Mode transformer of waveguide and microstrip line, and receiving converter comprising the same

Abstract: The invention presents a mode transformer of waveguide and microstrip lines capable of obtaining a sufficient cross polarization distinction, and a receiving converter comprising the same. By orthogonal first and second microstrip lines and a circular waveguide, transformers of horizontal and vertical polarized waves are composed, and by disposing a metal bar connected to the waveguide between the first and second microstrip lines, the metal bar has the earth potential same as the potential of the circular input waveguide, and the electric fields excited in the first and second microstrip lines go toward the direction of the metal bar, so that coupling of the first and second microstrip lines by electric fields may be avoided.

Description: FIELD OF THE INVENTION The present invention relates to a mode transformer of waveguide and microstrip line for receiving signals from, for example, a communication satellite, and a receiving converter comprising this mode transformer. BACKGROUND OF THE INVENTION Broadcasting and communication utilizing communication satellites are becoming common recently. Radio signals sent from communication satellites consist of both horizontally polarized waves and vertically polarized waves depending on the changing direction of the electric field for transmitting two programs by one channel. A coupling structure of conventional waveguide and microstrip line used in the satellite receiving converter for receiving the both polarized waves is disclosed, for example, in Japanese Laid-open patent publication no. 3-36243. In the coupling structure of waveguide and microstrip line disclosed in this publication, as shown in Fig 1A relating to a coupling structure of waveguide and microstrip line and Fig 1B showing section A-A' in Fig 1A, probe antennas 31, 32 for receiving horizontal and vertical polarized waves are disposed so as to be orthogonal to each other in a circular waveguide 1. In a satellite receiving converter comprising a coupling structure of waveguide and microstrip line, as other prior art, as shown in Fig 2A relating to the structure of a satellite receiving converter comprising a mode transformer of waveguide and microstrip line, and Fig 2B showing a sectional view in arrow B-B' direction in Fig 2A, a dielectric substrate 4 is inserted into a circular waveguide 1, mode transformers 51, 52 of waveguide and microstrip line are formed by microstrip lines on the dielectric substrate 4 respectively for horizontal polarized waves and vertical polarized waves, and these mode transformers 51, 52 of waveguide and microstrip line are disposed orthogonally to each other. In such conventional coupling structure of waveguide and microstrip line and satellite receiving converter comprising it, when receiving radio waves from a communication satellite emitting both horizontal polarized waves and vertical polarized waves, both probe antennas for receiving horizontal polarized waves and vertical polarized waves, or both microstrip lines for forming mode transformers of waveguide and microstrip line for horizontal polarized waves and vertical polarized waves are mutually coupled by electric field, and a sufficient cross-polarization distinction as converter cannot be obtained. EP-A-0350324 discloses a capacitively coupled printed patch as a high efficiency device to couple orthogonally polarized energy between a stripline and a waveguide. Coupling between the stripline and the patch is achieved by the stripline terminating in a narrow strip probe the end of which lies close to, but not in contact with, an edge of the patch. Two separate probes arranged mutually orthogonally are used to effect independent polarized couplings to produce independent linear orthogonal signals or independent left- and right-handed circularly polarized signals. The striplines and patch are supported on a common substrate which extends transversely through the waveguide. The waveguide wall has a quarter-wavelength thickness so that its inner edge appears continuous to energy passing through the substrate. A further construction of a waveguide coupling structure is shown in JP-A-5-226906 where first and second conductors are arranged at right angles to each side in the same plane perpendicular to the axis of a circular waveguide. A third, stub conductor is arranged in the same plane but at an angle of 45° to each of the first and second conductors in order to obtain a sufficient cross-polarized wave characteristic. A coaxial-waveguide converter is disclosed in DE-A-4213539 where a circular waveguide is open at one end for the introduction of electromagnetic waves and closed at its other end by a short-circuiting wall and a conducting rod is attached to the short-circuiting wall so that the rod is disposed along the axial line of the circular waveguide. Two probes to pick up coaxial-mode electric signals from waveguide-mode waves are disposed on a plane perpendicular to the axial line of the circular waveguide so that the axial lines of the respective probes intersect at right angles on the axial line. When waveguide-mode vertically polarized and horizontally polarized waves come into the circular waveguide, a coaxial-mode electric signal is induced from the vertically polarized wave in one of the probes and another coaxial-mode electric signal is induced from the horizontally polarized wave in the other of the probes. SUMMARY OF THE INVENTION The present invention provides a mode transformer for a waveguide and microstrip lines comprising: a circular waveguide, first and second conductors disposed in the waveguide orthogonally relative to each other corresponding to a horizontal polarized wave and a vertical polarized wave, with both ends closely isolated on a plane perpendicular to the axis of the circular waveguide, and a third conductor at the same potential as the circular waveguide being disposed closely to and isolated from each end of the first and second conductors, characterised in that: the first and second conductors are microstrip lines formed on an insulating substrate which is disposed on the plane perpendicular to the axis of the waveguide, and the third conductor extends in parallel with the axis of said circular waveguide from an end of the waveguide and passes through an opening formed on said insulating substrate. The satellite receiving converter of the invention comprises the mode transformer of waveguide and microstrip line described above. In this constitution, the third conductor is at the earth potential same as the potential of the circular waveguide, and the electric fields excited in the first and second conductors disposed orthogonally corresponding to the horizontal polarized wave and vertical polarized wave come to have a polarity going toward the direction of the third conductor, so that coupling of the first and second conductors by electric field does not occur. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1A is a structural side view of coupling of waveguide and microstrip line in prior art; Fig 1B is a sectional view of arrow A-A' direction in Fig 1A; Fig 2A is a structural side sectional view of a satellite receiving converter having a coupling structure of waveguide and microstrip line in other prior art; Fig 2B is a sectional view of arrow B-B' direction in Fig 2A; Fig 3 is a structural side sectional view of a satellite receiving converter comprising a mode transformer of waveguide and microstrip line in an embodiment of the invention; and Fig. 4 is a sectional view in arrow C-C' direction in Fig. 3. PREFERRED EMBODIMENT OF THE INVENTION Referring now to the drawings, a satellite receiving converter comprising a mode transformer of waveguide and microstrip line in a preferred embodiment of the invention is described in detail below. Fig. 3 is a structural diagram of the satellite receiving converter comprising the mode transformer of waveguide and microstrip line of the embodiment. In Fig. 3, a converter casing 3 is disposed closely to an input waveguide 1 which is a circular waveguide, and an insulating substrate 4 is disposed, with one end inserted in the input waveguide 1 and the other end straddling in the converter casing 3. The insulating substrate 4 is arranged to have a plane perpendicular to the axial direction of the input waveguide 1. In the portion of the insulation substrate 4 within the input waveguide 1, microstrip lines 51, 52 which are first and second conductors corresponding to horizontal polarized waves and vertical polarized waves respectively are formed orthogonally, with their ends isolated from each other, thereby composing a mode transformer of waveguide and microstrip lines. A metal bar 2 which is a third conductor is connected at same potential as the input waveguide 1, and penetrates through the insulating substrate 4 and is arranged so as not to contact any isolated end portion of the microstrip lines 51, 52. The insulating substrate 4 is generally composed of dielectric material, and composes a converter circuit in a certain portion in the converter casing 3. The output of the converter circuit is issued from a signal output plug 6. Fig. 4 is a sectional view in arrow C-C' direction of the satellite receiving converter shown in Fig. 3, and shows the configuration of metal bar and mode transformer by waveguide and microstrip lines corresponding to horizontal polarized waves and vertical polarized waves on the dielectric substrate, respectively. In Fig. 4, the microstrip line 51 forms the mode transformer for receiving horizontal polarized waves together with the waveguide 1. The microstrip line 52 forms the mode transformer for receiving vertical polarized waves together with the circular waveguide 1. The microstrip line 51 and microstrip line 52 are arranged orthogonally in a plane perpendicular to the axis of the circular waveguide 1 on the dielectric substrate 4. A hole is opened in the dielectric substrate 4, and the metal bar 2 connected at same potential as the circular waveguide 1 is inserted between the microstrip lines 51 and 52. In thus constituted satellite receiving converter, the operation is described below. A radio wave containing horizontal and vertical polarized wave components from a communication satellite enters the circular input waveguide 1, the horizontal polarized wave components are received by the microstrip line 51 for receiving horizontal polarized waves, and the vertical polarized wave components are received by the microstrip line 52 for receiving vertical polarized waves. Corresponding to the received polarized wave components, electric fields are excited respectively in the microstrip lines 51, 52, and between the microstrip lines 51, 52, there is the metal bar 2 of same potential as the input waveguide 1, close to but not contacting with the microstrip lines 51, 52, and hence the excited electric fields of the microstrip lines 51, 52 are immediately directed to the nearby metal bar 2, so that the microstrip lines 51, 52 are not coupled with each other through electric fields. In this operation, when receiving radio waves from communication satellite containing both horizontal polarized waves and vertical polarized waves, a sufficient cross polarization distinction as converter may be obtained. Thus, according to the invention, the metal bar is at the earth potential same as the potential of the circular waveguide, and the electric fields excited in each microstrip line for composing the mode transformer between the waveguide and microstrip lines corresponding to horizontal polarized waves and vertical polarized waves come to have a polarity going toward the metal bar, so that mutual coupling of the microstrip lines through electric fields may be avoided. As a result, when receiving radio waves from communication satellite containing both horizontal polarized waves and vertical polarized waves, a sufficient-cross polarization distinction as converter may be obtained. In the embodiment, the first and second conductors may be also realized by conductors in the prior art, instead of the microstrip lines formed on an insulating board. The case of receiving radio waves from communication satellite is explained herein, but the invention is not limited to reception from communication satellite, but may be modified within the claimed scope, not limited to the illustrated embodiment alone.

Claims: A mode transformer for a waveguide and microstrip lines comprising: a circular waveguide (1), first and second conductors (51, 52) disposed in the waveguide orthogonally relative to each other corresponding to a horizontal polarized wave and a vertical polarized wave, with both ends closely isolated on a plane perpendicular to the axis of the circular waveguide (1), and a third conductor (2) at the same potential as the circular waveguide (1) being disposed closely to and isolated from each end of the first and second conductors (51, 52), characterised in that: the first and second conductors (51, 52) are microstrip lines formed on an insulating substrate (4) which is disposed on the plane perpendicular to the axis of the waveguide, and the third conductor (2) extends in parallel with the axis of said circular waveguide (1) from an end of the waveguide (1) and passes through an opening formed on said insulating substrate (4). A mode transformer of waveguide and microstrip lines of claim 1 wherein the insulating substrate (4) is a dielectric material. A mode transformer of waveguide and microstrip lines of any one of claims 1 or 2, wherein the third conductor (2) is a metal bar. A receiving converter having a mode transformer according to any one of the preceding claims, wherein a converting circuit is disposed outside of the circular waveguide.

Inventors: KINOSHITA AKIRA, YOSHIMURA YOSHIKAZU, KINOSHITA, AKIRA, YOSHIMURA, YOSHIKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H01P1
IPC codes: H01P5, H01P1