GB2174251A

GB2174251A - Coupler for transmission lines

Info

Publication number: GB2174251A
Application number: GB08531268A
Authority: GB
Inventors: Martin Alexander
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1984-12-20
Filing date: 1985-12-19
Publication date: 1986-10-29
Also published as: GB8523076D0; GB2171257A; GB8432186D0; GB2174251B; GB8531268D0

Abstract

A conductive element 11 is used to suppress unwanted modes introduced at a transition between a coaxial line and a triplate line. The element 11 is U-shaped and partly encloses the end of the triplate line. The arms of the U are spaced from the triplate line by a distance which is ideally about twice the ground plane spacing according to conventional theory. The centre of the U shape is however spaced by a smaller distance. This matches the transition over a broad frequency band without resort to complicated matching aids. <IMAGE>

Description

1 GB2174251A 1 SPECIFICATION for such features as the Krauss step. It is

believed that the ability to obtain such a A device for coupling energy from a co- match is due to the relatively close spacing of axial line to a Tripiate line the said conductive element and the triplate 70 inner conductor at the end of the latter. This This invention relates to a device for coupling match can be achieved experimentally by mon energy from a co-axial line to a triplate line. itoring energy passing between the transition Such devices are well known and are corn- from co-axial to triplate lines whilst adjusting mercially available. They usually include a the position of the conductive element.

semi-circular arrangement of pins joining the 75 The conductive element preferably has a U two ground planes of the triplate. Each pin is shaped inner surface having a curved base spaced from the centre conductor of the tri- portion, the centre of which lies closest to the plate by an equal distance of at least twice inner conductor; and two arms extending from the ground plane spacing, this spacing being the base portion, parallel to the inner conduc- necessary to conform with the established 80 tor. The latter preferably has a convex curved theory that any conductor must be placed a end.

sufficient distance from the centre conductor One way in which the invention may be per not to interfere with its field lines. This series formed will now be described with reference of pins acts as a mode suppressor i.e. it pre- to the accompanying drawings in which:- vents the propagation of unwanted modes in 85 Figure 1 is a side view of a vertical triplate the triplate. In another known arrangement, constructed in accordance with the invention described by Nygren et a] in U.S. patent spe- and shown broken away and with one of its cification 4383226, U-shaped conductive earth planes and one of its dielectric sheets layers 28, 36 co-planar with layers 22, 24 removed to reveal a central conductor; and forming inner conductors of a triplate, are 90 Figure 2 is a crosssection through the line used for a similar purpose as the aforemen- XX of Fig. 1.

tioned pins. Referring to the drawings, the illustrated tri In the known systems described above plate structure, which forms part of a radar careful measures must be taken to match the antenna, comprises two identical earthed con- impedance of the co-axial line to that of the 95 ductive sheets 1 and 2 spaced by about 8 triplate line. One such matching technique is mms and forming the earth planes. One of the use of a so called---KraussStep- in the these earth planes is removed in the case of co-axial line. This employs staggered changes Fig. 1. Between the earth planes 1 and 2 is a in the diameters of the centre and outer conconductive strip 3 separated from the sheets ductors of the co-axial line to counteract the 100 1 and 2 by insulating layers 4 and 5 of foam susceptances at the point where the inner plastics material. Layers 1, 2, 4 and 5 are conductor of the co-axial line joins the centre connected together by bolts, (one of which is conductor of the triplate. The arrangement de- shown at 6) arranged to establish electrical scribed by Nygren in U.S. specification contact between the earth planes 1 and 2.

4383226 has a Krauss step defined by mem- 105 Energy to be transmitted (in this example of bers 5 and 40 as shown on Fig. 6 of Nygren. 270 mms wavelength mid-band) is fed from a This invention provides a coupler for cou- co-axial line (not shown) to a co-axial socket pling electrical energy from a co-axial 7 shown in Fig. 2. This socket comprises an transmission line having co-axial inner and earthed outer bush 8 and a central pin 9. The outer conductors to a triplate transmission line 110 bush 8 is in electrical contact with the earth having a central conductor between two paral- plane 1. The central pin 9 is connected elec lel earth planes the coupler comprising: trically to the conductive strip 3 and passes (a) an aperture in an earth plane of the tri- through a circular hole 10 in the earth plane plate adjacent one end of an inner conductor 1. In the illustrated arrangement it is notable thereof between the earth planes;. 115 that the hole 10 is in fact defined by a re (b) means for connecting an inner conduc- bated portion of the socket 7 which fits into a tor of the co-axial line to said end of the inn-er larger hole in the sheet 1. Thus, from an elec conductor of the triplate through the said trical point of view the rebated part of the aperture; and socket 7 forms part of the earth plane.

(c) a conductive element between and mak- 120 A conductive element 11, which will be re ing electrical contact with the earth planes of ferred to as a--- modesuppressor-, is clamped the triplate, the conductive element partially by bolts such as that shown at 6 between the enclosing the said end of the inner conductor ground planes 1 and 2 and forms an electrical and lying closer to the inner conductor at the connection between them. The mode suppres said end than at parts adjacent and leading to 125 sor 11 is U shaped and extends around the the said end. pin 9 on three sides. Its inner surface has a It has been found that by employing the concave semicircular part 12 whose centre of invention a match can be obtained over a curvature 13 lies on the conductive strip 3 broad bandwidth between the impedances of leading directly to the pin 9 and is offset from the co-axial and triplate lines without '---need 130 the position where it joins the pin 9. This 2 GB2174251A 2 offset, which contrasts with known similar ar- the base portion, parallel to the inner conduc rangements in which the centre of curvature is tor.

coincident with the pin 9, is such that the 3 A coupler according to claim 2 in which curved surface 12, which is of greater radius the inner conductor has a convex curved end.

than the hole 10 just touches the edge thereof 70 4. A coupler substantially as described and has a common tangent therewith. The with reference to the accompanying drawings curved portion 12 of the inner surface of the and substantially as illustrated therein.

mode suppressor 11 merges into straight por- Pijnted in the United Kingdom for tions 14 which extend parallel to the part of Her Majesty's Stationery Office, Dd 88189.35, 1986, 4235.

the line 3 leading to the pin 9, so as to Published at The Patent Office, 25 Southampton Buildings, enclose a length of line 3, as measured from London, WC2A 1 AY, from which copies may be obtained.

the pin 9, approximately equal to one eighth of a wavelength (which is 34 mms in this particular example).

Whilst the straight portions 14 should ideally be spaced from the conductor 3 by twice the earth plane spacing, i.e. by sixteen mms, in practice a lesser spacing of about one and a half times the earth plane spacing is practic- able where there is limited space available. In the illustrated example the spacing is 11.5 mms.

The purpose of the mode suppressor 11 is to create, in co-operation with the earth planes 1 and 2 and the central conductor 3, a portion of transmission line which is unable to sustain unwanted modes generated at the transition point between the co-axial line and the triplate. The offset arrangement of the il- lustrated mode suppressor additionally provides an improved impedance match between these two portions of the path over a broad bandwidth of at least the order of an octave.

From the line 3 energy is transmitted along circuitous paths to each of an array of dipole elements (not shown) from which the energy is radiated in the form of electromagnetic radiation.

Claims

1. This invention provides a coupler for coupling electrical energy from a co-axial transmission line having co-axial inner and outer conductors to a triplate transmission line having a central conductor between two parallel earth planes the coupler comprising:

(a) an aperture in an earth plane of the triplate adjacent on end of an inner conductor thereof between the earth planes; (b) means for connecting an inner conductor of the co-axial line to said end of the inner conductor of the triplate through the said aperture; and (c) a conductive element between and mak- ing electrical contact with the earth planes of the triplate, the conductive element partially enclosing the said end of the inner conductor and lying closer to the inner conductor at the said end than at parts adjacent and leading to the said end.

2. A coupler according to claim 1 in which the conductive element has a Ushaped inner surface having a curved base portion, the centre of which lies closest to the end of the inner conductor; and two arms extending from