Abstract: A rate sensor, preferably comprised of a tuned gyro, has a generally cylindrical outer housing and an inner, large diameter rotary cylindrical support defining the gyro's rotary axis. The rate sensing apparatus is mounted in the volume between the outer housing and the rotary cylindrical support whereby the interior of the rotary cylindrical support defines a large diameter hollow for receiving and supporting apparatus, other than rate sensing components of the rate sensor. The rate sensor may be advantageously used with a homing device, the seeker apparatus of the device being mounted in said hollow. This produces a homing device with a high packing density and, as the rotary axis of the gyro gimbals and that of the seeker coincide, detrimental unbalance and unwanted increase in inertial movement is eliminated.

Abstract: A high bandwidth microwave antenna feed comprises a spigot (4) of relatively high dielectric constant protruding from a conventional hollow waveguide (3) and a generally conical member (12) of relatively low dielectric constant fitted coaxially onto, so as to expand from the protruding spigot. The base portion (2) of the conical member is silvered and constitutes a sub-reflector. In use, microwave radiation from the hollow waveguide is refracted at the spigot/conical member interface and is reflected from the sub-reflector to a facing main reflector (1). The two-part dielectric construction enables the dimensions of the sub-reflector to be reduced, thereby reducing blockage of the main reflector and increasing the bandwidth of the system.

Abstract: A method and apparatus are provided for transmitting a microwave signal as a beam having a wavelength small as compared to the size of the reflecting surfaces, wherein a main reflector is stationary with respect to a sub-reflector and the main reflector is motional with respect to the feed. The invention comprises rotating the sub-reflector about a first rotational axis transverse and preferably orthogonal to the guide axis, the first rotational axis being on the vertex of a secondary reflector, thereby directing radiation from a fixed feed to the motional sub-reflector. The primary reflector and sub-reflector are disposed relative to one another to share a common focus or confocal point.

Abstract: A dual mode antenna that allows both millimeter wave and infrared radiation o enter a single aperture and propagate through a common transmission device to a point where the respective energies are divided to follow separate paths for subsequent processing. An electromagnetic transmission guide or waveguide for the system comprises a tubular waveguide member, having a beam directing lens or lenses therein, for maintianing low-loss, gaussian beam propagation of millimeter wave energy therethrough. A waveguide fiber array is coupled into or adjacent to the tubular member at one end and retrieves the infrared radiation but allows the millimeter wave radiatin to pass. Cassegrainian optics are positioned adjacent the other end of the tubular member for directing incoming millimeter and infrared energy into the guide. Subsequently, the energy impinges on the beam directing lens means that separates the respective energies, dividing them into two separate and distinct paths.

Abstract: A dual mode antenna that allows both millimeter wave radiated energy and rared radiated energy to enter a single aperture and propagate through a common transmission device to a point where the respective energies are divided to follow separate paths for subsequent processing. An electromagnetic transmission guide for waveguide comprises first and second parallel tubular members having beam directing reflectors and lenses therein for maintaining low-loss, gaussian beam propagation of radiated energy therethrough. Cassegrainian optics of a cassegrainian antenna system are positioned for directing the millimeter and infrared energy into a common, circular end of said tubes where the energy impinges on beam directing means that separates the respective energies, directing them into the respective parallel tubes along two separate and distinct paths.

Abstract: Antenna structure 10 has a primary reflective surface 12 over which is positioned a subreflector surface 14. Support webs 32, 34 and 36 are made of low dielectric loss insulating material and are adhesively secured in slots in the reflectors. The support webs are positioned on planes through the axis so that the small thickness of the webs is in the projected area and results in an unobstructed and undistorted electromagnetic radiation pattern. The webs firmly support the subreflector in the desired position. The antenna can be configured as a Gregorian system as shown or alternatively as a Cassegrainian system.

Abstract: An inverted Cassegrain antenna utilizing polarization rotation. The polarizer-reflector or mirror utilized comprises two or more polarizer-reflector elements joined together around hinges perpendicular to the widening direction required. A sheet comprising parallel metal wires covers the whole of the elements. For each element, a metal film is separated from the sheet by a first dielectric layer. Each hinge is formed, on one side, by the flexible sheet of metal wires which straddles the gap between elements. The other side of the hinge includes a second dielectric layer backed by a rigid metal strip, also straddling the gap. One end of the metal strip is fixed to the metal film of one adjacent element while the other end of the metal strip is in moveable electrical contact with the metal strip of the other adjacent element. The antenna is particularly applicable to multiple function radars.

Abstract: An antenna system having a main reflector, a sub-reflector, and a plurality of horns for radiating different frequencies includes a beam waveguide system which cancels cross polarization otherwise inherent in the system. If the antenna system uses a non-rotationally-symmetric sub-reflector the cross polarization caused thereby is cancelled by the beam waveguide system having at least two focusing reflectors and selected parameters. Alternatively the beam waveguide system can be used with a rotationally symmetric and stationary sub-reflector by being positioned to reflect said beam on the axis of the main reflector. Either the horns or the focusing reflectors may be rotatably switched, the other group being stationary.

Abstract: The present invention relates to a multibeam antenna arrangement comprising a main focusing reflector, a doubly curved subreflector disposed confocally with the main reflector and a plurality of feeds disposed on a doubly curved focal surface of the antenna on which an image of the far field of view is formed. The subreflector is doubly curved in orthogonal directions to introduce a predetermined amount of barrel distortion for transforming a three-dimensional, non-rectangular, matrix in the far field of the antenna arrangement into a substantially rectangular matrix on the doubly curved focal surface of the antenna arrangement. Feeds are aimed such that a central ray from each feed reflected by the subreflector impinges a common point on the main reflector.

Abstract: In a dual-band antenna of the beam wave-guide type, first and second electromagnetic waves from first and second feed horns, respectively, are directly provided to a frequency selective reflector along different axes, with both electromagnetic waves being provided from the frequency selective reflector along the same axis. The two electromagnetic waves are then reflected in common by first and second concave mirrors and then provided substantially along an axis coincident with the azimuth axis to the remainder of the beam wave-guide section which is itself rotatable about the azimuth axis.

Abstract: A short-focus microwave reflector is located a short distance away from the feed horn and is positioned to reflect the beam of microwave radiation away from the feed axis and toward a large primary reflector antenna for transmission. A conical shield extends from the feed horn to the secondary reflector, joining them together as a unit and preventing sidelobe radiation. The beam of radiation emerges from the shield through an aperture in the sidewall thereof. The short focus of the secondary reflector causes the beam to be focussed approximately at the plane of the aperture, such that the beam diameter is small and the aperture can also be small, reducing stray radiation from the shield structure. Diffraction at the edges of the aperture is reduced by a radially surrounding choke structure which further suppresses sidelobe radiation.

Abstract: A receiver for counterclockwise and clockwise circularly polarized microwave signals of the type comprising a receiving antenna with a feeder system, a polarization converter, a polarization filter and a circuit for converting the microwave signals of both polarization directions from the high frequency to the intermediate frequency plane. A portion of the feeder waveguide belonging to the feeder system of the receiving antenna is designed as a bandpass filter which is effective for both polarization directions. A microstripline substrate, which carries the frequency converting circuit, is connected with the output of the feeder waveguide and is provided with an arrangement for coupling in the energies of the waveguide modes of both polarization directions. The polarization converter is either directly integrated in the feeder waveguide or the polarization conversion is effected by coupling the waveguide modes into the microstripline circuit.

Abstract: A two-band multimode microwave source for an antenna of a low-elevation-tracking radar comprises a higher-frequency section nested in a lower-frequency section, the two sections having E-planes perpendicular to each other. The lower-frequency section includes two outer pairs of waveguides separated by a block which convergingly projects beyond their output ends and is bisected by the E-plane of that section. The higher-frequency section includes two inner pairs of waveguides disposed within that block and separated by an obstruction lying in the last-mentioned E-plane. The higher-frequency wave emitted by the inner waveguides is made planar by a lens disposed at n output aperture of the structure which is transparent to the lower-frequency wave.

Abstract: An antenna system having a main reflector, a sub-reflector, and a plurality of horns for radiating different frequencies includes a beam waveguide system which cancels cross polarization otherwise inherent in the system. If the antenna system uses a non-rotationally-symmetric sub-reflector the cross polarization caused thereby is cancelled by the beam waveguide system having at least two focusing reflectors and selected parameters. Alternatively the beam waveguide system can be used with a rotationally symmetric and stationary sub-reflector by being positioned to reflect said beam on the axis of the main reflector. Either the horns or the focusing reflectors may be rotatably switched, the other group being stationary.

Abstract: An antenna, such as Cassegrain or parabolic antenna, intended for use in the microwave or millimeter wavebands having improved wide angle radiation characteristics. At least one edge portion, in first embodiments, of a supporting post structure which supports a primary radiator or subreflector is modified to randomly scatter waves so that the field strength of scattered waves due to the presence of the supporting post is decreased.

Abstract: The present invention relates to an antenna arrangement for suppressing selected sidelobes where the arrangement comprises a main focusing reflector (10), a main feed arrangement (14) and at least two auxiliary feed arrangements (20 and 21) disposed on opposite sides of, and a predetermined equal distance from, the main feed arrangement in the plane of the sidelobes to be suppressed. All feed arrangements radiate the same signal and their individual beams are radiated in a direction whereby the central ray thereof impinge the same point on the main reflector. By proper adjustment of the phase, amplitude and transverse off-axis distance of the auxiliary feed arrangements from the main feed arrangement, the combined E-field distribution of the auxiliary feed arrangements at the edge of the reflector in the plane of the sidelobes to be suppressed will be equal and opposite to the E-field distribution of the main feed arrangement.

Abstract: An earth station antenna is employed for telecommunicating with a satellite in combination with an automatic tracking apparatus. The antenna is a dual reflector antenna which comprises a main reflector, a sub-reflector and a primary radiator, and the used frequency band of the antenna is at least 1.8 octaves. A parameter t of the antenna, defined by a configuration of the primary radiator is less than 0.3 at the lowest operating frequency. The phase center of the primary radiator, at a low frequency within the operating frequency range, is made to correspond with the focus of the sub-reflector.The above parameter t is defined by: ##EQU1## where: .lambda.: wavelength in free space,D.sub.h : aperture diameter of the primary radiator,L.sub.h : center axial length from the radiator apex to the radiator aperture plane, andL.sub.s : length from the radiator aperture plane to the bottom surface of the sub-reflector.

Abstract: The present invention relates to a wide-field-of-view antenna arrangement which permits the angular range over which multiple beams may be transmitted or received to be increased by taking into consideration the location of the tangential and sagittal focal regions associated with multibeam systems. The subreflector (18,24) is positioned relative to the main reflector (10,20) such that the tangential and sagittal focal regions lie behind the subreflector of a Cassegrainian arrangement, or alternatively in front of the subreflector of a Gregorian arrangement for a predetermined wide-field-of-view.

Abstract: The present invention relates to an antenna arrangement capable of correcting for astigmatism over a broadband range, the antenna arrangement comprising a main focusing reflector arrangement (10), such as, for example, a Cassegrain antenna system, a feed (12) and astigmatic correction means (14) disposed between the feed and the main focusing antenna arrangement. The astimatic correction means comprises a first and a second doubly curved subreflector (16, 18) or lens (30, 32) which are curved in orthogonal planes to permit the launching of an astigmatic beam of constant size and shape over a broadband range.

Abstract: Disclosed is a conically scanning antenna emitting a beam of radiation which is modulated in phase but not in amplitude. In certain embodiments the antenna comprises a source of radiation including a horn illuminating a reflector, the horn being either inclined to the reflector axis or juxtaposed with a prism imparting such inclination to its direction of maximum radiation. The inclined horn or the prism is rotated about the axis by drive means supplying a reference scanning signal to a processor.

Abstract: A radar antenna of the Cassegrain type comprises a paraboloidal principal reflector confronting a hyperboloidal or flat auxiliary reflector of smaller radius directing linearly polarized radiation from a source on their common axis toward the principal reflector along lines passing through the focal point thereof to produce an axially extending outgoing beam whose central part is intercepted by the auxiliary reflector. The latter consists of an array of linear conductors generally parallel to the direction of polarization while the principal reflector is formed by another conductor array generally inclined at 45.degree. to that direction, the latter array being backed by a solid mirror separated therefrom by a distance smaller than a quarter wavelength at the midfrequency of the emitted radiation.

Abstract: A subreflector and reflectors in a primary radiation system, these constituting an offset dual-reflector aerial, are subjected to the so-called reflector-surface shaping. The main reflector is formed as a part of a revolutional paraboloid, and is a combination of a number of congruent reflector-segments. With such an arrangement, the manufacturing cost of a large-sized main reflector is reduced. The production error, and the aerial gain reduction and the deterioration of a wide-angle directivity due to the production error are minimized.

Abstract: An antenna system of the Cassegrain type, used with a tracking radar and adapted to conically scan incoming waves reflected by outlying targets, comprises a paraboloidal main reflector and two hyperboloidal auxiliary reflectors confronting same. One of the auxiliary reflectors is semitransparent and coaxial as well as cofocal with the main reflector whereas the other is solid with an axis inclined to that of the main reflector about which it rotates while keeping its foci in the focal planes of the semitransparent reflector.

Abstract: A reflector antenna system is described suitable for ground stations used in communication with geostationary satellites. Dual beams or multi-beams can be directed at several satellites spaced angularly from 5.degree. to 20.degree. apart and these beams are scanned by feed motion keeping a single main reflector surface fixed. Offset feed geometry is used for low aperture blocking and shaping of subreflectors and main reflector results in very high aperture efficiencies, low sidelobes and symmetric low cross-polarization patterns needed for satellite links. A novel method for shaping subreflectors using the ratios of ray lengths squared and variable focal lengths is applied in the optimally tilted offset geometry results in almost uniform aperture power distributions.

Abstract: A corrugated horn radiator which includes a transition zone made up of a number of parts between the smooth-wall feed wave guide 1 having a cross-section which is constant and a horn radiator with regular periodic corrugations 10 with the depth of the corrugations being less than one-quarter wave length relative to the lowest frequency to be transmitted. The transition zone starting at the constant diameter feed wave guide comprises a first section which is conical shaped and then joins a second section which is cylindrical in shape and which serves as a phase drift space and then connects to a second conical smoothed walled portion and then connects to an exponential horn having a pair of corrugations with re-entrant ring-shaped ends (6, 7) which are narrower than the regular periodic corrugation structure 10.

Abstract: A five-horn Cassegrain antenna is disclosed comprising a main dish, an up-taper subreflector set on the boresight axis of the main dish, and including a sum horn positioned on the boresight axis between the subreflector and the main dish with four error horns set therearound. The aperture of the sum horn is relatively large to substantially eliminate radiation spillover at the up-taper subreflector. The error horns are positioned around this large aperture sum horn such that the radiation patterns of paired error horns will crossover in their sidelobes. These error horns are provided with a high aspect ratio with the narrow dimension of each error horn being located in the plane of its tracking crossover with the radiation pattern of the other error horn with which it is paired.

Abstract: A multi-spectral detection system for detecting radiation within frequency bands that are in diverse portions of the electromagnetic spectrum is disclosed.

Abstract: A system for, and a method of, scanning a collimated beam of electromagnetic radiation with a very large primary reflector of an antenna system without loss (i.e., with zero phase error) and without moving the primary reflector. The system is a two dimensional scan, loss-free, Cassegrain antenna system which comprises a stationary, parabolic-shaped, primary reflector, and a smaller, movable, parabolic-shaped subreflector, and also a movable, plane wave, electromagnetic radiation beam source interposed between the two reflectors. It is shown that, without moving the primary reflector, the beam can be scanned over wide angles with no phase error in the plane of scan, provided that the source and the subreflector are rotated about the focal point of the antenna in a specific manner. A unique angular relationship between the source and the subreflector is required for each scan angle, and is taught.

Abstract: A dual-band antenna for the transmission and reception of radiant energy has a Cassegrain-type reflector assembly, rotatable about an azimuth axis and pivotable about an elevational axis, to which two outgoing beams are directed in a transmitting mode by a periscopic supply system including a set of four mirrors, two of them flat and two of them concave. The lower-band beam originates at a first feed comprising a horizontally radiating horn and two confronting concave mirrors, one of these mirrors training the beam onto a slanting dichroic mirror reflecting same upward into the periscopic supply system. The higher-band beam originates at a second feed also comprising a horizontal horn which radiates onto a concave mirror reflecting the beam upward through the dichroic mirror into the periscopic supply system along a path coinciding with that of the other beam.

Abstract: Inverse Cassegrain antenna making it possible to use on the one hand the qualities of a conventional fine beam for look-out and tracking functions and making it possible on the other hand to have a widened beam either in the elevation plane or in the bearing plane. According to one embodiment, this antenna is provided with a mirror constituted by two reflector - polarizer elements joined to one another by a hinge which permits the articulation thereof, a remote control device regulating their relative orientation.

Abstract: The present invention relates to an antenna arrangement capable of generating one or more linearly scanning spot beams which are scanned over separate strip portions of the entire field of view of the antenna. The antenna arrangement comprises an optical system comprising an aperture and a focal plane, and a novel feed arrangement. The feed arrangement comprises a separate linear phased array disposed within a separate rectangular waveguide section for each linear scanning beam desired. An offset curved reflector, disposed in each waveguide section, converts a linearly scanning planar wavefront generated by an array into a converging beam forming a linearly moving point source on the antenna's focal plane with the principle ray of each instantaneous wavefront being directed at the center of an image of the aperture of the optical system as seen by the feed arrangement.

Abstract: A rectangular waveguide housing and a waveguide radiator have coaxial cavities that are contiguous. A pair of perpendicular walls intersect substantially on the axis of the housing to form therein four similar rectangular subwaveguides that may be excited to propagate electromagnetic waves in a TE.sub.10 mode. The walls have edges that are in the boundary between the contiguous cavities. A pair of intersecting tabs extend from the edges into the radiator. Additionally, a protrusion extends within the radiator from the intersection of the tabs. The polarities of the waves in the subwaveguides are selected to cause an electromagnetic wave to propagate through the radiator in either of two orthogonal difference modes or a sum mode that has desired beam shaping properties. The protrusion causes one of the difference modes to have a desired polarization.

Abstract: An offset antenna and feed combination for use in a satellite communication system is disclosed wherein an array of adjacent microwave energy feed elements is disposed within a predetermined shape on the focal surface of the antenna for delivering electromagnetic energy to a remote, correspondingly shaped zone. Electromagnetic energy beams are selectively radiated towards predetermined areas of the overall remote zone by associated cluster groups of feed elements typically arranged as a minimum of one central and six surrounding feed elements. Additionally, each surrounding type of feed element of a cluster may be shared by two or more overlapping clusters, the overlapping clusters generating contiguous beams of electromagnetic energy in different frequency subbands.

Abstract: A double reflector antenna mounted on a terrestrial mount with Azimuth and elevation shafts in which a primary field radiating and detecting horn is mounted within a housing behind a central opening of the main reflector dish so as to radiate and receive energy from a sub-reflector mounted at the focal point of the main reflector dish and such that the lead supplying energy to and from the horn can be very short and the horn can be substantially shielded from snow and rain.

Abstract: An aerial system (antenna system) comprisesdual-reflector aerial consisting of a main reflector and subreflector;a primary feed whose input and output ends are fixed for elevation and azimuth rotation of said dual-reflector aerial;a plane reflector which is turned together with said dual-reflector aerial around an elevation rotating axis;a first curved reflector for reflecting waves generated from said primary feed; andsecond and third curved reflectors which sequentially reflect to lead the wave reflected by said first curved reflector to said plane reflector.

Abstract: A reflector antenna is provided with two sets of perturbation reflectors, in the form of annular surface depressions, to reduce the effect of `external reflection` which is caused by energy re-entering a feed horn after reflection from the antenna. The two sets of perturbation reflectors are arranged to produce vector cancellation at two operating frequency bands.

Abstract: The present invention relates to novel launcher reflectors which are used with reflector antenna systems to compensate for the dominant aberration of astigmatism which was found to be introduced in the signals being radiated and/or received at the off-axis positions. A major portion of such phase error is corrected by using, with each off-axis feedhorn, an astigmatic launcher reflector having a curvature and orientation of its two orthogonal principal planes of curvature which are chosen in accordance with specific relationships, the launcher reflector being fed by a symmetrical feedhorn.

Abstract: An antenna is made operative for two frequency bands, .DELTA.F.sub.B and .DELTA.F.sub.H, the latter higher than the former, by means of a frequency selective absorber, covering, in its edge region, a reflector of the antenna in order to reduce the diameter of the reflecting surface of the reflector and therefore the gain of the antenna for frequency band .DELTA.F.sub.H.

Abstract: A reflex feed system for a dual frequency antenna which may be, for example, one which transmits and receives both S and X-band signals, uses, in place of the dichroic plate normally employed for directing X-band radiation away from the X-band horn a flange about the opening of the X-band horn.

Abstract: An aperture antenna comprising at least one reflector, a primary radiator, and a plurality of movable beam-wave-guide reflectors arranged between the reflector and the primary radiator for directing electric waves from the primary radiator towards the reflector, in which at least one of the movable beam-waveguide reflectors is controlled to displace a feeding point to the reflector, and in which wavefront defocusing distortion of a radiated electric wave resulting from the displacement of the feeding point is corrected by moving at least one of the remaining movable waveguide reflectors.

Abstract: An antenna apparatus comprising a primary radiator having an axially symmetric field pattern, and a main reflector and a subreflector coupled to the primary radiator through a beam wave-guide path. The antenna rotating axis is aligned with the axis of the primary radiator but a little deviated from the polar axis or the earth. At least one movable beam waveguide reflector or electromagnetic lens is provided in a radio wave path between the primary radiator and the subreflector. The antenna rotating axis is adapted to be rotatable only within a small angular range. The beam wave-guide reflector or electromagnetic lens is moved to thereby deflect the antenna beam.

Abstract: A Cassegrainian antenna having a feed horn, a subreflector and a main reflector exhibits subreflector reflections back toward the feed horn. Some of this energy is reflected by the feed horn, and some by the main reflector adjacent to the feed horn, back toward the subreflector. This doubly reflected energy constitutes an echo which is reduced in the prior art by using a flat reflector plate mounted near the subreflector to cancel energy radiated by the subreflector back toward the region of the feed horn. Herein is disclosed the addition of a frequency sensitive reflecting wire grid between the flat plate and the feed horn. A combined reflection from the grid, plate, and subreflector provides echo cancellation in two frequency ranges. The plate may be recessed in a hole in the subreflector. A guard ring surrounding the plate prevents leakage through the subreflector hole.

Abstract: In a Cassegrainian antenna, radiation transmitted from the feed system is reflected back as an echo to the feed from the subreflector. In the prior art, a flat plate has been located near the subreflector to return an echo cancelling reflection to the feed in one frequency range. In this disclosure, operation of an echo cancelling device which may be mounted externally on the feed side of the subreflector is extended to two frequency ranges by adding a reflective outwardly-flared wall-like ring around the perimeter of the flat plate. The ring may include two conical segments sized to adjust the relative reflection amplitudes to obtain substantially complete cancellations over broad bandwidths at both frequency ranges. The ring may also be stepped, or serrated, in cross section to avoid unwanted specular reflections and resonances.

Abstract: A dual- band Cassegrain antenna system operable at any polarization is described wherein the hyperbolic subreflector is made to reflect signals at a first band of frequencies and to transmit or pass signals at a second lower band of frequencies. The hyperbolic subreflector according to one embodiment is a square grid mesh with conductive rings centered along the connecting legs of the square grid mesh. The rings are approximately one-third wavelength in diameter at the first band of frequencies and act capacitively at the second lower band of frequencies. The inductive reactance provided by the conductive connecting legs of the grid mesh together with the capacitive reactance provided by the rings at the lower band of frequencies causes the subreflector to transmit signals at the second lower band of frequencies.

Abstract: A reflector antenna used for radar, etc, has a subreflector so as to give a relatively large aperture diameter for a primary radiator, and to decrease the effect of the outer atmosphere.A subreflector having a different curvature in a vertical plane than a curvature in a horizontal plane is provided whereby an antenna having a different beam width in the vertical plane to that in the horizontal plane is provided even though a primary radiator having a rotationally symmetrical beam is employed.

Abstract: A Cassegrain antenna for use in terrestrial communication systems. The antenna has the conventional subreflector including a central area which provides tapered illumination of the entire main reflector, and a peripheral area which directs spillover from the central area onto the peripheral portion of the main reflector to improve the uniformity of illumination of the main reflector, thereby improving illumination efficiency and reducing gain loss due to spillover. The central and peripheral areas of the subreflector are preferably both surfaces of revolution of conic sections of the same kind, such as hyperboloids, ellipsoids or paraboloids, although the peripheral area may be a frustoconical surface if desired. The virtual focal points of the two conic sections of the subreflector are both coincident with the phase center of the feed horn.

Abstract: In an antenna of the Cassegrainian or Gregorian type including a periscope arrangement, the auxiliary reflector has a longer focal distance than the auxiliary reflector of a conventional antenna of this type and the length of the optical path between the auxiliary reflector and the primary source of the antenna is choosen substantially equal to the distance between the auxiliary reflector and that one of its two foci which is the more remote therefrom. This makes it possible to use only plane mirrors in the periscope arrangement.