GB2314688A

GB2314688A - Hollow waveguide antenna

Info

Publication number: GB2314688A
Application number: GB9613351A
Authority: GB
Inventors: Robert Ian Henderson
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1996-06-26
Filing date: 1996-06-26
Publication date: 1998-01-07
Also published as: GB9613351D0

Abstract

An antenna feed 30 comprises a waveguide 31 and an insert 32 formed from a dielectric material. The insert 32 is fitted into an aperture 33 defined by one end of the waveguide 31. The insert 32 comprises two portions, a first portion 34 having dimensions corresponding with the internal profile of the waveguide 31 and a second portion 35, extending from the end of the waveguide, and having reduced dimensions extending from within the waveguide to the external end of the insert 32. The second section 35 has a coaxial recess 37 extending from an edge and reducing to an apex 38 which terminates in the vicinity of the aperture 33. The arrangement provides reduced cross-polarisation and increased gain characteristics.

Description

ANTENNA FEED ARRANGEMENT This invention relates to an antenna feed arrangement.

At present, an antenna feed arrangement, as used in a splash-plate or Cassegrain type antenna, usually comprises a solid cylinder of dielectric material inserted into a waveguide having a circular cross-section. The cross-sectional diameter of the dielectric insert may remain constant over the length of the insert or alternatively may gradually taper to a reduced diameter over the length of the insert to form a cone.

A radiation pattern produced by this type of antenna feed arrangement typically comprises a forward pointing single lobe with side lobes at a level approximately 20dB below the single lobe peak. The gain of the antenna depends on several factors including the length of the antenna, but the gain is usually in the range of 10 to 20dB.

A disadvantage of this type of antenna is the relatively high level of cross polarisation as measured using Ludwig's 3rd definition.

It is the object of this invention to provide an antenna feed arrangement, with reduced cross polarisation and increased gain characteristics.

According to this invention, there is provided an antenna feed arrangement comprising a hollow waveguide arranged to propagate a high frequency signal along its longitudinal axis towards an aperture defined by one end of the waveguide, and a dielectric insert having a first portion located inside the aperture and a second portion extending from the end of the waveguide and defining a recess extending substantially along the longitudinal axis of the waveguide.

Preferably the recess may be substantially conical with its larger diameter positioned furthest from the waveguide and reducing to an apex within the inset.

An outer profile of the second portion may be substantially constant, or alternatively an outer profile of the second portion may taper inwardly from the aperture towards the larger diameter of the recess.

The outer profile of the second portion may be smaller than the outer profile of the first portion and a surface may define the transition between the first and second portions, the surface may be substantially normal to the longitudinal axis. Preferably, the surface may be positioned inside the aperture to define a space between the profile of the second portion and the internal profile of the aperture or alternatively the surface may be positioned flush with the end of the waveguide.

At least part of the first portion may preferably have substantially the same profile as the internal profile of the aperture so that the insert first portion may abut against the internal profile of the aperture.

The apex of the conical recess may terminate in the plane of the aperture.

The first portion may further include a conical impedance matching portion, situated coaxially within the waveguide, the conical portion may have a taper which increases from a point towards the second portion.

Preferably the dielectric material may be polytetrafluoroethylene.

The waveguide may also include a choke arranged to surround the aperture.

The choke may be formed from a conducting material and may be arranged to fit about the external profile of the waveguide, one end of the choke may terminate in a short circuit within the waveguide, the other end may terminate flush with the end of the waveguide. Preferably, the internal profile of the choke may be larger than the external profile of the waveguide so as to define a space between the choke and the waveguide.

Preferably the waveguide may be substantially circular in cross-section or alternatively the waveguide may be substantially square in cross-section.

The arrangement may be used to illuminate a sub reflector of an antenna system.

Preferably the arrangement may be used within a splash-plate type antenna system or alternatively the arrangement may be used within a Cassegrain antenna system.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an axial cross-section through a known conventional antenna feed with a tapered dielectric insert; Figure 2 illustrates a radiation pattern and associated cross polarisation for a conventional antenna feed of the type shown in Figure 1 provided with a 75mm long tapered dielectric insert with a frequency of interest of 8GHz; Figure 3 is an axial cross-section through a first embodiment of an antenna feed in accordance with the present invention having a conical recessed dielectric insert; Figure 4 illustrates a radiation pattern and associated cross polarisation for the antenna feed shown in Figure 3 and having a 75mm long conical recessed dielectric insert with a frequency of interest of 8GHz; Figure 5 is an axial cross-section through a second embodiment of an antenna feed in accordance with the present invention having a conical recessed dielectric insert with improved retaining feature, and, Figure 6 is an axial cross-section through a third embodiment of an antenna feed in accordance with the present invention having a conical recessed dielectric insert and a quarter wave choke.

Referring to Figure 1, a conventional polyrod type antenna feed 10 for use in an antenna comprises a cylindrical waveguide 11 and a solid cylinder insert 12 formed from a dielectric material. The cross-sectional diameter of the insert 12 can either remain constant over its length or, as depicted in Figure 1, comprise a tapered section 13 which tapers gradually to a point. The insert 12 is fitted into an aperture 14 at one end of the waveguide 11 carrying the lowest waveguide mode TE 1 b the tapered section 13 remaining external of the waveguide 11.

A radiation pattern, as illustrated in Figure 2, measured for a conventional antenna feed 10 as described above, exhibits a forward pointing single lobe 15 of radiated energy and has side lobes 16 and 17 at a level of about 20dB below the peak value of the single lobe 15. The gain of this type of antenna feed 10 is typically in the range of 10-20dB and depends on several factors, the most predominant being the length of the insert 12. The radiation pattern was measured for the antenna feed 10 operating with a frequency of interest of 8GHz using a 75mm length dielectric tapered insert. The radiation pattern was taken using conventional techniques for cross polarisation measurement taken in a plane through the axis of the feed 10 inclined at a 45 degree angle to the plane of polarisation.

A drawback of the conventional antenna feed 10, is that there is a relatively high level of cross polarisation 19 owing to the circular symmetry of the antenna, which when excited by a linearly-polarised TEI, mode, creates no cross-polarised field on axis 20, but two lobes 21 and 22 appear in the +45 planes and in the +135 planes each with maxima near the -lOdB level of the co-polarised field.

The conventional antenna feed 10, as illustrated in Figure 1, exhibits a cross polar level of approximately -28dB relative to the radiation pattern, this is shown as reference line 25 in Figure 2. When this type of antenna feed 10 is used to feed a reflector type antenna system, the cross polar level of -28dB will appear at approximately the same level in the far-field pattern of the antenna.

Figure 3 illustrates an antenna feed 30 comprising a cylindrical waveguide 31 and a generally cylindrical insert 32 formed from a dielectric material, typically polytetrafluoroethylene (PTFE). The insert 32 is fitted into an aperture 33 at one end of the waveguide 31 carrying the lowest waveguide mode TE1 r The insert 32 comprises two portions, a first portion 34 having a diameter corresponding and co-operating with the internal profile of the waveguide 31 such that the insert 32 is retained within the waveguide 31 and a second portion 35 having a reduced diameter extending from within the waveguide 31 to the external end of the insert 32. The first and second portions 34,35 can be formed from one piece of material having a shoulder 36 to defme the interface between their different diameters.

Alternatively, the first and second 34, 35 could be formed from separate pieces of material and joined to define the shoulder 36 between their different diameters.

The shoulder 36 is housed within the waveguide 31 at a distance set back from the aperture 33.

The second section 35 has a coaxial conical recess 37 extending from a circular edge and reducing to an apex 38 which terminates in the plane of the aperture 33.

Figure 4, illustrates the radiation pattern 39 and corresponding cross polarisation pattern 40 for the antenna feed 30 of Figure 3. The operating frequency of interest in this particular design is 8GHz and the conical recessed insert 32 is 75mm in length. The radiation pattern was again taken using conventional techniques for cross polarisation measurement taken in a plane through the axis of the feed 30 inclined at a 45 degree angle to the plane of polarisation.

It will be understood that the frequency of interest could be changed by altering the design parameters of the dielectric insert 32 and the waveguide 31.

The radiation pattern 39 exhibits a forward pointing single lobe 41 of radiated energy and has side lobes 42 and 43. On comparison with the radiation pattern 18 of Figure 2, the width of the single lobe 41 of the radiation pattern 39 is narrower than the lobe 15 of the radiation pattern 18 thereby providing greater directivity of the radiated energy on axis 44.

On comparison with the cross polarisation pattern 19, of Figure 2, the cross polarisation 40 of Figure 4, is greatly reduced within the area about axis 44 to typically 40dB relative to the single lobe 41 peak as shown by reference line 45 in Figure 4. The low cross polarisation characteristic is maintained over a useful frequency range, typically 30%.

Furthermore the apparent phase centre of the antenna, that is the centre of the spherical wavefront in the far-field, remains close to the aperture 33 of the waveguide 31.

The gain of this antenna feed 30 is also increased by about 1.5dB providing a more compact design than the conventional antenna feed 10, as the gain of the conventional antenna feed 10 can be increased by increasing the length of the dielectric insert 12.

In Figure 5 a second embodiment of the present invention illustrates an antenna feed 50 which comprises a cylindrical waveguide 51 and a generally cylindrical insert 52 formed from a dielectric material, typically PTFE. The insert 52 is fitted into an aperture 53 at the end of the waveguide 51 carrying the lowest waveguide mode TE11 The insert 52 comprises two portions, a first portion 54, having a diameter corresponding and cooperating with the internal diameter of the waveguide 51 such that the insert 52 is retained within the waveguide 51, and a second portion 55 having a reduced diameter extending from the plane of the aperture 53 to the end of the insert 52.

The first and second portions 54,55 can be formed from one piece of material having a shoulder 56 to define the interface between their different diameters. Alternatively, the first and second portions 54, 55 can be formed from separate pieces of material and joined to define the shoulder 56 between their different diameters.

The shoulder 56 is housed within the waveguide 51 such that it is aligned with the plane of the aperture 53.

The second portion 55 has a coaxial conical recess 57 extending from a circular edge and reducing to an apex 58 which terminates in the plane of the aperture 53.

This embodiment provides a greater area of contact between the internal surface of the waveguide 51 and the first portion 54 with only a small degradation in the cross polarisation results.

A further improvement to the design can be achieved by the addition of choke about the waveguide. This third embodiment of an antenna feed 60 is illustrated in Figure 6, wherein an insert 61 may be of the design described above with reference to either the first or the second embodiments of the antenna feed 30 or 50.

The choke 62 consists of a concentric conducting tube which is of a slightly larger diameter than the waveguide 63 and surrounds the end of the waveguide 63. The choke 62 is coaxial with the waveguide 63 to terminate in a short-circuit with the waveguide 63 at a depth of approximately one quarter of the wavelength of the frequency of interest, ie. 0.25 A. The choke 62 extends from the short-circuit with the waveguide 63 to alignment with the end of the aperture 64.

The addition of the choke 62 acts to suppress external currents on the waveguide 63 and thereby reduces rearward radiation from the antenna feed 60.

It should be understood that the antenna feed designs (10, 30, 50 and 60) of Figures 1, 3, 5 and 6 illustrate a transition from the dielectrically-loaded waveguide to air-filled waveguide in the form of a simple tapered back cone in the dielectric insert (12, 32, 52 and 61). This is not strictly part of the antenna feed (10, 30, 50 and 60) and merely provides impedance matching between the air filled waveguide (11, 31, 51 and 63) and the dielectric insert (12, 32, 52 and 61). There are other means of launching the TEI, mode, for example, a dielectric load could be used throughout the waveguide (11, 31, 51 and 63) instead of air.

It will be understood that the present invention also applies to square cross section waveguides employing square cross section dielectric inserts and is not limited to circular cross sectional waveguides and associated dielectric inserts. Indeed any shaped cross section waveguide employing a substantially similar shaped cross section dielectric insert can be used.

Claims

1. An antenna feed arrangement comprising a hollow waveguide arranged to propagate a high frequency signal along its longitudinal axis towards an aperture defined by one end of the waveguide, and a dielectric insert having a first portion located inside the aperture and a second portion extending from the end of the waveguide and defining a recess extending substantially along the longitudinal axis of the waveguide.

2. An antenna feed arrangement, as claimed in Claim 1, wherein the recess is substantially conical with its larger diameter positioned furthest from the waveguide and reducing to an apex within the insert.

3. An antenna feed arrangement, as claimed in Claim 1 or 2, wherein an outer profile of the second portion is substantially constant.

4. An antenna feed arrangement, as claimed in Claim 2, wherein an outer profile of the second portion tapers inwardly from the aperture towards the larger diameter of the recess.

5. An antenna feed arrangement, as claimed in any preceding claim, wherein the outer profile of the second portion is smaller than the outer profile of the first portion and a surface defines the transition between the first and second portions.

6. An antenna feed arrangement, as claimed in Claim 5, wherein the surface is substantially normal to the longitudinal axis.

7. An antenna feed arrangement, as claimed in Claim 5 or 6, wherein the surface is positioned inside the aperture to define a space between the profile of the second portion and the internal profile of the aperture.

8. An antenna feed arrangement, as claimed in Claim 5 or 6, wherein the surface is positioned flush with the end of the waveguide.

9. An antenna feed arrangement, as claimed in any preceding claim, wherein at least part of the first portion has substantially the same profile as the internal profile of the aperture so that the first portion abuts against the internal profile of the aperture.

10. An antenna feed arrangement, as claimed in any of Claims 2 to 9, wherein the apex of the conical recess terminates in the plane of the aperture.

11. An antenna feed arrangement, as claimed in any preceding claim, wherein the first portion further includes a conical impedance matching portion, situated coaxially within the waveguide, the conical portion having a taper which increases from a point towards the second portion.

12. An antenna feed arrangement, as claimed in any preceding claim, wherein the dielectric material is polytetrafluoroethylene.

13. An antenna feed arrangement, as claimed in any preceding claim, wherein the waveguide includes a choke arranged to surround the aperture.

14. An antenna feed arrangement, as claimed in Claim 13, wherein the choke is formed from a conducting material and arranged to fit about the external profile of the waveguide, one end of the choke terminates in a short circuit with the waveguide, the other end terminates flush with the end of the waveguide.

15. An antenna feed arrangement, as claimed in Claim 14, wherein the internal profile of the choke is larger than the external profile of the waveguide so as to define a space between the choke and the waveguide.

16. An antenna feed arrangement, as claimed in any preceding claim, wherein the waveguide is substantially circular in cross-section.

17. An antenna feed arrangement, as claimed in any of Claims 1 to 15, wherein the waveguide is substantially square in cros s-section.

18. An antenna feed arrangement, as claimed in any preceding claim, wherein the arrangement is used to illuminate a sub reflector of an antenna system.

19. An antenna feed arrangement, as claimed in any preceding claim, wherein the arrangement is used within a splash-plate type antenna system.

20. An antenna feed arrangement, as claimed in any of Claims 1 to 18, wherein the arrangement is used with a Cassegrain antenna cystem.

21. An antenna feed arrangement substantially as described with reference to any of the accompanying drawings.