Variable attenuator for rectangular waveguides

Abstract

 Disclosed is a variable attenuation device for a rectangular waveguide. It comprises a substantially plane plunger element (5) with a rounded shape designed to be introduced perpendicularly into said waveguide (3) through a longitudinal slot (7) made along the axis of propagation of the waves in a first face of said guide, the plunger element (5) being rotationally mobile about an off-centered axis (6) so that the portion of the plunger element introduced into the waveguide is a function of the angular position of said plunger element. To increase the portion of plunger element introduced into the waveguide, the plunger element (5) and said longitudinal slot (7) are positioned at the level of an elbow (11) formed by said waveguide.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an attenuation device for rectangular waveguides. It can be applied especially in the field of microwave equipment.

[0002] It is relatively common to use attenuation devices for the attenuation, on a waveguide portion, of the electrical power transmitted in order to regulate the level of power received by a circuit that is positioned downline to the waveguide in the direction of propagation of the waves. There are different types of attenuators, especially attenuators based on PIN diodes and plate attenuators. The present invention relates especially to plate attenuators.

[0003] According to the principle of this type of attenuator, a plate is introduced into the waveguide by a longitudinal slot parallel to the axis of propagation of the microwaves. The plate is made of a resistive material that absorbs the electrical energy propagated in the waveguide. The incident energy that gets propagated in the guide is attenuated when it passes into the guide portion crossed by the plate.

[0004] The amplitude of the attenuation is a function of the plate portion inserted into the waveguide, which is given the task of acting as an obstacle to the propagation of the wave in the guide. The amplitude of the attenuation is also a function of the position of the plate with respect to the plane of symmetry of the guide parallel to the electrical field (the plane where the amplitude of the electrical field is the maximum). The attenuation is the maximum when the plate is introduced into this plane, namely the center of the waveguide.

2. Description of the Prior Art

[0005] A device of this kind is described in the U.S. patent No. 2 619 538. The device has a plunger disk with a substantially plane shape designed to be inserted into a waveguide through a longitudinal slot along the axis of propagation of the waves. This slot is made in the upper face of the guide, preferably in its middle. The plunger disk is mounted on a driving shaft whose axis is orthogonal to the axis of propagation of the waves. It is thus in a plane parallel to the electrical field of the electromagnetic waves. The shaft on which the disk is mounted is connected to an adjustable dial plate. The contour of the disk is not circular with respect to the center of the rotation shaft so that the rotation of the disk causes a variation in the surface of the disk inserted into the waveguide. A rotation of the dial plate drives a rotation of the disk which may be thus set at different angular positions corresponding to various amplitudes of attenuation.

[0006] The aim of the invention is to resolve the problems of miniaturization encountered with existing devices. Indeed, to obtain attenuation values of several tens of decibels, the disk portion inserted into the waveguide must be relatively large. To this end, the existing devices are provided with a plunging disk having a large radius to increase the disk portion submerged in the guide.

SUMMARY OF THE INVENTION

[0007] The present invention seeks to mitigate this major drawback of the prior art by proposing a relatively compact attenuation device with a simple design.

[0008] Thus, an object of the invention is a variable attenuation device for a rectangular waveguide, of the type comprising a substantially plane plunger element with a rounded shape designed to be introduced perpendicularly into said waveguide through a longitudinal slot made along the axis of propagation of the waves in a first face of said guide, the plunger element being rotationally mobile about an off-centered axis so that the portion of the plunger element introduced into the waveguide is a function of the angular position of said plunger element,
   wherein said plunger element and said longitudinal slot are positioned at the level of an elbow formed by said waveguide.

[0009] The result thereof is an increase in the proportion of plunger element introduced into the guide in relation to a rectilinear waveguide. The attenuation sought may then be obtained with a small-sized device.

[0010] Advantageously, the shape of the contour of the plunger element and the position of its axis are designed so that the attenuation is linearly dependent on the angular position of the plunger element.

[0011] The contour of the plunger element and the position of its rotational axis are then defined so that, in a first extreme angular position, the contour of the plunger element is tangential to the first face of the elbow in such a way that the portion of the plunger element introduced into the waveguide is zero and, in a second extreme angular position, the plunger element completely goes through the waveguide at the level of the elbow so that the portion of the plunger element introduced into the waveguide is the maximum. Advantageously, a scalloping is made, at the level of the elbow, in the face opposite the first face of the waveguide so that the plunger element can completely cross the waveguide.

[0012] In a preferred embodiment, the variable attenuation device comprises a body formed by two symmetrical parts each comprising a groove and a blind hollow leading into the interior of said groove so that, once the two parts are assembled, the grooves form said waveguide and the blind hollows form a cavity designed to receive the plunger element, which is introduced into said waveguide at the intersection between said grooves and said blind hollows corresponding to said longitudinal slot.

[0013] In a preferred version, the contour of the plunger element comprises:

• first and second rectilinear parts designed to be tangential, in said first extreme angular position, to said first waveguide face of each side of said elbow, and
• a first curved part between said first and second rectilinear parts designed to be tangential, in said first extreme angular position, to said first waveguide face at the level of said elbow.

[0014] Furthermore, the rotational axis of the plunger element may be positioned so that the distance between the first rectilinear part and the plunger element is greater than the distance between this first rectilinear part and the second rectilinear part.

[0015] The variable attenuator according to the invention is also distinguished by its system for the driving of the plunger element. Thus, according to the invention, the plunger element is mounted on a rotational driving shaft which has an end gear wheel to be coupled externally to a motor borne on an L-shaped support fixed by its base to the body. This plunger shaft is engaged in a neck collar going through the body and the base of said motor support by means of through bores adjusted to its external diameter, the collar comprising an internal end shoulder designed to take support on the edge of said bore and being furthermore interposed on the shaft between an internal shoulder of this shaft and said drive gear wheel, the gear wheel being engaged in the external end of the shaft and being adjusted and held axially on it by means of a screw entering the end of the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Other features and advantages of the invention shall appear from the following detailed description made with reference to the appended drawings, of which:

• Figure 1 shows an exploded view of the device of the invention;
• Figures 2 and 3 shows partial sectional views of the device according to the invention;
• Figure 4 shows a preferred embodiment of a plunger element;
• Figure 5 shows a front view of a preferred embodiment of the attenuation device according to the invention;
• Figure 6 shows a top view of the attenuation device of Figure 5;
• Figure 7 shows a partial view of the attenuation device of Figure 5;
• Figure 8 shows a sectional view along the axis A-A of the attenuation device of Figure 5;
• Figure 9 shows a sectional view, along the axis A-A, of the body of the device of Figure 5 provided with a motor and its L-shaped support; and
• Figure 10 shows an attenuation curve generated by an attenuation device corresponding to the preferred embodiment of the invention as a function of the angular position of its plunger element.

MORE DETAILED DESCRIPTION

[0017] Figure 1 shows an exploded view of an attenuation device according to the invention. This device consists of a body formed by two symmetrical parts 1 and 2. These parts 1 and 2 each have a groove 3 (not shown in Figure 2) with a depth equal to a and a width equal to b and a blind hollow 4 (not shown in the part 2) with a depth equal to c opening into the interior of the groove. Once the two parts 1 and 2 are assembled, the two facing grooves form a waveguide with a width 2*a and a height b and the two blind hollows form a cavity enclosing a plunger element 5 that is rotationally mobile around an off-centered axis 6 orthogonal to said parts 1 and 2. These parts 1 and 2 are made by machining a block of conductive material such as aluminum. The intersection between the grooves 3 and the blind hollows 4 forms a longitudinal slot 7 through which the plunger element 5 is introduced into the waveguide. In the example of Figure 1, the cavity is circular and its center coincides with the rotational axis 6 of the plunger element 5. Holes 8 and 9 are made in the parts 1 and 2 respectively to receive a rotational shaft 10 of the plunger element 5. Naturally, the radius of the cavity is chosen to be greater than the greatest distance between the rotational axis 6 and the contour of the plunger element 5, and the thickness of the plunger element is smaller than 2*c.

[0018] According to a major characteristic of the invention, said grooves 3 comprise at least one curved section so that the waveguide constituted by the two grooves form an elbow and the plunger element 5 is inserted into the waveguide at this elbow. The longitudinal slot 7 is at this elbow.

[0019] The insertion of the plunger element into this curved section of the guide enables an increase in the portion of the plunger element submerged in the guide without any need to increase the size of this element. For one and the same value of attenuation, it is thus possible to very substantially reduce the size of the plunger element with respect to a rectilinear waveguide.

[0020] The elbow shown in Figures 1 to 9 of the present application is a 90° elbow. It is possible to consider the use of elbows creating a different change in direction. The utility of the gain in size of the attenuator would then vary depending on the value of the angle of this elbow.

[0021] The pivoting of the plunger element 5 around its rotational axis 6 is illustrated in Figures 2 and 3. With a view to simplifying the explanation, the reference 11 designates the curved section of the groove 3, and the references 12 and 13 designate the rectilinear sections of the groove 3 on either side of the curved section 11.

[0022] In a first extreme angular position shown in Figure 2, the contour of the plunger element 5 is tangential to the internal face of the guide at the curved section 11 and the rectilinear sections 12 and 13, and the plunger element portion in the waveguide is zero. In this angular position, the waveguide formed by the grooves 3 approaches the state of a perfect waveguide (the plunger element plugging the slot without extending into the guide). The attenuation generated by the attenuator in this position is then zero.

[0023] If the plunger element 5 is made to pivot around its axis, it is then inserted gradually into the waveguide and the value of the attenuation increases gradually up to another extreme angular position, in which the plunger element 5 goes completely through the waveguide 3 at the elbow and for which the plunger element portion introduced into the waveguide is the maximum. This new angular position of the plunger element is illustrated in Figure 3 and corresponds to a pivoting of the plunger element by 180° with respect to the angular position of Figure 2. This angular position, although the plunger element portion 5 submerged in the guide is the maximum portion therein, does not correspond exactly to the maximum attenuation value. Indeed, phenomena other than the absorption of the signal by the plunger element must be taken into account to explain the effect of attenuation. Explanations in greater detail shall be provided in the descriptive part pertaining to Figure 10.

[0024] Preferably, the attenuation is chosen so as to be linearly dependent on the angular position of the plunger element. This dependence is related to the shape of the contour of the plunger element 5 and the position of its rotational axis 6 with respect to the groove 3. Thus, if D1 is the reference given to the distance between the rotational axis 6 and the inner face of the rectilinear section 12 of the guide and D2 is the reference given to the distance between the rotational axis 6 and the inner face of the rectilinear section 13 of the guide, then preferably D1>D2 will be chosen to obtain a linear dependence, given that the plunger element 5 rotates in the anticlockwise direction. This off-centered position of the axis 6 with respect to the rectilinear sections 12 and 13 of the guide will enable the plunger element to be introduced more deeply into the waveguide for the low angular values of the plunger element.

[0025] As a consequence of this offsetting of the rotational axis, a scalloping 14 is made in the external face of the waveguide, at the curved section 11 and the rectilinear section 13, so that the plunger element can completely go through the waveguide and pivot without being encumbered about its axis of rotation. The scalloping 14 has the shape of an arc of a circle having the same radius and the same center as the cavity formed by the two blind hollows 4. The scalloping 14 is made at the same time as the blind hollow 4 in one and the same machining operation.

[0026] The linearization of the attenuation curve can be made more precise by acting on the shape of the contour of the plunger element. A preferred embodiment of the plunger element is shown in Figure 4. This contour shape has been determined empirically. In this preferred embodiment, the contour of the plunger element 5 has two orthogonal rectilinear portions 15 and 16 whose neighboring ends meet by an elbow 17 matched with the elbow of the waveguide. The remaining part of the contour between the distant ends of the portions 15 and 16 is an essentially convex, curved segment 18. The center of rotation of the plunger element 5 is not equidistant from the two rectilinear portions 15 and 16. In the embodiment shown it is closer to the portion 16.

[0027] By way of an indication, a prototype was made with the following dimensions which are particularly useful for defining the shape of the curved segment 18. R1 refers to the distance between the rectilinear portion 15 and the center of rotation 6, and R2 refers to the distance between the rectilinear portion 16 and the center of rotation.

[0028] The curved segment 18 is defined by a sequence of values R(α) each representing the distance between a point of the curved segment 18 and the center of rotation, where α designates the angle between the distal end of the rectilinear portion 15 and the point of the curved segment considered and varies between 0° and 180°. The values R(0°) and R(180°) correspond respectively to the distance between the distal ends of the rectilinear portions 15 and 16 and the center of rotation of the plunger element. The set of values R(α) pertaining to the prototype are contained in the appended table. These values are indicated every 9°.

[0029] The variable attenuator according to the invention is also distinguished by the rotational driving system of its plunger element which has been designed especially with a view to optimum reliability of operation, taking into account all conditions of use. These conditions of use are often severe, especially as regards the variations of temperature which may go up to more than 100°C and give rise to a major risk of mechanical jamming.

[0030] Referring to Figures 5 to 9, it can be seen that the plunger element 5 is fixedly joined to a shaft 21 going through the body part 2 by a through bore 24 to be driven externally by a motor 19. To this end, the plunger shaft 21 is provided at one end with a gear wheel 20 driven by the output shaft of the motor 19. It must be noted that preferably, as shown, the gear wheel 20 has a concave rim designed to partially envelope the shaft of the motor 19 on a portion of this motor forming an endless screw, engaged with a complementary set of teeth of said concave rim.

[0031] In accordance with the invention, the motor 19 is mounted on the wing of an L-shaped support 22 whose base is applied and fixed to the external wall of the body part 2, the output shaft of the motor 19 going through said wing. Furthermore, the plunger shaft 21 is mounted without clearance in a neck collar 23 that is shouldered so as to take support at its internal end on the edge of the through bore 24, cross said body part 2 by this bore and then cross the base of the L-shaped support through a bore 25, the two bores 24 and 25 being adjusted to its external diameter. The bores 24 and 25 are shown in Figure 9.

[0032] On the shaft of the plunger 21, the neck collar 23 is interposed between an internal shoulder of this collar 26 and the gear wheel 20 which is engaged on the external end of the shaft and adjusted and kept in position by means of an end screw 27.

[0033] The collar 23 also determines the positioning of the plunger 5 in the direction of this rotational axis. Indeed, as can be seen clearly in Figure 8, the plunger element 5, engaged on its drive shaft 21, is held against a shoulder 28 of this shaft and the internal end of the collar 23. Hence, it is held at a distance from the bottom of the blind hollow 4 in the body part 2 that corresponds to the axial distance between the internal end of the collar 23 and its shoulder 28.

[0034] It can therefore be seen that all the mechanical clearance values of the system are carried into the diameter and length of the collar. The result of this is that it is almost impossible to have jamming caused by the effects of thermal expansion. Furthermore, it must be noted that this arrangement is extremely simple to mount and dismantle.

[0035] Figure 10 shows some values of the attenuation generated by an attenuation device corresponding to the preferred embodiment of the invention as a function of the angular position of its plunger element. The plunger element used is the one shown in Figure 4 with a thickness of 2 mm. The attenuation values are given for an incident signal with a frequency of 13 GHz. The attenuation is shown for the values of angular position ranging from 0°, corresponding to the case where the contour of the plunger element is tangential to the internal face of the waveguide, to 235° corresponding to a pivoting of the plunger element by 235° in the anticlockwise direction (the position for which the attenuation is the maximum). This attenuation curve shows that the attenuation of the incident wave does not result solely from the absorption of its energy by the plunger element 5. Otherwise, the maximum value of the attenuation would be located around the position 180° (it would not be exactly at 180° owing to the non-uniformity of the curved portion 18 of the plunger element 5). It would appear that phenomena of reflection of the incident wave explain this shift in the angular position of the maximum.

Claims

1. A variable attenuation device for a rectangular waveguide, of the type comprising a substantially plane plunger element (5) a rounded shape designed to be introduced perpendicularly into said waveguide (3) through a longitudinal slot (7) made along the axis of propagation of the waves in a first face of said guide, the plunger element (5) being rotationally mobile about an off-centered axis (6) so that the portion of the plunger element introduced into the waveguide is a function of the angular position of said plunger element,
   wherein said plunger element (5) and said longitudinal slot (7) are positioned at the level of an elbow (11) formed by said waveguide.

2. A variable attenuation device according to claim 1, wherein the shape of the contour of the plunger element (5) and the position of its axis (6) are designed so that the attenuation is linearly dependent on the angular position of the plunger element.

3. A variable attenuation device according to claim 1 or 2 wherein, in a first extreme angular position, the contour of the plunger element is tangential to the first face of said elbow in such a way that the portion of the plunger element introduced into the waveguide is zero

4. A variable attenuation device according to any of the claims 1 to 3 wherein, in a second extreme angular position, the plunger element completely goes through said waveguide at the level of the elbow so that the portion of the plunger element introduced into the waveguide is the maximum.

5. A variable attenuation device according to claim 4, wherein a scalloping (14) is made, at the level of the elbow (11), in the face opposite the first face of the waveguide so that the plunger element can completely cross the waveguide.

6. A variable attenuation device according to one of the preceding claims, wherein said elbow (11) is a 90° elbow.

7. A variable attenuation device according to one of the preceding claims, comprising a body formed by two symmetrical parts (1, 2) each comprising a groove (3) and a blind hollow (4) leading into the interior of said groove so that, once the two parts are assembled, the grooves form said waveguide and the blind hollows form a cavity designed to receive the plunger element (5), said plunger element (5) being introduced into said waveguide at the intersection between said grooves (3) and said blind hollows (4) corresponding to said longitudinal slot (7).

8. A variable attenuation device according to claim 7, wherein the contour of the plunger element comprises:

- first and second rectilinear parts (15, 16) designed to be tangential, in said first extreme angular position, to said first waveguide face of each side of said elbow (11), and

- a first curved part (17) between said first and second rectilinear parts (15, 16) designed to be tangential, in said first extreme angular position, to said first waveguide face at the level of said elbow (11).

9. A variable attenuation device according to claim 8, wherein the distance (R2) between said first rectilinear part (15) and the rotational axis of the plunger element (6) is greater than the distance (R1) between said second rectilinear part (16) of the plunger element and its axis of rotation (6).

10. A variable attenuation device according to claim 8 or 9, wherein the part (18) of the contour of the plunger element located opposite said first curved part between said first and second rectilinear parts is determined by the appended table pertaining to figure 4.

11. A variable attenuation device according to any of the preceding claims, wherein the plunger (5) is mounted on a rotational driving shaft (21), said shaft comprising an end gear wheel (20) to be coupled externally to a motor (19) borne on an L-shaped support (22) fixed by its base to the body (2), and wherein this plunger shaft (21) is engaged in a neck collar (23) going through the body (2) and the base of said motor support (22) by means of through bores (24, 25) adjusted to its external diameter, the collar (23) comprising an internal end shoulder designed to take support on the edge of said bore (24), and being furthermore interposed on the shaft (21) between an internal shoulder (26) of this shaft and said drive gear wheel (20), this gear wheel being engaged in the external end of the shaft (21) and being adjusted and held axially on it by means of a screw (27) entering the end of the shaft (21).

12. A device according to claim 11, wherein the plunger is engaged on the shaft (21) held between a shoulder (28) of this shaft and the internal end of the collar (23).

13. A device according to one of the two preceding claims, wherein said gear wheel (20) has a concave rim partially enveloping a portion of the output shaft of the motor (19), said portion forming a worm-screw and being engaged with a complementary set of gear teeth of said concave rim.

Drawing