High gain crossed field amplifier tube and radio transmission system equipped with such a tube

The invention relates to a high gain crossed field amplifier tube. Such a tube comprises in a vacuum enclosure 1 a cathode 2 and a delay line 3, itself constituted by an input line 31 whose height is less than that of the output line 32. The invention is applied to radio transmission systems.

BACKGROUND OF THE INVENTION 
The present invention relates to a high gain crossed field amplifier tube. 
Crossed field amplifier tubes are generally used in the power stage of 
radar transmitters. The invention also relates to radio transmission 
systems equipped with such a tube. 
These tubes are essentially constituted by two cylindrical, concentric 
electrodes placed under vacuum between which a potential difference is 
produced, which creates a d.c. field E.sub.o. A magnetic field B is 
applied parallel to the tube axis and therefore perpendicular to the 
electric field. 
The internal electrode is a cathode forming an electron current source. The 
external electrode is a delay line, whose function is to propagate the 
high frequency wave with a phase velocity V.sub..phi. of the order of a 
fraction of the speed of light. 
Under the interconnected actions of the electric field and the magnetic 
field, the electrons from the cathode follow cycloidal trajectories with 
an average azimuth speed V.sub.e. 
It has been shown that the amplification of the high frequency power occurs 
when V.sub.e =V.sub..phi.. A distinction is made between crossed field 
tubes with a forward or backward wave, as a function of whether the high 
frequency energy flows in the direction of the electron beam or in the 
reverse direction. The gain of crossed field amplifiers is given by the 
expression g=101og(Ps/Pe). 
Increasing the gain means decreasing the input lower Pe or increasing the 
output power Ps. In connection with the first solution there is a minimum 
value of Pe below which the tube does not operate because the power is 
insufficient for creating the first space charge branch. This value is 
dependent on the geometrical characteristics of the delay line, electrical 
and magnetic characteristics and the secondary emission coefficient of the 
cathode. 
If Pe is equal to the minimum Pe the gain can reach 18 dB, but in this case 
the signal to noise ratio is too low (&lt;20 dB). To bring this ratio to an 
acceptable value (approximately 40 dB) it is necessary to slightly 
increase the input power. In this case the value of the gain hardly 
exceeds 13 dB. 
Another way to increase the gain is to increase the output power Ps, which 
is given by the relation Ps=.eta..I.Uc, .eta. being the overall efficiency 
of the tube which is approximately 50%. 
The output power Ps is consequently essentially proportional to the 
electron current I, the operating voltage Uc varying only very slightly 
with the current. The total current I is proportional to the number N of 
space charge branches, each branch transmitting a current substantially 
equal to I/N. 
If an attempt is made to increase the gain of a crossed field amplifier by 
doubling, for example, the length of the line the improvement is at the 
most 3 dB. Thus, this operation is ineffective and can lead to a lack of 
stability, because the interfering modes are sensitive to the line length. 
On attempting to obtain the same result by increasing the operating current 
so as to double the total current, there is a multiplication by two of the 
current transmitted in each space charge branch, including the first and 
it is necessary to increase the input power in the same proportions for 
stabilizing said branch. Thus, it is not advantageous to increase the gain 
in this way. 
BRIEF SUMMARY OF THE INVENTION 
The present invention proposes to increase the gain of crossed field 
amplifiers by reducing the value of the current transmitted by the first 
space charge arm with the object of proportionally reducing the high 
frequency power necessary for the formation and for the stabilization of 
said branch. To this end the structure of the delay line is modified to 
reduce the current calculated level with the high frequency input. 
Therefore the present invention relates to a crossed field amplifier tube 
comprising in a vacuum space a cylindrical cathode and a delay line 
concentric thereto and which faces it over its entire height, said tube 
also comprising an input located at one of the ends of the line and an 
output located at the other end and separated by a degrouping space, said 
line receiving by said input the signal to the amplified and supplying by 
said output the amplified signal, wherein the height of the delay line is 
less at the input than at the output.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a cross-sectional view of a crossed field amplifier tube 
according to the prior art having a cylindrical structure. It comprises 
two concentric electrodes 2 and 3 in a vacuum enclosure 1. A not shown 
d.c. source produces an electric field Eo between the electrodes. 
The positive electrode 3 is constituted by a delay line having a periodic 
structure with a series of fingers having a constant pitch. They face the 
negative electrode or cathode 2, which is itself constituted by a 
molybdenum support 21, covered by e.g. an impregnated tungsten emissive 
part 22. 
Two connections 4 and 5 are provided on the delay line for the entry and 
exit of the high frequency wave and they are separated from one another by 
a degrouping space 9. A magnetic field B is produced in a plane 
perpendicular to the drawing. 
The tube shown in FIG. 1 is an amplifier with electron emission distributed 
by a cathode 2 and whose space charge branches are represented by 
reference numeral 6. It is a backward wave tube because the electron beam 
rotates in the direction indicated by arrow 7, which is the opposite to 
that of the electromagnetic energy flowing in the direction of arrow 8. 
FIG. 2a is a cross-sectional view of a crossed field amplifier tube 
according to the invention in the case of a tube with two uniform delay 
lines and forward or direct transmission. 
If the characteristic current I.sub.01 calculated at the level of the HF 
input is less than that of I.sub.02 calculated at the HF output, the 
corresponding power gain increase is substantially equal to 10 log 
(I01/I02). 
It is pointed out that the characteristic current Io is given by the 
formula: 
##EQU1## 
Thus, the characteristic current I.sub.0 is always very close to the 
operating current, the ratio I/I0 being between 0.3 and 1.2. Formula (1) 
shows that the characteristic current I.sub.0 is dependent on the 
following geometrical parameters: 
h: common width of the delay line and the cathode, 
ra: radius of the anode (delay line and degrouping space), 
rc: radius of the cathode, 
as well as the phase velocity of the wave along the delay line, which is 
itself dependent on the pitch of said line. 
According to the invention the characteristic current is reduced at the 
level of the first branch of the space charge and consequently reducing 
the dimensions of the delay line of this point. Now, any modification in 
the delay line leads to variations in the phase velocity V.sub..phi. of 
the wave. Thus, as it is necessary to maintain the synchronism 
Ve=V.sub..phi. and ve=(Eo/B) applies, Eo being the d.c. field, it is 
therefore necessary to additionally vary the magnetic field B applied 
along the delay line. 
The crossed field amplifier tube according to the invention shown in FIG. 
2a differs from the prior art crossed field amplifier tube by the fact 
that it comprises two delay lines of different dimensions in series, but 
which are uniform and separated by two degrouping spaces 91 and 92. 
The first line 31 has a width h.sub.1, which is less than the width h.sub.2 
of the second line 32, h.sub.1 and h.sub.2 being chosen in such a way that 
the average transmitted powers can, for example, be in a ratio of 20, 
corresponding to a gain increase of 13 dB. 
The magnetic field is stronger on input line 31 than an output line 32. The 
appropriate pole pieces are used for obtaining the desired result. 
The second degrouping space 12 makes it possible to separate the two delay 
lines. The effect of this space is to prevent spurious oscillations from 
propagating in the electron beam. 
The HF power is transmitted to the input of delay line 32 by a connection 
11 located within the vacuum enclosure 1. This connection can 
advantageously be constituted by a wave guide containing a ferrite. All of 
these are placed in the magnetic field of the tube and form an insulator, 
which absorbs the power reflected by the output line. 
By means of this device it is possible to obtain gains of approximately 26 
dB. Beyond this value it would be more difficult to obtain an adequate 
decoupling between the HF input and the HF output. 
The aforementioned tube uses forward transmission, but similar results can 
be obtained with backward transmission tubes. 
The invention is also applicable to the case of crossed field amplifier 
tubes containing a forward leakage line and a backward leakage line, in 
series with the first mentioned line. 
The advantage of such a device is that the facing HF inputs and outputs 
have HF powers which only differ by 13 dB, although the gain of the system 
is 26 dB. Thus, the tube would have little tendency to oscillate by direct 
coupling between the two ends of the line. However, it could oscillate on 
the output standing wave ratio and it would also be necessary to 
incorporate a ferrite between the input line output and the output line 
input. 
FIG. 2b shows an example of a cathode used in the case of a tube with two 
uniform delay lines. Such a cathode is constituted by a molybdenum support 
covered by an e.g. impregnated tungsten emissive part 22, provided with 
negatively polarized deflectors 23 serving to focus the electron beam. 
Only the emissive parts have the variable shape adopted by the delay line. 
The emissive 221 facing the input line has a height h1 which is less than 
the height h2 of the emissive part 222 facing the output line, h1 and h2 
being respectively equal to the widths of the input line and the output 
line. 
FIG. 3 shows an example of a cathode used in the case of a tube with a 
continually variable delay line. In such a tube four parameters are varied 
between the HF input and the HF output, namely: 
height h which is common to the cathode and the delay line, 
the cathode-line d spacing, 
the delay constant c/V.sub.100 (i.e. the pitch of line p), 
the magnetic field B. 
The variation of these parameters is chosen in such a way that the current 
transmitted by the space charge branch varies e.g. in a ratio of 20 
between the HF input and the HF output. 
The cathode of a continually variable delay line tube shown in FIG. 3 has 
an emissive part 22 whose height continually increases from the HF input 
to the HF output. The emissive part 225 facing the degrouping space 
ensures the continuity between parts 223 and 224. 
FIG. 4 is a cross-sectional view of an embodiment of a crossed field 
amplifier tube according to the invention in the case of a tube with two 
operating modes. By adding a grid 100 to the tube, e.g. with two forward 
transmission lines, facing output line 32, a tube with two operating modes 
is obtained. 
This grid, which is electrically insulated from cathode 2, can be 
negatively polarized relative to the latter (-Vg). There is no need to 
completely block the current, it merely being a question of adequately 
reducing the cathode emission to reduce the tube gain by 10 dB, whilst 
retaining the resistance of the beam. 
Two operating modes are obtained for a given input power Po: 
EQU Vg=0 peak output power: Po+26 dB (1) 
EQU Vg.noteq.0 peak output power: Po+16 dB (2) 
In the second mode and in the case of pulsed operation it is possible to 
increase the repetition frequency so as to equal out the average power of 
the first mode. 
The grid can be formed by pyrolitic carbon bars connected to the same 
potential. It covers all or part of the cathode surface facing the output 
line. 
It cannot be placed in front of input line 3, because that would decrease 
the power available at the input of the output line, which could prevent 
the formation of the space charge branch.