FR2678107A1 - MAGNETRON STRAPE WITH FREQUENCY STABILIZATION. - Google Patents

Abstract

The frequency drift during the heating of a straped magnetron is minimized or canceled by a choice of materials used for the realization of the anodes, fins or walls, and the straps. For a pair of straps arranged concentrically, the use of a strap of a material having a coefficient of thermal expansion relatively lower than that of the other strap of the pair, results in the straps deviating during heating, reducing their mutual electrical capacity and canceling the drift observed in the prior art. According to various characteristics of the invention: the fins or walls can be produced with shapes having an effect on their thermal resistance; the fins or walls are of different materials, having different thermal expansion coefficients, or different thermal resistances. The resulting differential expansion effect cancels out the frequency drift.

Description

STABILIZING STRAP MAGNETRONS

FREQUENCY

  The invention relates to electron tubes of the magnetron type and more particularly to so-called "straped" magnetrons, the anode elements of which are connected together by metal rings (the "straps") to impart certain properties to the frequency spectrum of operation

of these magnetrons.

  A straped magnetron, well known in the prior art, comprises a central cathode of circular section, surrounded by an anode comprising a multiplicity of resonant cavities

  (1) tuned to the operating frequency of the magnetron.

  These resonant cavities are hollow spaces inside the anode can have different shapes, for example circular or trapezoidal or others, when seen in section perpendicular to the axis of symmetry (see Figure 1) The cavities are separated by the anode material, copper for example In an example of a simple geometry (fig lb), the cavities

  trapezoidal are separated by fins (21,22).

  The shape and dimensions of the cavities determine the resonant frequencies thereof. A problem already encountered in the prior art is that many operating modes of such a device, having a multiplicity of (almost) identical cavities, are possible at frequencies very

  neighboring, because these modes are practically degenerate.

  Among the solutions to remove this degeneration, are known in particular the "rising sun" anode (fig le), and the straps (3) The straps are pairs of metal rings (3) which electrically connect the fins, respectively pairs

  22 between them, and the odd fins 21 between them.

  The straps (3) can be of the same diameter and arranged on either side of the anode as shown in Figures la and lb, or the straps can be of different diameters, arranged concentrically on one side or on both

  sides of the anode block (see Figure 2).

  These magnetrons operate normally in 1 Â mode, which means that the oscillation phases in two neighboring cavities different from Âf radians, in other words the even fins 22 are at the same tension at a given moment, and the odd fins 21 are they too to another tension which

  is the same for all of these odd fins.

  The addition of the straps therefore has the effect of modifying the capacity of the magnetron by the capacitive load represented by the capacity between the straps (The equivalent electrical diagram is given in FIG. 3) This has the effect of removing the unwanted modes from the mode main in the spectrum of possible operating frequencies, and so, to remove the

degeneration.

  For many applications, in particular those where a magnetron is frequency controlled or those where a receiver system incorporating the magnetron is at fixed frequency, it is desirable that the frequency drift of the magnetron due to the heating of its elements during the transitional start-up period is as short as possible, or even canceled This requirement becomes absolutely imperative for

  certain systems, such as seeker for vehicles, for example.

  However, the heating of the various constituent elements of a magnetron generates differential thermal expansions which modify the natural frequency of

resonance of the cavities.

  We can express the frequency in the Âà mode in terms of the capacities and inductive inductances of the cavities and straps (see figure 3): (j) Âi = 2 ÂTf = lL (C + (1) L = the inductive inductance and C = capacity of the cavity without straps, Y and) are, respectively, the capacity and the inductive inductance per cell of the straps alone This equation reflects that in mode  1, one can neglect the influence of the inductive inductance 2 which is connected to equipotential points This formula shows that it is possible to act on the frequency of the mode 1 T by acting on the capacity ô /, the inter-straps capacity per cell A variation at \ J of this capacity t will give a relative variation of the angular frequency If  according to the relation: 5 â) = / \ g- (2)

U) IY 2 C + &

  During the transitional period of start-up of the magnetron, the heating of the internal elements and their expansion reduce the differences between these parts and therefore increase the capacity C, decreasing the frequency (in the case of a

  sufficiently homogeneous heating).

  An object of the invention is to remedy this problem, by minimizing or canceling the frequency drift due to the thermal expansion of the elements internal to a magnetron. Another object of the invention is to propose a simple system to

  realize and inexpensive to cancel this frequency drift.

  For these purposes, the invention proposes a device for stabilizing the frequency of the strapped magnetrons, in particular for the purposes of stabilization in transient states, to cancel frequency drifts due to heating and thermal expansion t of the internal parts of the magnetron, characterized in that the capacitance (J) between the straps is forced to vary in equal amplitude and in opposite direction to the

  variation of the capacity (c) of the non-strapped cavities.

  In a variant of the invention, the straps are concentric circular rings, made of different metals with different coefficients of thermal expansion with the coefficient of the internal ring very much lower than that of the external ring, so that that the heating of the two increases the gap between the two and therefore reduces the capacity (ô)

between the two.

  In another variant of the invention, the straps which are fixed respectively to the even and odd anode fins, are forced to move apart because of the differential expansion between the odd and even fins. In another embodiment, the variation of the capacity between the straps, of equal amplitude and in opposite direction to the variation of the capacity (c) of the non-strapped cavities is obtained by a combination of the means used in the others

variants.

  Other characteristics and advantages of the invention and of

  its main achievements will emerge from the description

  detailed which follows, with its appended figures, of which: FIG. 1, already mentioned, shows schematically and in perspective, three magnetron anodes known from the prior art; FIG. 2 a, schematically and in plan perpendicular to the axis, shows another example of the strap-on anode known from the prior art; Figure 2b, shows schematically and in perspective a detail of Figure 2a; Figure 3 shows an electrical diagram equivalent to the structure of Figure 2; Figure 4 shows schematically and in perspective a detail of a straped magnetron produced according to the invention; FIG. 5, shows a detail of an example, in section perpendicular to the axis of the magnetron, of another embodiment

according to the invention.

  Figures 4 and 5 are given by way of non-limiting examples of embodiments according to the principles of the invention; other embodiments according to these same principles will be easily imagined by those skilled in the art The same references

  denote the same elements in the different drawings.

  In FIG. 1, we see three magnetron anodes known from the prior art which incorporate means for deviating parasitic frequencies from the frequency of

desired operation.

  In Figure la, we see an anode whose cavities (1) have the shape of circular holes parallel to the axis of the magnetron, coupled by radial slots (4) separated by thick walls (11,12) having a shape particular between these circular holes and these radial slots Two straps (3), metallic circular rings, are arranged on either side of the anode (up and down in the figure), and centered on the axis The strap (3) at the top of the anode in Figure la is electrically connected to the even walls (12), and the strap from below is electrically connected to the odd walls (11) The capacity between these two rings introduces the

  gamma term in equations 1 and 2 above.

  In FIG. 1b, we see an anode whose cavities (1) have a trapezoidal shape, (4) separated by radial fins (21, 22) Two straps (3), circular metal rings, are arranged on both sides. 'other of the anode (up and down), and centered on the axis The strap (3) at the top of the anode in figure lb is electrically connected to the paired fins (22), and the strap from below is electrically connected to the odd fins (21) The capacity between these two rings introduces the term gamma in equations 1 and 2 above. Another example of an anode known from the prior art is shown in FIG. 11c, the so-called "rising sun" anode. In this embodiment, the cavities (111, 112) have the form of radial slots, coupled by the opening on one side towards the central cathode (not shown) In this construction, the parasitic frequencies are separated from the operating frequency by the different length in the radial direction of the even slots (112), longer than the odd slots (111) This configuration does not use straps, and therefore does not concern us in the

part of the present invention.

  In FIG. 2 a, we can see schematically and in plan, from a point on the axis of the magnetron, the anode of a magnetron very similar to that of FIG. 1b, with a pair of straps (3) on each side of the anode Having respectively different diameters, the two straps visible in the figure are arranged concentrically, one making electrical contact with the even fins (22), and the other making electrical contact with the odd fins ( 21) These contact points are indicated on the drawing by small circles. This example will be useful later on to explain our invention in a

  particularly simple embodiment which is shown in FIG. 4.

  In Figure 2b, we see schematically and in perspective a detail of Figure 2a, which represents a paired fin (22) We see two pairs of straps (3) on either side (upper and lower) of the '' fin (22), with on the upper part, an electrical contact between the fin (22) and the larger diameter strap of the upper pair, and on the lower part, an electrical contact between the fin (22) and

  the smaller diameter strap of the lower pair.

  In Figure 3, we show the equivalent electrical diagram for the structure of Figure 2 The inductive inductance L and the capacitance C are values per cavity L and C placed in parallel therefore represent a cavity, constituting the basic cell of the diagram equivalent which will be repeated as many times as there are cavities in the magnetron Between the ends of each pair of cells (representing two cavities), there are connections which represent the two straps, connected to the even and odd fins respectively (the fins do not appear on the equivalent diagram) Each of these connections will have a selfic inductance of 211 clambda (per cell), but in the ÂT mode, lambda is substantially zero as we described above Between the two straps of each pair , we also have the gamma capacity per cell. In Figure 4, we see in section and in perspective a detail of a strapon magnetron anode according to the invention It is an anode having substantially the same shape as the anode of Figure 2, with a difference close up: on the fin shown in the detail in Figure 4, the inner strap (of smaller diameter than the outer strap) is of a metal with a low coefficient of thermal expansion, such as molybdenum, tungsten, titanium, or similar, while the external strap is of a metal with a high coefficient of thermal expansion such as copper for example. The fin represented in FIG. 4 is an "odd" fin because the internal strap makes electrical contact with this fin, then that the external strap does not make electrical contact with the fin The principle of the invention can be applied to only one side of the anode, or to both sides at the same time We limit our discussion here to only one side, but the invention can be applied to ux two faces with an arrangement similar to that shown in FIG. 2 b To overcome the mechanical stresses due to the differential expansion between the internal strap and the anode fin, this strap is joined to the fin by means of d 'a flexible tongue, a kind of metal base sufficiently ductile to deform under these constraints without breaking, copper for example Some dimensions are given in Figure 4 to allow the calculation of the theoretical capacity between the two straps: average radial distance between the axis of the tube and the diameter of the straps (rs), the distance between the two straps (e), and the height of the straps (a), except for the flexible tongue When heating the together, the expansion of the external strap is greater than that of the internal strap in this embodiment according to the invention, therefore the straps move apart (the distance e increases), and the capacity between the straps decreases By a jud choice here of dimensions, this reduction in capacity can cancel the increase in capacity due to the heating of the rest of the magnetron If the importance of the capacity of cancellation required deserves it, two pairs of straps can be used, of share and d other

  the anode as shown in Figure 2 b.

  In FIG. 5, we see in partial section and in perspective, another example of an embodiment of a strapron magnetron anode according to the principles of the invention, in which the spacing of the straps during the heating of the anode is obtained in another way In this example of an embodiment, there is only a pair of straps, of the same

  material, arranged concentrically on the same side of the anode.

  The fins are then profiled so as to present an equal surface towards the cathode in the center of the magnetron, but a different thermal resistance because of their different widths: the even fins are thinned to increase their thermal resistance, while the odd fins are thickened to reduce their thermal resistance Since the even and odd fins have the same surface facing the cathode they will be heated in the same way, but the heat will be evacuated more quickly from the odd thickened fins than from other even thinned fins The thermal expansion will be more large for even thinned fins than for odd thickened fins, so the straps will spread when the magnetron heats up The dimensions of the straps will be calculated as in the previous case to give a variation in inter-straps capacity during the warm-up to cancel the drift in freque due to

  thermal expansion of the other parts of the magnetron.

  In this example of an embodiment according to a characteristic of the invention, the flexibility tongues and the straps in rings of different metals are not necessary to obtain the desired effect. However, it is possible to make an embodiment according to the invention. which presents a combination of the characteristics of the example of the

  Figure 4 and those of the example in Figure 5.

  According to the same principles, in other exemplary embodiments (not shown) according to the invention, the fins can be made of different metallic materials, having either different thermal expansion coefficients, or similar thermal expansion coefficients, but with different thermal resistance coefficients The result is the same as in the other embodiments shown in the previous figures: the heating causes a variation in capacity between the straps which cancels the frequency drift due to the thermal expansion of the other parts of the magnetron. In all the forms and embodiments according to the invention o its principles, the assembly methods used are substantially the same as those used conventionally in the strapped magnetrons known from the prior art: mainly by brazing and or welding between the different metal parts The examples of embodiments according to the invention given above do not require a higher number of parts than the embodiments of the prior art, nor of the methods or methods significantly more complicated than those of the prior art L The invention therefore makes it possible to obtain significantly better results in terms of microwave performance during the heating of the magnetron compared to those of the prior art, and this practically without

  extra costs, neither of raw material, nor of industrial process.

Claims (12)

RESELL I CATIONS   1 so-called "strapé'r" magnetron, having at least one cathode and one anode arranged coaxially with respect to the axis of the magnetron, this anode having an even number of resonant cavities which are hollow spaces inside the anode, these resonant cavities being separated by an even number of walls or fins arranged radially around the cathode, the arrangement of these cavities and these fins or walls being of an azimuthal symmetry about the axis of the magnetron, the strapped magnetron having in addition one or more pairs of metal rings (called "straps") which electrically connect certain fins or certain walls to each other through electrical contacts between the straps and the fins, the two straps of each pair being arranged concentrically in a perpendicular plane at the axis of the magnetron, a first strap of smaller diameter being disposed inside a second strap of larger diameter, and each a pair of straps having an electrical capacity which depends on the dimensions and the relative arrangement of the two straps, characterized in that the embodiment of the anode, of the fins or walls, and of the straps is such that, during the heating of the magnetron, the thermal expansion of these above-mentioned parts results in a variation of said electrical capacity of the straps, so as to reduce or cancel the frequency drift due to the thermal expansion of the magnetron known from the prior art.   2 Magnetron according to claim 1, characterized in that at least one pair of straps is made of two different metallic materials, a first strap of said pair having a smaller diameter being made of a material having a relatively low coefficient of thermal expansion to the second strap of a larger diameter made of a material having a high coefficient of thermal expansion, the two il straps of said pair being arranged concentrically, so that, during the heating of said parts mentioned above, said straps move apart, reducing said   capacity between said two straps.   3 Magnetron according to claim 1, of which said walls or said fins are produced so as to present the same surface towards the cathode, at their end closest to the latter, so as to ensure equal heating for all the fins or walls, characterized in that the thermal resistance of the fins or walls is different for one in two alternating, therefore between two fins with a relatively higher thermal resistance, a fin is placed with a relatively lower thermal resistance , and between two fins with a lower thermal resistance, a fin is placed with a higher thermal resistance, so as to obtain a differential thermal expansion between the fins with high thermal resistance and those with resistance low thermal.   4 Magnetron according to claim 3, characterized in that said fins with relatively higher thermal resistance are produced by thinning these fins, taking care however to keep the same section in front of and near the cathode, and in that said fins with relatively lower resistance are made by thickening the latter, however taking care to keep the same section in   opposite of, and near the cathode.   5 Coaxial magnetron according to claim 3, characterized in that said fins with relatively higher thermal resistance are made of a metallic material having a relatively higher thermal resistance, and in that the fins with relatively lower thermal resistance are made of '' another metallic material having a   relatively lower thermal resistance.   6 coaxial magnetron according to claim 1, characterized in that said fins or walls are made of two different metallic materials in alternation, a material having a relatively higher coefficient of thermal expansion and another material having a relatively lower coefficient of thermal expansion , each fin or wall having a lower coefficient of thermal expansion being disposed between two fins with walls having a higher coefficient of thermal expansion, and each fin or wall having a higher coefficient of thermal expansion being disposed between two fins or walls having a lower coefficient of thermal expansion, so as to cause a separation of the straps during the heating of the magnetron. 7 Coaxial magnetron according to any one of   Claims 3 to 6, characterized in that at least one pair   of straps is made of two different metallic materials, a first strap of said pair having a smaller diameter being made of a material having a low coefficient of thermal expansion relative to the second strap of a larger diameter made of a material having a high coefficient of thermal expansion, the two straps of said pair being arranged concentrically, so that, during the heating of said aforementioned parts, said straps move apart, reducing said capacity between said two straps.   8 Coaxial magnetron according to claims 1 and 2,   characterized in that one or more flexibility tab (s) is (are) used to ensure said electrical contacts between said fins and said strap (s) having a coefficient of thermal expansion different from the fins, so as to overcome the mechanical constraints between the fins and   the straps due to differential dilation.   9 Coaxial magnetron according to any one of   Claims 5 to 7, characterized in that one or more   flexibility tab (s) is (are) used to ensure said electrical contacts between said strap (s) and said fins having a coefficient of thermal expansion different from (the) straps, so as to overcome the constraints mechanical between the fins and the strap (s) due to the differential expansion.   Magnetron according to any one of claims 1   to 4, characterized in that the anode is made of copper.   11 Magnetron according to any one of claims 2,   , 6 or 7, characterized in that said coefficient material   relatively strongest thermal expansion is copper.   12 Magnetron according to any one of claims 8   or 9, characterized in that said flexibility language is made of copper.   13 Magnetron according to any one of claims 10   to 12, characterized in that said material with a coefficient of   the lowest thermal expansion is tungsten.   14 Magnetron according to any one of claims 10   to 12, characterized in that said material with a coefficient of   the lowest thermal expansion is molybdenum.   Magnetron according to any one of claims 10   to 12, characterized in that said material with a coefficient of   the lowest thermal expansion is titanium.