GB2139818A

GB2139818A - High frequency transmission device

Info

Publication number: GB2139818A
Application number: GB08313061A
Authority: GB
Inventors: Paul Leslie Lowbridge; Christopher Edward Burnett
Original assignee: Marconi Electronic Devices Ltd
Current assignee: Marconi Electronic Devices Ltd
Priority date: 1983-05-12
Filing date: 1983-05-12
Publication date: 1984-11-14
Also published as: GB2139818B; GB8313061D0

Abstract

A microstrip circuit (5) is coupled to a waveguide (2). The microstrip circuit is formed on a dielectric substrate (4) which is used to hermetically seal the waveguide channel. The line (5) extends across the waveguide in the direction of the E field in the vicinity of an e-m. antinode. <IMAGE>

Description

SPECIFICATION High frequency transmission device This invention relates to high frequency transmission devices which operate at frequencies which are commonly termed "millimetric". In practice, the frequency band is normally taken to be upwards of about 20 GHz. At these very high frequencies, the wavelengths are correspondingly small, and the conventional waveguides which are used to transmit the energy are extremely difficult and intricate to manufacture, particularly the interface devices which connect a waveguide to another kind of circuit. The invention seeks to provide a high frequency transmission device in which the transition between a waveguide and a microstrip can be effected in a particularly convenient manner.In particular, it also seeks to overcome the difficulty of providing an hermetic seal to the interior of a waveguide structure when it is connected to such a microstrip circuit.

According to this invention a high frequency transmission device includes a microstrip transmission line comprising an elongate conductor which is carried by a dielectric substrate; and a hollow waveguide channel, the end of which is sealed off by said dielectric substrate which is arranged such that the elongate conductor is aligned with the E-plane of the waveguide so as to couple said waveguide to said transmission line.

Preferably, the end of said conductor extends partway across the width of said channel so as to constitue an E-plane probe. Although the conductor can extend across the full width of the channel, this is not preferred.

Thus, the substrate itself constitutes a waveguide window which provides an hermetic seal for the interior of the waveguide channel. Typically, the waveguide would be connected to a radiating horn antenna or the like, or alternatively to a source of very high frequency energy operating at millimetric frequencies. The microstrip circuit is particularly advantageously used for constituting various circuit components, such as mixers, combiners and frequency changes which are very cumbersome and difficult to implement in wavegide form.

Preferably, means are provided for causing an anti-node in the waveguide mode in the vicinity of said probe. In this way the probe couples to a maximum value of the E-field so that the greatest possible transfer of energy takes place between the waveguide and the microstrip circuit. The coupling is of bi-directional mature so that energy can be coupled in either direction as required. Preferably, said means is constituted of the waveguide channel which extends beyond the substrate and which is terminated in a short circuit, with the length of the extension to the short circuit being controllable so as to set up the required anti-node.

The invention is further described by way of example, with reference to the accompanying drawings, in which Figure 1 shows a sectional view of a device in accordance with the invention, and Figure 2 shows an exploded perspective view.

Referring to the drawings, a waveguide structure 1 comprises an extremely small rectangular hollow waveguide channel 2 formed within a brass block 3.

The waveguide channel 2 has a broad wall of dimension X and a narrow wall of dimension Y, where X is greater than Yin conventional manner to support propagation of an electromagnetic wave having a cut-offfrequencywhich is determined by the dimension X, and the impedance and power handling capacity is determined by the magnitude of the dimension Y. The waveguide structure 1 is designed to operate at approximately 90 GHz, and thus the dimension X is of the order of 2.56 mm and the dimension Y is about 1.27 mm. Structures having critical dimensions as small as this can be extremely difficult to manufacture accurately, but since in the present example the waveguide channel 2 is simply a smooth straight rectangular shape this presents no particular difficulty.

A microstrip circuit 7 is connected to the upper face of the brass block 3, and it comprises a dielectric substrate 4, composed of quartz, which carries a thin electrical conductive track 5 on its upper surface and a continuous electrically conductive groundplane 6 on its lower surface. The microstrip is positioned such that the end of the track 5 partly overlies the edge of the channel 2 so that it constitutes a probe 12. Because of the orientation of the microstrip circuit relative to the broad wall of the waveguide channel, the end of the probe 12 is aligned with the E-plane of the electric field within the waveguide channel and is positioned about midway along the broad wall of the channel.Although the dielectric substrate 4 is continuous, and extends over the end of the channel 2 so as to close it, the groundplane 6 is provided with a small aperture at the region of the waveguide channel so that it does not interfere with the propagation of electromagnetic energy along the waveguide channel 2. As the quartz substrate 4 is an impermeable material, the substrate itself provides an hermetic seal which separates the microstrip circuit from the waveguide channel 2 which may be open to the external environment. The groundplane 6 is secured to the brass block 3 by means of a solder seal which extends entirely around the periphery of the channel 2 so as to encircle the end which is closed by the substrate.This serves not only to maintain the required hermetic seal but also to precisely and accurately retain the location of the microstrip relative to the position of the waveguide channel, and ensures that the probe is accurately aligned with the electric field within the channel. The thickness of the quartz substrate is about 120 microns, and that of the track 5 is only about 5 microns. In practice, the microstrip circuit is mounted within a sealed container (not shown) which may, if desired, contain an inert gas.

A further brass block 8 is mounted on top of the microstrip circuit and it, too, has an open channel 9 which is aligned with that of the waveguide channel 2. It contains a small rectangular brass plug 10 which is slideable within the channel and whose position can be accurately adjusted in the longitudinal direction. The purpose of the small plug 10 is to provide a short circuit a controllable distance from the probe 12. This short circuit sets up a standing wave in the electromagnetic energy within the waveguide so that an anti-node of the electric field is produced precisely in the region of the probe itself. This permits maximum coupling of energy between the microstrip circuit and the waveguide. Typically, the lower surface of the plug 10 will be a quarter wavelengh from the probe 12.So as to ensure that the block 8 does not adversely affect the properties of the microstrip circuit a recess 11 is formed in its lower surface so as to provide considerable clear ance for the track 5. Typically, the recess 11 is of square cross-section, about 1 mm by 1 mm.

Typically, the width of the electrically conductive track 5 is about .2 mm and its width largely determines the characteristic impedance of the microstrip line, in conjunction with the thickness of the dielectric substrate in conventional manner.

Typically, the impedance of the microstrip circuit is 50 ohms, whereas the characteristic impedance of the waveguide is more typically about 300 ohms.

The arrangement illustrated in the drawings not only provides a very efficient coupling between two circuits but provides a very satisfactory impedance transformation. It is found that well in excess of 90% of the energy is transferred across the transition.

Typically, a waveguide horn would be connected directly to the lower end of the waveguide channel, and any associated circuits, such as circulators, detectors, frequency changers, filters and the like are implemented in microstrip form and formed upon the substrate 4.

Although the waveguide channel 2, and the extension channel 9 are both shown as having the same rectangular shape, this need not be the case, and one or both could have a circular shape. In the latter case, it is necessary to ensure that the orientation of the E-plane is aligned with the probe 12.

Claims

1. A high frequency transmission device including a microstrip transmission line comprising an elongate conductor which is carried by a dielectric substrate; and a hollow waveguide channel, the end of which is sealed off by said dielectric substrate which is arranged such that the elongate conductor is aligned with the E-plane of the waveguide so as to couple said waveguide to said transmission line.

2. A device as claimed in claim 1 and wherein the end of said conductor extends partway across the width of said channel so as to constitute an E-plane probe.

3. A device as claimed in claim 1 or 2 and wherein means are provided for causing an antinode in the waveguide mode in the vicinity of said probe.

4. A device as claimed in claim 3 and wherein said means is constituted by an extension of the waveguide channel which extends beyond the substrate and which is terminated in a short circuit, with the length of the extension to the short circuit being controllable so as to set up the required anti-node.

5. A device as claimed in any of the preceding claims and wherein said substrate is composed of a thin plate of quartz.

6. A device as claimed in any of the preceding claims and wherein said substrate carries an electrically conductive ground plane having a window which aligns with said channel, with the ground plane being in contact with the material which defines the end of the waveguide channel so as to form an hermetic seal therewith.

7. A device as claimed in claim 6 and wherein the waveguide is formed of conductive metal which is soldered to said ground plane, with the solder encircling the end of the waveguide channel.

8. A high frequency transmission device substantially as illustrated and described with reference to the accompanying drawings.