US2898561A - Electrical attenuators - Google Patents

Description

Aug. 4, 1959 A. G. HAYES ELECTRICAL ATTENUATORS Filed Dec. 14, 1954 @EHEEIEBunnn R M V 1N.

ATTORNEY United States Patent ELECTRICAL ATTENUATORS Alfred George Hayes, Wembley, England, assignor to The General Electric Company Limited, London, England Application December 14, I1954, Serial No. 475,084

Claims priority, application Great Britain December 17, 1953 3 Claims. (Cl. 333-81) The present invention yrelates to electrical attenuators.

In a waveguide it is well known to provide a member having a uniform resistive surface, for example a portion of card having a surface layer of carbon, for the purpose of reducing the amount of energy propagated through the waveguide. The member may project into the waveguide through a slot in the waveguide wall, the member being pivoted and shaped so that it is turned the amount thereof within the waveguide is varied with the result that the attenuation is changed. In order, however, to ensure that the standing wave ratio within the waveguide does not differ appreciably from unity, it is necessary for the member not to present a large impedance discontinuity within the waveguide. ln order to satisfy the requirements of variable attenuation and impedance discontinuity, the member usually has a compleX shape so that it cannot easily be reproduced in large numbers.

One object of the present invention is to provide an alternative construction of attentuator in which the movable part does not have to be accurately shaped.

According to one aspect of the present invention an electrical attenuator comprises a waveguide and a member moveably mounted so that at least part thereof may lie within the waveguide, the member carrying a nonuniform resistive layer so that the attenuation of the waveguide may be varied by moving the said member and the resistance of the layer per unit area over the member being such that, in any position, the member presents little impedance discontinuity within the waveguide.

Preferably the member is mounted solely for rectilinear movement in a direction perpendicular to the waveguide axis.

According to another aspect of the present invention, a memberfor use in a variable waveguide attenuator carries a resistive layer for the purpose of effecting the desired attenuation, the said layer being non-.uniform over the member so that when in use the member presents little impedance discontinuity within the waveguide for all values of attenuation.

One construction of an attenuator in accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which Figure l shows an isometric view of the attenuator, Figure 2 shows a diagrammatic representation of part of the attenuator, and Figure 3 shows an alternative arrangement of the part shown in Figure 2.

Referring to Figure 1 of the drawings, the attenuator comprises a waveguide 1 of rectangular cross-section having a longitudinal slot in the wall 2. A mica or glass plate 3 is mounted between a pair of guides 4 and 5 so as to pass through this slot and lie partially within the waveguide 1, the guides 4 and 5 being located wholly outside the waveguide 1 and enabling the plate 3 to be moved in a direction perpendicular to the waveguide wall 2. The plate 3 may alternatively be of ceramic, plastic or any other suitable insulating material.

The plate 3 is moved relative to the waveguide 1 for ICC the purpose of varying the attenuation presented by a resistive layer on the surface 6 of the plate 3 to electromagnetic energy being propagated through the waveguide 1. In fact the resistive layer on the surface 6 ydoes not have a uniform resistance per unit area over that `surface and the Variation is such that, .for all positions of the plate 3, the portion thereof within the waveguide 1 presents substantially no impedance `discontinuity to the flow of electromagnetic energy through the waveguide. For this purpose, the resistance per unit area. varies over the surface 6 both in the direction of movement of the plate 3, so as to give the required relationship and attenuation and plate position, and along the length of the waveguide 1, so as to effect impedance matching. In the first of these two directions, the conductivity, that is to say the reciprocal of the resistance, is least along the edge 7 and increases towards the opposite edge of the plate 3, while in the second direction it is greatest in the centre of the plate 3 and tapers off to the edges 8 and 9.

The attenuator described above may be modified by providing a second slot in the 'waveguide wall 10 opposite the previously mentioned slot in the 'wall so that the plate 3 may then project from both sides of the waveguide 1. In this case the plate 3 may be substantially larger with a more gradual change of resistance per unit area in the direction in which it is arranged to be moved.

The said resistive layer on the surface 6 of the plate 3 may consist of a continuous` film of resistive material such as Nichrome or carbon. Alternatively it may, as shown in Figure 2 (which shows only one half of the resistive layer, the other half being a mirror image abuts the centre line) consist of a grid 11 of resistive material in accordance with copending United States patent application Serial No. 475,085. In this case the required gradation of resistance is obtained by varying the size of the holes 12 in the grid 11. It will, of course, be understood that in Figure 2 only a relatively small number of holes 12 are shown for convenience and that in practice there would be an appreciably larger number.

Another form of the resistive layer is shown in Figure 3 (which again only illustrates half the layer). In this case the resistive layer consists of a mosaic of small pieces 13 of resistive or conductive material that are insulated from one another. I-f the resistive material of the grid 11 of Figure 2 is a noble metal or alloy thereof, it may be produced by the process described in British patent specification No. 689,795. Alternatively the grid may be formed by the process described in British patent specication No. 573,798. Each of these processes invol-ves printing from a master which may itself be either a positive or a negative depending upon the process used. It will be appreciated that once the master has been prepared it is possible to print a resistive layer on a plurality of plates such as the plate 3 having the required variation of resistance per unit area.

I claim:

1. An electric attenuator comprising a waveguide having a longitudinal slot in a wall thereof, a member of insulating material which has a planar surface and which projects into the said waveguide through the slot, means to mount said member for movement into and out of said waveguide in a direction such as to enable the said surface to move in the plane of that surface thereby varying the portion of the surface that lies Within the waveguide, and a resistive layer which is carried by the said member on the said planar surface thereof and which is non-uniform over the surface, the resistance of the resistive layer varying gradually and without any abrupt change in value, in a first direction which is parallel to the longitudinal axis of the waveguide, so that it has a relatively high value in the region of each edge of the layer and a relatively low value between t-l1e edge regions and, in a dierction which is perpendicular to the irst direction, from a relatively low value to a relatively high value whereby, i-n any position of the member, the re- -sistive `layer presents 'little impedance discontinuity within the waveguide.

2. An electrical attenuator according to lclaim 1 Wherein the said member is mounted ysolely for rectilinear movement in a `direction perpendicular to the waveguide axes.

3. An electrical attenuator according to claim 1 Wherein the said member is of glass.

References Cited in the le of this patent UNITED STATES PATENTS A im," Pingu-

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