GB2265759A - Horn antenna - Google Patents

Abstract

A T.E.M. horn includes open sides and comprises a pair of divergent spaced apart plates 1, 2. A closed loop surrounding the horn serves to band limit the signal radiated from the horn. The loop may be made by members 5 joining the plates 1, 2. These may be adjustable along the horn length to vary the cutoff frequency. For support the horn may be mounted within a waveguide section or have shaped reinforcing members 50 along the plates. <IMAGE>

Description

ANTENNA The invention relates, in one aspect, to an antenna and in particular to a T.E.M. horn suitable for radiating a T.E.M., or unbounded plane wave, typically in the range 100 MHz to microwave frequencies.

This invention provides a T.E.M. horn comprising a pair of opposed electrically conductive plates, the plates being divergent with respect to each other from an input end of the horn, and an electrically conducting closed loop surrounding the horn so as to provide a low frequency cut-off for the radiated signal.

In one embodiment the closed loop may be provided by a pair of members each electrically connecting together the plates at opposed sides of the horn.

In a U.W.B. (Ultra-Wide Band) pulsed radar system, for example, the low frequency components are less directional than the higher frequency components of the radiated signal. The closed loop can ensure that these low frequency components of the signal are not radiated.

Furthermore, where the pulse repetition rate is high, the trailing low frequency components can interfere with the following pulses. When a T.E.M. horn is being used for testing purposes within an enclosure, induced room resonances can be a problem unless frequency components in the radiated signal corresponding to such resonances are removed.

In each case above the use of a T.E.M. horn according to the invention, by not radiating such relatively low frequency components, can be of advantage.

The electrically conductive members are preferably secured to the plates and have sufficient rigidity to provide mechanical support and space the plates apart from each other.

This additionally gives a constructional advantage as compared to the known horns in which the plates are spaced apart by insulating posts.

The cut-off frequency is related, as is well known from waveguide theory, to the area of the window bounded by the closed loop. In another embodiment the area of the closed loop is adjustable so as to vary the cut-off frequency. In a preferred instance, conductive members may be movable longitudinally along the sides of the horn.

The two plates may be, at least partially, located within a length of waveguide, the closed loop being provided by the walls of the waveguide. The closed loop need not be square or rectangular in section but could be circular or elliptical, depending on the modes or the polarisation of the signal which it is required to radiate.

When a length of transmission line is connected to a T.E.M. horn, the requirement of matching the impedance between free space and the transmission line means that the shape of each plate, in the direction of divergence of the two plates, can be of a complex shape, typically following an exponential curve. Because of the cost it is usual to form each plate from a relatively thin sheet of metal and it is difficult to construct each plate so that it can be bent to the required shape and remain in that shape.

In another aspect the invention provides a T.E.M.

horn comprising a pair of opposed electrically connective plates, the plates being divergent with respect to each other and each being secured to a respective rigid support member having a surface contiguous with the plate which surface is profiled according to the required shape of the plate.

In a preferred embodiment, for ease of construction, the or each support member is of elongate shape and comprises a sheet of metal folded about a fold line, profiled surfaces being provided on opposing edge regions of the sheet on either side of the fold line. In alternative embodiments, the surface may be formed from a block of material such as wood. In each case the profiled surface may be simply formed, e.g. by cutting along a line which follows an equation defining the required shape of the horn.

In another aspect the invention provides a method of making a T.E.M. horn comprising the steps of forming one or more support members each having a profiled surface, and securing an electrically conductive plate to the profiled surface so that itMlies contiguous with the surface.

In order that the invention may be well understood, embodiments thereof will now be described with reference to the accompanying diagrammatic drawings, in which; Figure 1 is a perspective view of a T.E.M horn according to one embodiment of the invention; Figure 2 is a perspective view of a modification of the horn shown in Figure 1; Figure 3 is a longitudinal sectional view along a horn according to another embodiment of the invention; Figure 4 is a sectional view along line A-A of Figure 3; Figure 5 is a perspective view of yet another embodiment of the invention; Figure 6 is a perspective view of the horn shown in Figure 1 including a support member according to a further aspect of the invention; Figure 7 is a plan view of a blank suitable for making the support member shown in Figure 6; and Figure 8 is a perspective view of conventional T.E.M.

horn having a support member as shown in Figures 6 and 7.

As shown in Figure 1, a T.E.M. horn comprises two plates 1, 2 each formed from a sheet of electrically conductive material such as aluminium. Each plate 1, 2 includes sides la, 2a respectively which diverge from the input end 3 of the horn. The plates 1, 2 also diverge with respect to each other from the input end 3, where they are spaced apart and are deformed so that, in longitudinal section, each plate is of generally exponential shape, the actual shape depending upon the frequency and power of the signal to be radiated. The sides of the horn are open for the majority of their length so as not to bound the radiated T.E.M. wave. At the output end 4, conductive members 5 extend between the two plates 1, 2 at opposite sides of the horn The conductive members 5 ensure that the end of the horn behaves like a closed structure such as a waveguide.

As is well known, the minimum frequency of a signal which will propagate through a waveguide is related to the cross-section of the waveguide and thus the members 5 ensure that only signals above a predetermined frequency can be radiated. The members 5 are secured e.g. by brazing and also have sufficient rigidity and mechanical strength that they are able to support the end 4 of the plates 1, 2 in spaced relation.

In the version shown in Figure 2, conductive members 10 include enclosed slots 11. Flanges 12 attached to the sides la, 2a of the plates also include enclosed slots 13 and the conductive members 10 are secured by a nut and bolt 14. In this way the conductive members 10 are movable in a longitudinal direction so that the lower most cut-off frequency can be varied. In an alternative embodiment (not shown) a concertina or bellows arrangement may be attached to the members 11 for enclosing the horn between the output end 4 and the members 11. The width of the conductive members 5, 11 in an axial direction, is of the same order of magnitude as the wavelength of signals to be radiated.

Figures 3 and 4 show another embodiment in which a pair of plates 1,2 are enclosed within a length of rectangular section waveguide 15. The output ends 4 of the plates 1,2 are retained within a length 16 of insulating material and are thus insulated from the waveguide 15.

Alternatively, the ends 4 may be electrically connected to the waveguide, if required. Insulating posts 17 are also present for supporting the plates intermediate the ends of the horn.

The closed loop for band limiting the radiated signal is determined by the cross-sectional area of the guide. In addition, because the guide encloses the sides of the T.E.M.

horn, the unwanted emission of radiation from the sides is reduced.

Figure 5 shows a version in which the output end 4 of the horn is of circular cross-section. The gain of such a horn can be relatively higher than for the rectangular section horns shown previously, since it is easier to evenly illuminate a circular aperture. Such a horn is useful in radiating modes having circular symmetry or polarisation.

Alternatively the end 4 may be of elliptical shape.

The horn shown in Figure 6 is generally similar to that shown in Figure 1 and the same parts are identified by the same reference numerals. In order to provide mechanical support for the horn and to define the shape, the outermost surface of each respective plate 1, 2 is mounted on a rigid support member 50. The support member is formed from a sheet of material such as aluminium folded about a central fold line 51. The member 50 includes opposed profiled or shaped edges 50a. Their exact shape can be determined using base geometry knowing the required shape of each plate 1, 2 of the horn. Also, the shape of the edges 50a will vary according to the desired angle between the two halves of the sheet 50. When folded, the fold line 51 provides the rigidity for the support member.Each plate 1, 2 is deformed so as to be contiguous with the edges 50a and is secured thereto, e.g. by brazing or by means of a suitable adhesive. The support member both defines the shape of each respective plate 1, 2 and maintains that shape during use of the horn. Such a support can be used with a conventional horn in which the two plates are spaced apart by means of insulating posts 52, as is shown in Figure 8.

The support member may include longitudinally spaced profiled surfaces so that spaced regions only of the plate are secured to the support member. In each case the surface can be simply formed by cutting along a line drawn according to the required shape of the horn. In an alternative embodiment (not shown) the support member may extend substantially the full width of each plate 1,2 and transversely extending shaped cross-members may be sandwiched between the support member and the respective plate.

Claims (15)

1. A T.E.M. horn comprising a pair of opposed electrically conductive plates the plates being divergent with respect to each other from an input end of the horn, and an electrically conducting closed loop surrounding the horn so as to provide a low frequency cut-off for the radiated signal.

2. A T.E.M. horn, according to claim 1, in which the closed loop is provided by a pair of electrically conductive members each electrically connecting together the plates at opposed sides of the horn.

3. A T.E.M. horn, as claimed in claim 2, in which the electrically conductive members are secured to the plates and have sufficient rigidity to provide mechanical support for the horn.

4. A T.E.M. horn, according-to claim 1, in which the two plates are, at least partially, located within a length of waveguide.

5. A T.E.M. horn, according to claim 1, in which the closed loop is of circular or elliptical cross-section.

6. A T.E.M. horn, as claimed in any preceding claim, in which the area enclosed by the loop is adjustable so as to vary the cut-off frequency.

7. A T.E.M. horn according to claim 6 as dependent on claim 2 in which the members are movable longitudinally along the sides of the horn.

8. A T.E.M. horn, as claimed in any preceding claim, in which the plates are each secured to a rigid support member having a surface contiguous with the respective plate which is profiled according to the required shape of the plate.

9. A T.E.M. horn, as claimed in claim 8, in which the support member comprises a sheet of material folded about a fold line, edge portions of the sheet on either side of the fold line defining the profiled surface.

10. A T.E.M. horn comprising a pair of opposed electrically conductive plates, the plates being divergent with respect to each other from an input end of the horn, the plates each being secured to a rigid support member having a surface contiguous with the plate which surface is profiled according to the required shape of the plate.

11. A T.E.M. horn according to claim 10, in which the or each support member is of elongate shape and comprises a sheet of metal folded about a fold line, profiled surfaces being provided on opposing edge regions of the sheet on either side of the fold line.

12. A method of making a T.E.M. horn having a pair of opposed electrically conductive plates, the plates being divergent with respect to each other from an input end of the horn, the method comprising the steps of forming one or more rigid support members each having a profiled surface, and securing an electrically conductive plate to the profiled surface so that it lies contiguous with the surface.

13. A method, according to claim 12, in which the support member is formed by cutting along a line defining the required shape of the plate.

14. A T.E.M. horn substantially as described with reference to the accompanying drawings.

15. A method of making a T.E.M. horn substantially as described with reference to any of Figures 6 to 8 of the accompanying drawings.