US2944234A

US2944234A - Adjustable impedance for use in waveguides

Info

Publication number: US2944234A
Application number: US666058A
Authority: US
Inventors: Ronde Frans Christiaan De
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1956-07-11
Filing date: 1957-06-17
Publication date: 1960-07-05
Anticipated expiration: 1977-07-05
Also published as: FR1178323A

Description

y 1950 F. c. DE RONDE 2,944,234

ADJUSTABLE IMPEDANCE FOR USE IN WAVEGUIDES Filed June 17, 1957 INVENTQR FRANS CHRISTIAAN DE RONDE AGEN United States Patent 2,944,234 ADJUSTABLE IMPEDANCE FOR USE IN WAVEGUIDES Frans Christiaan de Ronde, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed June '17, 1957, Ser. No. 666,058 Claims priority, application Netherlands July 11, 1956 1 Claim. (Cl. 33398) This invention relates to adjustable impedances for use in waveguides. Such impedances may, for example, be employed for terminating a waveguide in a reflectionfree manner or as a comparison impedance in a measuring arrangement for measuring unknown impedances. With known adjustable impedances, the modulus and the argument of the reflection factor are, in general, not adjustable independently of each other and these impedances are not so well suited for use in wave systems for millimetre waves.

The present invention provides a simple adjust-able impedance for waveguides, in which the modulus and the argument of the reflection factor are adjustable independently of each other and which is particularly suitable for use in measuring arrangements, since the modulus and the argument are simple functions of two variable factors, so that the impedance can be calibrated in an absolute manner. With the adjustable impedance according to the invention, the waveguide comprises a first flat strip, which consists at least at its surface of resistive material and is fixedly positioned in a plane extending through the axis of the waveguide. Furthermore, a second fla-t strip, which also consists of resistive material at least at its surface, is secured at right angles to the surface of a short-circuiting piston which, together with the second strip, is rotatable and also is movable in an axial direction of the waveguide.

A variable attenuator is known per se, in which two flat strips of resistive material are fixedly arranged in a waveguide and, between these strips, a third strip of resistive material is rotatable about the axis of the waveguide.

This device permits only the value of the attenuation tobe controlled, but the phase of the waves is not variable.

In order that the invention may be readily carried into effect, an example will now be described in detail with reference to the accompanying drawing, in which Fig. 1A shows a circular waveguide G in which a strip S1 is arranged in a plane through the axis of the wave guide. The strip may, for example, consist of a mica plate which is provided with a layer of resistive material, for example finely divided carbon. The waveguide G contains a short-circuiting piston Z which, in order to prevent the emission of energy, is in known manner composed of three parts Z1, Z2 and Z3, the length of which corresponds to one quarter wavelength. Onthe left-hand surface C of the piston Z rests another strip S2 which may also consist of a mica plate provided with a layer of resistive material. The strip S2 extends at right angles to the surface of the piston in a plane through the axis of the piston. The piston Z together with'the strip S2 is rotatable about the axis of the waveguide and also is movable in a longitudinal direction of the waveguide. The position of the piston is read off scale divisions (not shown). In order to reduce reflection of the waves against the strips S1 and S2, the ends of the strips are bevelled (see strip S2).

Fig. 1B is a cross-sectional view of the waveguide on the line 1B-1B. The left-hand end of the waveguide G may, for example, through a gradual transition, be connccted to a wave-guide of rectangular cross-section.

This device operates as follows:

The incident waves at the input B are linearly polarized at right angles to the strip S1, as indicated by a vector E in Fig. 2. Hence, they canpass unhindered through the strip Si. The strip S2 subtends, for example, an angle a with the direction of polarization of the incident waves E. The incident waves E may be thought of as being decomposed into two components, a first component Ea, the sense of polarization of which is parallel to the plane of the strip S2, and a component Eb, the sense of polarization of which is at right angles to the plane of the strip and the strength of which is equal to E sin a. The first component Ea is entirely absorbed by the strip S2. The component Eb, however, passes unhindered through the strip S2 and is completely reflected by the piston Z. The reflected component may in turn be decomposed into two components, that is to say a component Be, the sense of polarization of which is parallel to that of the strip S1, and a component Ed, the sense of polarization of which is at right angles to that of the strip S1 and consequently corresponds to the sense of polarization of the incident waves E. The component E0 is fully absorbed by the strip S1. However, the component Ed will pass unhindered through the strip S1 and emerges to the left. The strength of this component corresponds to Eb sin a=E sin a. By varying the angle a, the strength of the reflected wave, that is to say the modulus sin a of the reflection coefiicient is consequently adjustable to any desired value in a simple manner.

The phase difference between the incident waves E and the reflected waves Ed corresponds to twice the spacing between the inlet B and the surface C of the piston Z expressed in electric degrees plus since a phase change of 180 occurs upon reflection against the piston surface. The phase difference between the incident waves E and the reflected waves Ed, that is to say the argument of the reflection factor, is consequently adjustable to any desired value by moving the piston Z either to the left or to the right.

The device may, for example, *be employed for calibrating unknown impedances by means of a so-called magic- T, the unknown impedance and the variable impedance being, in accordance with the invention, connected to arms opposite each other of the magic-T, a third am being connected to a generator, and a fourth am being connected to a measuring device. The variable impedance is varied in such manner that no wave energy appears in the fourth arm. In this case, the unknown impedance is equal to the variable impedance, and the modulus and the argument can be read oil the scale-division of the variable impedance.

What is claimed is:

A variable impedance arrangement comprising a hollow waveguide, a first flat strip composed at least in part of resistive material positioned in said waveguide in a plane passing through the axis of said Waveguide, a shortcircuiting piston positioned in said waveguide and adapted to be independently rotated and moved in the axial direction of the waveguide, and a second flat strip composed at least in part of resistive material attached to the end of said piston which faces said first flat strip and being rotatable with said piston, said second flat strip being aligned along the axis of said waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 2,088,749 King Aug. 3, 1937 2,603,709 Bowen July 15, 1952 2,692,977 Koppel Oct. 26, 1954 2,808,571 Cohn Oct. 1, 1957 FOREIGN PATENTS.

691,939 Great Britain May 27, 1953