 
aluminum oxide, boron nitride, quartz and polytet- 
rafluoroethylene disposed between said top cover 
and said fins, said electrically insulating layer being 
in contact with said fins and said top cover and 
comprising at least two strips of the dielectric ma- 50 
terial extending perpendicular to the rectangular 
fins and separated sufficiently to avoid interaction 
with the electron beam; 

and wherein said electrically insulating layer has been 
disposed on the top cover by vapor deposition, 55 
electrodeposition, sputtering or ion beam deposi- 

2. The SWS of claim 1 wherein the insulating layer 
has a dielectric constant preferably less than 10. 

3. The SWS of claim 1 wherein said electrically insu- 60 
lating layer comprises two rows of inwardly extending, 
opposed rungs bonded to a bottom surface of said top 
cover, said rungs being spaced to be in register with said 
fins and wherein the respective opposed rungs extend 
inwardly no farther than a distance at which they 65 
would begin to interact with the electron beam. 

4. The SWS of claim 1 wherein said base member said 
fins and said top cover are copper. 


5. The SWS of claim 1 wherein the center of each 
aperture is at about 0.9 the distance from the base mem- 
ber to a top edge of the fin adjacent the electrically 
insulating material. 

6. A method of making a miniature traveling tube 
(TWT) utilizing a slow wave structure (SWS) and an 
electron beam comprising the steps of: 

providing an elongated block of electrically conduct- 
ing material having a height greater its width; 

drilling a passageway through said block from end to 

removing material transversally to the length of said 
elongated block at a plurality of predetermined 
locations between from a top surface toward a 
bottom surface thereby forming a base member and 
fins having apertures which are aligned, the center 
of each aperture being positioned a measured dis- 
tance from a top edge of its respective fin, said 
measured distance being between 0.1 and 0.3 times 
said height of the respective fin; 

disposing by electrodeposition, vapor deposition or 
sputtering strips of an electrically insulating dielec- 
tric material lengthwise along the edges of a bot- 
tom surface of a top cover plate to insulate the top 
cover plate from the fins, said strips being sepa- 
rated sufficiently to avoid interaction with the elec- 
tron beam; 

disposing said top cover plate with the electrically 
insulating material contacting each of said fins, said 
cover plate being juxtaposed on said fins thereby 
forming a SWS; and 

disposing the SWS in a TWT electrically insulating 
housing with the apertures aligned on the electron 

7. The method of claim 6 wherein said electrically 
insulating material has a dielectric constant preferably 
less than 10. 

8. The method of claim 6 wherein the dielectric strips 
are selected from the group of materials consisting of 
diamond, aluminum oxide, boron nitride, quartz, and 

9. The method of claim 6 including the steps of 

applying a mask to said one surface of said cover 

plate before disposing said electrically insulating 
material thereon, said mask having a plurality of 
slits extending inwardly from opposite edges of the 
cover plate, said slits being positioned to be in 
register with said fins when the cover plate is dis- 
posed on said fins, said electrically insulating mate- 
rial being deposited on the cover plate through the 
slits to form rung members; and 

removing said mask prior to disposing said cover 
plate on said fins with the rungs in register with the 

10. The method of claim 9 wherein said rung mem- 
bers extend inwardly only sufficiently so that the insu- 
lating layer deposited in said slits will have no signifi- 
cant interaction with the electron beam of the TWT. 

11. The TWT of claim 1 and including an elongated 
flange on each side of the SWS base member to main- 
tain the structural integrity of the SWS when the cou- 
pling ports are the same width as the fins. 

12. The method of claim 6 wherein said block of 
elongated electrically conductive material is substan- 
tially wider for that portion of the block which will 
serve as the SWS base whereby forming coupling ports 

in the base will not affect the integrity of the SWS.