US2742640A - Aerial systems - Google Patents

Description

April 17, 1956 D. E. CRONIN 2,742,640

AERIAL SYSTEMS Filed March 18, 1952 INVENTOR Dew/ ER c (RON/N lTTORNEY Hired This invention relates to aerial systems. I

It is well known that on open-ended waveguide may beused for launching electro-magnetic' radizitidtiinto space. -If now the waveguide is formed'by a holethrough a metal body so that there is an' electrically conducting surface extending for an appreciable distance from the waveguide'aperture in a plane inclined, for example at right angles, to the waveguide axis, it is found that-currents set -up in the conducting surface tend to distort the radiation polar diagram from that of the waveguide alone since there is radiation both from the waveguide and from the conducting surface.

=Iti'isone object of the present invention to provide an improved aerial system in which this distortion of the radiation polar diagram is reduced.

According to the present invention, an aerial system comprises a waveguide which extends into a metal body and has its open end flush with the outside surface of the body, there being a gap between the body and a metal wall of the waveguide, the end of this gap remote from the said surface being bridged electrically and the gap being of such depth that, at the operating frequency of the aerial system, there is presented a high, preferably infinite, impedance between the outside surface of the body and the said end of the waveguide wall.

If the waveguide is of circular cross-section,'it is preferred that the gap shall completely encircle the waveguide.

One arrangement of an aerial system for launching circularly-polarised electromagnetic radiation will now be described by way of example with reference to the two figures of the accompanying drawing. Figure 1 shows a plan view of the aerial system while Figure 2 shows a cross-section at the line II--II in Figure 1.

Referring now to the drawing, the aerial system comprises a waveguide 1 which is adapted to launch; the circularly-polarised radiation in the direction of the arrow 2. The waveguide 1 is of circular cross-section and is formed in a brass member 3. The member 3 has an annular flange 4 on which is seated a metal, for example brass, plate 5.

The metal of the member 3 surrounding the waveguide wall 6 is cut away so as to form an annular gap 7 surrounding the waveguide 1. The end 8 of the waveguide wall 6 is flush with the outside surface 9 of the plate 5. The sides 11 and 12 of the gap 7 are parallel to the waveguide axis 13, and the sides 11 and 12 provide a co-axial transmission line which is shorbcircuited at its end remote from the surface 9. The depth of the gap 7 is such that, at the frequency at which radiation is to be launched from the waveguide 1, the effective impedance presented by the open end of said transmission line is substantially infinite. For this purpose the depth of the gap 7 has an electrical length approximately equal to one quarter wavelength at the operating frequency.

It will. be appreciated, therefore, that there is a substantially infinite impedance between the end 8 of the 2,742,640 Patented Apr. 17, 19 56 waveguide wall 6 and the surface 14 of the 'part'15 of the member 3 which surface is co-planar with the surface 14 so that the surfaces 9 and 14 effectively form a'single conducting surface. This high impedance tends to prevent any appreciable current being set up in the metal surrounding the waveguide 1, when the aerial system is radiating, which would affect the-polar diagram of the systernz I 5 The open end of the waveguide 1 may be closed by a member which forms a window across the waveguide.

' Moreover'the waveguide 1' and/or the gap 7 may be filled with solid dielectric'material, for example polythene.

l Although theoretically it is necessary for the gap 7 to have an electrical length of a quarter wavelength, it is found that, in the aerial system described above, there is noappreciable distortion of thepolar diagram due to currents in the surface 9 when the operating frequency lies anywhere in a-band of up to ten per cent of the optimum frequency. I

Instead, as in the above arrangement, of providing the waveguide and its surrounding gap in a unitary member,' the waveguide may alternatively be provided by a metal tube-which fits into a metal body. In this arrangement the'open end of the waveguide member'is again flush with the outside surface of the body and the body itself is' shaped so as to provide a gap surrounding the waveguide member,-the waveguide member and the body being'secured together, for example by soldering, to provide the required short-circuit across the end of the gap.

Aerial systems in accordance with the invention may be used for either transmitting or receiving electromag netic radiation.

I claim:

1. An aerial system comprising a metal body having an external electrically conductive surface facing unobstructed space, a metal waveguide which extends into the body from said external surface, said waveguide having an open end which is flush with' said external surface of the body and launches electro-magnetic radiation into space, the shape and size of the cross-sectional configuration of the open end of the waveguide being the same as the shape and size of the cross-sectional configuration of the waveguide within the body, said external surface of the body extending for an appreciable distance radially outwardly away from the open end of the waveguide, there being a gap between the body and the wall of the waveguide immediately adjacent the open end thereof, and means electrically bridging the end of the gap re mote from said external surface at a depth such that at the operating frequency of the aerial system a high impedance is presented between said external surface of ,the body and the wall of the waveguide at the open end thereof.

2. An aerial system comprising a metal body having an external electrically conductive surface facing unobstructed space, a metal waveguide which extends into the body from said external surface, said waveguide having an open end which is flush with said external surface of the body and launches electro-magnetic radiation into space, the shape and size of the cross-sectional configuration of the open end of the waveguide being the same as the shape and size of the cross-sectional configuration of the waveguide within the body, said external surface of the body extending for an appreciable distance radially outwardly away from the open end of the waveguide, there being a gap between the body and the wall of the waveguide immediately adjacent the open end thereof, and means electrically bridging the end of the gap remote from said external surface at a depth equal to approximately one quarter wavelength of the operating frequency of the aerial system so as to present a high impedance between said externalsurface of the body and the wall of the waveguide at the open end thereof.

3. An aerial system comprising a metal body having an external electrically conductive surface facing unobstructed space, a metal waveguide which extends into the body from said external surface, said waveguide having an open end which is flush with said external surface of the body and launches electromagnetic radiation into space, the shape and size of the cross-sectional configuration of the open end of the waveguide being the same as the-shape and size of the cross-sectional configuration of the waveguide within the body, said external surface of the body extending for an appreciable distance radially outwardly away from the open end of the waveguide, there being a gap between the body and the wall of the waveguide immediately adjacent the open end thereof, and means electrically bridging the end of the gap remote from said external surface at a depth within ten per cent of one quarter wavelength of the operating frequency of the aerial system so as to present a high impcdance between said external surface of the body and the wall of the waveguide at the open end thereof.

4. An aerial system according to claim 3 wherein the waveguide is of circular cross section and the gap is a full annulus completely encircling the waveguide.

5. An aerial system according to claim 4 wherein the waveguide is elongated and its longitudinal axis is perpendicular to the external electrically conductive surface of the body.

6. An aerial system as set forth in claim 4 wherein the waveguide and body are unitary.

7. An aerial system'comprising a metal body having an external electrically conductive surface facing unobstructed space, a metal waveguide which extends into the body from said external surface, said waveguide having an open end which is flush with said external surface of the body and constitutes the operative end of the aerial system, the shape and size of the cross-sectional configuration of the open end of the waveguide being the same as the shape and size of the cross-sectional configuration within the body, said external surface of the body extending for an appreciable distance radially outwardly away from the open end of the waveguide, there being a gap between the body and the wallof the waveguide immediately adajcent the open end thereof, and means electrically bridging the end of the gap remote from said external surface at a depth such that at the operating frequency of the aerial system a high impedance is presented between said external surface of the body and the wall of the waveguide at the open end thereof.

Great Britain Feb. 7,

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