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Патент USA US3074000

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Jan. 15, 1963
J. F. HENl-:Y
3,073,990
RADIO FREQUENCY ATTENUATOR
Filed June 25, 1958
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By
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AgenÍ
United States Patent O "ice
l
3,@7339@
Patented Jan. 15, 1963
2
cathode, acts to vary the plasma frequency of an elec
3,073,990
RADIO FREQUENCY ATTENUATGR
John F. Heney, Bloomûeld, NJ., assigner to International
Telephone and Telegraph Corporation, Nutley, NJ.,
a corporation of Maryland
Filed June 23, 1958, Ser. No. 743,676
4 Claims. (Qi. 315-39)
This invention relates to radio frequency transmission
line attenuators and more particularly to electron dis 10
charge attenuators for employment in radio frequency
transmission lines.
a With the advent of long range, high speed aircraft, it
1s necessary to increase the range of obstacle detection
tron discharge across a gap disposed in the center con
ductor of a coaxial line in a manner to vary the attenua
tion of the radio frequency energy being conducted across
the conductor gap by said electron discharge.
The above-mentioned and other features and objects
of this invention ’will become more apparent by reference
to the following description taken in conjunction with
the accompanying drawings, in which:
FIG. 1 is a longitudinal cross sectional view of a co
axial line attenuator in accordance with the principles of
this invention.
FIG. 2 is a graphical representation of attenuation vs.
plasma frequency illustrating the relationship between the
systems to enable identification and maximum opportunity 15 attenuation of a radio frequency signal in decibels (db)
for interception. The range of obstacle detection systems
and the plasmo frequency of an electron beam.
may be increased by the use of much higher power and
Referring to FIG 1, there is shown therein a section of
lower frequency radiated energy than has heretofore been
coaxial line 1 having an outer conductor 2 and a hollow
required.
This approach has brought about changes
inner conductor 3.
Inner conductor 3 has a gap 4 dis
in the components required for these systems and has 20 posed therein which is defined by the ends 5, 6 of inner
brought to light the fact that components which were con
conductor 3. Two electrodes 7, 8 are shown in juxta
sidered conventional in the lower power, high frequency
position to ends 5, 6 of center conductor 3 and means are
systems are not suited for high power operation or are
shown which permit the selective energization of elec
not operable at the lower frequencies used in these
trodes 7, S to provide an electron discharge between
relatively low frequency systems. This critique is par 25 these electrodes toV provide a conductive path for radio
ticularly applicable to attenuating or switching devices
frequency energy from source 9 to load 9’ across the gap
such as gas tube “TR” and ferrite switches.
4. The electrodes 7, 8 and the gap 4.are shown to be
Gas tube “TR” devices are limited in their use as far
hermetically sealed by dielectric envelope 10 to enable
as high power operation is concerned in that there is a
the establishment of a relatively high vacuum electron
limit to the amount of radio frequency power which can 30 discharge across gap 4.-
be employed. If this radio frequency power limit is ex
In FIG. l, electrodes 7, 8 which are an electron emis
sive electrode or cathode and a collector, respectively,
are disposed in gap 4 of inner conductor 3 in juxtaposition
to the ends 5, 6 of inner conductor 3. The cathode 7 is
low frequency limit at which their non-reciprocal char 35 shown as the type which emits a hollow annular beam of
acteristics disappear and they are limited in their power
electrons 11. It should be noted, at this point, that while
handling capacity.
the use of a hollow beam is preferable, one need not be
The structures described and claimed herein are re
limited by this fact. A solid cylindrical beam may be
ferred to as attenuators, either fixed or variable, to bet 40 used, but the diameter of the beam, whether it is hollow
ter define the scope of this invention. A switch, such
or solid, should have a diameter which substantially ap
ceeded, losses in the switch can destroy the tube or
seriously shorten tube life. Ferrite type switches are
limited in their utility in that they are known to have a
as a conventional “TR” device, is considered to be a
fixed attenuator having substantially infinite reactive at
tenuation when in a conducting or operative condition.
proaches the diameter of inner conductor 3. By this
means, large discontinuities between the ends 5, 6 of inner
Íconductor 3 and the beam 11 are avoided and lower
It is therefore, an object of this invention to provide a
45 insertion loss can be expected. The hollow beam, how
radio frequency transmission line attenuator which over
ever, has been found to be preferable not only because the
comes the limitations of prior art devices.
electrons on the outer surface of the beam are the only
Another object of this invention is to provide a radio
ones which contribute to the conduction of radio frequency
frequency transmission line attenuator which is operable
energy across the gap but also by reason of lower heater
under high power, low frequency conditions.
50 requirements and better collector cooling obtained.
A further object of this invention is to provide a novel
The cathode 7 and collector element 8 are shown in
'coaxial line radio frequency attenuator.
FIG. 1 disposed perpendicular to the longitudinal axis
Still a further object of this invention is to provide
of coaxial transmission line 1. Cathode 7, which is
a novel coaxial line radio frequency attenuator which is
heated by filament 12, is further shown displaced from
leither a switch or variable attenuator.
55 gap end 5 by a small distance d to insure that hollow
A feature of this invention is the provision of a radio
annular electron beam 11 provides a conductive path
frequency attenuator including a transmission line con
across gap 4. Cathode 7 is shown in FIG. 1 as an in
ductor having two ends defining a gap therebetween, an
directly heated cathode but it may be directly heated
electrode disposed in juxtaposition to each of said ends
without affecting the operation of this device.
land means to selectively energize said electrodes to pro 60
In FIG. l, a third electrode, focusing and accelerating
vide an electron discharge therebetween. The electron
anode 13 is shown disposed in the path of electron beam
discharge is along a given path to provide a conductive
11 and contains an annular cutout portion 14 to permit
path for radio frequency energy across the gap. A vacu
passage of hollow annular beam 11 therethrough. Sup
um envelope is provided to maintain a vacuum between
port wires 1S which may be tungsten or some other metal
the electrodes.
65 which can withstand high temperatures are connected to
Another feature of this invention is the utilization of an
portion 16 of electrode 13 to support the central por
electron beam whose outer diameter is substantially equal
tion 13a of electrode 13 in its desired coaxial relation
-to diameter of the center conductor of a coaxial line
ship. Portion 16 of electrode 13 extends beyond the
to provide a conductive path between the ends of a gap
dielectric envelope 10 which acts to maintain a vacuum
lin the center conductor.
70 around electrodes 7, 8, 13 and filament 12. The extended
Still another feature of this invention is the provision
portion 16 of electrode 13 acts as a flange against which
of a source of variable power which, when applied to the
center conductorend 5 is firmly butted thereby main
3,073,990
3
taining electrode 13 at the same potential as that of center
conductor 3. Collector electrode 8 likewise contains a
flanged portion 16a which extends beyond dielectric en
velope 10. This flanged portion 16a is butted firmly
,against center conductor end 6 and this flange in coopera
tion with the ñange on end S of center conductor 3 holds
the electrode assembly in fixed relation to center con
11.
d.
This means that radio frequency energy will be
transmitted without attenuation to load 9’ when puiser
20 energizes cathode 7. Thus, during the pulser ofï=
time, attenuation, either substantially infinite or of fixed
value dependent on gap length l, is introduced and, as in
prior art devices such as gas discharge coaxial switches,
no means for changing the attenuation during the con
duction cycle has been provided. However, it has been
ductor 3. Collector electrode 8, in order to dissipate
found, using the device of PIG. 1, that variable attenua
heat rapidly, -may be cooled by water or some other well
known coolant and in FIG. 1, ports for the ingress and 10 tion can be obtained during the conduction cycle by
varying the power of variable pulser 20 to vary the
egress of coolant are designated 17, 1S, respectively. 'The
plasma frequency of electron beam 11.
coolant means may be introduced into center conductor
3 by the well known means of a matching stub which is
not shown in the drawing.
Cathode 7 is heated by filament 12, the power to fila
ment 12 being supplied by source 19. Cathode 7 is selec
tively energized for electron emission by means of pulser
20 which .can be adjusted to provide pulses of any ampli~
tude and duration. Pulser 20, therefore, can supply pulses
in the range of microsecond duration up to and includ
ing continuous wave operation. VTo provide a pulsed
electron beam, a negative potential of given amplitude
and duration from pulser 2t? is applied to cathode 7.
Beam focusing means 21, which may be a solenoid for
applying a magnetic field parallel to the longitudinal axis
of the tube, maintains electron beam 11 in a given path
as it traverses gap 4. Beam focusing means 21 may also
be electrostatic in nature and would use a number of elec
FIG. 2 is a graphical representation of attenuation vs.
plasma frequency illustrating the relationship between
the attenuation of radio frequency signal in db as the
plasma frequency of the electron beamis increased. Re
ferring to FÍG. 2, it may be seen that as the plasma fre
quency increases the attenuation decreases from point A
to practically 0 db at point B. Investigation has shown
that if the gap length l is adjusted such that 40 db re
active attenuation, for instance, is present when there is
no conductive electron beam present, it is possible by ap
plying increasing power to pulser 20 to gradually de
crease the attenuation of an RE. signal in accordance
with the curve of FIG. 2. With the filament power
constant to make electrons available for instantaneous
operation, the power to pulser 20 is increased from zero,
electrons are emitted from cathode 7 and are accelerated
‘across gap d at a relatively low value of plasma fre
trodes with proper applied voltages to maintain electron
30 quency. At point A, where the plasma frequency is in
beam 11 in a given path.
the lorder of one-tenth the frequency of the radio fre
In operation, as a substantially infinite attenuator or
quency energy, the attenuation starts to decrease. As the
switch, pulser Ztl applies a negative pulse of given ampli
plasma frequency in increased by increasing the power
tude, for a given duration, to cathode 7 which cause the
to cathode 7, the attenuation continues to decrease until
`|cathode to emit an electron beam 11 which is accelerated
across the gap d and caused to drift at a constant velocity 35 poin B is reached where a further increase in plasma
frequency provides no further decrease in attenuation. At
'across the distance l to impinge on collector electrode
point B the plasma frequency is greater than ten times
«the frequency of the radio energy being transmitted.
Thus, by varying the amplitude of the pulses, for instance,
' 'frequency across the gap, the plasma frequency of the
‘electron beam must be substantially h’gher than the fre 40 from pulser 20, it is possible to obtain variable attenua
tion of radio frequency energy during the conducting
'quen'rìy of the radio energy vbeing transmitted across the
cycle. This type of attenuation is particularly useful at
¿gap and the electrons must drift across the distance l at
frequences of the order of 20 megacycles and is only
a constant velocity. The plasma frequency of an electron
-"8. In order to insure proper operation of this device
‘when the electron beam is acting as a conductor for radio
Vbeam amy be defined as the frequency of radiation due
limited in utility when it is not practical to obtain the re
to the motion of electrons oscillating about their point of
quired electron densities necessary to keep the plasma fre
quency high with respect to the signal frequency being
-least energy. A more complete exposition on this sub
ject may be found in the hook “Gaseous Conductors,”
by I. D. Cobîne, p. 131, first edition, McGraw-Hill Book
Co. If these criteria are not adhered to, the radio fre
-'quency energy attempting to pass the gap by means of
‘the electron beam 11 will be attenuated. The plasma
`¿frequency is directly proportional to the power applied
-by variable power pulser 20 and when’the device is act
ing as a switch, a constant amount of power, sufiicient
to cause the plasma frequency to be substantially higher
than the radio frequency, is applied. When the puiser
20 removes the negative potential from cathode 7, elec
used.
While I have described above the principles of my ‘in
vention in connection with specific apparatus, it is to be
clearly understood that this description is made only by
way of example and not as a limitation to the scope of my
invention as set forth in the objects thereof and in the
accompanying claims.
I claim:
`
1. A radio frequency variable attenuator comprising a
coaxial line having an inner and outer conductor for trans
mission of radio frequency energy therealong, said inner
conductor having two ends defining a fixed gap therebe
tron emission Vstops and the conductive path across gap
tween, a cathode disposed at one end of said gap, a collec
4 is removed. Radio frequency energy being transmitted
along coaxial line 1 from source 9 upon reaching gap 4 60 tor disposed at the other of said ends, heater means caus
ing said cathode to emit electrons, beam forming means to
is reactively attenuated, i.e„ reflected, and this attenua
provide a longitudinal hollow beam of electrons between
tion increases exponentially as a function of the gap
>said cathode and said collector, a dielectric vacuum en
length designated l in FIG. 1. From this, it may be seen
velope to maintain a vacuum between said cathode and
that the gap length l may be set to a predetermined point
and a fixed amount of attenuation introduced other than 65 said collector and adjustable pulse means coupled to said
cathode to variably control the power and plasma fre
the substantially infinite attenuation which is introduced
quency of said electron beam and the attenuation of said
when the attenuator is being used as a switch.
When the device'of FIG. 1, as just described, is being
gap to said radio frequency energy.
2. A radio frequency attenuator comprising a coaxial
used as a switch or as a fixed attenuator, the plasma
70
line having an inner and outer conductor for transmis
frequency of the electron beam 11 must be substantially
sion of radio frequency energy therealong, said inner con
higher than the frequency of the energy being carried
ductor having two ends defining a predetermined gap
by coaxial line 1. For switching and introducing a fixed
therebetween, a first electron emission electrode disposed
amount of attenuation, the radio frequency energy must
encounter a large discontinuity which is provided when 75 at one of said ends, a second collector electrode forming
the other of said ends, means heating said first electrode
the gap 4 iS dçVQid 0f the conductive electron beam
3,073,990
5
to emit electrons, beam forming means to provide a lon
gitudinal focused electron beam between said first and
second electrodes, a third electrode disposed in the plane
of and adjacent said ñrst end to provide said beam with a
constant diameter and to accelerate said electron beam to
a given velocity, means coupled to said first electrode to
provide a source of adjustable pulsed power to Vary the
plasma frequency and attenuation of said electron beam
and a vacuum envelope to maintain a vacuum between
said first and second electrodes.
3. A radio frequency attenuator comprising a coaxial
line having an inner and outer conductor for transmis
sion of radio frequency energy therealong said inner con
ductor having two ends defining a fixed gap therebetween,
6
conductor having two ends defining a fixed gap there
between, a cathode disposed at one of said ends, a col
lector disposed in the plane of the other of said ends, »
means to provide a focused cylindrical, electron beam
between said cathode and said collector as a collinear eX
tension of said inner conductor, a beam forming anode
disposed in the plane of said one of said ends between
said cathode and said collector to provide said beam with
a constant diameter and to accelerate said electron beam
to a given velocity, a dielectric vacuum envelope to main
tain a vacuum between said cathode and said collector
and means applied to said cathode to provide a variable
amplitude and duration pulse to adjustably control the
plas-ma frequency of said electron beam with respect to
a first electrode disposed at one of said ends, a second 15 the radio frequency and thereby, to Vary the value of
electrode disposed in the plane of the other of said ends,
attenuation of said radio frequency energy.
means to provide an electron discharge in the form of a
focused, cylindrical electron beam between said first and
second electrodes along a given path collinear with and
having substantially the same diameter as said inner con 20
ductor, a third electrode disposed in the plane of said
one of said ends between said first and second electrodes
to provide said beam with a constant diameter and to
accelerate said electron beam to a given velocity, a vacu
um envelope to maintain a vacuum between said ñrst 25
and second electrodes and variable pulsed power means
coupled to said first electrode to adjustably control the
plasma frequency of said electron discharge to a higher
frequency than said radio frequency and to vary the value
30
of attenuation of said radio frequency energy.
4. A radio frequency variable attenuator comprising a
coaxial line having an inner and outer conductor for trans
mission of radio frequency energy therealong, said inner
References Cited in the file of this patent
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Southworth __________ __ Apr. 1l,
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