GB2105112A - Horn antenna - Google Patents

Description

1 GB 2 105 112 A 1

SPECIFICATION

Horn antenna This invention relates to a horn antenna which has a main beam with a broad beamwidth in the H-plane of the antenna, said beamwidth having a variation with frequency substantially less than the usual nominal inverse proportionality to frequency.

More particularly, the invention relates to a horn antenna wherein, with reference to a cylindrical co-ordinate system with parameters z, cp and r, z representing distance measured parallel to a rectilinear z-axis from a plane normal thereto, cp repre- senting an angle measured aboutthez-axis from a datum and r representing radial distance from the z-axis, the horn has a wide angle of flare about the z-axis in the cp, r plane, said angle being less than 360 degrees, and is bounded by conductive surfaces spaced apart in the z-direction, wherein the aperture of the horn is concave in the cp, r plane as seen from the z-axis, and wherein the antenna comprises means for launching electromagnetic energy into the horn towards the aperture. Since an antenna is reciprocal in nature, said launching means are to be understood to mean additionally or alternatively means for receiving electromagnetic energy propagating in the horn.

Such an antenna may be used in a broad-band direction-finding system comprising a set of N adjacent similar such antennae whose respective main beam axes are spaced at regular angular intervals of (360/N) degrees (normally in azimuth). An R.F. source whose direction relative to the system is to be found may be detected by summing the output signals of all the antennae, and said direction may be established by comparing the magnitudes of the output signals of a suitable pair of adjacent antennae of the set. In order to provide substantially the same probability of detection of an R.F. source for all angles in azimuth and in order to provide optimum accuracy in establishing the direction of thesource it is desirable that the power level of an antenna main beam (relative to its peak level) in a direction corresponding to the main beam axis of an adjacent antenna, i.e. at an angle of (360/N) degrees to its own main beam axis, should lie approximately in the range of -8 dB to -15 dB over the operating frequency range of the system.

An antenna as setforth in the second paragraph of 115 this specification in disclosed in the Applicants' co-pending U.K. Patent Application 8041126 (G.B. 2090068M. In that antenna, electromagnetic energy is launched into the horn towards the aperture (or mouth) of the horn by a rectangular waveguide having a pair of opposed E- plane ridges. In order to obtain a substantially constant beamwidth over an operating frequency range of 3:1 which includes a band of frequencies immediately above the cut-off frequency of the TE30 mode, the ridges are spaced along the waveguide from the throat of the horn: the generation of the TE30 mode by the ridged waveguide is so phased with respect to the horn as to minimise variations of beamwidth with frequency in said band immediately above the TE30 cut-off fre- quency. The antenna is suitable for an abovementioned direction-finding system wherein N=8.

It is an object of the invention to provide an improved horn antenna. It is particularly desirable to provide such an antenna for a broad-band directionfinding system such as set forth in the third paragraph of this specification whereby the system may operate over a greater bandwidth with the same number (i.e. eight) of antennae or, especially, where- bythe system may require fewer antennae while operating over a similar or possibly a greater bandwidth.

According to the invention, a horn antenna as set forth in the second paragraph of this specification is characterised in that the launching means act substantially as a line source coincident with thez-axis and are adapted to launch a substantially pure lowest order radial mode having its electric field in a direction parallel to thez-axis.

Such an antenna may have a flare angle in the cp, r plane substantially greater than 130 degrees, the highest value referred to in the abovementioned U.K. Patent Application, thus providing a substantially greater beamwidth which may be relatively con- stant over a broad frequency range.

It may be mentioned that the known biconical antenna has a H-plane flare angle of 360 degrees and produces an omnidirectional radiation pattern. However, the effect on the radiation pattern over a broad-band of reducing the H-plane angle of flare, for example by introducing two conductive walls extending radially between th cones in planes including the axis of the cones, could not readily be predicted; the beamwidth might reasonably be expected to decrease with increasing frequency as usual.

To assist in obtaining the desired substantially pure mode, said conductive surfaces spaced apart in the z-direction may extend substantially from the aperture to the z-axis, the launching means may be situated substantially at thez-axis, and the extent of the launching means in the T, r plane may be small compared with a wavelength over the the operating fequency range. Alternatively or additionally, the launching means may be substantially rotationally symmetrical about thez- axis over substantially the whole of said angle of flare. The launching means may comprise an electric probe extending substantially along the z- axis. Such a probe inherently acts as a line source in the horn, and is particularly suitable for use with a coaxial line as a transmission line feeder forthe antenna. With this form of launching means, the antenna suitably comprises a cavity of which at least part is radially opposed to the horn and which is such that, over at least the majority of the operating frequency range of the antenna the impedance presented to the launching means; the cavity is substantially greater than the impedance presented by the horn. Electromagnetic energy which is coupled out of the launching means but which initially travels away from the horn aperture may thus be mainly reflected back for radiation at the aperture. The cavity may extend around the z-axis over an angle of substantially 360 degrees minus said angle of flare, so that energy 2 GB 2 105 112 A 2 which does not initially travel towards the aperture may be reflected back towards the launching means.

For each point at the aperture of the horn, the phase length between that point and the launching means may be substantially the same as for all other points at the aperture in the same plane normal to the z-axis, thereby providing a suitable wavefront at the aperture for constant beamwidth over a very broad frequency band. Such performance may be obtained with an embodiment in which alternatively or additionally the aperture is part of a substantially cylindrical surface having thez-axis as its axis of revolution.

To improve the match between the launching means and free space, the spacing parallel to the z-axis of said conductive surfaces may increase progressively with increasing r, suitably exponentially.

Embodiments of the invention will now be de- scribed, by way of example, with reference to the accompanying diagrammatic drawings, in which:Figure 1 is a front view of a horn antenna embodying the invention, the Figure showing the z-axis of a cylindrical co-ordinate system; Figures2 and 3 are cross-sectional view of the antenna respectively in a plane normal to thez-axis and in a plane including thez-axis, Figure 3 being at twice the scale of Figures 1 and 2; Figure 4 is a graph of beamwidths and beam power levels, and Figure 5 shows radiation patterns.

Referring to Figures 1-3, Figure 1 is a front view of a horn antenna looking into the aperture of the horn along the axis of the main beam of the antenna, i.e.

the line defined byz = 0,(f = 0. The plane z = 0 normal to the z-axis is a plane of mirror symmetry of the horn. Figure 2 is a cross-sectional view of the antenna in this plane. Figure 3 is a cross-sectional view of the antenna in a plane including the z-axis at cf = 0. Thez-axis is shown in Figure 3 as well as in Figure 1, and the parameters cf, rfor an arbitrarily selected point at the aperture of the horn are shown in Figure 2.

The horn is bounded by spaced, curved conduc tive surfaces 1, 2 which (as indicated in Figure 2) when projected onto a plane normal to the z-axis each extend over a semicircle centred on the z-axis; as shown in Figures 1 and 3, the spacing between the surfaces in the z-direction increases progressive ly, in this case exponentially, with increasing dis tance from the z-axis. In this embodiment, the horn is further bounded by conductive surfaces 3, 4 which lie in planes including the z-axis at (p = +90 degrees and (f = -90 degrees respectively, i.e. they are coplanar. The horn thus has a flare angle of 180 120 degrees in the cp, r plane (which is the H-plane of the antenna). The portions of the peripheries of the surfaces 1, 2 lying in the range -90'--cp--90' deter mine the shape of the horn aperture which is thus concave in the cp, rplane as seen from thez-axis.

More specifically, in this embodiment said portions of the peripheries are semicircles which have the same radius, which lie in respective planes normal to the z-axis, and which are centred on thez-axis, and the aperture of the horn is thus half the surface of a cylinder having thez-axis as its axis of revolution.

Electromagnetic energy can be supplied to or derived from the horn along a coaxial line feeder 6 centred on thez-axis. To form launching means, the inner conductor 7 of the line extends beyond one end of the line across the gap between the conductive surfaces 1 and 2 and is conductively connected to the surface 2. The other end of the line is connected to an SMA connector 8 (shown in outline).

As shown, the conductive surfaces 1, 2 in this embodiment extend from the aperture of the horn substantially to thez-axis and beyond it to a semiannular cavity 9 radially opposed to the horn. The axial length of the cavity, i.e. its dimension parallel to the z-axis, is chosen to be much greater than the spacing between the surfaces 1 and 2 adjacent the cavity so as to have a much higher characteristic impedance. The radial distance between the short- circuiting back wall 10 of the cavity and the conductor 7 is chosen to be approximately a quarter wavelength nearthe top of the operating frequency range of the antenna.

In operation, the portion of the conductor7 extending between the surfaces 1, 2 constitutes an electric probe for launching electromagnetic energy into the horn towards its aperture, the probe acting as a mode transducer from the TEM mode of the coaxial line 6. Since the probe is located on the axis of rotational symmetry of the horn, has a very small extent in the (f, r plane compared both with the aperture of the horn and with the wavelength throughout the operating frequency range, and extends in the z-clirection, it is particularly suited to launching substantially only the lowest order radial mode having its electric field in a direction parallel to the z-axis and having no variation of electric field in that direction. Radial modes with such electric fields have components E, H(f and H, only, where E, varies with (f as cospef, wherep = (2m - 1) 904 for an aperture extending between (D degrees and m being an integer; for the lowest order radial mode, m It should be noted that while the horn of course has an aperture or mouth, it does not have a distinct throat as found in conventional horns. Such a throat usually both results in a cut-off frequency, at which the width of the throat in the H-plane of the antenna is half a wavelength and below which the horn will not propagate electromagnetic energy in the desired mode, and at higher frequencies constitutes a generator of undesired higher-order modes in the horn. The absence of such a throat is therefore especially helpful in obtaining consistent performance over a broad frequency band, and while the lowest order radial mode in the horn is fairly similar to the fundamental TE10 mode in rectangular waveguide, the antenna does not appear to have a similar cut-off frequency.

A horn antenna of the form shown in Figures 1-3 has been constructed and found to show surprisingly little variation of beamwidth with frequency over a very broad frequency range. Figure 4 is a graph of beamwidths in degrees (left-hand vertical scale) and power levels in dB relative to peak (right-hand vertical scale) against frequency, plotted at intervals 3 GB 2 105 112 A 3 of 2 GHz overthe range of 2-16 GHz. LineA is 3 dB beamwidth, line 8 is 10 dB beamwidth, line Cisthe power level at 9) = 60 degrees (corresponding to N = 6 antennae for the above-mentioned direction finding system), and line D is the power level at (p 72 degrees (corresponding to N = 5). It will be seen that the variations with frequency are far less than would be expected from the usual inverse proportionality, especially between 2 GHz and 14 GHz, i.e. a frequency range of 7: 1. In that range, the 3 dB beamwidth only varies between 78 degrees and 96 degrees, and the power level at 72 degrees lies between -9 dB and -13 dB, so that a very broad band direction-finding system comprising five such antennae would effect a substantial reduction in the required equipment by comparison with a system comprising eight antennae as disclosed in the above-mentioned U.K. Patent Application 8041126, both by reducing the number of channels required to cover the total azimuth of 360 degrees and by reducing the number of sub-systems required to cover a frequency range substantially greater than 3: 1. The radiation patterns of the constructed embo diment were found to be of high quality at each frequency of measurement in the range 2-16 GHz, indicating that a substantially pure lowest order radial mode is launched in the horn.

Figure 5 shows three of these radiation patterns in dB relative to peak power, the circles being spaced at intervals of 10 dB and the radial lines being spaced at 95 intervals of 30 degrees. Line a was obtained at 2 GHz, line b at 8 GHz and line c at 14 GHz. The remarkable similarlity of the patterns over the majority of the forward sector can be seen.

The dimensions of the constructed embodiment 100 were as follows:

the radius of the horn aperture was 100 mm; the spacing of the conductive surfaces 1 and 2 was 2 mm at the launching probe and 70 mm at the aperture, the spacing of each surface from the plane z = 0 of mirror symmetry increasing between r = 3 mm and r = 100 mm according to the function z = exp [0.036653 (r - 3)l; the inner and outer diameters of the semi-annular cavity 9 were 6 mm and 12.5 mm, and its axial length was 20 mm. The spacing of the axial end walls of the cavity was thus 10 times the spacing of the surfaces 1 and 2 at the probe, providing a much higher characteristic impedance, while the increasing separation of the surfaces 1 and 2 towards the aperture of the horn helped to provide a match between the horn at the launching means and free space. The inner and outer diameters of the coaxial line feeder were 1.3 mm and 4.1 mm respectively.

Experiments with a horn antenna of the kind disclosed in the abovementioned U.K. Patent Application 8041126 indicate that the use of an aperture which forms part of the surface of a cylinder rather than a conventional rectangular aperture prevents deterioration of the radiation pattern only towards the bottom of its operating frequency range, e.g. in the case of an antenna having an operating frequen- cy range of 12-10.5 HGz with a cylindrical aperture, only below about 4 GHz. It is therefore thought that the aperture of a horn antenna embodying the present invention may for example be less sharply curved than a cylindrical surface centred on the z-axis and tangential to at least part of the aperture.

Other forms of electric probe launching means may be used. For example, the probe may extend only part of the way across the gap between the conductive surfaces 1 and 2 and may be thickened at its free end to form a -doorknob-. Measurements on an embodiment with such a probe indicate good radiation performance at frequencies up to 18 GHz. Such other forms of probe may introduce a variation into the electric field in the z-clirection.

The cavity 9 is thought to constitute an important factor limiting the operating bandwidth of the antenna. By selecting the spacing between the launching probe and the short-circuiting semi-cylindrical wall of the cavity to be a quarter wavelength at or near the top of the operating frequency range, the cavity will present a very high impedance to the launching probe at the top of the range. However, as the operating frequency decreases and this spacing become a progressively smaller fraction of a quarter wavelength, the impedance presented to the probe by the cavity will progressively decrease. This effect may be mitigated by making the characteristic impedance of the cavity much greater than that of the horn at the launching probe, as indicated above, so that the impedance presented by the cavity to the probe may be greater than that presented by the horn over at least the majority of the operating frequency range, but as the frequency is decreased, the impedance presented by the cavity will inevitably decrease to a small value and constitute a significant mismatch.

An alternative form of launching means is a circular waveguide extending into the horn along the z-axis, the waveguide having an aperture in its cylindrical wall within ffie horn. The aperture may be a slot extending circumferentially over substantially the whole of the angle of flare of the horn, the slot being bounded by a pair of edges in respective cp, r planes spaced by less than the spacing between the bounding conductive surfaces of the horn that are spaced apart in the z- direction. The launching means are then substantially rotationally symmetrical about the z-axis over substantially the whole of the angle of flare (as of course is the electric probe). No cavity radially opposed to the horn is required. The circular waveguide may be terminated in a short-circuit approximately a quarter wavelength beyond the slot, or a mitred bend may be used to effect a smooth transition from the circular to the radial waveguide.

Other forms of launching means may be used, for example a magnetic loop. This may have the advantage that it is suitable for use immediately adjacent a short-circuiting conductive wall perpendi- cular of the plane of the loop, and may therefore alleviate the limitation on the operating bandwidth imposed by a cavity in combination with an electric probe, as described above.

Where energy is to be coupled into or out of the antenna at R.F., the antenna is suitably used with a 4 GB 2 105 112 A transmission line feeder extending from the launching means and supporting a TEM or quasi-TEM mode, as with the coaxial feeder 6 in the abovedescribed embodiment. However, where for exam- ple an antenna is to be used only for the detection of radiation, a suitable diode may be located substantially on thez-axis and be provided with means for coupling itto a substantially pure lowest order radial mode in the horn, these means then constituting the launching means of the invention.

It will be seen that as a result of the rotationally symmetrical form of the above-described embodiment (for -90'--cf--90') and since the launching means acts as a line source on the axis of symmetry, the phase length between the launching means and any point at the aperture is substantially the same as for all other points atthe aperture in the same plane normal to the z-axis. This may also be achieved for apertures which do notform part of a substantially cylindrical surface, for example by including dielectric material in the horn anclior by suitable shaping of the conductive surfaces 1 and 2.

A mathematical model has been devised for the radiation patterns of antennae embodying the inven- tion, using the assumption that the E-plane and H-plane radiation patterns are separable and that the H-plane radiation pattern may therefore be predicted by taking the spacing in the z-direction between conductive surfaces bounding the horn to be uni- form, as in a sectoral horn. The model gives results in good agreement with the measured performance of the above-described constructed embodiment, and may be used to predict the radiation patterns of embodiments having H-plane flare angles other than 180 degrees; for example, it is expected that a flare angle of 300 degrees will result in a 3 dB beamwidth of approximately 120 degrees.

An antenna embodying the invention may be used with a polarisation twister adjacent the horn at its aperture so that, for example, when used in an azimuth direction-finding system, the system can respond to both horizontally and vertical ly- polarised radiation by rotating the plane of polarisation of the incident radiation through 45 degrees.

Claims (15)

1. A horn antenna wherein, with reference to a cylindrical co-ordinate system with parameters z, cf and r, z representing distance measured parallel to a rectilinear z-axis from a plane normal thereto, (f representing an angle measured about the z-axis from a datum and r representing radial distance from the z-axis, the horn has a wide angle of flare about the z-axis in the (f, r plane, said angle being less than 360 degrees, and is bounded by conductive surfaces spaced apart in th zclirection, wherein the aperture of the horn is concave in the cr,, r plane as seen from the z-axis, and wherein the antenna comprises means for launching electromagnetic energy into the horn towards the aperture, characterised in that the launching means act substantially as a line source coincident with thez-axis and are adapted to launch a substantially pure lowest order radial mode having its electric field in a direction 4 parallel to the z-axis.

2. An antenna as claimed in claim 1, characterised in that said conductive surfaces spaced apart in th z-direction extend substantially from the aperture to thez-axis, in that the launching means are situated substantially at the z-axis, and in thatthe extent of the launching means in the cp, r plane is small compared with a wavelength over the operating frequency range.

3. An antenna as claimed in claim 1 or2, characterised in that the launching means are substantial ly rotational ly symmetrical about the zaxis over substantially the whole of said angle of flare.

4. An antenna as claimed in claim 3, characte- rised in that the launching means comprise an electric probe extending substantially along the z-axis.

5. An antenna as claimed in claim 4, characterised in that the antenna comprises a cavity of which at least part is radially opposed to the horn and in that over at least the majority of the operating frequency range of the antenna, the impedance presented to the launching means by the cavity is substantially greater than the impedance presented by the horn.

6. An antenna as claimed in claim 5, characterised in that the cavity extends around the z-axis over an angle of substantially 360 degrees minus said angle of flare.

7. An antenna as claimed in any preceding claim, characterised in that for each point at the aperture of the horn, the phase length between that point and the launching means is substantially the same as for all other points at the aperture in the same plane normal to the z-axis.

8. An antenna as claimed in any preceding claim wherein the aperture is part of a substantially cylindrical surface having the z-axis as its axis of revolution.

9. An antenna as claimed in any preceding claim wherein the spacing, parallel to thez-axis, of said conductive surfaces increases progressively with increasing r.

10. An antenna as claimed in claim 9, characte- rised in that said spacing increases exponentially with r.

11. An antenna as claimed in any preceding claim, characterised in that said angle of flare is substantially greaterthan 130 degrees.

12. An antenna as claimed in claim 11, characte- rised in that said angle of flare is substantially 180 degrees.

13. An antenna as claimed in any preceding claim wherein the operating bandwidth of the anten- na is at least 3: 1.

14. An antenna as claimed in any preceding claim in combination with a transmission line feeder extending from the launching means and supporting in operation a TEM or quasi-TEM mode.

15. An antenna substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon. Surrey, 1983. Published by The Patent Office. 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.