US3763493A

US3763493A - Antenna device applicable for two different frequency bands

Info

Publication number: US3763493A
Application number: US00186911A
Authority: US
Inventors: S Shimada; M Koyama; H Kumazawa; M Karikomi
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 1970-10-17
Filing date: 1971-10-06
Publication date: 1973-10-02
Anticipated expiration: 1990-10-02
Also published as: FR2112310B1; FR2112310A1; DE2151488A1; GB1370669A; JPS5119742B1

Abstract

A branching filter using dielectric elements is disposed on the side of one of a pair of reflectors for reflecting emitted or incident waves, the branching filter being capable of reflecting one of the two frequency group components and of transmitting the other frequency group component to branch and converge the two components at different points.

Description

343-9 1 1 R AU 0 Umted States Patent 1 1 1 1 3,763,493 Shimada et al. Oct. 2, 1973 ANTENNA DEVICE APPLICABLE FOR TWO [56] References Cited DIFFERENT FREQUENCY BANDS UNITED STATES PATENTS 1 lnvemrs= Shim, Kosanei-shi, 3,281,350 l0/l966 Hannan 343 9119 T y Maslki y 3,394,378 7/l968 Williams et al...... Sayama-shi; Hiroyukl Kumazawa, 2,972,743 2/1961 Svensson et al. 343/909 Saitama-ken, Tokorozawa-shi; Mmhim xmkomi, suginambku, FOREIGN PATENTS 0R APPLICATIONS Tokyo, all of Japan 335,425 2/1959 Switzerland 343/837 562,602 9/1958 Canada 343/756 [73] Assigneez Nippon Telegraph and Telephone Pubhc Corponuon Tokyo Japan Primary Examiner-Eli Lieberman [22] Filed: Oct. 6, 1971 Attorney-Milton J. Wayne Appl. N0.; 186,911

Foreign Application Priority Data Oct. 17, 1970 Japan 45/91422 US. Cl 343/755, 343/781, 343/837, 343/911 Int. Cl. H0lq 19/14 Field of Search 343/840, 909, 755,

[57] ABSTRACT A branching filter using dielectric elements is disposed on the side of one of a pair of reflectors for reflecting emitted or incident waves, the branching filter being capable of reflecting one of the two frequency group components and of transmitting the other frequency group component to branch and converge the two components at different points.

7 Claims, 13 Drawing Figures Patented Oct. 2, 1973 3,763.493

4 Sheets-Sheet 1 PRIORART 2 PRIORART FIG. 5 (b) 4 Sheets-Sheet 2 FIG. 6

-- FREQUENCY (GHZ) Patented Oct. 2, 1973 O O O 4 Patented Oct. 2, 1973 4 Sheets-Sheet 3 FIG. 8

FIG. 9

fezo

Patented Oct. 2, 1973 3,763,493

4 Sheets-Sheet 4 FIG.||

ANTENNA DEVICE APPLICABLE FOR TWO DIFFERENT FREQUENCY BANDS BACKGROUND OF THE DISCLOSURE The present invention relates to an antenna device comprising a pair of reflectors and more particularly an antenna device in which a branching filter using dielectric elements is disposed adjacent to one of a pair of reflectors so that the waves of two different frequency groups may be directly branched or composed in the antenna section.

In the telecommunication systems utilizing the telecommunication satellites as the relay stations, there has been proposed the use of two or more frequency bands such as the microwave frequency band of 4 6 GI-Iz and the quasi-millimeter frequency band of i7 30 GHz. The antenna devices used in such telecommunication system must have the capability of simultaneously composing or branching the frequency groups in the microwave and quasi-millimeter frequency bands. This is accomplished by use of the frequency group branching filters connected to the Cassegrain antennas which are widely used in the microwave communication systems. In antenna devices of the type described, two wave guides are coupled through many slots that are equidistantly spaced apart to constitute a wave-guide type frequency group branching filter which in turn is coupled to the feeder section of the antenna device. In case of transmission the frequency group branching filter composes the two signal components of the microwave and quasi-millimeter frequency bands which are transmitted by the two different wave guides respectively, and the composed signals are transmitted to a primary horn through a feeder section, reflected by a subreflector and a main reflector and emitted into the space. In case of reception, the incident waves are reflected first by the main reflector and then by the subreflector to be transmitted into the feeder section through the primary horn, and are branched into the microwave and millimeter frequency group signal components by the frequency group branching filter to be fed into the two different wave guides respectively.

However, the number of slots in the frequency group branching filter must be increased as the frequency of the signals transmitted is increased. Furthermore since the signals of the quasi-millimeter frequency band which is four to five times higher than the microwave frequency band are transmitted or received simultaneously with the signals of the microwave frequency band, the feeder section coupled to the primary horn of the antenna device tends to become oversized relative to the signals in the quasi-millimeter band. Therefore the undesired high order mode of the quasimillimeter band tends to be excited in the frequency group branching filter and the feeder section and so the quality of the transmitted signals is much deteriorated. The undersired high order mode produced causes tracking error when the antenna device is steered to track a communication satellite. Furthermore, there is heat loss in the walls of the wave guides of the frequency group branching filter when the signal current flows. This heat loss is a cause of the deterioration of the gain-noise temperature ratio (6/1) of the antenna device. Therefore it is not preferable to use a frequency group branching filter of the type comprising wave guides in order to branch or compose the wide band signal waves extending from a relatively lower frequency group to a relatively higher frequency group which is several times higher than the lower frequency group in frequency.

One of the objects of the present invention is therefore to provide an antenna device in which the frequency group branching and composition may be carried out directly in the antenna section without using the wave-guide type frequency group filter.

Another object of the present invention is to provide .an antenna device incorporating a branching filter using dielectric elements which is capable of reflecting a high frequency beam of the two different frequency group components and of transmitting therethrough the other frequency group component so that the frequency group branching and composition of the beam may be effected.

According to one embodiment of the present invention, first and second reflectors are disposed in opposing relation, and a third reflector or a branching filter using dielectric elements capable of reflecting the waves in the higher frequency group and of transmitting therethrough the waves in the lower frequency group is disposed adjacent to one of the first and second reflectors, for example the second reflectorqln reception the incident waves in the low frequency group are first reflected by the first reflector, transmitted through the third reflector and reflected again by the second reflector to converge toward a primary horn for low frequency group. The waves in the high frequency group are first reflected by the first reflector and then by the third reflector or branching filter to converge toward a primary horn for high frequency group. In case of transmission the direction of the propagation or transmission of the waves in the high and low frequency groups are reversed.

In the antenna device of the present invention, at least one of the first, second and third reflectors is a paraboloidal reflector while the other two reflectors are hyperboloidal reflectors. Alternatively one of the three reflectors is a plane reflector which is disposed at an angle relative to the axes of the paraboloidal or hyperboloidal reflectors. The former type antenna device in accordance with the present invention is especially adapted for use in an earth station in the satellite telecommunication system using two or more frequency bands, while the latter type antenna device is adapted to be mounted on a telecommunication satellite.

BRIEF DESCRIPTION OF THE DRAWINGS FIGJ is a schematic sectional view of a prior art Cassegrain antenna device incorporating therein a waveguide type frequency group filter;

FIG .2 is a fragmentary longitudinal sectional view, on enlarged scale, of the frequency group filter thereof;

FIG.3 is a sectional view taken along the line 3-3 of FIGJ;

FIG.4 is a schematic sectional view illustrating one embodiment of an antenna device in accordance with the present invention;

FIGS-(a) is a perspective view of a branching filter using dielectric elements used in the antenna device in accordance with the present invention;

FIGS-(b) is a diagram for explanation of the principle of operation thereof;

FIG.6 is a graph illustrating the frequency characteristic curves of the branching filter shown in FIGS;

FlGS.7-9 are schematic sectional views of some variations of the antenna device in accordance with the present invention;

FlG.10 is a sectional view of the antenna device of the present invention used as an antenna for an earth station;

FIG.11 is a schematic sectional view illustrating the antenna device in accordance with the present invention used as a satellite antenna;

FlG.12 is a view for explanation of the mode of operation thereof; and

FIG. 13 is a cross sectional view of a reflector which may be employed in the antenna device of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior Art Referring to FlG.1, a prior art Cassegrain antenna device generally designated by 10 comprises a main reflector 11, a subreflector 12, a primary horn 13 located at the center of the main reflector 11, a feeder section 14, and a frequency group branching filter including a pair of

wave guides

16 and 17. In case of transmission, two signal components of different frequency bands such as quasi-millimeter frequency band and the microwave frequency band which are respectively transmitted through the

wave guides

16 and 17 are composed by the frequency group branching filter 15 and led to feeder section 14 and transmitted from the primary horn 13. The electromagnetic waves emitted from the primary horn 13 are reflected by the subreflector 12 and the main reflector 11 in the order named and transmitted into space. In case of reception, the incident waves are reflected by the main reflector 11 to the subreflector 12 from which they are led into the primary horn 13, the feeder section 14 and the frequency group branching filter 15 where they are branched into the signal components of the quasi-millimeter frequency band and of the microwave frequency band respectively. The branched signals are transmitted through the

wave guides

16 and 17 respectively to a reception apparatus (not shown).

Next referring to FlGS.2 and 3 illustrating the enlarged sectional views of the frequency group branching filter 15, the two

wave guides

16 and 17 are coupled to each other through a plurality of slots 18 which are substantially equidistantly spaced apart from each other, and the transmitted wave frequency band characteristic of the coupler is so selected as to comply with the quasi-millimeter frequency band. Therefore in case of the reception, the incident waves fed from the feeder section 14 in the direction indicated by the arrow 19 into the frequency band branching filter 15 are so branched that the signal components in the quasimillimeter frequency band are transmitted through the slots 18 into the wave guide 16 while the signal components in the microwave frequency band are transmitted through the wave guide 17. In case of transmission, the propagation of these signal components is of course reversed, and the signal components in the quasimillimeter and microwave frequency bands which are composed by the group frequency branching filter 15 are led toward the primary feeder 14.

However, as stated previously, it is not preferable to couple the frequency group branching filter of the type described comprising a pair of

wave guides

16 and 17 to the feeder section of the Cassegrain antenna in order to branch and compose the waves widely extending in frequency from the microwave frequency to the quasimillimeter frequency band, because of many associated problems.

THE INVENTION FlG.4 is a schematic sectional view of an antenna device in accordance with the present invention for explanation of the underlying principle thereof. A main reflector 101 is a paraboloidal reflector with the focal point indicated by 102, and a first subreflector 103 that is a paraboloidal reflector with the focal point indicated by 102 and a second subreflector 104 which is a hyperboloidal reflector are disposed in opposing relation with the main reflector 101. The second subreflector 104 comprises a branching filter using dielectric elements to be described in more detail hereinafter, so that the electromagnetic waves in higher frequency bands such as the quasi-millimeter frequency band may be reflected while the waves in the lower frequency bands such as the microwave frequency band may be transmitted therethrough. A primary horn 106 for high frequency band is located at the conjugate focal point 105 of the focal point 102 of the hyperboloidal subreflector 104, and another primary horn reflector 107 for lower frequency band is located at the center of the main reflector 101.

Now it is assumed that the waves in the low frequency band indicated by the solid lines 108 and the waves in the higher frequency band indicated by the dashed lines 109 arrive at the same time at the antenna device 100. The waves 108 in the lower frequency band are reflected by the main reflector 101, transmitted through the second subreflector 104 or the branching filter using the dielectric elements and then reflected again by the subreflector 103 to be made incident onto the primary horn 107 as plane waves. The incident waves are converged at the focal point 110 of the primary horn reflector 107 and derived in the direction indicated by the arrow 111. The waves in the higher frequency band 109 are first reflected by the main reflector 101 and then by the second hyperboloidal subreflector 104 or branching filter using dielectric elements, converged at the focal point 105 and made incident onto the primary horn 106, from which the waves are derived in the direction indicated by the arrow 112. It is of course understood readily that in case of transmission the propagation paths of the waves are reversed.

As described above according to the present invention the waves in the higher and lower frequency bands are branched or composed directly at the antenna section so that oversized wave guides coupled to the

primary horns

106 and 107 are not required. Furthermore the branching and composition of the waves over the wide frequency band may be effected with a negligible loss and the minimum of undesired higher order mode.

FIG .5 illustrates a branching filter using dielectric elements for branching two waves in the two frequency bands. For the simplicity of explanation, the frequency bands used are the quasi-millimeter frequency bands of 18 GHz and 26 GHz and the microwave frequency bands of4 GHz and 6 GHz. Referring to FIGS-(a), the branching filter 200 is shown as comprising a plurality of sheet-shaped dielectric elements 201-205 of two types having different dielectric constants. The thickness of the elements 201-205 is substantially equal to,-

for example, one fourth of the wave length of the center frequency (23.5 6112), of the two quasi-millimeter frequency bands of 18 and 26 61-12. The filter 200 is inclined as shown in FlG.5-(b), and the composed waves 206 in the above stated quasi-millimeter and microwave frequency bands are made incident to the filter 200. The substantial components in the quasimillimeter frequency bands are reflected by the first dielectric element 201 as indicated by 211, and the remaining components are transmitted through the element 201 as indicated by 207. The transmitted waves 207 are then reflected by the second dielectric element 202 so that the substantial components in the quasimillimeter frequency bands included in the transmitted waves 207 are reflected as shown by 212. The remaining components are transmitted through the second dielectric element 202 as indicated by 208. In a manner similar to that described the waves are successively reflected by and transmitted through the successive dielectric elements 203-205. As a consequence the substantial components in the quasi-millimeter frequency bands are derived as the reflected

waves

211, 212 and 213 while the substantial components in the microwave frequency bands are derived as the transmitted waves 210. FIG. 13 is a cross sectional view of the combination of the first subreflector 103 and second subreflector 104 of FIG. 4 employing the branching filter as illustrated in FIG. 5(b).

The frequency characteristic curves of the filter 200 shown in F165 are illustrated in FlG.6. The solid curves indicate the theoretical values while the dotted line curves, those marked with measured," indicate the measured values. The frequency is plotted against the abscissa while the transmission loss (T) and reflection loss (R) against the ordinate. From FIG.6, it is seen that the filter shown in FIGS has the excellent capability of branching the waves in the quasi-millimeter frequency band of 18-26 61-12 from those in the microwave frequency band of 4-6 61-12.

The present invention is not limited to the arrangement shown in FlG.4, and various variations and modifications can be effected as will be described hereinafter by reference to FlGS.7-9 without departing from the scope of the invention.

In a variation illustrated in FlG.7, both of the first and second subreflectors for reflecting the waves in the lower and higher frequency bands are hyperboloidal reflectors at one focus or focal points of which are located the primary horns respectively. More specifically, one of the focal points of the first hyperboloidal subreflector 303 is located to be coincident with the focal point 302 of the paraboloidal main reflector 301. The other focal point or focus of the hyperboloidal subreflector 303 is indicated by 308. The axis connecting the points or focal points 302 and 308 of course makes an angle with respect to that of the paraboloidal main reflector 301. The primary horn 310 for reception of the lower frequency bands is located at the focal point 308 of the first subreflector 303 in opposed relation therewith. One focal point of the second hyperboloidal subreflector or branching filter 304 similar in construction to the filter shown in FIGS coincides with the focal point of the main reflector 301. The other focal point is indicated by 309. The axis of the second hyperboloidal subreflector 304 is also inclined at an angle relative to the axis of the main reflector 301, but on the opposite side relative to the axis of the first subreflector 303. The primary horn 311 for reception or transmission of the higher frequency band is located at the focal point 309 of the second subreflector 304 in opposed relation therewith and is aligned with the axis thereof.

The incident waves in the lower frequency band indicated by the solid lines 314 are first reflected by the main reflector 301 to converge toward the focal point 302 and then transmitted through the second subreflector or branching filter 304 and again reflected by the first subreflector 303 to converge toward the focal point 308. Consequently the waves in the lower frequency band are made incident into the primary horn 310 and derived in the direction indicated by the arrow 313. The incident waves in the higher frequency band indicated by the dashed lines 315 are first reflected by the main reflector 301 and then by the second subreflector or filter 304 to converge toward the focal point 309 at which the primary horn 311 is located so that the waves may be derived in the direction indicated by the arrow 314.

An antenna device generally designated by 400 in FIG.8 comprises a plane reflector 401 which is disposed at about 45 relative to the axes of a first and a second

paraboloidal reflectors

402 and 403. The second paraboloidal reflector 403 comprises a branching filter using dielectric elements. The incident waves in the lower frequency band indicated by 410 are first reflected by the plane reflector 401, transmitted through the second paraboloidal reflector 403 and then reflected again by the first paraboloidal reflector 402 to converge toward the focal point 404, at which is located a primary horn 408 so that the waves in the lower frequency band may be derived in the direction indicated by the arrow 406. The incident waves 411 in the higher frequency band are first reflected by the plane reflector 401 and then by the second paraboloidal reflector 403 to converge toward the focal point 405, at which is located a primary horn 409 so that the incident waves may be derived i the direction indicated by the arrow 407.

A still further variation of an antenna device in accordance with the present invention illustrated in FIG .9 is similar in construction to that shown in FlG.8 except that a branching filter 502 is flat and is disposed on the side of a first plane reflector 501 in spaced apart relation therewith. A paraboloidal reflector 503 is disposed in opposed relation with the first plane reflector 501 and the second plane reflector or the branching filter 502. As in the case of the antenna devices described hereinbefore, the antenna device 400 is provided with a primary horn 504 for reception and transmission of the lower frequency band and another primary horn 505 for the reception and transmission of the higher frequency band. The waves in the lower frequency band are propagated as indicated by 506 while the waves in the higher frequency band are propagated as indicated by 507. The mode of operation is similar to that of the antenna device shown in FlG.8 so that no description will be made.

As described above, the antenna devices of the present invention may have various arrangements, and those shown in FlGS.4 and 7 are especially adapted for use in the earth stations of the satellite communication systems, while those shown in FIGS.8 and 9 are adapted for use as the satellite antennas.

An antenna for an earth station in accordance with the present invention is schematically illustrated in FlG.10. The antenna device comprises a main paraboloidal reflector 601 with a focal point indicated by 623, a first paraboloidal subreflector 602 made of metal with its focal point indicated by 623 and a second hyperboloidal subreflector 603 or filter of the type described comprising a plurality of dielectric elements. One of the foci or focal points of the second subreflector 603 coincides with that of the first subreflector 602. The first and second subreflectors 602 and 603 are supported by stays 604. The antenna device is provided with a horn reflector 605 for reception of the lower frequency band and a primary horn 624 for reception of the higher frequency band which is located at the other focus 606 of the hyderboloidal reflector 603. Lower and higher frequency

band reception apparatus

607 and 608 are coupled through

rotary joints

609 and 610 to the

primary horns

605 and 624 respectively. Since the antenna device is elevated about the axis 616, the transmission and

reception apparatus

607 and 608 which are fixed, are coupled to the

primary horns

605 and 624 through the

rotary joints

609 and 610 coaxial with the axis 616.

When the waves in the higher and lower frequency groups arrive simultaneously, the waves in the lower frequency group are reflected by the main reflector 601, transmitted through the second subreflector 603 and reflected by the first subreflector 602 to form the plane waves which are converged by the horn reflector 605 and transmitted to the transmission and reception apparatus 607. The waves in the higher frequency group are reflected by the main reflector 601 and then by the second hyperboloidal reflector or dielectric filter 603 to converge toward the focal point 606 at which is located the primary horn 624. The waves are then transmitted to the transmission and reception apparatus 608. In case of transmission, the wave propagation paths are reversed.

The main reflector 601 is supported by the

stays

617 and 618 securely fixed to the main reflector and an altitude-azimuth mount 619 respectively, and is adapted to rotate about the altitude axis 616 as described previously by a motor 614 through a gear 613 which is carried by a motor drive shaft 615 and is in mesh with a gear 612 carried by the altitude shaft 616. The motor 614 is securely fixed to the mount 619 which in turn rides on a rail 620 on a foundation 621 to rotate about the axis 622. Therefore the elevation or altitude and azimuth of the antenna device may be selected to track a communication satellite.

FlG.11 is a schematic view illustrating a mechanical despun antenna of a satellite. In general, the geostationary satellite is spun in order to stablize its hovering and then the satellite antenna must be rotated in the di rection opposite to the direction of spin of the satellite in order to direct the antenna beam toward the earth. in case of the satellite antenna in accordance with the present invention which is not symmetrical about the axis of spin, both of the primary horns and transponders must be rotated as the antenna device is rotated. The antenna device of this type is called a platform despun antenna.

The satellite antenna illustrated in FIG.11 comprises a metal paraboloidal reflector 701 with an axis 703 and a focus or focal point 706, another paraboloidal reflector 702 comprising a filter using dielectric elements with an axis'704 and a focus or focal point 705, a lower frequency group primary horn 708 located at the focal point 706, a higher frequency group primary horn 707 located at the focus or focal point 705, a plane reflector 709 disposed at about 45 relative to the axes 703 and 704 and supported by stays 711 and an arm 710 or supporting the

reflectors

701 and 702. The satellite 712 carries a solar cell 713, a despun motor 714 whose rotary shaft 715 is securely fixed to the

reflectors

701 and 702, the

primary horns

707 and 708 and a platform 718, high and low frequency group transponders 716 and 717 and an apogee motor 719.

In case of reception, the waves in the lower frequency group are reflected by the plane reflector 709, transmitted through the reflector 702 and reflected again by the reflector 701 to converge toward the focal point 706. The converged waves are fed to the transponder 717 through the primary horn 708 which is located at the focal point 706. In like manner waves in the higher frequency group are reflected first by the plane reflector 709 and then by the reflector 702 to converge toward the primary horn 707 which is located at the focal point 705, and are transmitted toward the transponder 716.

The rotation of the antenna device is effected by the stator 714 fixed to the satellite proper and the rotary shaft 715 fixed to the platform 718 and the primary horns of the antenna device, the stator and the rotary shaft constituting a despun motor. Therefore, the antenna elements, the primary horns and the transponders may rotate in unison in the direction opposite to the direction of spin of the satellite proper so that the beam may be always directed toward the earth.

The following advantages may be accrued from the satellite antenna shown in FIG. First there is no blocking because the primary horns and their stays are not disposed in the propagation paths. Secondly the reflector shaping of the two reflectors may be effected relative to each other so that the high antenna efficiency and the easy beam shaping may be attained.

FlG.12 schematically illustrates a geostationary satellite 804 having an antenna device comprising a reflector assembly 801 including a pair of paraboloidal reflectors one of which is a dielectric filter of the type described, and a plane reflector 802 which is disposed at an angle relative to the axis 803 of the reflector assembly 801. The geostationary satellite 804 spins about the axis 803 which is perpendicular to a plane including the equator of the earth 805 so that when the plane reflector 802 is inclined at 45 relative to the axis 803, the antenna beam is directed toward the equator as indicated by 806. In order to direct the beam to, for example, Japan as indicated by 806', the plane reflector 802 is inclined as indicated by the dashed line 802'. In such a simple manner as described above, the beam may be directed to any desired direction without adversely affecting the antenna characteristics.

Although the dielectric filter has been described as being capable of transmitting the waves in the lower frequency group and of reflecting the waves in the higher frequency group, it may be designed to reflect the waves in the lower frequency group and to transmit the waves in the higher frequency group.

What is claimed is:

1. An antenna for reflecting emitted and incident wave components of first and second frequency groups, comprising first and second reflectors positioned in opposed relation to sequentially reflect wave components, and a third reflector positioned adjacent one of said first and second reflectors, said third reflector being comprised of a branching filter having a plurality of dielectric layers positioned to reflect wave components of said second group and transmit wave components of said first group, said first and second reflectors being positioned to converge wave components of said first group at a first point, and said third and otherof said first and second reflectors being positioned to converge wave components of said second group at a second position spaced from said first position, said branching filter being comprised of a plurality of alternate layers of materials of two different dielectric constants with thickness substantially equal to one-quarter of the wave length of the center frequency of the second group.

2. The antenna of claim 1 wherein said one reflector is said second reflector, said first and second reflectors are paraboloidal with a common focal point, and said third reflector is positioned between said first and second reflectors and is hyperboloida] with a focus at said common point.

3. The antenna of claim 1 further comprising a horn for said second group at the conjugate focal point of said common focal point, and a second horn for said group positioned at the center of said first reflector.

4. The antenna of claim 3 wherein said first group is in the microwave frequency band and said second group is in the quasimillimeter band.

5. The antenna of claim 1 wherein said one reflector is said second reflector, said second and third reflectors are hyperboloidal, said first reflector is paraboloidal with a focus common with one focus of said second reflector a horn at the other focus of said second reflector with the axis of the two focal points of the second reflector at an angle to the axis of said first reflector, one focus of the third reflector coinciding with the one focus point of said second reflector, and a horn at the other focus point of said third reflector, the axis of the focal points of said third reflector being at a different angle to the axis of the first reflector.

6. The antenna of claim 1 wherein said one reflector is said first reflector, said first and third reflectors are paraboloidal, and said second reflector is a plane reflector at an angle to the axis of the first reflector, the axes of said first and third reflectors being spaced apart.

7. The antenna of claim 1 wherein said one reflector is said second reflector, said first reflector is paraboloidal and said second and third reflectors are plane reflectors at different angles to the axis of said first reflectOT.

##tit

Claims

2. The antenna of claim 1 wherein said one reflector is said second reflector, said first and second reflectors are paraboloidal with a common focal point, and said third reflector is positioned between said first and second reflectors and is hyperboloidal with a focus at said common point.

7. The antenna of claim 1 wherein said one reflector is said second reflector, said first reflector is paraboloidal and said second and third reflectors are plane reflectors at different angles to the axis of said first reflector.