GB2165097A

GB2165097A - Biconical antennae

Info

Publication number: GB2165097A
Application number: GB08305478A
Authority: GB
Inventors: Heinz Ludiger
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1982-05-18
Filing date: 1983-02-28
Publication date: 1986-04-03
Also published as: GB2165097B; IT8320225A1; IT1160821B; DE3218690C1; FR2573576B1; FR2573576A1; IT8320225A0

Abstract

A biconical antenna having two cones (1,2) whose tips face one another is provided in the innermost half of the annular horn with one or more coaxial polarisation grids 8, 9, 10, the or each grid comprising a plurality of parallel, narrow conductors which are inclined relative to the axis of symmetry. Preferably the inclination of the conductors of the outermost grid or of the single grid amounts to 45 DEG . The antenna can be used to transmit and/or receive in two widely separated frequency bands, the waves in the lower band being linearly polarised, and those in the upper frequency band possibly being arbitrarily polarised, depending upon the grid configuration. If the conductors of the outermost grid or of a single grid if only one is provided, are inclined by 45 DEG relative to the horizontal plane, the antenna can receive horizontally, vertically, anti-clockwise or clockwise polarised signals in the upper frequency band with a maximum loss of 3 dB. The antenna can be used as a secondary radar transponder antenna for a plurality of frequency bands and for various polarisations. <IMAGE>

Description

SPECIFICATION Biconical antennae The invention relates to biconical antennae of the rotation-symmetrical omnidirectional type, which may be fed via a coaxial line introduced centrally along the antenna axis from one end, the coaxial line inner conductor being connected to the tip of that cone remote from its entry end, and the outer conductor being connected to the tip of the cone through which the line enters, which tip faces the tip of the other cone.

Antennae of this kind, which are also known as biconical horn antennae, are described for example on page 10 of the text book entitled "Microwave Antenna Theory and Design" written by Silver and published by Mac Graw-Hill Book Company in 1949. They can operate with wide band-width over a large frequency range, but serve for only one plane of polarisation, i.e. vertically poiarised electro magnetic waves for an antenna having a vertical axis of symmetry and fed by a co-axial line.

One object of the present invention is to provide a biconical antenna of this kind that is capable of transmitting and/or receiving in two frequency bands spaced apart, using one linear polarisation in the lower frequency band and any other type of polarisation in the upper frequency band. Until now this sort of requirement has only been able to be satisfied by the use of a separate omnidirectional antenna for each frequency band.

In accordance with a preferred embodiment of the invention there is provided a biconical antenna of the omnidirectional rotation-symmetrical type fed via a coaxial line introduced along the axis of symmetry from one end, the inner coaxial conductor being connected to the tip of the cone remote from said one end of the outer conductor being connected to the tip of the cone through which the line enters, and at least one polarisation grid comprising a plurality of narrow, parallel electric conductors inclined to the axis and co-axial therewith to lie between the two cones in the innermost half of the annular horn.

Radiation of waves of the lower frequency band is then uninfluenced, whereas radiation of the waves of the upper frequency band will have a selected or arbitrary polarisation corresponding to the outermost grid, or to the sole grid if only one is present. Therefore an antenna arrangement constructed in accordance with the invention may operate in the manner of two separate omnidirectional antenna for two different planes of polarisation, although the antenna for the upper frequency band is integrated into the antenna for the lower frequency band.

If two grids composed of conductors in a serpentine array are used, a circular polarised radiation can be obtained in the upper frequency band.

If the plurality of polarisation grids are used to produce linear polarisation in the upper frequency band, the conductors of the respective grids expediently exhibit different inclinations such that the inclination towards the axis of symmetry is smallest for the innermost grid and greatest for the outermost grid.

When only one single polarisation grid is used to produce linear polarisation in the upper frequency band, the conductors thereof are advantageously inclined by 45" relative to the axis of symmetry. If a plurality of polarisation grids is used to produce linear polarisation in the upper frequency band, then preferably only the outermost of the grid conductors are inclined by 45" relative to the axis of symmetry.For example it is possible to provide three polarisation grids, where the conductors of the innermost grid are inclined by an angle of substantially 74" relative to the axis of symmetry, the conductor of the central grid by an angle of substantially 58" and the conductors of the outermost grid by an angle of substantially 45 . An antenna of this kind is suitable to transmit only signals polarised in the planes containing the axis of symmetry in the lower frequency band but to receive any arbitrarily selected polarised signals in the upper frequency band, e.g. horizontally, vertically, anticlockwise or clockwise, the 45" inclination of the grid conductors resulting in all these received signals only being subject to a theoretical maximum loss of 3 dB in comparison to an antenna specifically adapted to the particular received signal polarisation.

An antenna designed in accordance with the invention is particularly suited for use as a secondary radar transponder antenna for sending vertically polarised signals in the S-band region, say from 2 to 4GHz and receiving arbitrarily polarised signals in the X-band to Ku -band region, say from 8 to 18 GHz.

The invention will now be described with reference to the exemplary embodiment illustrated in the drawing.

The drawing shows an axial cross-section of a rotation-symmetrical, omnidirectional, biconical antenna for a secondary radar transponder. This antenna is to receive signals in the X-band to Ku band region, and to transmit signals in the S-band region As drawn, the antenna consists of an upper metallic cone 1 and a lower metallic cone 2. The tips of the two cones 1 and 2 face one another.

The inner conductor 3 of a coaxial feed line is connected to the upper cone 1 in the region of its tip, whereas the outer conductor 4 of this coaxial feed line is conductively connected to the lower cone 2 at or adjacent the tip of that cone. Around the vertically extending (as drawn) longitudinal axis of the antenna, which is identical to the axis of symmetry, there are arranged three coaxial cylindrical carriers, 5,6 and 7, which each consist of foam material and whose upper and lower edges match the biconical shaping. These carriers 5, 6 and 7 serf two support three respective polarisation foils 8, 9 and 10, which thus lie concentrically about the longitudinal axis. A fourth carrier ring 11, also of foam material matching the biconical shaping, is provided for stabilisation.The polarisation foils 8, 9 and 10 each comprise parallel, conductive strips, and the strips of the inner most polarisation foil 8 are inclined by approximately 16 relative to the horizontal plane, (74O to the axis of symmetry) the strips of the central polarisation foil 9 are inclined by approximately 32", (58 to the axis of symmetry) and the strips of the outer polarisation foil 10 are inclined by 45 . The polarisation foils 8, 9 and 10 provide that the polarisation of the electro-magnetic waves is rotated by 45 in three stages in the upper frequency region i.e. the X-band to K -band region.The polarisation foils 8, 9 and 10 are only effective in this higher frequency band, and have virtually no influence upon S-band signals. The apertural extension, vertical in the represented exemplary embodiment, amounts to 0.65at, and the cross-section of each of the two cones 1 and 2, horizontal as drawn, amounts to 0.77to, wheres, represent the longest operating wavelength in the S-band. This results in an apertural angle of approximately 80 between the two cones 1 and 2.

The biconical antenna is arranged at a specific distance from a circular metallic base plate 12 on which there is arranged a ring 13 of absorbent material which extends from the edge of the metallic base plate 12 to the outer edge of the lower cone 2, and whose height is such that it lies flush with the outer edge of the lower cone 2. The outer edge of the lower cone 2 is connected via a vertically extending conductor zone 14 to the base plate 12 to define the distance of the biconical antenna from the base plate 12.

The entire antenna is covered by a dielectric protective hood 15.

The antenna illustrated in the exemplary embodiment is able to meet the requirement for vertical polarisation in the S-band and a 45 polarisation in the X- and K, -band. Thus it can be used as a transponder antenna which receives signals in the X band to K, -band region, say from 8 to 18 GHz, and transmits signals in the S-band region, say from 2 to 4 GHz. The received signals can be horizontally, vertically, anti-clockwise or clockwise polarised, as an antenna polarised at 45 receives all these signals with a theoretical maximum loss of 3 dB in comparison to an antenna specifically adapted for the particular polarisation of the received signals.

Claims

1. A biconical antenna of the omnidirectional rotation-symmetrical type fed via a coaxial line introduced along the axis of symmetry from one end, the inner coaxial conductor being connected to the tip of the cone remote from said one end and the outer conductor being connected to the tip of the cone through which the line enters, and at least one polarisation grid comprising a plurality of narrow, parallel electric conductors inclined to the axis of symmetry being arranged around said axis and co-axial therewith to lie between the two cones in the innermost half of the annular horn.

2. A biconical antenna as claimed in Claim 1, in which a plurality of said poiarisation grids is used, and the conductors of the respective grids exhibit manually different angles of inclination, the inclination to the axis of symmetry being greatest for the innermost grid and lowest for the outermost grid.

3. A biconical antenna as claimed in Claim 1, in which there is one single polarisation grid having conductors inclined at an angle of 45 .

4. A biconical antenna as claimed in Claim 2, in which the conductors of the outermost polarisation grid are inclined at an angle of 45".

5. A biconical antenna as claimed in Claim 4, in which there are three coaxial polarisation grids, the conductors of the inner grid being inclined at an angle of substantially 74 relative to the axis of symmetry, the conductors of the central grid are inclined at an angle of substantially 58 , and the conductors of the outer grid are inclined at an angle of substantially 45 .

6. A biconical antenna as claimed in any preceding Claim, in which the or each polarisation grid is in the form of a polarisation foil carrying strip conductors.

7. A biconical antenna as claimed in Claim 6, in which the or each said polarisation foil is arranged between a respective pair of annular carriers of insulating material, shaped to match the biconical formation at their edges.

8. A biconical antenna as claimed in Claim 7, in which said carriers are of a foam material.

9. A biconical antenna as claimed in any preceding Claim, in which an attachment supports the antenna at a specific distance from a metallic base plate.

10. A biconical antenna as claimed in Claim 9, in which the metallic base plate carriers a ring of absorbant material which extends out from the outer edge of the metallic base plate to the outer edge of the lower cone and up from the base plate to the outer edge of the lower cone.

11. A biconical antenna as claimed in any preceding Claim, in which a dielectric protective hood is provided as a cover.

12. A biconical antenna as claimed in any preceding Claim, in which there is an apertural angle of substantially 80 between the two cones.

13. A biconical antenna as claimed in any preceding Claim, in which axial extent of the annular aperture between the outer edges of the two cones is substantially 0.65 times the length of the longest electro-magnetic wave to be transmitted.

14. A biconical antenna as claimed in any preceding Claim, dimensioned for use as a secondary radar transponder antenna for vertically polarised signals in the S-band region and for arbitrarily polarised signals of frequencies from the X-band region to the K, -band region.

15. A biconical antenna substantially as described with reference to the drawing.