US2467578A - Electromagnetic horn - Google Patents

Description

April 19, 1949. w. L. BARROW ELECTROMAGNETIC HORN Original Filed Jan. 9, 1939 INVENTOR Y LBW/0w ATTORNEY I-Irctype; or more generally, the Him-type;

Patented Apr. 19, 1949 ULNITED STATE S PAT EN T 7 .0 F Fl CE ELEG'EROMAGNETIC HORN Wilmer-manier- Barrow, Newton, Massl, assignor ate-Research Corporation, New Kerk, N. Y., a v "corporation of New :York

. -,:.'0'riginal application January 9,. 1939, Serial No.

$219,910, nowlatent No. 2,316,151, dated April 13,1943; Divided and this application Decem- TbBlfIO', 1941,.Sfiri2tl No. 422,402

191=ClaimsQ (Cl. 250,'33.63)

I The present invention relates to electromagnetic horns. The -present --application is a a diviantenna orthelikeis'positioned in'the throat of the horn.

It is by nomeans true that any'horn will transmit or receive any wave, and an object of the present invention is to assure the'production near the mouth of a sectoral horn -ofa-wave of the The type of horn-known as-sectoral and the 'Hn,0 waves will be more f-ully descr-ibedhereinafter.

I A On the other hand; aplurality-of different types of horn waves may be generated; depending upon the configurationof- "the "exciting means in the throat of the -horn-,-or-'-upon the nature ofthe waves delivered there by a hollowepipe transmission system, and-also upon the flare "angle 0 and the: cut-o-fiiength pc from the-apex to'the-throat, or small 'or irefiectingend, or the back of the horn. Special "cases' may arise-wherein several wave-types -may be used'*"simultaneously.- In

vmany-applications, howeverra' single waveshould obtain-near the mouth of the-horn, as-theradiation patter-nwil1-be distorted bythe presence of other waves.

A further objectpthereforeis to exclude waves of other types'than those-above describedin the 'sectoral'horn.

To the attainment ofthese endsgit is desirable to position the excitin or absorbing means properly, or properly to'designthersize of .the' throat "of the sectoralhorngor-both.

Another object is 'toprovide a sectoral horn havingza predetermined performance.

In, an electromagnetic *horn provided -with a pair of oppositely-disposed"flaring sides-substanother},objectaiofsthezpreseniziin ention is to assure the production of the desired "H waves -near-the mouth of thehorn and to exclude other than -the desired waves.

Other andfurther objects will be explained hereinafter and will be particularly pointed-out inthe appendedclaims.

The invention will now be described in connectio-n"with the accompanying drawings, in

whichFig, l is a perspective, partly broken away, of a sectoral electromagnetic horn embodying the invention, and fedby a ihollow-pipe line; Figs. -,2=and 3 are similarperspectives of modifications; and-Fig. .4 is a diagrammatic perspective of a sectoral horn,-disposed in Cartesian =and'cylindrical systems of-co-ordinates, and carrying symbo ls useful indesclibll'lgth6= il1VeIltlOn.

The horn 16 is illustrated as ofrectangular cross section, fiaringsmoothly and-continuously from the throat toamouth or-apertureorqlarge end of the horn, at its-front; The principal'or central axis of the horn extendsbetweenthe smaller and larger endsof-thehorn. At its throat, it isishown, in Figs. 1--and;2,- connected to a hollow-pipe transmissionsystem comprising an'elongated hollow pipe or tubular body-portion orsec- :tion 43 that is connected to thethroat of--the horn .so. as to extend over any desired=dis-tance fromrthe horn, totheleft. In'F-i'g. 3; the horn is shown unprovided with-the hollow pip l8.

, Thehornmay be constituted of a formed-sheet ofc'conducting,material like metal -such as copper or, aluminum, or .it may beconstituted ofother material if its inner Wall i is otherwise rendered a conductor of thesaid' waves, The-hollow-pipe body portion l8, to-which thehornl6 -is--connected, may be of any desired material-,-conducting or dielectric, or it may otherwise beaprovided with an inner conducting-wall. It may'con'tain .air or other. gas, or: it zmay beevacuated.

:Two of the sides,: shown as the top and' bottom sides, may be substantially parallel, and theother two'sidesfiared, as illustrated in Figs. -1 an'd 3; or the. horn may beslightly pyramidal, Withthe sides of the horn corresponding tothe saidsubstantially parallel sides alsoq flarin-g, butslightly only, as illustrated in Fig. 2, LThe-hOIIISOfFigS.

1 and 3 maybedescribed as of sectoral-shape, and the horn of Fig. 2, as :more fully-explained hereinafter, may be termed-substantially sectoral. .In the horn of Fig, 2, thesaidslightly flaring sides will, ior convenience, be reierred to as substantially parallel, toconform to'the'terminology, em-

ployed in connection with the sectoral ghorns.

Horns provided with-straight sides inlongitudinal cross-section, as illustrated-are economical and easy to construct. The invention is, however, applicable also to horns the sides of which are of varying flare, such as exponential or hyperbolic.

The Hn,0 waves inside these horns, at a distance several wave-lengths from the throat, are

governed, among other things, by the dimensions of the parallel sides of the born.

A projecting metal exciting or absorbing antenna rod I 4 or other energy-translating apparatus is shown in Fig. 1 disposed approximately centrally in the hollow pipe I8, substantialy at right angles to the axis of the horn, but it may be disposed unsymmetrically in the horn, to give a modified directive pattern for the radiant energy.

Sending apparatus (not shown) may be connected to a coaxial-line system I0, I2, or to a parallel-wire system, or to any other desired connecting system. The conductor l2 may be extended into the bell of the born, as illustrated in Fig. 3, or into the tube 18, parallel to the top and bottom walls. In the latter case, the translating apparatus [4 is positioned in the pipe 18, at the rear of the throat of the horn. In the former case, it is positioned directly in the throat of the horn to constitute the antenna I l.

The horn of the present invention may be used for transmission or reception of ultra-high-frequency electromagnetic waves. In transmitting with the system of Fig. l, the antenna [4 first excites electromagnetic waves in the hollow pipe, which are transmitted through the pipe I8 to the throat of the horn, and thence propagated through the horn to the mouth as horn waves of the Hn,0 type. At the mouth, substantially all of this energy is radiated into free space as ordinary radio waves. The horn thus constitutes a directive electromagnetic radiator. As shown in Fig. 3, however, the translating apparatus may be positioned directly in the throat f the horn, in

order directly to excite the horn itself, without the use of a hollow-pipe transmission line. Substantially the same radiation pattern may be produced with either arrangement.

In receiving, a similar, but reverse, process takes place, the electromagnetic waves being received by the horn l6, and communicated to a receiving system (not shown),

The horn operates eiliciently to receive substantially all of the incident energy, when used as a receiver; or, when used as a transmitter, to excite waves of the horn type, and thereby to radiate a beam of character appropriate to the horn rather than to the apparatus and the antenna itself.

An important mode or wave type is the lowestorder transversely polarized horn wave H1,o, with the electric vector mainly parallel everywhere to the horizontal direction, obtained with an exciting rod l4 transverse to the axis and in the horizontal plane. This and the Ho,1 wave types are probably the best for sending a single beam of radiant energy and are the waves most naturally adapted for receiving.

Beams transmitted from directive antenna systems are usually accompanied by small amounts of radiation in directions other than those intended. These small amounts of radiation are referred to as secondary lobes. Beams radiated from sectoral horns may be remarkably free from secondary lobes. The configuration of the field of the Hn,0 waves, among others, inside the sectoral horn, is such that the lines of electric-field intensity are everywhere substantially parallel to the substantially parallel sides of the horn. The flaring sides of the horn are substantially normal to the said lines of electric-field intensity. The result of this configuration is to produce substantially single-lobe beams of linear polarization in the radiated waves. These waves have constant phase on cylindrical surfaces about the axis within the horn. For this reason, the use of sectoral horns is particularly advantageous in certain kinds of applications where the shape of the beam plays an essential role in the operation. In the blind-landing of airplanes, for example, it is desirable, not only that the beam be very sharp, but also that it be peculiarly free from secondary lobes. In one such application, a smooth straightline intersection is formed between two overlapping beams; systems of this type are commonly referred to as equal-signal" systems. Horn radiators can provide such smooth overlapping beams without waviness or spurious components that would affect the straightness of this path of intersection. No other types of antenna has been found to produce so smooth beams with such small secondary lobes. Patterns of this character are useful also in other applications, such as direction-finding and obstacle-detection.

In general, there are two distinct groups of waves: E-waves, having a radial component of electric intensity, but no radial component of magnetic intensity, in the horn; and H-waves, having a radial component of magnetic intensity, but no radial component of electric intensity, in the horn. Two subscripts are needed to define the waves of different orders. The subscripts n and m are used, each representing a positive integer denoting the number of half-sinusoidal variations in the field between the top and bottom and the two side walls, respectively. Thus, we have the Hn,m and the En,m types of waves.

The properties of the H1,o wave will not be further described herein. A description thereof may be found, for example, in Reissue Letters Patent 23,003, of May 25, 1948, which makes reference to a paper by L. J. Chu and W. L, Barrow, entitled, Electromagnetic waves in hollow metal tubes of rectangular cross section, Proceedings of the Institute of Radio Engineers, vol. 26, No. 12, December 1938, commencing at page 1520, a paper by W. L. Barrow, entitled Electromagnetic-horn radiators, Union Radio Scientifique Internationale, No. 79, page 277, containing a revision of a paper presented at the joint meeting of the said Union and the Institute of Radio Engineers, at Washington, D. G, April 30, 1938, a paper by W. L. Barrow and L. J. Chu, entitled, Theory of the electromagnetic horn, Proceedings of the Institute of Radio Engineers, vol. 27, No, 1, January 1939, commencing at page 51, and also a paper by W. L. Barrow and F. D. Lewis, entitled, The Sectoral electromagnetic horn, Proceedings of the Institute of Radio Engineers, vol. 27, No. 1, January 1939, commencing at page 41.

Though the analysis will be hereinafter given in connection with sectoral horns, it applies also very approximately to horns that are somewhat pyramidal in which the two substantially parallel sides are not strictly parallel but diverge to some extent. The term substantially sectoral" will be employed in the claims to include somewhat pyramidal horns of thisnature.

The invention is not, of course, limited to the use of an exciting or absorbing rod l4. Other radiating or absorbing means, such as a vacuum tube, may also beemployed,'as described, for

example," in "Letters Patent 2,367;011-- *and 2,307,012,is"sued January 5, 1943; "Asis- 'al's p'lained in-thesaid Letters "Patent, epumumcondit-ions maybeobtaine d by adjusting apiston (not shown) at-the back or throat of the horn, thusto resonate or tune the throatof the-horn, thereby rendering the throat ofthehorn'more responsiveto-aparticular frequency or "a narrow band of frequencies than to other '-fi'i1uencies, and also in otherways.

Symbols that will be used inth' sp'ecifioation will'be understood by rererencet Fig. 4. The lower ofthe two parallel 'sides o Figs-l is here assumedto lie inth XZ'piarie its f non-parallel or flaring "sidesfxtended, I v se cting on'," and making equal'lan'gles'with, the 'Y axis. The sy tem-may; ib 'regarue as jsymliblizalso cylindrical coordinates "y, P3111115. The upper parallel side of *the'hornfis parallel to 'the X2 "plane, and at a distance a therefrom; "It is assumed'thatthe throat ofth'e'hor'n is-disposed along a vertical cylindri'cal surfa'ce having aradillS'pc-and with its" axis Edihcidhtfivithth "Y axis. The non-parallel-sides, "'e'achof length ps 0, are'symmetricallydisposd inpranes per#- pendicular to the 'XZplane, formingwiththe XY plane'a dihedral angle equal to and parallel to the line's of"elctric 'iritehsity of the wavespr'dpagatedwithin the Horn. -Th 1id1n is regarded as having a'ior'war'd directi the positive direction or the X axis. Theh length of the mouth or aperture of t I assumed to have a value b; and-the ttrres'ptnding lerigth of'the'throat to have a-'va1ue'-l3b. -The symbols illustrated in-Fig. 4 havefitlierforel the following'meaning:

(it re resents "the fiar'e 'an'g'le or the horn inns- "tr'ated in'Fig. 1; t

b represents the-horizontal dimension of "the mouth dr'apeimre ofthis horn, orthe distance between the opposite "vertically A disposed sides of-the horn, at the mouthofthe horn;

a represents the corresponding 1 vertical dimen- "sion', at right angles to the horizontal dimension, or the 'di s'tan ce between the opposite horizontally dis-posed sides "of-thehprn, In the case of the slightly pyramidalhorn, the di mension'a-should be taken at the throat of'the born. This vertical dimension isat-rightangIeS to the" lines of electric intensity ofthe waves propagated within the horn p1 represents the radiallength of the-horn,-n'ieas ured along one of the flaring sides, from the point or apex of the horn to the mouth or aperture; and V p represents the cut-01f length, from the said 'apexto the free end of-the'th-roat of the horn.

These "symbols will have approx'irriatelfthe' saine or corresponding meaningsin the case efi -pyramidal horns two of the opposite ides fwhi'ch may be more or" less parallel. -It is cdii-vnient to mealsure the'len'gths a,'-b, o '-an'd; 1'in--*terrns of 'the wave-length A they mayf-there'forefif the same unit oflength be used foralI-'dinieiisions'-*and for the wave-length A, be represented by the-symbols "The space" wav'e radiated-bi ths 'h'orn in beams or'difier'ent' configurations," and theresponse of a receiving'h'orn towaves- 'arr'iviri'g""at differefit As-before explained, it-is -an object o'f the presentinvention to design the-throat or-the'exciting means 14,01" both," so that a'des'ired Hum wave and; moreparticularly, anHro wave, and also, in cases,the desiredwave only, shall exist in the horn,'-'when 'i'ised'for transmitting; or so that the horn shall be responsive to the desired wave, such as'the -Hi;o wave, or the desired wave alone, when used for receiving. The -H1-,o type of Wave-maybe excited bythe current carr'ying "antennarod f4 disposed 'centrallyin -the throat parallel to the XZ plane, as illustrated in"Figs. '2 and 3, or'by feeding an Hrb'waVe into the throat from a' rectangular hollow pipe, as illustrated in Fig. 1.

In the H1,o wave, as appears from'the beforementioned papers, and as is illustrated in Figs. 8 to 10 of the said Reissue Letters Patent 23,003, of May 25, 1948, the electric lines of force lie along "arcs concentric with the Y axis between the two flared sides; they have a uniform distribution along the arcs, but a half sinusoidal distribution in'thedirection of the Y axis. The magnetic lines'lie inplanes passing through the Y axis.

- An'alysis,"verified by experiment; has demonstrated that the attenuation of-the H wave is 'r'egioush-rinks and is-cenfined substantially to the-throat. The 'radialdistance; from the apex radially outward, to which the region of relati'vely-high-attenuation extends, is progressively greater for waves of higher order than for waves of lower order. v

Analysis, verified by experiment, has also established-that there is abest position for the antenna-rod ifi, the horn I6 being-carried back far-enough toward the apex of the horn so as toa'tt'enuate 'undesirable-Wavesand to send forth only the desired waves.

=As explained in the said'Letters Patent 2,316,- -151 ,-in'-ahorn of a given flare angle (to, and for a "'given"wave'-length A, a particular value for the cut-off length poirom the apex of the horn to 'the' exciting -antenna It may be adopted that will permit the-H0 wave to form and travel freely throughthe horn to be radiated into space from its mouth substantially unattenuated, but i that will eflect-almost complete attenuation or filtration-of higher-order Has, Hm, etc., wavesduring propagation from the I exciting antenna toward the mouth.

se'ctoral --or islightlyi pyramidal horns desired,

9; hei bsieeii lln P al l s d --b 1 s m tially. parallel, .tol thei' lirisfof Ilctiid intensity of thes'aid wal witliin tli horn, thehom has; ing exciting or bs, rb g nslposi'tionedwhere the transverse dimension ofiitlie throa't, between the substantially parallel sides of warmth; is between substantially \/2 and,3 \/2.

5. An electromagnetichorn of substantially sectoral shape adapted for the propagation therein of the lowestorder, H1,o, waves-of waveslength A havinga radialiconipon sity but no \radialcompon 't v in th hqrni hemj s pair of oppositely dispose fl tially normal, and "am M Q I k I epsesicisicus posed sides substan'tially p allel, to'thflineso'f electric intens't of the .'said waves within .the horn, and exc i ng or absorbingmealis positioned in t thrqeto'fflthehorn where, theitransverse dimension of thet c 't, betweentheIlastT-named ir-oi sid se g h hera is b n siibstelii'ially Mz ndBM .H A l 6, An electromagnetic; horn of substantially sectoral shape. having'a pairl'of oppositely dis posed substantially parallel sides and a 'pair' of oppositely disposed fiaring sides, the angle between the flaringsides being different from the angle, bet-Ween, the substantially parallel sides, the horn being adapted ior the propagation there: in of heiowest orderiHm; W ves .oilwavez enst A having a radialcomponent ofinagneticinten sity but-no. radial component of electric. inte'n sity. in the horn, thegflaring, sides being substan tial-lyf normal, and )thesubstantiallv parallel: sides being substantially parallel, to the-lines ofe1ec tric, intensity of the, said wayesl within thehorn, the hornhaving a cut-cit throat, portionlthe transverse. dimension of which, between thel'sub; stantially parallelsides of thehorn, is between substantially M2 and,3 ,\/2.

'7. An electromagnetic; horn of substantially sectoral shape havinga, pair of cppesitelyndis posed substantially parallel sides and; a P EK-of oppositely disposed flaring, sides, the angle between the flaring sidesgbieing, differenttron; the; angle between the substantially parallel sides, the horn being adapted for the propagation therein of Hn,0 waves oi wave-length A having a radial component of magnetic intensity but no radial component of electricintensity in the horn, where n is a positive integer, the flaring sides being substantially normal,andthesubstantiallyparal. lel sides being substantiallyv parallel; to the lines of electric intensity of thesaid waves within the horn, the horn having exciting or absorbingmeans positioned where. thettransverse dimension, of, the throat,, between the substantially parallelsides of thehorn, is between substantially A weed a l0 last-namedv painof sides} of the horn, is between b tantially, a.

A A 12: and (n+2) angle between the substantiallyparallel sides, the

horn being adaptedhfor the. propagation therein of; Hut .waves of wavedengthk having a radial component of magnetic intensity but no radial component of electric intensity in the horn, where n is a positive integer, the, flaring, sides, being 'substantiallynormal, and the. substantially parallel: sides being substantially parallel, to the lines, of electric :intensity of the said waves Within the,horn,. thehornhaving a cut-01f throat pore tion the transverse dimension of which, between the substantially; parallelflsides of the horn, is betweeni'sub stantially' A n and (n+2) 5 1.0,, n e sc rqmesn i s rn... of s b tan a sectoral shape having a pair, of oppositely'disposed substantially parallel sides and spar of opposite y d sp s d flar s id she e sl ib tween" the flaring sidesbeing different from the angle betweenthe substantially parallel sides, thehorn being adapted for ,the propagatio'njthere in of l lrntwaves of wavelen gth x having a radial eom qne it me n ie i ten it b not as 1 i electric intensity in the horn, where n is a, po v'e integer, the'fiaring sides being bs anti ll no m andthe ubstantia lv p ed.- lel sides being substantially parallel, to the lines of electric intensity ofthe, said waves within the h rn ihehor hev ns 'I ns or ab orb ea positioned where the transverse dimension of the throat, betweenthe substantially parallel sides of the horn, is greater than a value within about fifty percent ofsubstantially \/2.

n ect qinasnsii h r of substantially ect l s ape depted o the p o agation were in oi l-l waves' of wave-length x having "a radial component of magnetic intensity but no radial component of electric intensity in the: horn, Where; .11 is a positive integer, the horn being providee'w th, a a r ,Q epmsitdy d s d flaring sideslsubstantially. normal, and a pair of apps: sitely disposed sides substantially parallel, to the linesof electric. intensity o'f'the said'wa'ves within the horn, and exciting or absorbing means positioned in the throat ,of the horn'wh'ere the transverse. dimension of the throat, between the last-named pair of sides of the horn 'is' greater than a value within abgut fifty per cent of substantially M2.

12. electromagnetic horn of substantially ei a s aps ha in a s ir o q s if l posed, substantially parallel sides and a pair of oppositely disposed flaring sides'f the angle between the, flaring sides being diiierent' iron} the angle'between thesubstantially parallel sides, ehb nb ipe edepiedi r the rere n t in of ,Hmwaves of wave-lengthixhaving a radial componen oimaenetie intensity but no radial wrap ent o f electric intensityjin the'horn', where n is apositive integer, "the; flaring sides being substantiallynorrnal, and the substantially parallel" sides being substantially parallel, to" the lines of electric intensity of the said waves Within the "11 horn, the horn having a cut-off throat portion the transverse dimension of which, between the substantially parallel sides of the horn, is greater than a value within about fifty per cent of substantially A/2. r

13. An electromagnetic horn of substantially sectoral shape having a pair of oppositely disposed substantially parallel sides and a pair of oppositely disposed flaring sides, the angle between the flaring sides being different from the angle between the substantially parallel sides, the horn being adapted for the propagation therein of the lowest order,-H1,o,waves of wave-length A having a radial component of magnetic intensity but no radial component of electric intensity in the horn, the flaring sides being substantially normal, and the substantially parallel sides being substantially parallel, to the lines of electric intensity of the said waves within the horn, the horn having exciting or absorbing means positioned where the transverse dimension of the throat, between the substantially parallel sides of the horn, is within about fifty per cent of a value between substantially A/2 and 3A/2.

14; An electromagnetic horn of substantially sectoral shape adapted for the propagation therein of the lowest order, Hi,o, waves of wave-length A having a radial component of magnetic intensity but no radial component of electric intensity in the horn, the horn being provided with a pair of oppositely disposed flaring sides substantially normal, and a pair of oppositely disposed sides substantially parallel, to the lines of electric intensity of the said waves within the horn, and exciting or absorbing means positioned in the throat of the horn where the transverse dimension of the throat, between the last-named pair of sides of the horn, is within about fifty per cent of a value between substantially A/2 and 3A/2.

15. An electromagnetic horn of substantially sectoral shape having a pair of oppositely disposed substantially parallel sides and a pair of oppositely disposed flaring sides, the angle between the flaring sides being different from the angle between the substantially parallel sides, the horn being adapted for the propagation thereinof the lowest order, H1,o, waves of wave-length A having a radial component of magnetic intensity but no radial component of electric intensity in the horn, the flaring sides being substantially normal, and the substantially parallel sides being substantially parallel, to the lines of electric intensity of the said waves within the horn, the

horn having a cut-off throat portion the transverse dimension of which, between the substantially parallel sides of the horn, is within about flity per cent of a value between substantially 72 and 3A/2. s V

16. An electromagnetic horn of substantially sectoral shape having a pair of oppositely disposed substantially parallel sides and a pair of oppositely disposed flaring sides, the angle between the flaring sides being different from the angle between the substantially parallel sides, the horn being adapted for the propagation therein of Hn,0 waves of wave-length A having a radial component of magnetic intensity but no radial component of electric intensity in the horn, where n is a positive integer, the flaring sides being substantially normal, and the substantially parallel sides being substantially parallel, to the lines of electric intensity of the said A i n and (ml-2) 17. An electromagnetic horn of substantially sectoral shape adapted for the propagation therein of H waves of wave-length A having a radial component of magnetic intensity but no radial component of electric intensity in the horn, where n is a positive integer, the horn being provided with a pair of oppositely disposed flaring sides substantially normal, and a pair of oppositely disposed sides substantially parallel, to the lines of electric intensity of the said waves within the horn, and exciting or absorbing means positioned in the throat of the horn where the transverse dimension of the throat, between the lastnamed pair of sides of the horn, is within about fifty per cent of a value between substantially A X 12- and (n+2) 18. An electromagnetic horn of substantially sectoral shape having a pair of oppositely disposed substantially parallel sides and a pair of oppositely disposed flaring sides, the angle between the flaring sides being diiferent from the angle between the substantially parallel sides, the horn being adapted for the propagation therein of Hn,0 waves of wave-length A having a radial component of magnetic intensity but no radial component of electric intensity in the horn, where nis a positive integer, the flaring sides being substantially normal, and the substantially parallel sides being substantially parallel, to the lines of electric intensity of the said waves within the horn, the horn having a cut-off throat portion the transverse dimension of which, between the substantially parallel sides of the horn,

- is within about flfty per cent of a value between substantially and 3A/2.

and (n+2)% WILMER L. BARROW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,206,923 Southworth July 9, 1940 2,283,935 King May 26, 1942 2,316,151 Barrow Apr. 13, 1943 Certificate of Correction Patent No. 2,467,578. April19, 1949.

WILMER LANIER BARROW It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 8, line 57, claim 3, before the Word substantially, second occurrence,

insert the lines 70 and 71, claim 4, strike out there-adapted for the propagation;

column 10, line 35, claim 10, for not read no;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 4th day of October, A. D. 1949.

THOMAS F. MURPHY,

Assistant Oommz'ssz'oner of Patents.

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