US2893003A

US2893003A - Antenna feed

Info

Publication number: US2893003A
Application number: US668273A
Authority: US
Inventors: James S Arnold; Ralph W Dressel; Herbert W Haas
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-06-26
Filing date: 1957-06-26
Publication date: 1959-06-30
Anticipated expiration: 1976-06-30

Description

' i 1111/1/11]: ELECTRIC FIELD-E PLANE J June 30, 1959 J. s. ARNOLD ET AL 2,893,003

ANTENNA FEED Filed June 26, 1957 -11 Sheets-Sheet 1 I 1 J F/GZ a III/III!IIIIIIIIIIIIIIIIII/IIIIIIII IIIIIIIIIIIIIIIIIIIIIIIII,

IIIIIlIIIIII/I II III,

FIG. I. 33

JAMES S. ARNOLD RALPH n. DRESSEL HERBERT n. HAAS INVENTORS ATTORNEYS June 30, 1959 J. 5. ARNOLD ET AL ANTENNA FEED 11 Sheets-Sheet 2 Filed June 26, 1957 FIG. 4.

E PLAN 5 PLANIE 60 8Q ANGLE FROM AXIS-DEGREES JAMES $.AR/VOLD ANGLE FROM AXIS- DEGREES RALPH W DRESSEL FIG. 5. m H445.

HERBERT INVENTORS BY 05W June 30, 1959 J. 5. ARNOLD ET AL 2,

ANTENNAFEED Filed June 26, 1957 11 Sheets-Sheet a POLARIZATION JAMES S. ARNO RALPH W DRE'S L HERBERTW HAAS' INVENTORS BY @i;

ATTORNEYS 111M 1959 J. s. ARNOLD ET AL 2,893,003

ANTENNA FEED ll Sheets-Sheet 4 Filed June 26, 1957 'E PLANE ----H PLANE DEGREES 5 m mJUEOUO IFGZUEPm ANGLE FROM AXIS wfim wmmm MW RR AD $.5 MM %w ATTORNEYS J. S. ARNOLD ET AL June 30, 1959 ANTENNA FEED 11 Sheets-Shet 5 Filed June 26, 1957 FIGS/3 5 m m m ANGLE FROM AXIS- DEGREFS mJmmawo zhwzmmhm ANGLE FROM AXIS-DEGREES L m wx OS W NM m mam SWW. wm Mam MRH IIIIIIl'IIIA IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII ,,,,,,,,,,,,,,,,,,,,,,,,.,,,,.W.

BY ,glflflan FIG. 12.

RELATIYE FIELD STRENGTH DECI BELS RELATIVE FIELD STRENGTH DECIBELS June 30, 1959 Filed June 26, 1957 ANTEINN FEED ll Sheets-Sheet 6 W Y \Y\ V /'\\V 20 4o Vo 80 I00 I20 I40 I60 ueo ANGLE FROM AXIS-DEGREE 0 .ZA & Lil.

v q .BA f L H PLANE \0 IX //%d/ b J f R 2o 40 so so I00 I I I I ANGLE FROM AXIS-DEGREES INVENTORS FIG. /6. JAMES s. ARNOLD RALPH W DRESSEL HER EH7 9!. H445 M ATTORNEYS June 30, 1959 J. s. ARNOLD ET AL ANTENNA FEED ll Sheets-Sheet '7 Filed June 26, 1957 E PLANE 4o so ANGLE FROM AXIS DEGREES m m m omo-x 5zmEw 3!. mwi um H PLANE mimawai gzmmhw 3!. was]? INVENTORS' ANGLE FROM AXIS DEGREES L m. DQEJA I- A O OfiH W A R w T Hm M BW W E mm w F June 30, 1959 J. 5. ARNOLD ET AL 2,

ANTENNA FEED Filed June 26, 1957 ll Sheets-Sheet 8 III III!

Ill

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' s F/ .2 EQUIPHIASE CONTOURS JAMES a g v olz RALPH W. DRESSEL H RBERTW. HAAS BY 00% RELATIVE FIELD STRENGTH DECIBELS June 30, 959 J. s. ARNOLD'ET AL 2,393,003

I ANTENNA FEED I Filed June 26, v 1957 -l1 Sheets-Sheet 9 FIG-22 E PLANE RELATIVE FIELD STRENGTH DEGIBELS N 60 80 I I ANGLE FROM AXIS-DEGREES 6O '80 I00 I20 I40 I I ANGLE FROM AXIS DEGREES FIG. 23.

JAMES 5. ARNOLD RALPH M. DRESSEL HERBERT I4. HAAS INVENTORS ATTORNEYS RELATIVE FIELD STRENGTH-DEOIBELS June 30, 1959 I J. 5. ARNOLD ET AL 2,893,003

' ANTENNA FEED Filed Jun 26, 1957 I 11 Sheets-Sheet 10 FIG. 25.

E PLANE 20 40 V 60 80 I00 I20 I40 I60 I80 ANGLE FROM AXIS DEGREES H PLA E RELATIVE FIELD STRENGTH-DECIBELS O 20 4O 6O 80 I00 I20 I40 I I ANGLE FROM AXIS-DEGREES FIG. 26. 4a 43 JAMES S. ARNOLD 45- 4/ RALPH W. DRESSEL HERBERT n. HAAS K INVENTORS ATTORNEYS June 30, 1959 Filed June 26, 1957 RELATIYE FIELD STRENGTH 'DECIBELS u I I "-IO l l I RELATIVE FIELD STRENGTH DEGIBELS J. s. ARNOLD ET AL 2,893,003

ANTENNAFEEID ll Sheets-Sheet 11 -a'o 2 o -16 b +l o +2'o 1S8 ANGLE FROM AXIS- DEGREES E PLANE, EXTREME RIGHT I ------EPLANE, EXTREME LEFT PLANE MES. 5, ARNOLD RALPH u. DRESSEL HERBERT m H443 INVENTORS ANGLE FROM AXIS-DEGREES ATTORNEYS .eter extending along the axis of the feed.

front in the aperture.

United states Patent ANTENNA FEED James S. Arnold, Stanford, Calif., and Ralph W. Dressel,

Las Cruces, and Herbert W. Haas, State College,

N. Mex., assignors to the United States of America as represented by the Secretary of the Navy Application June 26, 1957, Serial No. 668,273 Claims. (Cl. 343-781) The present invention relates to microwave'antenna feeds. More particularly, it relates to an antenna feed of "a type particularly suited for use with parabolic dish reflectors.

a A common form of antenna used at centimeter wavelengths consists of a parabolic reflector illuminated by a radio frequency power source located at thereflector focus. The power source is frequently referred to as the'antenna feed. The basic function of the feed is to extract power from a radio frequency transmission line and direct it toward the reflector as an electromagnetic field. The properties of the secondary radiation field generated by the subsequent reflection of energy from the paraboloid are directly determined by the space distribution, the polarization, and the phase pattern of the primary radiation from the feed. Close control over the primary field is therefore'necessary in order to produce a secondary field having predetermined characteristics.

The use of a parabolic reflector as part of an antenna system places very specific requirements upon the space configuration of the primary field established by the feed.

It is frequently desired that. the distant field be plane polarized. The radiation from the feed, in that case, must be so polarized that after reflection from the paraboloid theresulting electric field vectors are all parallel. It may be demonstrated that the required primary field be of such form that lines describing the direction of the electric vector form circles on the surface of a sphere whose center is located at the feed head. These circles ..are defined by the intersection of the sphere with a set of vertical planes passing through the end of the diam- It is further known that the equiphase surfaces in the primary field must be spherical.

In principle, it is possible to design a reflector to match any arbitrary phase surface supplied by a feed so that the phase surface will be transformed into a plane phase However, the difliculties involved in obtaining phase measurements and the accuracy with which the phase must be known before an appropriate reflector contour can be calculated render the design .diflicult. It is more practical therefore to construct the feed to supply the required spherical phase front for control of a three dimensional field, proper terminations could not always be obtained.

It is desirable that the feed have no preferential axis of polarization. The feed may then be rotated on its axes without modifying the wave form or the polarization of the microwave power passing through it. Such feeds are termed rotationally symmetrical feeds.

Among the known rotationally symmetrical feeds is the 2,893,003 Patented June 1959 ICC direct radiating type comprising, for example, a horn placed in front of the reflector. A horn feed is highly effective, but the waveguide carrying power to the feed, together with supporting structure, must pass in front of the aperture of the reflector there creating interference inbthe secondary field and scattering energy into the side The rear feed is another of theknown symmetrical types. The rear feed comprises a waveguide terminated by a reflecting disk which serves to direct the radiation back along the outside of the waveguide. A rear feed may be inserted through the center of the reflector leaving all of the support and the mechanism for rotation behind the reflecting surface. As a' result, interference with the secondary field in the reflector aperture is minimized.

Cutler earlier constructed a geometrically symmetrical rear feed comprising a short section of circular waveguide terminated by a flat disk of metal. The dimensions of the disk and the spacing from the end of the waveguide were experimentally chosen to give an optimum .power transfer from the waveguide. However, asthe dimensions of the disk were'of the order of a wavelength, the disk became strongly excited by the radiation issuing "from the end of the waveguide. It is therefore-more'approprr ate to describe the disk as a secondary radiator rather than as a simple reflector.

One consequence of the excitation of Cutlers disk is that the equiphase surfaces of the reflected radiation are toroidal rather than spherical as required in an, ideal point source. The Cutler rear feed has therefore been occasionally referred to as a ring focus feed. ,Rotationally symmetrical radiation patterns are not obtained with a ring focus feed, nor is the secondary-fieldplari'e polarized. a i

Accordingly, it is an object of'the present invention to .provide in a microwave antenna a rotationally symmetrical rear feed for exciting a parabolic reflector in such a manner that the distant field will possess the characteris- Another object of the present invention is'to provide a rotationally symmetrical rear feed for a parabolic'reflector in which back radiation from the feed isminimized.

A further object of the present invention is to provide a rear feed exhibiting the properties of a resonant cavity and thereby affording greater control over the resulting pattern of radiation.

An additional object of the present invention -is to provide an antenna feed providing modulation of the secondary field by rotation of the antenna feed.

Still another object of the present invention is to provide an antenna feed Well balanced mechanically to permit high speed rotation of the feed about its longitudina axis for modulating the secondary field.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. l is an axial section of one embodiment of the feed of the present invention illustrating the parameters which may be varied to control the primary radiation pattern;

Fig. 2 schematically represents the feed of Fig. l and illustrates the spatial electric field for a fourth ordercurrent distribution;

Fig. 3 is a perspective view of the feed of Fig. 1 illustrating the charge and current distribution on the outer surface of the feed for a fourth order charge distribution;

Fig. 4 is a chart which illustrates radiation patterns ob- 3 tained with the feed of Fig. 1 operating in the TE mode for various values of cap diameter y;

Fig. 5 is a chart illustrating radiation patterns ob tained withtheieed of Fig. 1 operating in the TE mode for various values of cap side length x;

'Figs. .6 and 7 are schematics which illustrate the electric polarization of the radiation from the feed of Fig. 1 operated in the TE mode with fourth order charge distribution;

Fig. 8 illustrates the spatial electric field of the feed ofFig. l operated in the TM mode with sixth order charge distribution;

Fig. 9 is a chart which illustrates the radiation pattern of the feed shown in Fig. 8;

Figs. 10 'and 11 illustrate the polarization of the electric field of the feed of Fig. 8;

- Fig.-" 1-2 is an axial section of a second embodiment of th'e'present invention;

Figs. 13 and 14 are charts illustrating the E andH plane radiation patterns, respectively, of the feed of Fig. 12 forvarious values of cap side length x;

f-Figs. 1-5 and 16 are charts showing the E and H plane radiation patterns, respectively, of the feed of Fig. 12 for'var'ious insertion lengths Z with a cap side length equal to one wavelength;

Figs. 17 and 18 are pattern charts similar to those of Figs. and 16 except that the side length is equal to "056 wavelength;

Figs. 19 and 20 are schematics which illustrate equiphase contours in the E and H planes, respectively, from a feedsinjilar to that of Fig. 12 and having a side length 'of ,016 wavelength;

' Fig. 21 is an axial section of a modification of the 'feedof Fig. 12;

'Figs. 22 and 23 are E and H plane radiation pattern charts, respectively, of the feed of Fig. 21 for various values of insertion length Z;

Fig. 24 is an axial section of a further modification of the feed illustrated in Fig. 21;

" Figs. 25 and 26 are radiation pattern charts in the E andHplanes, respectively, of the feed of Fig. 24 for various values of spacer diameter k;

Fig. 27 is an axial section of a feed similar to that of Fig. 24, illustrating details of its construction;

Fig. 28 is a distant field radiation pattern obtained with .-face is.referred to as the secondary pattern.

Cap 33 is ofcylindrical shape and is formed ofan end disk 34, having a diameter of critical dimension y, secured. to a circular side wall 35 having a critical length x. Cap 33 is supported coaxially with waveguide 32 by adielectric rod36 inserted therein and secured to the .ce'nt'en'ofdisk34 by any suitable means. Rod 36 also enables thediameter of waveguide 32 to be reduced withldut' encountering cutoff. The cap dimensions x and y are selected so as to provide a resonant cavity terminationof waveguide 32. The depth Z of insertion of waveguide 32 into cap 33 is critical and afiords one means of ,controlling the primary radiation pattern, as

described more sp ecific'ally hereinafter with reference to "another embodiment of the invention.

For matching purposes, however, it is desirable that the depthZ be "'equal to a quarter-wavelength.

Because of the large radiating aperture 37 and because of the small size of the cap 33, little energy is actually stored within the resonant cavity of the feed. Nevertheless, there is a resonant mode or combination of modes that serve to transform the TE mode of the circular waveguide into a wave progressing back toward the reflector along the outside of the Waveguide. The resonant modes within the cavity are controlled by choice of the variables x, y, and z to achieve the desired radiation patterns.

The electromagnetic field surrounding thefeed of Fig. l is a function of the resonantmode within cap 33 as well as the surface currents on the outside of the cap and the exterior of waveguide 32. Fora rotationally symmetrical cap, two different modes can be excited by the principal or TE mode in the waveguide 32. The existence of one or the other mode depends upon the diameter y of cap 33.

Fig. '3 illustrates the field distribution within the cap 33. It will be seenthat the field distribution is quite complex andrepresents a transition from.the TE circular waveguide mode of the TE coaxial mode by means of the resonant cavity within cap 33. The electric field distribution is illustrated at only one instant of time. As time advances the waves will also advance through space in such a manner that the envelope tends toward a sphere. However, close to the 'feed cap, it is apparent that the envelope must depart significantly from a spherical shape.

From Fig. 2 it is apparent that the surface currents circulating over the outside of the cap play an important part in determining the space distribution of the field. Standing waves of current and charge are excited by the radiation passing over the edge of the cap and are an integral part of the total radiating system. Fig. 3 further illustrates the charge and current distribution on the surface of cap 33. Since the cap is symmetrically excited, all of the charge antinodes occur in the E plane.

Depending upon'the dimensions of the cap, the current and charge distribution over its surface may vary. The distributions can be classified according to the number of charge antinodes appearing on the outer surface of the cap. The minimum-number of charge antinodes that may occur is two since the dipole is the lowest order radiator. Only the even orders of antinodes may appear as odd orders are excluded by the form of cap excitation. By varying the diameter y and side length x of the cap, one order oranother of charge distribution may be enhanced. If the dimensionsof the cap are intermediate between those characteristic to two adjacent charge distribution orders, a mixture of orders occurs and the resultant radiation field isa superpositionof the fields associated with each order.

Fig. 4 illustratesthe primary, field distribution for various cap diameters 'y with the length x of side 35 held constant. The dimensional units are the wavelength of field distribution of Fig. 2. As the diameter y of cap 33 is decreased, the radiation from disk 34 becomes stronger indicating an approach to the characteristic dimensions for the fourth order current'distribution. The resulting influence upon the'jprirriary' lobe in the E plane is marked but very little change occursin the primary lobe for the H plane. i

Fig. 5 illustrates, the, radiationpatterns resulting from varying the length x'of'side 35with the diameter y of disk 35 constant at 1.2 wavelengths. The primary lobes in both the Band H planes remainrelatively unaffected. However,the radiation from theback of the feed changes from a lobe. to a null at the 180 position. The range of variation of the sideQIength x lies intermediate'io the characteristic dimensions'jfor .currenfdist'ributions 05 orders 2 and -4. Sincefthe relative strengthof the normal modes is determined byx, at a particular value of x, i.e.

35am contributions from eachof themodes are at 1801 relative phase and are equal, resulting in complete cancellation. v I Figs. 6 and- 7 illustrate the space distribution of electric-polarization for the feed of Fig. 1 operated in the TE mode and having a fourth order surface current distribution. It will be seen that the'feed of Fig. 1 does not produce ideal polarization for all parabolic reflectors. However, the polarization is satisfactory for reflectors of limited aperture.

The radiation patterns of Figs. 4 and 5 and the polarization diagrams of Figs.r6 and 7 concern operation of the feed in the TE mode. The cut-off, wavelength for the TE mode is expressed by the approximate formula E2(b --a), TM mode For. agiven operating frequency, the difference between the diam eter'of waveguide 32 and the diameter of cap 33 must be atleast one-half a wavelengthto support the TM 'mode. l

vA complete change in the structure of the radiation field aboutthe feed takes place when the TM mode is dominant within the resonant cap 33. Fig. 8 illustrates the electric field distribution in the E plane. Fig. 8 refers to an instant of time when the electric field is a maximum within the cap cavity. .The field structure is modifiedbothby the change in mode inside cap 33 and by the change in the current distribution over the exterior. The dimensions are sufliciently large to support a sixthorder current distribution causing six nulls in the radiation from disk 34 and two nulls in the radiation from aperture 37.

.Fig. 9 illustrates the radiation pattern obtained by operating in the TM mode- The radiation from disk 34 is nearly as strong as thatfrom aperture 37. Figs. 1 0 and 11 illustrate the polarization of radiation from the feed of Fig. 1 operated in the TM mode. The angle subtended at the feed by the reflector must be less than 30 to provide plane polarization of the distant field.

- Neither the pure TE nor TM modes provide symmetry in the E and H plane radiation patterns. For the TE mode, the E plane pattern is broader than the H plane. It is necessary that the primary radiation intensity be circularly symmetrical to obtain symmetry in the distant field. Symmetry may be provided by appropriately mixing the radiation fields associated with the two charaeteristic modes in the cap. I nFig. 12, a folded coneresonant cap feed is illustrated'which permits adjustment of the relative intensity of the TE and TM modes therein. The resonant cap.4-1 comprises a-conical end plate 42 secured at its apexto a dielectric rod 36 inserted into the circular waveguide 32 which conveys power for radiation from the transmitter (not shown). End plate 42 is arranged coaxially with waveguide 32. A sidewall 43 extending rearwardly toward the reflector (not shown) and having a length x is joined perpendicularlyto the periphery of endplate 42-thus assuming the shape of a truncated cone. The side length of conical end plate 42 is somewhat greaterthan one half wavelength (from 0.6)\ to 0.7)) and-thefincluded angle at the vertex thereof is substantially 160. The length x of side 43 is variable, as is the 'de'pthZ of the insertion of waveguide 32 into theresonant cap 41 to control the radiation pattern of the feed. A dotted line 44 marks the distance of one half wavelength from the apex of end plate 42.- That portion of 6 TM mode. 'That portion of the feed to the leftline- 44 is sufliciently large to support both modes. Thus, the distance z of insertion of waveguide 32 into cap 41 controls the attenuation of the TM mode and hence the relative mixture of the two characteristic modes.

In order that the TM mode be cutoff, the length x of side 43 must be a substantial part of a wavelength. As the length of side 43 is made less than a wavelength, cut-. off dimensions calculated on the basis of infinitely longer waveguides no longer apply. Instead, the T M mode is gradually released in spite of the fact that the dimensions of end plate 42 appear too small to support it. Figs. 13 and 14 illustrate radiation pattern obtained with an insertion Z of 0.2), a value ordinarily sulficient to guarantee cutoff for the TM mode. With a constant cap diameter of 0.68)., a variation in side length x from 1.0) to 0.3). shows the release of the TM mode. For a side length of x=0.6)t reasonably good symmetry between the E and H plane patterns are obtained through the angular interval of 0 to 50.

Figs. 15 and 16 illustrate radiation patterns obtained by varying insertion Z of waveguide 32 into cap 41. The side length x is equal to one wavelength. Small values of z yield a radiation pattern typical of the TE mode, since the dimensions are beyond cutoff for the TM mode. However, as z is increased, the E plane pattern grows progressively narrower and as z passes the critical value of 0.5) there is a rapid change in pattern structure. For all values of z greater than 0.67\, the ratiation pattern is typical of' the TM mode. A progressive change with increasing z occurs in the H plane, but the patterns fold over themselves so that the total variation is small. By interpolation between the curves of Figs. 15 and 16, a setting of z=0.42)\ is obtained as the optimum for circular symmetry.

Figs. 17 and 18 illustrate the radiation patternsobtained with a feed having a side length x of only a fraction of a wavelength. Increasing the value of 2 no longer narrows the primary pattern in the E plane as in Fig. 15, since the TM mode is present for all values of z. Instead, both the E plane and H plane primary patterns grow in angular width with increasing z.

Figs. 19 and 20 illustrate the equiphase contours of the radiation in the E and H planes respectively from the feed of Fig. 12 having a side length x of 0.6)., a cone side length of 0.68). for plate 42, and a waveguide insertion Z of 0.2). The white curves mark the loci of constant phase. The curvature of the equiphase contours in the E and H planes is not identical, and therefore the ideal reflector is not a paraboloid of revolution. However, if the reflector possesses a reasonably long focal length (about 10). or greater) satisfactory operation will be obtained. If the focal length is short, say of the order of 6a or less, a paraboloid of revolution produces serious phase errors in the resultant aperture field.

Fig. 21 illustrates a feed similar to the feed of Fig. 12 except that the resonant cavity within' the cap 41 is filled with a low loss dielectric material 45. The dielectrio material 45 supports the metallic cap 41 and accurately maintains its position with respect to the circular waveguide 32. The wavelength of the radiation passing through the dielectric medium is reduced according to the index of refraction of the dielectric. As a result, the cutoff of the TM mode is altered and the effective dimensions of cap 41 are increased. As the radius of the waveguide is fixed, it is inappropriate merely to alter dimensions proportionately with the index of refraction to preserve the desired balance between the TM and TE modes. It is necessary to reduce the cone side length of plate 42 in accordance with thenew cutofi dimensions specified in the following formula:

TM mode- E feed to the right ofline 44 is beyond cutofi for the 7 where a and b are the outside radius of the waveguide 7 and the inside radius of the cap, respectively; is the velocity of lightin vfree space, v is the frequency of the radiation ands is the dielectric constant of the fillin-g material.

Figs. 22 and 23 illustrate radiation patterns obtained with the feed of Fig. 21 for various values of waveguide insertion Z. Due to the reduction in cap diameter and to the refraction of the radiation at the dielectric-air interface, the radiation patterns are broadened as compared with those of Figs. 17 and 18. The E and H plane patterns demonstrate that reasonable symmetry will be obtained for small values of z. Further improvement in circular symmetry can be achieved by shaping the dielectric in the aperture of the feed.

The primary radiation patterns for the feeds of Figs. 1, 12, and 21 show the presenceof undesired radiation from the sides and back of the cap. Radiation from the back of the feed must be suppressed before low side lobe levels can be realized in the secondary pattern. In Fig. 24, double cap feed is illustrated which not only reduces the radiation from the back of the feed, but also improves the circular symmetry of the primary radiation pattern.

The double cap feed comprises a feed of the type of Fig. 21 combined with an outer cap 46 similar to cap 41 but spaced apart therefrom by a conductive spacer 47 having a diameter 'k. The sides 48 of cap 46 preferably extend beyond the sides of inner cap 41. A resonant cavity 49 is thus formed in the space separating caps 41 and 46, the length of which is adjustable by varying the diameter k of spacer 47. If desired, the cavity 49 may be'filled with solid dielectric material to further alter the characteristics of the radiation pattern.

Figs. 25 and 26 demonstrate the effect of varying the length of cavity 49 by means of increasing the diameter k of spacer 47. The initial value of spacer diameter k is 0.2)., where A is the free space Wavelength of the radiation.

As k is increased, the field at the edge of cap 46 progressively changes fromelectric to magnetic and back again to electric according to whether the number of quarter wavelengths included within the cavity 49 is odd or even. For values of k in the vicinity of 0.20). the length of cavity 49 is equal to approximately one mode wavelength and the field at the edge is purely magnetic. The primary pattern is nearly the same as that for a single cap feed except that the symmetry between the E and H planes is more pronounced and the radiation from the back of the outer cap is less. The interaction of the electric field of cavity 49 with the field from the aperture of cap 41 is apparent in the E plane patterns for larger values of k.. As the value of k passes through the critical resonance dimensions, reinforcement rapidly changes to partial cancellation. The deeper the cavity the more pronounced is the resonance.

Fig. 27 illustrates a modification of the feed of Fig. 24 in which the resonant cavity 49 is filled with solid dielectric material. Inner cap 41 and outer cap 46 are secured to dielectric rod 36 by a screw 51, the spacer 47 being constituted by a boss turned on the outer surface of cap 41. The dielectric material 45 filling the cavity of inner cap 41 is held in place by a pin 52 of similar dielectric material passed transversely through rod 36. Lower edge 53 of the dielectric filling of cavity 49 is tapered approximately at 20 to the vertical to intersect the lower lip of side 43 at an angle of approximately The lower lip 54 of the dielectric filling of cavity 49 intersects edge 53 at an angle of approximately 60 to the vertical.

' Polystyrene may suitably be used as the dielectric filling of cavity 49 while Teflon may suitably be used to fill the inner cavity 45.

Fig. 29 illustrates the secondary pattern of the feed of Fig. 27 combined with a reflector inches in diameter.-.and havinga focal-length of 8 inches. Circular symmetry of the pattern of Fig. 29 is demonstrated by the fact that every cross section plane through the beam is similar within 1 /2 db. In addition, the radiation within the main lobe is very nearly plane polarized. Measurements of the cross components of polarization indicate that these are less than --30 db with respect to the peak of the main lobe.

-While the double cap feed of Figs. 24 and 27 provides excellent circular symmetry, it is also possible by means of the feed to create an asymmetrical field pattern. As previously demonstrated, spacer 47 exerts a strong influence on the distribution of primary radiation from the feed. If spacer 47 is placed eccentrically between

caps

41 and 46 as asymmetrical primary pattern will result. For a given eccentricity, the greatest asymmetry will, of course, be obtained when the mean radius of spacer 47 corresponds to the resonance dimensions of cavity 49. A spacer having a radius of 0.31% placed ofi center by 0.02 yields the maximum asymmetry. Such a minor eccentricity does not add serious mechanical unbalance to the feed so that the feed may be rotated on axis at very high speed to produce a high frequency modulation of the radiation field.

As shown in Fig. 29, rotation of a feed containing an eccentrically placed spacer produces modulation solely in the "E plane, provided the feed is set at the reflector focus. The degree of modulation in the E plane is determined not only by the asymmetry of the primary pattern but also by the focal length and diameter of the reflector. Thus there are available ,a large number of parameters for adjusting both the field pattern and the modulation obtainable with a double cap feed having an eccentrically placed spacer.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. An antenna feed for controlling the emission of energy from the end of a waveguide, comprising a cap having a conical base and a side wall joined to said base, said side wall being in the form of a truncated cone, said base and side wall being joined at the smaller diameter of said side wall so as to enclose the apex of said conical base, and means for securing said cap with the apex of the base thereof spaced axially from the open end of the waveguide with said side wall extending along the waveguide so as to overlap the open end thereof. 2. An antenna feed as claimed in claim 1 with additionally, a second cap similar to said first mentioned cap but of larger dimensions than said first cap, and means securing said second cap to said first cap so as to surround said first cap.

3. An antenna feed as claimed in claim 2 wherein said last named means includes a conductive spacer joining said first and second caps at the apices of their respective conical bases.

4. An antenna feed for controlling the emission of energy from the end of a waveguide, comprising a cap including a conical base and a side wall joined to said base, said side wall being in the shape of a truncated cone and being joined to said base at the smaller periphery of said side wall so as to enclose the apex of said conical base, a solid dielectric material filling said cap, said cap being dimensioned so that the cavity enclosed by said cap is resonant at the frequency of the energy transmitted by the waveguide, and means supporting said cap over the open end of the waveguide so that said open end extends into the cavity of said cap with the apex of said base thereof spaced axially from said open end.

5. An antenna feed as claimed in claim 4 with additionally, a second cap similar to said first mentioned cap but of larger dimensions than said first mentioned cap and means supporting said second cap so as to contain said first mentioned cap, there being sufiicient spacing between the outer surface of said first mentioned cap and the inner surface of said second cap to provide a second cavity therebetween resonant at the frequency of the energy transmitted by the waveguide.

6. An antenna feed as claimed in claim 5 wherein said first mentioned cap and said second cap are aligned coaxially with the waveguide, and said means supporting said second cap includes a conductive spacer placed eccentrically to the axis of the waveguide.

7. An antenna feed as claimed in claim 6 with additionally, a solid dielectric material filling said second cavity between said first mentioned cap and said second cap.

8. An antenna feed for controlling the emission of energy from the end of a waveguide, comprising, a first cap having an end portion and a side wall, means securing said first cap spaced from the end of the waveguide and perpendicularly to the axis thereof with said side wall extending along said waveguide so as to overlap the end of said waveguide, a second cap similar to said first cap but of larger dimensions, and means for securing said second cap spaced from and surrounding said first cap.

9. An antenna feed for controlling the emission of energy from the end of a waveguide, comprising, a first cap having a conical base and a side wall, said wall having a length greater than one-half the wavelength in air of the energy transmitted by the waveguide and being joined to said base so as to enclose the apex thereof, means for supporting said first cap in spaced relationship to the open end of the waveguide and with said side wall extending along the waveguide so as to overlap the end of said waveguide, a second cap similar to said first cap but of larger dimensions, and means securing said second cap in enveloping relationship with said first cap.

10. An antenna feed as claimed in claim 9, wherein the side wall of said second cap is substantially equal in length to one wavelength in air of the energy transmitted by the waveguide and the spacing between the open end of the waveguide and the apex of the base of said first cap is between 0.4x and 0.5). of the wavelength in air of the energy transmitted by the waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 2,482,158 Cutler Sept. 20, 1949 2,566,900 McArthur Sept. 21, 1951 2,750,588 Hennessey June 12, 1956