GB2219438A

GB2219438A - Coupling transmission lines

Info

Publication number: GB2219438A
Application number: GB8812807A
Authority: GB
Inventors: Antony John Scammell
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-05-28
Filing date: 1988-05-28
Publication date: 1989-12-06
Anticipated expiration: 2008-05-28
Also published as: GB8812807D0; GB2219438B

Abstract

In order to connect a transmission line in the form of a strip conductor 2 separated by dielectric 4, 5 from opposed ground planes, such as a strip-line transmission line, to a transmission line in the form of a waveguide 10, such as a rectangular waveguide, a coupling comprises two conductors 12, 13 connected to the strip conductor 2 and an output 11 at which the signals are 180 degrees out of phase with each other, and a loop connected to the output to be inserted into the waveguide with its plane parallel to opposed walls of the waveguide. Such a coupling permits an end on connection of strip transmission line to a waveguide to be achieved. <IMAGE>

Description

Transmission Lines This invention relates to transmission lines, and especially to couplings for connecting transmission lines having a strip conductor separated by dielectric from opposed ground planes and transmission lines in the form of rectangular waveguides.

Such couplings are useful where power splitters are used. High input power into a waveguide is divided among several transverse branches each of which can then be coupled to a lower power handling strip transmission line.

It is often desirable, for example in confined spaces, to connect such strip transmission lines and waveguide branches axially, and co-axial cable links have been used for this purpose to join them. However, this introduces loss into the system.

A perpendicular connection between a strip transmission line and waveguide has been proposed, but this is unsuitable for such multiple branches if space is confined.

The invention provides a coupling for connecting a transmission line having a strip conductor separated by dielectric from opposed ground planes and a transmission line in the form of a rectangular waveguide, which coupling comprises two conductors connected to the strip conductor and to an output at which the signals are in use 180 degrees out of phase with each other, and a loop connected to the output for insertion into the waveguide with its plane parallel to opposed walls of the waveguide.

Such a coupling permits a well matched low loss endon connection of a strip transmission line to a waveguide to be achieved.

Advantageously, the length of one of the conductors between the strip conductor and the output is greater than the length of the other conductor between the strip conductor and the output by an amount which is such that the signals from the strip conductor are in use 180 degrees out of phase with each other at the output.

The coupling may include a stepped waveguide portion, an end of which into which the loop extends being square in section, and the other end of which having cross section such that one dimension is greater than the other dimension.

To assist in packaging, the plane of the loop may be perpendicular to the plane of the ground planes. The loop may surround a mass of dielectric material.

The coupling is suitable for all microwave frequencies, for example, from wavelengths of from 1 cm to 30 cm (30 GHz - 1 GHz).

The dielectric which separates the strip conductor from the opposed ground planes could be solid dielectric but could be partly air. Either space between the strip conductor and the opposed ground planes may be partly filled with solid material and partly separated by air.

It is intended that references to rectangular waveguides should include waveguides of square crosssection as well as waveguides the cross-section of which has a greater dimension in one direction than in the other direction.

Couplings constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which, Figure 1 is a perspective view of a first form of coupling in accordance with the invention; Figure 2 is a side view, partly in section, of the coupling shown in Figure 1; Figure 3 is a plan view of the coupling shown in Figure 1; Figure 4 is a perspective view of a second form of coupling in accordance with the invention; and Figure 5 is a perspective view of a third form of coupling in accordance with the invention.

Referring to Figures 1 and 3 of the accompanying drawings, the coupling is for connecting a strip transmission line indicated generally by the reference numeral l-to a standard size waveguide (not shown). The strip transmission line has a strip conductor 2 mounted on a substrate 3 sandwiched between layers of dielectric material 4, 5. These layers of dielectric material are in turn sandwiched between substrates 6, 7 carrying conductive ground planes on their inner surfaces across the whole width of the substsrate 6, 7.

The coupling comprises a half wave balun indicated generally by the reference numeral 8, a loop radiator 9 and a step taper waveguide indicated generally by the reference numeral 10.

The balun 8 consist of two conductors joining the strip conductor 2 to an output 11, wherein the length of the conductor 13 is greater than the length of the conducter 12, with result that, at 11, the signals on the conductors 12, 13 are 180 degrees out of phase-with each other.

The output is connected to the loop radiator 9, which extends through the closed end of the step taper waveguide portion 10, the cross section of which is square. The magnetic vector associated with the loop radiator 9 is orthogonal to the plane of the loop, the same as that of the waveguide itself, and hence the radiation from the strip conductor is coupled into the waveguide portion 10.

The waveguide portion is then stepped in lengths of one quater of the guide wave-length down to the standard waveguide size at the end 14, where only the dominant TE 10 waveguide mode can propagate, hence attenuating any higher order modes generated at the transition.

All discontinuities are optimised to give a well matched, low loss device. Preliminary measurements on a proto type device have shown an insertion loss of 0.25 dB with a match better than 1.2:1 over an 11% bandwidth, a figure comparable with that of a co-axial connection.

To minimise the reactive component of the loop impedance, the mean radius of the loop b is chosen to be b = 0.183 \ , where \ is the wavelength of the microwave radiation in air. For such a loop, the resistive component is around 200fL.

Such a size of loop is too large a diameter for insertion directly into a standard size of waveguide, and an advantage of the step taper section is to connect such a loop to a standard size waveguide.

Referring to Figure 4, the second form of coupling differs from the first in that the strip transmission line has been turned through 90 degrees about its axis leaving the plane of the loop radiator unchanged. Secondly, the loop radiator surrounds a dielectric support 15.

Such a configuration has the advantage that if it is desired to feed several strip transmission lines into the respective parallel waveguide branches from a power splitter the transmission lines can be stacked one on top of each other i.e. with their faces in contact with each other, rather than one edge on top of each other as would be the case with Figure 1. It will be noted that, in this embodiment, the direction of the electric vector is the same in the strip transmission line as in the waveguide i.e. vertical as seen in the drawing in both cases.

The coupling shown in Figure 5 differs from that shown in Figure 4 in that the dielectric material 15 has a high dielectric constant, and, since the loop radius is inversely proportional to the dielectric constant, a smaller loop can be used, enabling the step taper section to be omitted, so that the loop radiator 9 enters directly into the end of a standard size waveguide 16.

Such a form of coupling is very compact, enabling stripline transmission lines to be closely stacked but a reduction in power handling, and a slight increase in loss are produced.

Claims

1. A coupling for connecting a transmission line having a strip conductor separated by dielectric from opposed ground planes and a transmission line in the form of a waveguide, which coupling comprises two conductors connected to the strip conductor and to an output at which the signals are in use 180 degrees out of phase with each other, and a loop connected to the output for insertion into the waveguide with its plane parallel to opposed walls of the waveguide.

2. A coupling as claimed in claim 1 in which the length of one of the conductors between the strip conductor and the output is greater than the length of the other conductor between the strip conductor and the output by an amount which is such that the signals from the strip conductor are in use 180 degrees out of phase with each other at the output.

3. A coupling as claimed in claim 1 or claim 2, in which the coupling includes a stepped waveguide portion, an end of which into which the loop extends being square in section, and the other end of which having a cross section such that one dimension is greater than the other dimension, the plane of the loop being parallel to the shorter dimension.

4. A coupling as claimed in claim 1 or claim 2 in which the plane of the loop is perpendicular to the plane of the ground planes.

5. A coupling as claimed in claim 4, in which the loops surrounds a mass of dielectric material.

6. A coupling for connecting a transmission line having a strip conductor separated by dielectric from opposed ground planes and a transmission line in the form of a waveguide, substantially as herein described with reference to the accompanying drawings.

7. A coupling as claimed in any one of claims 1 to 6, in combination with a transmission line having a strip conductor separated by dielectric from opposed ground planes and a transmission line in the form of a waveguide.