GB2476787A - Microwave antenna - Google Patents

Abstract

A circular horn antenna having a wide beam and an extended bandwidth of 6-18 GHz, for use with signals having orthogonal polarization planes. The horn 15 is required to adapt to a standard square polarizer having differential delay ridges and is itself provided with corresponding metal ridges 18 which provide a step change in the internal ridge face at the polarizer horn interface. A good match is thus achieved. The good match is continued by tapered interfacing between the horn ridges and a dielectric ridged plug 25 followed by a solid dielectric plug 21. A wide beam angle is provided by an annular dielectric lens 27 mounted on the mouth of the horn the front face of which is slotted, concentrically and radially, to provide circular symmetry of the beam vis-a-vis azimuth and elevation beamwidth, while also providing a good match to space above 12 GHz.

Description

S

Microwave Antenna This invention relates to microwave antennas.

More particularly the invention relates to microwave antennas having a wide beamwidth in both azimuth and elevation and being operable over a wide frequency band. Such antennas find application, for example, in airborne ground surveillance radar systems where the antenna is mounted on the nose of the aircraft and directed ahead of the aircraft.

One object of the present invention is to provide a novel form of microwave antenna which has a significantly increased bandwidth relative to known antennas.

A further object of a preferred embodiment of the invention is to provide improved matching to free space and thus improved return loss performance.

According to the present invention, a microwave horn antenna, for use with signals having orthogonal polarization plane components, comprises a circular horn adapted to interface with a square polarizer having axial metallic ridges providing differential polarization plane phase shifting, the horn comprising corresponding metallic ridges adapted to abut the polarizer ridges, and the internal diameter of the horn ridges being greater than the corresponding dimension of the polarizer ridges.

The horn ridges preferably have the same radial dimension as the polarizer ridges so that the horn diameter exceeds the side dimension of the square polarizer.

The horn ridges preferably have a constant rectangular form from the polarizer horn interface throughout an axial extent of at least one-quarter wavelength at the highest operating frequency, the horn ridges then tapering down to zero radial dimension.

The antenna preferably includes matching means comprising a respective dielectric ridge for each of the horn ridges, each such dielectric ridge extending radially outward from the axis of the antenna to form a triangle which conforms to the tapered surface of a respective one of the horn ridges, the horn between the dielectric ridges and the horn mouth being fitted with dielectric material.

Each dielectric ridge may comprise part of a comon unitary element of conical form, the base of the cone so formed facing towards the mouth of the horn and abutting a cylinder of dielectric material. The antenna preferably includes a dielectric lens mounted on the mouth of the horn for increasing the beanMidth.

The dielectric lens may comprise a dielectric annulus having an internal shoulder in which the mouth of the horn fits snugly.

The radiating end face of the lens may be substantially planar and incorporate slots providing improved circular symetry of the antenna beam. These slots may be radial slots extending from the central hole to the outermost edge, and provide improved matching between the horn and space for signal polarizations transverse to the respective scanning planes at frequencies greater than an octave above the low frequency limit of the antenna bandwidth. Again, the slots may include at least one concentric annular slot providing improved matching between the horn and space for polarizations aligned with the respective scanning planes at frequencies greater than an octave above the low frequency limit of the antenna bandwidth.

One embodiment of microwave antenna according to the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is an exploded perspective view of an antenna horn and associated components; Figure 2 is a sectional view, on a longitudinal axis, of the assembled antenna; Figures 3 and 4 are sectional views on the lines 111-111 and IV-IV of Figure 2; Figure 5 is a view of the radiating end of the antenna; and Figure 6, (a) and (b), shows, very diagrammatically an axial view of an antenna beamwidth for different polarizations.

Figure 1 shows the components of the antenna horn, which is supported by a metal plate 1. This mounting plate is fixed to a corresponding plate 3, shown in Figures 2 and 3, of a polarizer 5, which is intimately associated with the antenna and to a large extent determines its design.

The antenna transmits (and in some applications may receive) signals having electric field components in both horizontal and vertical planes. Circularly polarized signals in space must be derived from (or must be converted to) plane polarized signals in the transmitter receiver associated with the antenna. This requires a 900 differential phase shift between the horizontal and vertical components and it is the function of the polarizer 5 to provide this shift. The polarizer consists of a square metallic waveguide 7 having longitudinal metal ridges 9 fixed centrally along each internal face.

One pair of opposing ridges 9 are plain while the other pair, ie in the orthogonal plane through the waveguide axis, are corrugated.

Components in the corrugated ridge plane' are thus delayed relative to those in the orthogonal plane and the length of the polarizer is chosen to produce a re1ative delay of 90°.

Such polarizers are well known. The present invention is concerned with the design of the antenna horn when the horn is required to couple to a standard polarizer of the above kind and is also required to be circular to obtain, as far as possible, rotational symetry of the radiation characteristic, ie the same radiation pattern in all axial planes.

A requirement of the presently described antenna is that the bandwidth should be extended downwards, from a band of 8-18 GHz that has been obtained from existing antennas, to 6-18 GHz.

Such extension might be expected to require an increase in the transverse dimensions of the polarizer and antenna but this possibility is prevented where an existing polarizer of fixed dimensions has to be employed. The square polarizer being considered has an internal side dimension of 19.9 millimetres which cannot be increased. The ridges have a radial depth of 5 millimetres.

In the design now described, the Internal diameter of the cylindrical metallic horn Is set at 22 millimetres. Metallic ridges 13 are provided in the circular horn 15 to correspond with and abut against those (9) in the polarizer. The ridges 13 are also of radial depth 5 millimetres and thus, when the horn is assembled against the polarizer and correctly rotated relatively, there is a step at the horn/polarizer interface 17, between the internal faces of the ridges 9 and ridges 13, as shown in Figure 2.

The ridges 13 improve the match at the interface 17, and they themselves provide a match into the horn by being tapered down to zero radial depth on their horn ends.

The ridges 13 may be formed integrally with the cylindrical horn 15, eg by extrusion and machining, or they may be fixed on to it by brazing. The inner face 19 of these ridges 13 has an axial length which is at least one quarter wavelength at the upper limit of the frequency band, ie 18 GHz. A length of 5 millimetres is suitable.

The long, ie outer, face of each ridge may suitably be 20 millimetres long, giving a taper angle of about 18°.

A cylindrical block of dielectric material 21, PTFE, fills the upper part of the horn 15. This contributes significantly to the increase in the bandwidth referred to above. Matching between the tapered ridges 13 and the dielectric cylinder 21 is provided by a generally conical, dielectric (PTFE), plug-like element 23 shown in elevation (fe not In section) in Figure 2 and in section in Figure 3.

It may be seen as four triangular ridges 25 extending radially outwards from the antenna axis. The angle of the cone (or of each triangle) is such that the sloping faces of the dielectric ridges conform to the tapered faces of the metal ridges 13.

* The dielectric cylinder 21 abuts against the base of the conical plug 23 and good matching between ridges 13 and cylinder 21 is provided.

Mounted on the outer end, the mouth of the horn antenna, is a dielectric lens 27, also of PTFE, shown in section in Figure 2 and in end view in Figure 5. This lens produces a widening of the beam (ie the radiation characteristic) to give a beam angle of about 900 at the 3dB points. The lens is 40 millimetres outside diameter, 16 millimetres inside diameter, 20 millimetres axial length and has an internal shoulder in which the horn cylinder 15 fits snugly to a depth of 10 mIllimetres.

The basic dielectric lens 27 inherently reacts differently to horizontally and vertically polarized signal components.

Considering a radiation pattern in an azimuth axial plane (Figure 6(a)), the beam pattern is widened for vertically polarized components (ie components transverse to this azimuth, beam, plane) as compared with its response to horizontally polarized components, because of the inherent nature of the characteristics of the circular horn.

A simple dielectric lens, without the grooves shown in Figure 5, would thus produce a departure from the desired rotational symmetry for both polarization components since, in the azimuth plane, Figure 6(a), the vertical component is transverse to the plane so that the vertical polarization' beam is widened, whereas, in the elevation plane, Figure 6(b), the horizontal component is transverse to the plane so that the horizontal polarization' beam is widened.

These effects are particularly apparent above about 12 GHz, ie the octave of the lower band limit.

Apart from this problem -of loss of rotational syrmietry -the lens 27 requires matching to free space to improve the return loss characteristic, ie to level out and reduce the peaks in the characteristic. The two problems are to a large extent overcome by grooving the radiating surface of the lens with, either, concentric equi-spaced grooves (or a single groove), or with radial grooves, or a combination of the two as shown in Figure 5. Slots generally do of course tend to reduce beamwidth. However, radial slots tend to reduce the beaim',idth for polarization transverse to the scanning plane, * eg for vertical polarization in the azimuth.plane, to a much greater extent than they affect the beamwidth for polarization aligned with the scanning plane. (Scanning' plane does not imply that there is any physical scanning of the antenna -it merely indicates azimuth or elevation.) In addition, radial slots improve the matching considerably, and particularly above 12 GHz, for polarization transverse to the scanning plane (eg for vertical polarization in the azimuth plane).

On the other hand, circular slots tend to reduce the beamwidth for polarization aligned with the scanning plane and have little effect on transverse polarization bea,mndth. The converse matching situation also applies in that good matching is achieved for aligned polarization above 12 GHz.

The arrangement adopted in the present embodiment to take best advantage of these various effects is a combination of eight radial slots 29, ie spaced at 450, and one concentric slot 31, as shown in Figure 2. These slots have a width of 2 millimetres and a depth of 5 millimetres.

Claims (11)

1. CLAIMS1. A microwave horn antenna for use with signals having orthogonal polarization plane components, the antenna comprising a circular horn adapted to interface with a square polarizer having axial metallic ridges providing differential polarization plane phase shifting, said horn comprising corresponding metallic ridges adapted to abut the polarizer ridges, and the internal diameter of the horn ridges being greater than the corresponding dimension of the polarizer ridges.
2. 2. An antenna according to Claim 1, wherein the horn ridges have the same radial dimension as the polarizer ridges so that the horn diameter exceeds the side dimension of the square polarizer.
3. 3. An antenna according to Claim 1 or Claim 2, wherein the horn ridges have a constant rectangular form from the polarizer/horn interface throughout an axial extent of at least one-quarter wavelength at the highest operating frequency, the horn ridges then tapering down to zero radial dimension.
4. 4. An antenna according to Claim 3, including matching means comprising a respective dielectric ridge for each of said horn ridges, each said dielectric ridge extending radially outward from the axis of the antenna to form a triangle which conforms to the tapered surface of a respective one of the horn ridges, and the horn between the dielectric ridges and the horn mouth being fitted with dielectric material.
5. 5. An antenna according to Claim 4, wherein each said dielectric ridge comprises part of a common unitary element of conical form, the base of the cone so formed facing towards the mouth of the horn and abutting a cylinder of dielectric material.
6. 6. An antenna according to any preceding claim including a dielectric lens mounted on the mouth of the horn for increasing the beam width.

   *
7. 7. An antenna according to Claim 6, wherein the dielectric lens comprises a dielectric annulus having an internal shoulder in which the mouth of the horn fits snugly.
8. 8. An antenna according to Claim 6 or Claim 7, wherein the radiating end face of the lens is substantially planar and incorporates slots providing improved circular symmetry of the antenna beam.
9. 9. An antenna according to Claim 8, wherein said slots include radial slots extending from the central hole to the outermost edge, said radial slots providing improved matching between the horn and space for signal polarizations transverese to the respective scanning planes at frequencies greater than an octave above the low frequency limit of the antenna bandwidth.
10. 10. An antenna according to Claim 8, wherein said slots include at least one concentric annular slot providing improved matching between the horn and space for signal polarizations aligned with the respective scanning planes at frequencies greater than an octave above the low frequency limit of the antenna bandwidth.
11. 11. A microwave dielectric-filled circular horn antenna substantially as hereinbefore described with reference to the accompanying drawings.