US2512468A - Wave guide with mode suppression means - Google Patents

Description

JuneZO, 1950 w. s. PERCIVAL 2,512,468

.WAVE GUIDE WITH MADE SUPPRESSION MEANS Filed Aug. 2, 1946 INVENTOR Wat/AM 6T pfAC/VAL ATTORNEY Patented June 20, 1950 WAVE GUIDE WITH MODE SUPPRESSION William Spencer Percival, Ealing, London,,E ng- ,land, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a corn+ 'pany of Great Britain v Application August 2, 1946, Serial No. 688,165

r In GreatBritain May 12, 1943 1 Section 1, Public Law 690, August 8,1946

Patent expires May 12, 1963 2 Claims. (Cl. 178-44) This invention relates to electric waveguides and has for its object to provide means whereby waves of the so-called E mode, such as E waves, can be propagated along a guide and undesired waves of the so-called H mode, such as H; waves, can be eliminated from the guide or substantially attenuated. The present inventionpro vides a simple and effective meanswhereby such elimination can be accomplished.--

The present invention is based on the fact that a narrow longitudinal slot in a guide will allow waves of H mode to escape while having a negligible effect upon waves of E mode, so that waves of the latter mode will be transmitted along the guide while Waves of H mode will be attenuated.

According to the invention there is provided an electric waveguide for the transmission of waves of the E mode coupled to a source of said waves and having undesired waves of the H mode present in said guide, wherein said guide is provided with one or a plurality of slots arranged longitudinally of said guide, the size of said slot or slots being such as to afiord substantial attenuation of said undesired waves. A plurality of said slots may be provided spaced equidistantly around the circumference of said guide and, if desired, a plurality of sets of said slots are provided each set being spaced longitudinally of said guide. Where the said slot or slots is or are provided in a guide formed of a conducting tube, a conducting sleeve or sleeves may be provided axially movable with respect to said tube for the purpose of adjusting the length of said slot or lots.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 illustrates a portion of a waveguide provided with means in accordance with the preferred form of the invention, and

Figure 2 illustrates a portion of a waveguide provided with means in accordance with another embodiment of the invention.

In the embodiments of the invention shown in Figures 1 and 2', the section of the waveguide illustrated is intended to transmit waves of E0 mode and to eliminate or substantially attenuate waves of H1 mode.

Referring first to Figure l of the drawings, the waveguide illustrated comprises a conducting tube 3 of circular form in cross-section having an internal diameter of one and one-eighth inches and a wall thickness of one-sixteenth of I an inch, and is,intended to mode which in air have .a-wavelengthgoi ,3,2 cms.,

The guide is provided witl;1;.;giour .filter sections each comprising aplurality of setsof longitudinal slots 4-, 5, 6 and], eight slots being provided in each sectionequidistantly spaced around the circumference of the guide. .The optimum length of each slot is found to be-about 1.5 times the wavelength of the waves to be propagatedin-the guide and where waves of a, wavelengthof 3.2 cms. are required to be transmitted, the optimum length of each slot is 4.5 cms. The width of each slot is one-sixteenth of an inch and the spacing between the slots in the adjacent sections is 3 cms., although this spacing is not critical.

H waves when travelling along a guide set up circumferential currents in the guide due to their axial magnetic field and the effect of the slots above described is to present a high impedance to the circumferential currents which thus set up a potential difierence across the slots and hence causes radiation of H1 waves. The filter acts like an impedance in the guide which has no measurable effect on E0 waves but reflects or absorbs nearly all the power of the H1 waves. Since a considerable fraction of the power of the H1 waves is absorbed this is advantageous compared with a filter which operates entirely by reflection since, in general, it is not possible to build up high H1 voltages in the guide on the source side of the filter. If desired, it is possible by suitably choosing the length of the slots forming the filter section nearest the source to adjust the impedance of the filter to match the impedance of the source of H1 waves. The length of the slots may be adjusted by the provision of an axially movable conducting sleeve 8.

In the embodiment of the invention shown in Figure 2 the guide 3 is provided with two filter sections each comprising a plurality of slots 9 and I0 spaced apart as indicated. Each section has four slots arranged longitudinally with respect to the axis of the guide and equidistantly spaced around its circumference. The slots 9 and ID are substantially longer than the slots shown in Fi ure 1 and are partially covered by conducting sleeves H and I2 which are arranged for axial movement along the length of the slots 9 and I ll. In one specific example for waves with an air wavelength of approximately 3.2 cms., the length of the slots 9 and I 0 are five and one-eighth inches, the two sections being spaced apart by one and one-quarter inches, although this latter distance is not believed to be critical. The length of each sleeve II and I2 is one inch and with transmit waves of the sleeves l I and I2 arranged 1.79 inches from the left and right hand ends of the sections respectivel'y,Imaximurri attenuation ofithe Hr'waves occurredalthoughmot'to" the same extent as the attenuation obtained with the arrangement of Figure 1.

The present invention is of particulanuse-:irr-

apparatus for measuring impedances. appreciated that in such apparatus it. is necessary to isolate the impedance meas'uringrapparatus from possible s0urces of"l- I1 "waves1in theff 7 source of E0 waves and in the impedance'to be measured. By inserting a filter as described above between the measuring apparatus and the; source and between the measuring apparatus and the impedance to be measured, thisrisolation can be readily accomplished.

Although in the above description the filters V described each employ a.. ,plurality of sections, it

moveable-with respect to said member to ad-' just the apertures of said slots to lengths at which undesired waves of the H mode present in said guide are substantially attenuated.

2; An electric wave guide for the transmission of waves of the E mode to the exclusion of waves 'of" the H mode, comprising a hollow conducting tube havinga plurality of sets of slots open to free space arranged longitudinally of said tube,

the slots of each set of slots being spaced equidistantly about the: circumference of said tube,

and conducting sleeves surrounding said tube,

said sleeves being axially moveable with respect to said tubes to present openings of said slots having lengths atwhich undesired waves of the H mode present in said guide are substantially completely attenuated.

' WILLIAM: SPENCER PERCIVAL.

REFERENGES CITED vFlhe:followingreferences are of record in the file of.-tthis; patent:-

I UNITED STATES PATENTS Number Name Date 2,151,118 King Mar. 21, 1939 2,239,905 Trevor 'Apr. 29, 1941 2,241,119 -Dallenbach Ma 6, 1941 2,423,526 Sontheimer July 8, 194'! Southworth Feb. 8,v 1949

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