US2596480A

US2596480A - Directive antenna for microwaves

Info

Publication number: US2596480A
Application number: US787190A
Authority: US
Inventors: Ernest W Guptill; Watson William Heriot
Original assignee: National Research Council of Canada
Current assignee: National Research Council of Canada
Priority date: 1946-11-20
Filing date: 1947-11-20
Publication date: 1952-05-13
Anticipated expiration: 1969-05-13

Description

Patented May 13, 1952 UNITED STATES PATENT OFFICE DIRECTIVE ANTENNA FOR MICROWAVES Ernest W. Guptil l, Montreal; Quebe andWill iarn Hcriot Watson, Saskatoon, Saskatchewan, Canada, assignors to National Research Council; Ottawa, Ontario, Canada, a body corporate of Canada Application November 20, 1947, Serial No. when In Canada. November 20,1946

13 Claims. 1

This invention relates to directive antennas for microwaves, that is electromagnetic waves having'a wave length in free space less than one metre.

The present inventors have previously proposed in application Serial No. 626,738, filed November 5 1945 (Patent No. $573,746 granted November 6. 1951), the production of antennas comprising wave guides formed with arrays of radiators consisting of slots in special arrangements. lhe antennas described in this previous application have many advantages over any microwave ahtennas previously used, but they are limited in the range of frequencies to which they will respend satisfactorily. An array according to the prior application, in which the slots are spaced at intervals of half the wave length in the guide (X9), is particularly sensitive to changes of frequency, but even one in which the slots are arrange at off half-wave spacing is limited by the fact. that the frequency must not alter either to such an extent as to bring the. slots effectively to half-wave spacing or to such an extent as to bring thesiot spacing above seven-eighths of the wave length in free space (As). 7

According tothe present invention these difiiculties are avoided by providing an array which will operate equally satisfactorily Whether the slots are at half-'wave'spacing or at off half f-wave spacing. This results from the discovery that ii an appropriate combination of a slot and'a probe is made in a guide which is terminated by its characteristic conductance, this conductance is transformed unchanged by the combination. For

this purpose theslot must be of such a length that when-it isinclinedat an appropriate angle to the longitudinal centre line of the broad face of a rectangular wave guide fed with a microwave of a given wave length and its centre is di placed by an appropriate distance from this line, it Will i transform the characteristic conductance of the guide to an admittance composed of a conductance having a value equal to the characteristic conductance of the guide and of a susceptance. A slot of such a length is termed in thisspecitlcation' a purely reactive slot, since its net effect, when the guide beyond it is terminated by the characteristic conductance, is to alter the input admittance solely by the addition of animaginary quantity. i

Accordingto the invention a directive antenna for microwaves comprises a wave guide formed with an array of slots substantially purely reactive for a microwave of a given wave length and of substantially greater length than Width. The

2' v centres of the slots are spaced along the length of the guide at intervals of less than substantially seven-eig'hths of said Wave length in free space. The centres of alternate slots along the length of the guide are displayed on opposite sides from a line at which the transverse current flow along the guide wall is zero in the absence of electrical discontinuities when theguide is fed with amicrowave. All the slots, mcreover, are inclined with respect to the line in question. The angleof inclination of each slot and the distance of displacement of the centre of each slotis such that the slot will radiate substantially no powerwhen it is shunted by a susceptance of a sign appropriate to the direction of inclination of the slot and of a value equal to the valueof the characteristic admittance of the guide. If the direction of inclination of the slot such that the end nearest theinput end of the guide is closestto the line of zero transverse current flow" referred to, the appropriate susceptance is negative, and if the slot is inclined inthe opposite direction, the appropriate susceptance is positive. Inside the guide and associatedw'ith" at least each group of consecutive slots which radiate substantially one-fifth of the total power supplied to such group are means for introducing a susceptance of substantially the same value as" but of opposite sign to the susceptance re'sultihg from such group when the guide beyond such group is terminated by the characteristic admittance; w

Wave guides of various shapes and cross sectional diinensions may be used. These haveheretofore been the subject of considerable study, and various forms have been used or proposed to be used in connection with previously known dipole arrays. The wave guideshape and crosssection forms no part of the present invention; Substantially the same considerations apply to the selection of a suitable shape and cross section for a guide provided with an array of slots according to the present invention a's'fo'r a guide provided with a-dipole array. The cross sectional shape mos-t generally used is rectangular but not square, themaximum electrical dimension being well under X3 and wellover y xs, generally about lpks, and the minimum electrical dimension being about half the maximum dimension. A reamgular wave guide of such cross sectional dimensicns will effectively propagate only thelowest mode'of atra'nsverse electric wave. Other cross sectional shapes in which the maximum electrical dimension in one direction is substantially'greater than the maximum electrical dimension in a direction at right angles thereto, both dimensions being of the order of is, may be suitable. A circular or square cross section, unless a radial septum is provided, is apt to give such difiiculties as to be unsuitable because the substantially equal dimensions in the two directions at right angles to each other tend to permit propagation with equal ease of modes polarized in either of these directions. Since the most usual cross section is a rectangular one with dimensions as indicated above, and since the invention has been principally developed in connection with a guide of such a cross section, the description which follows will be based exclusively on a guide of that cross section.

In a guide of such cross section the line of zero transverse current flow referred to is the longitudinal centre line of each broad face. The slots are, accordingly, cut in a broad face of the guide at angles to its longitudinal centre line, and the centres of alternate slots along the length of the guide are displaced on opposite sides of this line. The means associated with the slots for introducing susceptance may, if the required susceptance is positive, as is preferred, consist of probes of appropriate length extending from the inner surface of the opposite broad face opposite the centres of the slots. If the required susceptance is negative, the means in question may consist of fiat irises, i. e. thin metal sheets, extending an appropriate distance from the inner surfaces of the narrow faces of the guide.

The invention will be more fully described by reference to the attached drawings, which illustrate a suitable embodiment of it, and in which Figure l is a perspective view of a section of a slotted wave guide constructed according to the invention,

Figure 2 is a top plan view of portions of the upper broad face of a guide of the kind illustrated in Figure 1,

Figure 3 is a longitudinal section on the line 3-3 of Figure 2, and

Figure 4 is a schematic illustration of a slotted wave guide with its input and termination.

In the guide illustrated in the drawings the longitudinal axis is shown at 20, the u per broad face at 2|, the lower broad face at 22, and the longitudinal centre line of the broad face 2| at 23. The direction of the electric vector is illustrated by the arrow in Figure l, the wave being a transverse electric wave. All the slots shown are of substantially greater length than width, and have longitudinal axes denoted by the numeral 24. Wherever the direction or inclination of a slot is referred to, the direction or inclination of this longitudinal axis is meant. In Fi ure 1 the near end of the section illustrated is assumed to be nearest to the input end of the guide, and in Figures 2 and 3, the ends towards which the arrows point are assumed to be nearest to the input end of the guide.

The guide section illustrated in Figure 1 has four

slots

25, 26. 21 and 28, the centres of which are displaced alternately on opposite sides from the centre line 23 of the face 2|. As illustrated, the centres 25a and 21a of slots 25 and 2! respectively are displaced from the centre line 23 to the left hand side, and the

centres

26a and 28a of

slots

26 and 28 are displaced from this line to the right hand side, the centres of the slots being at increasing distances x, :01, 9:2, and as, from this line. All the slots are inclined to the centre line 23, with the ends facing the input end of the guide, for example the end 26b, of the slot 26 closer to this line than the opposite ends, for

example the end 280 of theslot 26. The four slots illustrated are inclined at successively greater angles 1, 52 and m. The displacements and angles are considerably exaggerated in the drawings in order to illustrate the principle.

Various shapes of slots may be used, but the length of the slot must always be substantially greater than its narrowest width. The most usual shape is substantially rectangular with either straight or rounded ends.

In the array of the present invention, a slot of a length equal to half the wave length in free space of the microwave whose frequency corresponds to the mean frequency of the band for which the array is designed is in practice satis-' factory. The exact length for a purely reactive slot must be determined separately for every different wave length and guide, though it depends primarily on the .wave length and only secondarily on the guide dimensions. The determination may be efiected by cutting a slot much shorter than one-half the wave length in free space and at a substantial angle which has an appropriate relation to the displacement as discussed below, and then lengthening this slot by successive increments until standing wave measurements show that the slot fulfils the required condition.

In any given wave guide, the best length for a purely reactive slot will vary slightly depending on the position of the slot in the guide. In the case of the arrangement'of Figure 1 it increases as the distance of the centre of the slot from the longitudinal centre line increases. However, when the distance from the centre line is small, as it is for all slots in most arrays, the required difference in length is so small that it may be disregarded in practice, and all the slots may be made of the same length. In such a case they are all so nearly purely reactive as to be quite satisfactoryin practice. As an aid in estimating the proper slot length, it may be indicated that for a wave length in free space of 10.7 cm., and for a rectangular guide having a wall thickness of 2.03 mm., an internal width of 7.2 cms. and an internal height of 3.4 cm. the length of a rectangular purely reactive slot near the centre line of the broad face of the guide is 2 inches.

The displacement and angle of each slot must be such that the slot will radiate substantially no power when it is shunted by a susceptance of a sign appropriate to the direction of inclination of the slot and of a value equal to the value of the characteristic admittance of the guide, that is, when it is shunted by a reflecting plunger \g or /941, as the case may be, away from its centre. In the case of the'slots illustrated, the ends of which nearest the centre line 23 are those nearest to the guide input, the appropriate susceptance is negative so that the slots should be put out of action by a reflecting plunger Ag (equivalent in operation to xg) away from their centres; For all cases in which the angles and displacements are small, as is the case for all but possibly a few terminal slots in most arrays, the angle of a slot which is to be displacedby a given distance from the centre line should be the angle of a resonant series slot, according to our said prior application, cut in the broad face of the guide with its centre on the longitudinal centre line of that face and presenting a resistance of a value equal to the value of the conductance presented by a resonant shunt slot according to that application cut said prior application is one whose longitudinal axis has projections extending transversely to *the longitudinal current "in the guide and which is excited solely by those currents. The "loads presented by a number of such slots are added according to the laws of series combination. The-shunt slot according to the saidprior application one whose longitudinal axis has projections extending transversely to the transverse currents in the guide and i which is excited solely 'by those currents. The loads presented by a number of such slots are added according --to the laws of shunt combination. should be noted that all measurements of resistances, conductances, etc.,in this specification are expressed as fractions of the characteristic --resistance, conductance, etc., of the guide.

The centres of alternate slots along the length of the guide are displaced on opposite sides from the centre line 23 of the broad face :in order that adjacent slots should be coupled in reversed phase relation and thus radiate in phase if the slots are at half wave spacing, or nearly so if they are atoif half wavespacing.

-The slots may be arranged at any spacing "less than about am, which their geometry will allow. Generally it is preferable to arrange them at a "spacing of kg/2 at the centre-of the band of frequencies to which the array is to respond, fin order that the beam "should, at the median frequency, be normal to the axis of the array. The more the spacing departs from 1972 the more the angle of the beam to the axis of the array departs "from a right angle.

nssociatedwith cache-f the slots illustrated is a means for introducing :a susceptance of the same value as but of opposite sign -'to the :sus-

ceptance resulting from the slot when the guide beyond the :slot is terminated by a match, i. e.

the characteristic admittance. fIT-hese'means, in t-the case of the slots illustrated, are probes 3.5, 35, 81 and *38 associated withthe slots 12-15, 26, 2] and 28 respectively. The probes are mounted :in :the lower broad face 22 directly "underneath the centres of the :slots with which they are associated and may conveniently be short metal rods of about "":;in diameter, efor example of silver, inserted through .the'broad face "22. The

length of each probe is determined by 1the susceptance which :it must cancel, this susceptance for any t given slot being given ,for practical purposes by the formula where .u is the value of the conductance which would be-presentedfbyaslot ;of the same dimensions and :same displacement ;as the .given slot .butdying parallel to the centre line of :the broad "face of the guide. The appropriate :probeiength lfOl the susceptance required for any given guide mayeasilybe-found by a series of experimental measurements. 'The :susceptance introduced by -:a long probe is measured and the length of the probe then reduced by successive small increments, the :susceptance being measured each time so that a graph of *probe length "and susceptance for the given "guide is established.

'There need not be a probe for every slot in an array, but there should 'be one for every group of consecutive slots which radiates substantiallyionerfiith of the powensu plied rt group. Thus, with "the "usual cabling, i. :e. :distribution of the excitation along the array there might be no probe until the tenth slot :fromthe input end of the uide and this probe would be v of a length such as to introduce a :susoeptance substantially compensating the susceptance resulting from the first ten slots. .As the load end o! the array is approached, the probes must become more "frequent as the slots are radiating a larger fraction of the power supplied to them. The groups of slots thus become smaller until. near the load end, they consist of one slot. *In a fifty slot-array, the last five to ten slots would be likely each to require a probe.

When the inclination and displacement of :a slot is small, as is true of most of the slots of a practical array, it is not essential that the probe be directly underneath the centre of the slot, provided that it is in the same transverse plane of the guide as the slot centre. Accordingly, construction of an array in practice may be facilitated by placing all the probes, except from the slots near the terminal end which radiate a substantial fraction of .the power ssupplied to them, along the-centre line of the lower face of the guide.

If the slots are inclined in the opposite direction to thatshown, i. e. with their ends which are furthest from the input end of the :guide nearest to the longitudinal centre line :of the broad face, :then the sign of the susceptances resulting from them is reversed, and these susceptances can be cancelled, by means of irises projecting into theguide from the narrow faces in the same transverse plane of the zguideaas ia slot centre. There appears, however, to ibeno, advantage in such an arrangement, and that shown is accordingly preferred.

The power radiated by any given slotarranged according to the invention is determined for practical purposes "by the product 1211 d lated) where a. is the value of the conductance which would be presented by a slot of the same divcriensions and same displacement as the given slot but lying parallel to thecentre line of the broadliace of the guide.

Each slot-probe,combination results in :a phase retardation of :the wave in the guideby approxi- .uuitely ,an angle of which the ta nt is (where a has the same meaning as indicated above), ,For slots near the input end .of the uide ,a is so small that the phase retardation is unimportant, but near the load .end where each slot is relatively substantially displaced from the longitudinal centre line of the broad face, the :angle of phase retardation "becomes a! preciable. The spacing of the slots mustaccordsubstantially accurate for all slots which extract less than about one-fifth of the power suppliedto them. In most practical arrays this is true of all slots. For any array in which the last two or three slots may extract more than onefifth of the power supplied to them, the necessary calculations for these slots may be made with quite satisfactory practical accuracy by one or two experimental measurements and extrapolation of the curves given by the formulae through the points determined by these measurements.

As is shown schematically in Figure 4, any array of slots according to the invention may be terminated by a non-reflecting load, for example a conventional sand and graphite load. A long array of about fifty slots or-more, in which the last slot radiates about one third or more of the power supplied to it, can, at'some slight sacrifice of band width, be very conveniently terminated by a reflecting plunger arranged 4 kg beyond the centre of the last slot. Another alternative for a long array in which the power passing the last slot is a very small frac tion, say 1% of the input power, may be simply to allow the power passing the last slot to be radiated into space from the end of the guide.

Radiation from a slot is polarized with the electric vector at any point tangent to the surface of a cylinder coaxial with the slot (i. e.

whose axis coincides with the longitudinal axis elements or more, the radiation from the array is polarized mainly with the electric vector perpendicular to the axis of the array.

. -What we claim as our invention is:

1. A directive antenna for microwaves, comprising a wave guide formed with an array of slots substantially purely reactive for a microwave of a given wave length and of substantially greater length than width, the centres of said slots being spaced along the length of the guide at intervals of less than substantially seven-eighths of said wave length in free space, said slots being inclined with respect to a line at which the transverse current flow along the guide wall is zero in the absence of electrical discontinuities when the guide is fed with a microwave, and the centres of alternate slots along the length of the guide beingclisplaced on opposite sides from said line, the angle of inclination of each slot and the distance of displacement of the centre of each slot being such that the slot will radiate substantially no power when it is shunted by a susceptance of a sign appropriate to the direction of inclination of the slot and of a value equal to the value of the characteristic admittance of the guide, and means in the guide associated with atleast every group of consecutive slots which radiates substantially one-fifth of the total power supplied to such group for introducing a susceptance of substantially the same value as but of opposite sign to the susceptance resulting from such group when the guide beyond such group is terminated by the characteristic admittance of the guide.

2. A directive antenna according to claim 1,

' in which the centres of the slots are spaced along the guide at intervals of half the wave length in the guide of the median wave length of the band of frequencies to which the antenna is designed to respond.

3. A directive antenna according to claim 1,

in which the array of slots is terminated by a non-reflecting load in the wave guide.

4. A directive antenna according to claim 1, in which the wave guide is formed with an. array of at least about fifty slots inclined in a direction such that their ends nearest the end from which the guide is adapted to be fed with a microwave are nearest to the line of zero transverse current flow, and in which the last slot of the array radiates not less than about one-third of the power supplied to it, comprising a reflecting plunger arranged in the guide at a distance from the centre of said last slot equal to fiveeighths of the wave length in the guide of the wave for which the slots are substantially purely reactive.

5. A directive antenna for microwaves comprising a wave guide of substantially rectangular cross section with two opposite faces broader than the other two faces, said wave guide having formed in a broad face thereof an array of slots substantially purely reactive for a microwave of a given wave length and of substantially greater length than width, the centres of said slots being spaced along the length of the guide at intervals of less than substantially seveneighths of said wave length in free space, said slots being inclined with respect to the longitudinal centre line of said broad face, and the centres of alternate slots along the length of the guide being displaced on opposite sides from said line, the angle of inclination of each slot and the distance of displacement of the centre of each slot being such that the slot will radiate substantially no power when it is shunted by a susceptance of a sign appropriate to the direction of inclination of the slot and of a value equal to the value of the characteristic admittance of the guide, and means in the guide associated with at least every group of consecutive slots which radiates substantially one-fifth of the total-power supplied to such group for introducing a susceptance of substantially the same value as but of opposite sign to the susceptance resulting from such group when the guide beyond such group is terminated by the characteristic admittance of the guide.

6. A directive antenna according to claim 5, in which the centres of the slots are spaced along the guide at intervals of half the wave length in the guide. of the median wave length of the band of frequencies to which the antenna is designed to respond. 7

'7. A directive antenna according to claim 5, in which the array-of slots is terminated by a nonreflecting load in the wave guide.

8. A directive antenna according to claim 5, in which the slots are inclined in a direction such that their ends nearest the end from which the guide is adapted to be fed with a microwave are nearest to the longitudinal centre line of the broad face of the guide, and in which the means for introducing susceptances consist of probes mounted in the opposite broad face of the guide to that in which the slots are formed, each of said probes being arranged in the same transverse plane of the guide as the centre of a slot.

9. A directive antenna according to claim 8, in which the wave guide is formed with an array of at least about fifty slots and the last slot of the array radiates not less than about onethird of the power supplied to it, comprising a reflecting plunger arranged in the guide at a distance from the centre of said last slot equal to five-eighths of the wave length in the guide of the Wave for which the slots are substantially purely reactive.

10. A directive antenna for microwaves, comprising a wave guide formed with an array of slots substantially purely reactive for a microwave of a given Wave length and of substantially greater length than width, said slots being spaced along the length of the guide at intervals of less than substantially seven-eighths of said wave length in free space and inclined with respect to a line at which the transverse current flow along the guide wall is zero in the absence of electrical discontinuities when the guide is fed with a microwave, and the centres of alternate slots along the length of the guide being displaced on opposite sides from said line, the angles of inclination of successive slots and the distances of displacement of the centres of successive slots along said wave guide being of progressively increasing values in the direction of travel of waves in said guide, and means in the guide associated with at least every group of consecutive slots which radiates substantially one-fifth of the total power supplied to such group for introducing a susceptance of substantially the same value as, but of opposite sign to, the susceptance resulting from such group when the guide beyond such group is terminated by the characteristic admittance of the guide.

11. An antenna for microwaves comprising a wave guide of rectangular section having a plurality of slots of greater length than width formed in one broad face thereof, the centres of said slots being spaced at regular intervals along the length of said guide, alternate slots being oppositely inclined at acute angles to the center line of said broad face, and having their centers displaced on opposite sides of said center line, the angles of inclination of successive slots and the distances of displacement of the centres of successive slots along said wave guide being of progressively increasing values in the direction of travel of waves in said guide.

12. A directive antenna according to claim 1. in which the angles of inclination of successive slots and the distances of displacement of the centres of successive slots along the wave guide are of progressively increasing values in the direction of travel of waves in said guide.

13. A directive antenna for microwaves, comprising a wave guide formed with an array of slots substantially .purely reactive for a micro- Wave of a given wave length and of substantially greater length than with, the centres of said slots being spaced along the length of the guide at intervals of less than substantially seven-eighths of said wave length in free space, and said slots being inclined with respect to a line at which the transverse current fiow along the guide wall is zero in the absence of electrical discontinuities when the guide is fed with a microwave, and the centres of alternate slots along the length of the guide being displaced on opposite sides from said line, the angle to said line of zero transverse current fiow of substantially every slot of which the centre is displaced by a given distance from said line being substantially the same as the angle to said line of a resonant series slot with its centre on said line and presenting a resistance of a value equal to the value of the conductance presented by a resonant shunt slot parallel to said line and displaced therefrom by said given distance, and means in the guide associated with at least every group of consecutive slots which radiates substantially one-fifth of the total power supplied to such group for introducing a susceptance of substantially the same value as but of opposite sign to the susceptance resulting from such group when the guide beyond such group is terminated by the characteristic admittance of the guide.

ERNEST W. GUPTILL. WILLIAM HERIO'I WATSON.

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

UNITED STATES PATENTS Number Name Date 2,405,242 Southworth Aug. 6, 1946 2,407,068 Fiske et a1. Sept. 3, 1946 2,408,435 Mason Oct. 1, 1946 2,414,266 Lindenblad Jan. 14, 1947 2,433,368 Johnson et a1. Dec. 30, 1947 2,477,510 Chu July 26, 1949 2,479,209 Chu Aug. 16, 1949 OTHER REFERENCES Journal of the Institution of Electrical Engineers, vol. 93. part III A, No. 4, March-May, 1946, pages 747 to 777.