EP0080319A1 - Combinaisons d'antennes - Google Patents

Combinaisons d'antennes Download PDF

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Publication number
EP0080319A1
EP0080319A1 EP82306095A EP82306095A EP0080319A1 EP 0080319 A1 EP0080319 A1 EP 0080319A1 EP 82306095 A EP82306095 A EP 82306095A EP 82306095 A EP82306095 A EP 82306095A EP 0080319 A1 EP0080319 A1 EP 0080319A1
Authority
EP
European Patent Office
Prior art keywords
antenna
reflector
radiation
cassegrain
assembly
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP82306095A
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German (de)
English (en)
Inventor
George Alan Hurd
James Matthew John Whellens
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
BAE Systems Electronics Ltd
Original Assignee
Marconi Co Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Marconi Co Ltd filed Critical Marconi Co Ltd
Publication of EP0080319A1 publication Critical patent/EP0080319A1/fr
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q19/00—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic
    • H01Q19/10—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces
    • H01Q19/18—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces having two or more spaced reflecting surfaces
    • H01Q19/19—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces having two or more spaced reflecting surfaces comprising one main concave reflecting surface associated with an auxiliary reflecting surface
    • H01Q19/195—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces having two or more spaced reflecting surfaces comprising one main concave reflecting surface associated with an auxiliary reflecting surface wherein a reflecting surface acts also as a polarisation filter or a polarising device
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q25/00—Antennas or antenna systems providing at least two radiating patterns
    • H01Q25/001—Crossed polarisation dual antennas

Definitions

  • This invention relates to antenna assemblies, and concerns in particular such assemblies as may be used for the simultaneous transmission and/or reception of electromagnetic radiation in the form of two distinct signals.
  • Radar is now a common way of employing electromagnetic radiation at radio frequencies to "see" distant target objects.
  • the technique involves emitting radiation (usually as a controlled, directed beam), and receiving any "echo” that may be returned by way of reflection off some target, the receiving apparatus being such that there may be determined at least the direction, and usually the delay time, of the returned signal, and hence that there may be calculated the relative position of the reflecting object.
  • Present-day radar systems operate at frequencies in the hundreds of megahertz, and even in the tens of gigahertz (thousands of megahertz), there being certain quite well user-defined frequency bands in this range referred to by letters of the alphabet - thus, L, X, S, J and Q-band radars cover the frequencies from about 220MHz to 50GHz.
  • the invention is primarily about the antennae employed to transceive radar signals at these frequencies.
  • a radar antenna depends to a considerable degree both upon the nature of the beam to be emitted by the antenna (and consequently upon its sensitivity pattern to received radiation) and upon the frequency of the signal to be transceived.
  • an antenna designed to emit a narrow, pencil-like beam tends to be a combination of a parabolic reflector (the surface of the solid generated by revolving a parabola about its axis) and an emitting element placed at the principal focus of the reflector.
  • the dimensions of the antenna aperture are determined primarily by the beamw ⁇ d Efi required of the antenna.
  • a typical radar antenna for transceiving tight beam electromagnetic radiation in the S-band region is a dish - a generally disc-shaped object - of parabolic section having a diameter of about 1 metre, a "depth" (from front to back of the dish) of about 0.25 m, with its emitting element about 0.25 m beyond the dish front.
  • Radar antennae are often of the Cassegrain reflector type (the name is derived, by analogy, from a particular variety of astronomical optical telescope).
  • Such an antenna is one wherein radiation from a focus is collimated from one surface and reflected by another, and in its most typical form it uses a main, parabolic reflector facing a secondary reflector of hyperbolic form with a radiation-emitting element in between so that it (the element) appears, by reflection off the secondary reflector, to be at the focus of the parabolic reflector (its actual position is known as the "Cassegrain focus").
  • a truly single antenna could be used to transceive two different signals if the antenna is of the Cassegrain type, a first emitting element (for signals of the first type) is at the Cassegrain focus, a second emitting element for signals of the second type) is at the main reflector's principal focus, and the secondary reflector is reflective to the first signal but transparent to the second signal. Even this arrangement has, however, a number of not insignificant problems associated with its use in a high power system.
  • the flux that has to be transmitted through the secondary reflector may be sufficient to cause heating (by dielectric absorption) of the secondary reflector to the point where it is destroyed, while another such is yet again the very real difficulty of ensuring adequate isolation between the two transceiving systems without the use of costly high performance filtering components.
  • the present invention seeks to deal with the requirement for the simultaneous transception of two high power signals by the simple, but apparently previously unconsidered, expedient of employing two antennae close together in tandem, a direct antenna at the fore and a Cassegrain antenna at the rear, the reflector of the former being transparent to the radiation transceived by the latter.
  • the invention provides a radar antenna assembly comprising two separate antennae arranged closely together in tandem, the fore antenna being a direct antenna and the rear antenna being a Cassegrain antenna, the reflector of the former being transparent to the radiation transceived by the latter.
  • tandem By the expression “in tandem” is meant that the two antennae are aligned on a common axis one behind the other and pointing in the same direction.
  • the expression “radiation transceived” as applied to a reflecting surface is meant either the radiation reflected off the surface during transmission or the radiation directed onto the surface during reception.
  • the expression refers to the radiation on the target side of the reflecting surface.
  • each antenna is such as to focus the radiation in some desired fashion.
  • each antenna may, as regards the general nature of its (or its primary) reflecting surface, be of any convenient variety.
  • each antenna has preferably a parabolic dish reflector (of a size suitable to the intended purpose of the antenna), and, as is explained in more detail hereinafter, the Cassegrain antenna primary reflector is advantageously a twist reflector while the direct antenna reflector is advantageously a parallel wire reflector.
  • the two antennae are mounted in tandem close together. It is not easy to define the scope of the word "close” as used in this context. Some indication may be given, however, by pointing out that it is an important requirement of the assembly that its overall volume and its inertia be kept low (so that it may be mounted and/or housed within a volume much the same as that occupied by any single antenna it may replace, and so that it may be employed - for example, in nutating, pivotting or rotating mode - in the same manner). In addition, an upper limit for the distance between the two antenna- specifically, between their two reflectors - can be said to be less than the focal length of the Cassegrain antenna's primary reflector.
  • the rear antenna in the assembly could be a direct antenna (like the front one), but that then any reasonable construction of the two antennae would necessitate the distance between the two reflectors being too large for the assembly to fulfil the requirements of relatively small volume and inertia. Accordingly, it is a major feature of the inventive assembly that the rear antenna be a Cassegrain antenna, the actual "depth" of the rear antenna as a whole then being small so that it can indeed be mounted close behind the front antenna.
  • the fore antenna is a direct antenna, having its emitting element in front of it and at the prime focus of the reflecting surface.
  • the emitting element may be of any convenient form appropriate to the radiation involved, and no more need be said about it here.
  • the rear antenna is a Cassegrain antenna, having its emitting element at the Cassegrain focus, the position of which is determined generally by the nature and position of the secondary reflector.
  • the secondary reflector may in general be of any convenient shape and size - it may be flat or convex or concave (towards the primary reflector), and it may extend across either the full aperture of the primary reflector or only a small central area of the aperture. In the latter case it may be totally opaque to the radiation involved, but in the former case it must naturally be reflectively opaque to the emitting element side radiation but transparent to the target side radiation (the radiation transceived).
  • the full aperture type of secondary reflector is preferred, and, as explained in more detail hereinafter, a particularly preferred variety transparent to the radiation transceived is a parallel wire reflector.
  • the emitting element of the Cassegrain antenna may be of any convenient form appropriate to the radiation involved, and no more need be said about it here.
  • the inventive antenna assembly requires that the reflective surface of the direct antenna, which is mounted in front of the Cassegrain antenna, be transparent to the radiation transceived by the Cassegrain antenna. While it is conceivable that this requirement could be met in other ways, it is presently the intention to meet it by constructing the direct antenna's reflector so that it is polarity selective, and then operating the antenna assembly in such a way that the radiation transceived by the direct antenna is of such a polarity that it is reflected while the radiation transceived by the Cassegrain antenna is of such a polarity that it is transmitted.
  • the direct antenna's reflector is constructed in essence of appropriately spaced parallel conductive elements (wires or strips), so as to reflect radiation plane polarised in a direction aligned with the elements but transmit radiation plane polarised transverse to the elements, and in operation the reflector is so disposed that its elements are horizontal, then if the direct antenna is operated so as to transceive horizontally plane polarised radiation while the Cassegrain antenna is operated so as to transceive vertically plane polarised radiation the direct antenna's reflector will reflect the former but transmit the latter.
  • the Cassegrain antenna may have a secondary reflector extending across only a small part of the aperture of the primary reflector, in which case the primary reflector may be quite opaque to the radiation transceived by the Cassegrain antenna (for it will block only a small part of that radiation).
  • the secondary reflector extends across the full aperture of the primary reflector, in which case it too, like the direct antenna's reflector, must be transparent to the radiation transceived by the Cassegrain antenna - and yet at the same time it must, of course, be reflective to the radiation on the emitting element side of the primary reflector.
  • the Cassegrain antenna's secondary reflector so that it has a polarity sensitivity matching that of the direct antenna's reflector, and constructing the primary reflector so that it is a polarity converter (changing the polarity of radiation reflectable by the secondary reflector to a mode to which the secondary reflector is transparent), and then operating the assembly so that each antenna's emitting element transmits or receives radiation of the same polarity mode, and the Cassegrain antenna's primary reflector changes that mode into one to which both the secondary reflector and the direct antenna's reflector are transparent.
  • the Cassegrain antenna's primary reflector can be a twist reflector, while its secondary reflector can be constructed in essence of appropriately spaced parallel conductive elements (wires or strips) aligned with and generally similar to those from which the direct antenna's reflector can be constructed.
  • the direct antenna is to be operated so as to transceive horizontally plane polarised radiation
  • its reflector will be reflective to that and transparent to vertically plane polarised radiation
  • the radiation transceived by the Cassegrain primary reflector will be vertically plane polarised
  • the secondary reflector will be transparent to that but reflective to horizontally plane polarised radiation
  • the Cassegrain antenna's emitting element will transceive horizontally plane polarised radiation which, after reflection from the secondary reflector, is twisted by the Cassegrain primary reflector into the vertical mode.
  • Reflectors that are selectively reflective/transparent to plane polarised radiation, or that twist the plane of such radiation, are well known. Such reflectors are discussed in detail in "Microwave Antennae derived from the Cassegrain Telescope", P.W. Hannan, IRE Transactions, Antennae and Propogation, Vol. AP-9, pp 140-153, March 1961.
  • a typical parallel wire reflector might comprise a rigid sandwich of a plastics foam filling with front and back plastics skins, having laid in the front skin a multitude of fine parallel wires - typically 6000 gauge (about 0.1 mm) at a spacing of roughly ten times their gauge or 5% of the wavelength to be transmitted; the higher the frequency (shorter the wavelength) the thinner and closer the wires.
  • the actual dimensions of the sandwich, including its thickness, are matched to the frequencies/wavelengths involved.
  • a typical "twist" reflector is very similar, except that it has a reflective back skin spaced X/4 from the wires, and in operation in a system using vertically and horizontally plane polarised radiation it is so arranged that the wires are at 45° to each polarisation plane.
  • the antenna assembly of the invention is of particular value as part of a high power radar system.
  • a problem with the earlier - and otherwise quite satisfactory - "dual" signal systems of the sort wherein there is used a single antenna with one emitting element at the principle focus and a second at the Cassegrain focus is that of necessity there must be a secondary reflector which is in the way of - and inevitably absorbs some of - the radiation emitted by both elements.
  • the assembly of the invention reduces power absorption in the secondary reflector by a very considerable proportion, for one of the radiated signals - that from the front, direct, antenna - never passes through it at all.
  • the inventive antenna assembly shown in Figure 1 comprises a closely-spaced tandem arrangement of direct antenna at the fore and Cassegrain antenna at the rear.
  • the Cassegrain antenna has a parabolic primary reflector . (11), a-secondary reflector (12) and, at the Cassegrain focus, an emitting element (13); the secondary reflector 12 is a small convex one, not covering the full aperture of the primary reflector 11, and thus is reflectively opaque.
  • the direct antenna has a parabolic reflector (14) in the form of many horizontally-disposed parallel wires (as 15) embodied within a radiation-transparent support structure (generally 16; see Figure 6), with an emitting element (17) placed at its principal focus.
  • the rear antenna's emitting element 13 is radiating vertically plane polarised “radar” waves (V) to which its own secondary and primary reflectors (12,11) are reflectively opaque, and to which the fore antenna's reflector 14 is transparent, while the fore antenna's emitting element 17 is radiating horizontally plane polarised “radar” waves (H), to which its own reflector 14 is reflectively opaque.
  • Figure 2 there is shown a similar system except that the fore antenna's reflector (21) is now a parabolic vertical strip reflector, and reflects vertically plane, polarised from/to an "off axis" emitting element (22), while the rear (Cassegrain) antenna now has a concave full aperture secondary reflector 23 (which, like the fore antenna's reflector 21, is a vertical strip reflector), and its primary reflector (24) is now a twist reflector (see Figure 5).
  • the fore antenna's reflector (21) is now a parabolic vertical strip reflector, and reflects vertically plane, polarised from/to an "off axis" emitting element (22), while the rear (Cassegrain) antenna now has a concave full aperture secondary reflector 23 (which, like the fore antenna's reflector 21, is a vertical strip reflector), and its primary reflector (24) is now a twist reflector (see Figure 5).
  • the rear antenna's emitting element 13 radiates vertically plane polarised “radar” waves (V) which are reflected off the vertical strip secondary reflector 23 onto the primary, twist, reflector 24, from which they are reflected in turn but as horizontally plane polarised waves (H). As such, they pass through the fore antenna's reflector 21, which appears to be transparent.
  • the fore antenna's emitting element 22 is also emitting vertically plane polarised “radar” waves (V), and these are reflected off its reflector 21.
  • FIG. 3 shows an antenna assembly like that of Figure 2 except that the former's fore antenna has a "cylindrical" parabolic reflector (31) - some of the vertical wires (as 32) have been drawn in - and a line emitting element (33) rather than the "spherical" reflector 21 and point emitter 22 of the latter.
  • Figure 4 has a rear antenna like that of Figure 2 and a fore antenna like that of Figure 1 (it should be emphasised that Figure 4 is a horizontal section, and that the fore antenna's emitting element is radiating vertically plane polarised "radar" waves). It needs little separate discussion, except to point out that the illustrated assembly is enclosed within an integral "radome” (with side and front portions 41,42,43).
EP82306095A 1981-11-19 1982-11-16 Combinaisons d'antennes Withdrawn EP0080319A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8134828 1981-11-19
GB08134828A GB2110003B (en) 1981-11-19 1981-11-19 Antenna assemblies

Publications (1)

Publication Number Publication Date
EP0080319A1 true EP0080319A1 (fr) 1983-06-01

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EP82306095A Withdrawn EP0080319A1 (fr) 1981-11-19 1982-11-16 Combinaisons d'antennes

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EP (1) EP0080319A1 (fr)
GB (1) GB2110003B (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998049750A1 (fr) * 1997-04-29 1998-11-05 Era Patents Limited Antenne torsade a reflecteur
WO2000028622A1 (fr) * 1998-11-12 2000-05-18 Raytheon Company Antenne cassegrain a balayage electronique avec reflecteur secondaire/radome pleine ouverture

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4769646A (en) * 1984-02-27 1988-09-06 United Technologies Corporation Antenna system and dual-fed lenses producing characteristically different beams
GB8628553D0 (en) * 1986-11-28 2004-11-10 Marconi Co Ltd Radar system

Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE582007C (de) * 1933-08-07 Ernst Gerhard Dr Anordnung zur Aussendung von mehreren voneinander unabhaengigen und verschiedenen Strahlenkegeln elektrischer Wellen
GB758957A (en) * 1954-03-23 1956-10-10 British Thomson Houston Co Ltd Improvements relating to ultra-high frequency aerials
FR1214296A (fr) * 1958-10-29 1960-04-07 Thomson Houston Comp Francaise Nouvelle antenne pour ondes ultra-courtes
US2990544A (en) * 1957-04-30 1961-06-27 Hazeltine Research Inc Radar antenna system providing improved resolution
US3049708A (en) * 1959-11-20 1962-08-14 Sperry Rand Corp Polarization sensitive antenna system
US3271771A (en) * 1962-02-15 1966-09-06 Hazeltine Research Inc Double-reflector, double-feed antenna for crossed polarizations and polarization changing devices useful therein
GB1048354A (en) * 1964-03-09 1966-11-16 Csf Frequency changer system for multiple channel receivers
US3820116A (en) * 1972-04-24 1974-06-25 Ericsson Telefon Ab L M Double reflector antenna with polarization rotating main reflector
US3898667A (en) * 1974-02-06 1975-08-05 Rca Corp Compact frequency reuse antenna

Patent Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE582007C (de) * 1933-08-07 Ernst Gerhard Dr Anordnung zur Aussendung von mehreren voneinander unabhaengigen und verschiedenen Strahlenkegeln elektrischer Wellen
GB758957A (en) * 1954-03-23 1956-10-10 British Thomson Houston Co Ltd Improvements relating to ultra-high frequency aerials
US2990544A (en) * 1957-04-30 1961-06-27 Hazeltine Research Inc Radar antenna system providing improved resolution
FR1214296A (fr) * 1958-10-29 1960-04-07 Thomson Houston Comp Francaise Nouvelle antenne pour ondes ultra-courtes
US3049708A (en) * 1959-11-20 1962-08-14 Sperry Rand Corp Polarization sensitive antenna system
US3271771A (en) * 1962-02-15 1966-09-06 Hazeltine Research Inc Double-reflector, double-feed antenna for crossed polarizations and polarization changing devices useful therein
GB1048354A (en) * 1964-03-09 1966-11-16 Csf Frequency changer system for multiple channel receivers
US3820116A (en) * 1972-04-24 1974-06-25 Ericsson Telefon Ab L M Double reflector antenna with polarization rotating main reflector
US3898667A (en) * 1974-02-06 1975-08-05 Rca Corp Compact frequency reuse antenna

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998049750A1 (fr) * 1997-04-29 1998-11-05 Era Patents Limited Antenne torsade a reflecteur
WO2000028622A1 (fr) * 1998-11-12 2000-05-18 Raytheon Company Antenne cassegrain a balayage electronique avec reflecteur secondaire/radome pleine ouverture
US6150991A (en) * 1998-11-12 2000-11-21 Raytheon Company Electronically scanned cassegrain antenna with full aperture secondary/radome

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Publication number Publication date
GB2110003A (en) 1983-06-08
GB2110003B (en) 1985-03-13

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Inventor name: HURD, GEORGE ALAN

Inventor name: WHELLENS, JAMES MATTHEW JOHN