EP0421757A2

EP0421757A2 - Microwave antenna

Info

Publication number: EP0421757A2
Application number: EP90310821A
Authority: EP
Inventors: Hari Jairam
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1989-10-04
Filing date: 1990-10-03
Publication date: 1991-04-10
Also published as: GB8922377D0; EP0421757A3; GB2236626B; GB2236626A; GB9021481D0

Abstract

A microwave horn antenna providing wide beam-width of approximately 95 degrees over a band width of 8 to 18GHZ. The antenna consists of a radiating cylinder (1) the front end of which is shaped to provide a central apex or ridge (6) with two faces sloping back from it. A conductor wire (5) extends to provide this ridge across the cylinder (1). Further matching wires (9, 11) extend across behind the ridge conductor (5). A choke ring (19) surrounds the radiating aperture, consisting of a plurality of concentric channels (21) of graded depth, this being one quarter wavelength at frequencies throughout the band. A wide beam, broad band antenna of high performance is thus provided.

Description

This invention relates to a microwave antenna particularly but not exclusively for use on an aircraft performing ground surveillance by radar. In such an application it is desirable that the antenna should have limited elevation beam width but wide azimuth beam width since the antenna would normally be mounted underneath the aircraft and directed ahead, perhaps with a small declination.
Linearly polarized circular horn antennas are known to give a maximum 3dB beamwidth in the E and H planes of approximately 70 degrees when operating near the cut-off frequency of the circular waveguide feed. While such a beamwidth would be satisfactory in many applications, if this figure is calculated for the low frequency end of a broad band system, of say 8 to 18 GHz, the beamwidth at the upper end would be unacceptably low, say below degrees.
Attempts have been made to broaden the beamwidth by loading the edges (azimuth-wise) of a cylindrical aperture with dielectric material, but this is difficult to shape and locate accurately. Other attempts have been made involving machining diametrically opposite slots extending axially from the edge of a cylindrical aperture. None of these designs has been entirely satisfactory for reasons of either, poor matching, difficulty of implementation, inadequate beam widening, or a combination of these.
An object of the present invention is to provide a broadband microwave antenna having a wide beam throughout the band.
According to the present invention a microwave antenna comprises a cylindrical radiator the radiating aperture of which slopes back on both sides of a diameter, and a ridge conductor extending across the radiating aperture along this diameter, the arrangement being such as to widen the radiation characteristic in a transverse axial plane lying transverse to the diameter, in respect of an E-field component parallel to the diameter.
There are preferably included further conductors extending across the radiating aperture parallel to the diameter and in an axial plane containing the diameter, the further conductors being spaced from the ridge conductor by one quarter wavelength successively, at approximately the mid-band operating frequency of the antenna.
The sloping faces of the radiating aperture preferably contain an angle of 60 degrees. A plurality of capacitive studs project from opposite internal surfaces of the radiating aperture towards the axial plane containing the diameter, the studs being spaced apart and of extent such as to widen the radiation characteristic in the transverse axial plane, in respect of an E-field component perpendicular to the diameter.
The antenna preferably comprises an annular choke member closely surrounding the cylindrical radiator, the choke member comprising a plurality of annular concentric channels open to the front of the antenna and centred on the axis of the cylindrical radiator, each of the channels providing a short-circuited quarter-wave stub and the depth of the channels being graded to correspond with the range of operative frequencies, the innermost channel being the shallowest.
One embodiment of a broadband microwave antenna in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a sectional elevation of the antenna radiating aperture and associated choke ring;
Figure 2 is a plan view of the antenna radiating aperture;
Figure 3 is a sectional side elevation of the radiating aperture; and
Figure 4 is an alternative form of choke ring.

Referring to the drawings, the radiating aperture consists of a cylindrical member 1 having a mounting flange 3 on the rear end and bevelled faces on the front end, the bevelled faces having an included angle of 60 degrees. 'Front' and 'rear' here are in relation to the direction of transmission.
The signal transmitted is circularly polarized and the flange 3 is fixed to a polarizer (not shown) which converts a plane (ie linearly) polarized signal into a circularly polarized signal.
Broadening of the beam in the plane of Figure 1, ie the azimuth plane, is achieved by, in effect, splitting the beam into two in respect of a vertically polarized E-field component by means of a conductor 5, a metal post 0.5mm diameter, extending along the apex, ie the ridge, of the bevelled faces of the aperture. The ridge coincides with a diameter 6 of the cylinder 1. The conductor 5 is parallel to the vertical component of the polarized signal and thus forms a boundary to each sloping face of the aperture. The resulting two beams are thus diverged significantly and they jointly constitute a beam of azimuth width in the region of 95 degrees as indicated at 7.
An alternative explanation of its operation is now given.
The effect of bevelling the front end of the radiating aperture is to alter the phase distribution across the aperture. The phase at the azimuth sides 13 now lags the phase at the ridge diameter. This occurs because the phase velocity in the waveguide is greater than the velocity in free space. Hence energy at the centre will be advanced in phase when compared to that at the edges. This lagging phase shift causes the beam to widen. The overall effect is the same as dielectric at the sides of the waveguide (see Figure 2).
Two further conductors 9 and 11 extend across the radiating aperture in the same plane as the conductor 5 but spaced from it by one quarter wavelength successively at centre frequency of 13GHz. The conductors form an impedance transformer in the matching of the radiating aperture to free space. In a practical embodiment of the invention the conductors 5, 9 and 11 could be an integral part of the radiating cylinder 1, machined out of a solid piece with it.
The above described beam widening effects are in respect of the vertical component of the circularly polarized signal. The horizontal E-field component, extending transverse to the conductors 5, 9 and 11 is substantially unaffected by these conductors. Beam widening in respect of the horizontal component is achieved by, in effect, narrowing the aperture. Capacitive studs 15, which may be of screw form for ease of adjustment or of pre-set length, are fitted to extend from the internal wall of the radiating aperture. There are three on each side of the ridge arranged as mirror images. The extent of each stud into the aperture must be less than one quarter wavelength at the highest band frequencies and preferably at 19GHz. Above 19GHz the studs become inductive and have no effect on the beam. 3 millimetres is found to be satisfactory. The studs of each set of 3 are mutually spaced at one quarter wavelength at 19GHz.
The studs effectively narrow the aperture in the azimuth plane and thus broaden the beam.
Broad band operation is enhanced by axial ridges 17 in known manner. The tapered ends of these ridges taper over a distance of one quarter wavelength at 18GHz, the upper frequency.
As so far described, a significant amount of radiated energy would spill over the edge of the radiating aperture and increase the side lobes of the radiation pattern relative to the main lobe. Such spillover energy is considerably suppressed by a novel form of choke which is fitted around the radiating aperture 1 as shown in Figure 1. The choke is an annular member 19 shown in Figure 1 and partially in Figure 3. This choke ring is of course metallic and consists of a thick 'disc' in the front face of which 7 annular concentric channels 21 are formed. The depth of these channels increases linearly with the radius in such manner that it is one quarter wavelength at 186Hz in the innermost channel and at 8GHz in the outermost channel.
At 18GHz the innermost channel is one quarter wavelength deep, the remainder are increasingly deeper and thus increasingly capacitive. The signal phase is thus advanced progressively from centre to outside. Any 'spillover' energy thus tends to be driven into the main beam.
At 8GHz the outermost channel is one quarter wavelength deep and the inner, shallower, ones are progressively inductive. The signal phase is thus retarded progressively towards the centre, again therefore tending to drive 'spillover' energy into the main beam.
In Figure 1 it can be seen that the gradation of channel depth is achieved by a plane front face and a conical contour to the bottoms of the channels. An alternative form of the choke ring is shown in Figure 4 in which design the bottoms of the channels lie on a plane and the front face of the channels lies on a conical contour. The same effect of graded frequency impedances is achieved but there is a slight difference in the effect on the form of the main lobe. In Figure 1 the effect is to increase the gain of the main lobe while the Figure 4 design tends to widen it by approximately 7.5 degrees at high frequencies. This occurs because the current is allowed to flow at the sides initially and then 'choked' by the quarter wave stubs. This small current flow tends to enhance the beamwidth at high frequency. The choke ring of Figure 4 represents the best option for constant wide beam performance. Both designs however, achieve considerable suppression of the side lobes. The choke ring is a close fit on the radiating aperture and is fixed in position as shown in Figure 1 so as just to expose the lowermost slopes of the bevelled surfaces.
The result of the above design is to produce an antenna operative over 8 to 18GHz with a VSWR of only 2:1, an azimuth 3dB beam width of 95 degrees plus/minus 7.5 degrees over the band, an average gain of 3.5dB and sidelobes better than 25dB down on the main lobe over the band.
It will be appreciated that where specific dimensions and frequencies are stated a small departure from the specified value will merely degrade the performance slightly.

Claims

1. A microwave antenna comprising a cylindrical radiator (1) the radiating aperture of which slopes back on both sides of a diameter (6), and a ridge conductor (5) extending across the radiating aperture along said diameter (6), the arrangement being such as to widen the radiation characteristic in a transverse axial plane lying transverse to said diameter (6), in respect of an E-field component parallel to said diameter (6).

2. An antenna according to Claim 1, including further conductors (9, 11) extending across the radiating aperture parallel to said diameter (6) and in an axial plane containing said diameter (6), said further conductors (9, 11) being spaced from said ridge conductor (5) by one quarter wavelength successively, at approximately the mid-band operating frequency (13 GHz) of the antenna.

3. An antenna according to Claim 1, wherein the sloping faces of the radiating aperture contain an angle of 60 degrees.

4. An antenna according to any preceding claim for use in circularly polarised transmission, wherein a plurality of capacitive studs (15) project from opposite internal surfaces of said radiating aperture towards the axial plane containing said diameter (6), said studs (15) being spaced apart and of extent such as to widen the radiation characteristic in said transverse axial plane, in respect of an E-field component perpendicular to said diameter (6).

5. An antenna according to any preceding claim, and comprising an annular choke member (19) closely surrounding said cylindrical radiator (1), said choke member (19) comprising a plurality of annular concentric channels (21) open to the front of the antenna and centred on the axis of the cylindrical radiator, each of said channels (21) providing a short-circuited quarter-wave stub and the depth of the channels being graded to correspond with the range of operative frequencies, the innermost channel being the shallowest.

6. An antenna according to Claim 5, wherein the front openings of said channels (21) lie in a plane and the bases of the channels (21) lie on a conical surface.

7. An antenna according to Claim 5, wherein the front openings of the channels (21) lie on a conical surface and the bases of the channels (21) lie in a plane.