US3427621A - Antenna system for a secondary radar installation - Google Patents

Description

Feb. H, 1969 ANTENNA Filed April 24, 1967 H. BRUNNER 3,427,621

SYSTEM FOR A SECONDARY RADAR INSTALLATION Sheet of 4 Fig.3 PRIOR ART CONTROL ANTENNA EIL TRNR .9 I I0 l7 l N j DIRECTIONAL /COUPLER EQ$ ATTENUATOR A\ RECEIVER PULSE GENERATORS nVentOr- Hum B Q fibbo vsegs Feb. M, 193% H. BRUNNER 3,427,621 ANTENNA SYSTEM FOR A SECONDARY RADAR INSTALLAT ION Filed April 24, 1967 Sheet 2 of 4 AMPLITUDE [/PI P 2 3 a, CONTROL ANTENNA INTERROGATION ANTENNA) OMNIDI 2 RECT|ONAL-- \DIRECTIONAL fATTENUATOR M RECEIVE}; F 5 I PHASE SHIFTER '72 9 C PULSE GENERATOR 8 (PULSE GENERATOR lnvemar:

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ANTENNA SYSTEM Filed April 24, 1967 H. BRUNNER 3,427,523

FOR A SECONDARY RADAR INSTALLATION Sheet 3 of 4 CONTROL ANTENNA INTERROGATION 7 ANTENNA) OMNID|- RECTIONAL DIRECTIONAL i ATTENUATOR 24 -//I0 HYBRIDIZER 41 Z PHASE /3C SHIFTER 1030 1090 DIPLEXER 27 25 l 1' ROTARY COUPLING M; 3% 22; M 3 DIPLEXER-\\ PULSE F I GENERATOR 1 30 1 TRANSMITTER RECEIVER Fig 7 Inventor- HaLmi3 M 5 F iqiikovmo s auft H. BRUNNER ANTENNA SYSTEM FOR A SECONDARY RADAR INSTALLATION Filed April 24, 1967 Sheet CONTROL ANTENNA INTERROGATION ANTENNA F I ATTENUATOR k h.l$\ ROTARY RECEIVER PULSE Fig.

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Hu in M m M United States Patent T 31,264 US. Cl. 343100 Int. 'Cl. H04!) 7/10; H01q 21/00 Claims ABSTRACT OF THE DISCLOSURE Antenna system for a secondary radar installation, consisting of an interrogation antenna ad a control antenna, in which the interrogation antenna is a directional antenna whereas the control antenna, the polar diagram of which having an indentation in the main radiation direction of the directional antenna, is mounted to one side of the directional antenna and approximately in the same height as the directional antenna. The control antenna itself consists of an omnidirectional antenna together with a further directional antenna which has the main radiation direction as the interrogation antenna. Said further directional antenna is connected in anti-phase with the omnidirectional antenna via a phase shifter and an adjustable attenuator, in order to produce an indentation in the polar diagram of the omnidirectional antenna.

State of the art and invention The invention relates to an antenna system for a secondary radar installation, consisting of an interrogation antenna and a control antenna.

The invention and the pertinent state of the art is explained in view of the drawings, of which FIG. 1 shows an antenna system for a combined primary and secondary radar installation, the antenna system for the secondary radar installationcomprising an interrogation antenna and a control antenna,

FIG. 2 shows both the polar diagrams of said interrogation and control antennae,

FIG. 3 is a block circuit showing the interrogation and control antennae as well as other circuit elements of an already known secondary radar installation,

FIG. 4-represents as an example a predetermined amplitude-time program of an interrogation pulse and two preliminary pulses as received by the transponders co-operating with the installation according to FIG. 3,

FIG. 5 is a block circuit showingan advantageous eX- ample of the system according to the invention,

FIG. 6 shows a preferred version of an omnidirectional antenna suitable for use in the system according to FIG. 5,

FIG. 7 is a block circuit showing another example of the system according to the invention,

FIG. 8 is a block circuit which is similar to that of FIG. 3, but adapted for use of the sum-difference method known per se.

The interrogation antenna of a secondary radar installation is usually a directional antenna capable of rotating or swinging synchronously with the primary antenna of the radar installation, the interrogation antenna having the same main direction of radiation as the primary antenna. FIG. 1 shows at 1 the primary antenna whose beamed sending and receiving of radar signals is used for locating the target. The figure also shows at 2 the interrogation antenna, which sends out beamed interrogation signals to the transponders mounted in the targets, and receives their answering signals. The two antennae 1 and 2 are rigidly fixed together by the mechanical connections represented symbolically by the double line 3, and the whole rotates on a turntable represented symbolically at 4. In this way the two antennae can swing around or rotate synchronously in azimuth, to the effect that the main axes of the polar diagrams of the two directional antennae are always directed in the same direction.

The polar diagram of the interrogation antenna 2 in the azimuth plane is shown in FIG. 2, in polar co-ordinate representation, by the full line 5. This polar diagram has unavoidable side lobes, as shown in FIG. 2 which could cause an undesired answering of transponders provided in targets in the near neighborhood of the secondary radar installation but not within the instantaneous coverage of the main lobe of the polar diagram 5.

To prevent this disadvantage it is known to use a method called side lobe suppression, which consists in providing a further antenna, in addition to the interrogation antenna, in the form of a control antenna whose typical azimuthal polar diagram is a circular diagram. In order to prevent any suppressing of nearby targets situated in the direction of the main lobe of the polar diagram 5 of the interrogation antenna, the circular diagram is indented, as represented by the broken line 6 in FIG. 2.

FIG. 3 is a block circuit showing the interrogation and control antenna as well as other circuit elements of an already known secondary radar installation (report by Stewart-Warner Electronics, Analysis and Comparison of the Setrin and Stewart-Warner ATC Radar Beacon Interrogation Path Side Lobe Suppression Reflection Suppression Systems, January 1960'). This apparatus has polar diagrams approximately as shown in FIG. 2. In order to produce these polar diagrams there is mounted centrally over the interrogation antenna 2, which is a slot antenna, an omnidirectional antenna 7 fed by a control pulse generator 8. This generator is also connected via a phase shifter 9, an attenuator 10, which can if desired be adjustable, and a directional coupler 11 in the direction of the arrow shown in FIG. 3 to the feed point of the interrogation antenna 2. The phase shifter 9 should be so adjusted that a control pulse from the generator 8 is radiated by the interrogation antenna 2 with a phase shift of relative to the same pulse radiated at the same time by the control antenna, that is to say by the omnidirectional antenna 7. With this arrangement the resulting azimuthal field strength diagram is the control diagram 6 shown in FIG. 2.

The depth of the indentation in this diagram is controllable by means of the attenuator 10, whose attenuation is of the order of magnitude of the difference between the antenna gains of the interrogation and control antenna, that is to say 20 db. Instead of the separate pulse generators 8 and 12 there is often used a common sender and generator.

When the arrangement shown in FIG. 3 is in operation there are radiated before each interrogation pulse P two preliminary pulses P and P The time intervals between the three pulses are predetermined quantities and furthermore all the pulses contain a predetermined coding. The pulses P and P are produced by the pulse generator 12, whereas the pulse P is produced by the control pulse generator 8. The transponders contain means which ensure that the transponder returns a coded reply signal, in reply to an interrogation pulse P only when the transponder has received all three pulses P to P on a predetermined amplitude-time programme, for example as represented in the diagram in FIG. 4. It should however be observed that, although the pulses P and P must differ in amplitude by at least a value of the order of magnitude of 9 db, as received by the transponder after being influenced by the different antenna gains of the interrogation and control antennae, this does not apply to the amplitude differences between these signals as sent by the transmitter. The reply signals from the transponder are received by the interrogation antenna 2 and are then evaluated and/or indicated by the circuit element 13. In the embodiment of the invention shown an amplitude difference of at least 9 db is ensured over an angular range of 7.5

However a disadvantage results from the previously known secondary radar installation as described above, in that the superposition of the control antenna 7 over the interrogation antenna 2 makes it impossible in principle to keep the indentation of the control diagram 6 independent of the angle of elevation. For example, assuming that the phase centres of the interrogation and control antennae arespaced apart by one operational wavelength, at distance which could in practice hardly be shortened, then at an angle of elevation of 30 the result would be a phase difference of between the two pulses v P radiated by the two antennae, whereby below this angle of elevation the indentation of polar diagram 6 is changed into a hump, so that transponders situated in nearby targets cannot be interrogated as long as these targets are within a specific zone about this angle of elevation. Furthermore with this arrangement the control and interrogation antennae are at different heights in the plane of elevations, and consequently their radiation characteristics peak in different ways, and this again impairs the side lobe suppression. A further disadvantage is that in these previously known secondary radar installations the indentation zone in the control diagram 6 in the azimuth plane is too narrow for many practical applications. For example these previously known secondary radar installations are incapable of satisfying the requirements oflicially specified at the present time for the proposed secondary radar installations in the German Federal Republic, in particular these installations cannot guarantee a minimum of 7 aircraft target hits when the antenna is rotating in azimuth at 24 revolutions per minute, the method of operation being the so-called three-mode interlace method (for example (1) recognition, (2) height, (3) civil or military target).

It is not practicable to position the control antenna 7 next to the interrogation antenna 2, instead of over it, with a view of preventing the different peaking, as mentioned above, of the radiation characteristics, because if this is done the phase centres of the two antennae in the azimuth plane become so far apart that the indentation in the azimuth plane of the control diagram would become intolerably narrow.

The object of the present invention is to eliminate these disadvantages in an antenna system for a secondary radar installation comprising an interrogation antenna and a control antenna, the interrogation antenna being a directional antenna having as idetatio in the direction omnidirectional antenna, the polar diagram of the omnidirectional antenna having an indentation in the direction of the main beam of the directional antenna, in order to prevent suppression of nearby targets. The invention is based in recognition of the fact that in order to obtain a sufficiently wide indentation (an indentation which is not too narrow) in the control diagram 6 in the azimuth plane, it is necessary to position the phase centres of the control and interrogation antennae close together in the azimuthal plane and furthermore to use a fairly wide polar diagram of the interrogation antenna, which must be capable of being connected in anti-phase with the control antenna.

This problem is solved by the invention by arranging the control antenna to one side of the directional antenna, by the control antenna itself consisting of an omnidirectional antenna together with a further directional antenna, which has the same main radiation direction as the interrogation antenna, and by said further directional antenna being connected in anti-phase to the omnidirectional an tenna by means of a pulse shifter and an adjustable at- 4 tenuator, in order to produce the indentation in the polar diagram of the omnidirectional antenna.

Advantageous embodiments of the invention will now be described in greater detail with reference to FIG. 5. In all the figures corresponding parts have been given the same index numbers, and these parts will not be described several times.

FIG. 5 shows an arrangement which is similar to that of FIG. 3 but modified according to the present invention, so that the disadvantages of the arrangement of FIG. 3 are eliminated. In this circuit the directional coupler 11 is omitted and instead the control antenna is positioned next to the interrogation antenna 2. The control antenna consists of an omnidirectional antenna 7' and an additional directional antenna 1 4, which can be of any of the already known kinds. As indicated by the double lines in FIG. 5, all three antennae 2, 7' and 14 are rigidly fixed together mechanically, so that they all three rotate or pivot together about the vertical central axis of the interrogation antenna 2. The interrogation antenna 2 is however electrically entirely separate from the control antenna (consisting of the antennae 7' and 14), an arrangement which greatly simplifies electrical assembly. The phase shifter 9 should be so adjusted that the signals radiated by the antennae 7' and 14 are opposite in phase. The depth of indentation in the control diagram is here again adjustable by means of the damping member 10.

FIG. 6 shows a preferred version of an omnidirectional antenna particularly suitable for use as the antenna 7 in the arrangement shown in FIG. 5. This omnidirectional antenna consists of an electrically conductive base plate 15 through whose central hole there projects a feed conductor in the form of a coaxial cable having an outer conductor 16 and an inner conductor 17. The outer conductor is centrally enclosed in a tube 18 made of an electrically conductive material, so'that the coaxial cable passes centrally through this tube 18. The outer conductor 16 terminates at the top end of the tube 18. The outer conductor 16 and the tube 1'8 are in electrical contact with each other. Above the tube 18 there is a further electrically conductive tube 19. The mechanical connection between the tube 18 and the tube 19 is provided by a supporting ring 20 made of an electrically insulating material. In this way the base plate 15, the tube 18 and the tube 19 together form an unsymmetrical vertical dipole. The inner conductor 17 is electrically connected to the upper tube 19 by means of an adjustment screw 21. The figure further shows a sealing ring 22 and a radome 23 of the already known kind.

In the arrangement shown in FIG. 5 the directional antenna 14 is a logarithmic periodic antenna. If the indentation in the control diagram is too wide, it is advisable to provide two or more adjacently arranged logarithmic periodic antennae as antenna 14. FIG. 7 shows a preferred version of the invention. This arrangement agrees in its main parts with the arrangement shown in FIG. 5. However, in order to increase the sharpness of the beam of the interrogation antenna this takes the already known form of a series of twenty-eight logarithmic periodic dipoles 2', which are operated by the sum-difference principle in conjunction with a hybrid arrangement of twenty-four sum signals and difference signals A and 2. Sum and difference diagrams are produced by alternately in-phase and anti-phase feeding the two antenna halves of the slot antenna 2' with a hybridizer 24. In order to be able to use only one two-channel rotary coupling 25, there are provided in the version of the invention shown in FIG. 7 two frequency switchpoints (diplexers) 27 and 26. In the figure, the numbers 1030 and 1090 relate to the interrogation frequency 1030 mHz and the responding frequency 1090 mHz.

It should be observed that if it were desired to use the sum-difference method in the case of the previously known arrangement shown in FIG. 3, there would have to be provided a circuit such as that shown in FIG. 8,

and this would require three channels in the rotary coupling 25 instead of the two channels in the rotary coupling 25 of FIG. 7.

What is claimed is:

1. In an antenna system for a secondary radar installation a control antenna mounted by the side of and approximately in the same height as a directional interrogation antenna, said control antenna comprising an omnidirectional control antenna and a directional control antenna having the same radiation direction as the directional interrogation antenna, said directional control antenna being connected to the omnidirectional control antenna in anti-phase via a phase shifter and an attenuator.

2. In an antenna system for a secondary radar installation a control antenna mounted by the side of and approximately in the same height as a directional interrogation antenna, said control antenna comprising an omnidirectional control antenna and a directional control antenna having the same radiation direction as the directional interrogation antenna, said directional control antenna being connected to the omnidirectional control antenna in anti-phase via a phase shifter and an attenuator, 'whereby said omnidirectional control antenna consists of a vertically positioned dipole and a horizontal conductive base plate.

3. In an antenna system for a secondary radar installation a control antenna mounted by the side of and approximately in the same height as a directional interrogation antenna, said control antenna comprising an omnidirectional control antenna and a directional control antenna having the same radiation direction as the directional interrogation antenna, said directional control antenna being connected to the omnidirectional control antenna in anti-phase via a phase shifter and an attenuator, whereby the directional control antenna consists of at least one logarithmic periodic antenna.

4. In an antenna system according to claim 1, said directional interrogation antenna providing a sum and a difference diagram and being connected to a hybird arrangement delivering signals when reception is obtained which signals are evaluated by a circuit for the purpose of narrowing the beam.

5. In an antenna system for a secondary radar installation a control antenna mounted by the side of and approximately in the same height as a directional interrogation antenna, said control antenna comprising an omnidirectional control antenna and a directional control antenna having the same radiation direction as the directional interrogation antenna, said directional control antenna being connected to the omnidirectional control antenna in anti-phase via a phase shifter and an attenuator, whereby a first diplexer is connected to the control antenna on the one hand and to that output terminal and a hybrid arrangement delivering the signals of the difference diagram provided by the directional interrogation antenna on the other hand, and whereby a second diplexer is provided for separating those signals which can be transmitted via a single channel between said two diplexers.

3,213,446 10/1965 Voyner 343- RODNEY D. BENNETT, Primary Examiner. RICHARD E. BERGER, Assistant Examiner.

US. Cl. X.R. 343--6.5

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