Orthomode transducer between a circular waveguide and a coaxial cable

Abstract: The orthomode transducer between a circular waveguide and a coaxial cable consists of a circular waveguide section (WG), into which two probes (PR1,PR2) externally connected to coaxial connectors penetrate. The probe (PR2) placed close to the input aperture of the waveguide is tuned by a screw (SC2) and a metal plate (LS) belonging to the same axial plane and the other probe (PR1) is tuned by a screw and a circular disc (TS) which closes the waveguide. The shapes of the plate and of the probes are such as to allow the best power coupling between the wave guide and the coaxial line over a wide operating band (10% of the mid-band frequency).

Description: The present invention concerns microwave devices for telecommunication systems and more particularly it refers to an orthomode transducer between a circular waveguide and a coaxial cable. To increase the capacity of transmission channels between terrestrial radio link stations or between earth stations and satellites, it is usual to use at the same time two carriers with equal frequencies and orthogonal polarizations, transmitted or received by the same reflector antenna with convenient characteristics. The carriers are generally separated by waveguide devices, the so-called orthomode transducers, which are an integral part of the antenna feed; the transmission of respective signals to station apparatus is effected by means of separated waveguides or coaxial cables. The orthomode transducers must satisfy two requirements at the same time: they must ensure a satisfactory coupling of the radiofrequency signal between the antenna and transmission lines, consequently presenting a low stationary wave ratio, and on the other hand they must ensure a good isolation between the two access ports over a frequency band being at least as wide as 10% of the mid-band frequency. These electrical performances ought to be obtained by satisfying the mechanical requirements of maximum construction simplicity and reduced encumbrance. The latter property is important if the orthomode transducer is used in an antenna feed installed on board a satellite, either individually or as a part of an array. In the latter case, by reducing feed size, and hence feed weight and encumbrance, satellite launching results simpler and cheaper. In addition, still in view of its use on board of satellites, the transducer structure must present mechanical properties permitting it to remain efficient in spite of shocks suffered during the launching. More particularly, the number of parts which in consequence of vibrations might change their positions ensuring the best electrical performance, such as the parts used for frequency tuning (namely screws), is to be minimized as far as possible. An orthomode transducer is described on page 410 of the book entitled "Antennes micro-ondes" by Nhu BUI-HAI, issued by MASSON, in which the central conductors of two coaxial connectors are used as probes, placed at 90° with respect to each other and connected by a waveguide section. A metal plate is secured into this guide for the tuning of the parallel probe, as it acts as a short-circuit with respect to the radiofrequency signal. Higher performances can be achieved by the orthomode transducer provided by the present invention which presents a stationary wave ratio less than or equal to 1.1 over a band of width equal to 10% of the mid-band frequency, a isolation higher than 50 dB between the input ports and insertion losses lower than 0.05 dB. In addition its longitudinal sizes are reduced to about two wavelengths and there is a single tuning element (screw) per each probe, which entails an easy and fast setting. In cross-linear polarization systems having waveguide branches to extract the cross-polarized fields, from US-A-3 162 828 a septum is known which is placed in front of the aperture of the waveguide branch and is tapered on the side facing the waveguide input. This septum is to match the impedence of two waveguides to each other. by converting the entire waveguide into two smaller waveguides. Also, a waveguide-stripline transducer is known (US-A 3 462 713) wherein the stripconcucter extending into the waveguide has sections of differnt widths. These prior art configurations of constructional details could not, however, suggest any solutions to the above problems of orthomode transducers. The present invention provides an orthomode transducer between a circular waveguide and a coaxial cable, consisting of a section of circular waveguide, into which two probes penetrate, which are placed along two diameters belonging to orthogonal axial planes and which at the outside are connected to standard impedance coaxial connectors through constant impedance transitions, the probe close to the input aperture of the waveguide being tuned by a screw and a metal plate situated in the same axial plane and the other probe being tuned by a screw and a circular buffer disc closing the waveguide, said orthomode transducer being characterized in that the side of said metal plate adjacent the probe parallel to it is rectilinear and the opposite side facing the other probe is tapered towards the middle, and in that said probes consist of different cylindrical sections with different diameters, the first section of which allows the probe to be supported by a dielectric washer inserted in a circular hole made in the wall of the waveguide and to form with said aperture a standard impedance coaxial line, a second section of larger diameter, surrounded by a section of the hole of inferior diameter, continues the standard impedance coaxial line, and a third section of larger diameter, a fourth section of even larger diameter and a final section with a diameter equal to that of the third section, as well as their lengths, are determined according to the optimum of power coupling between the waveguide and the coaxial lines over a wide operating band. As regards the sizes of probes, they were experimentally found taking into account the goals of obtaining both the best power coupling and the largest bandwidth and of matching the required encumbrance constraints. The foregoing and other characteristics of the present invention will be made clearer by the following description of a preferred embodiment thereof given by way of a non-limiting example, and with the annexed drawing in which a longitudinal section of the orthomode transducer is shown. The orthomode transducer consists of a circular waveguide section WG, which presents an inner diameter equal to about 0.7 times the mid-band free-space wavelength, so as to allow the propagation ot the only fundamental mode. This waveguide comprises two probes PR1 and PR2, placed along two diameters belonging to orthogonal axial planes, which allow two different signals with orthogona! polarizations propagating in the guide to be extracted, or to be generated, according to whether the antenna system comprising the orthomode transducer be used in reception or in transmission. The probes are fixed to the waveguide wal! by washers RT1 or RT2 of low-loss dielectric material, inserted in circular holes of diameter D1. The narrowing of the hole to diameter D2 allows formation of a step for the washer, which thus remains blocked between the wall itself and a conical transition TR2, which is generally screwed to the external wall of the waveguide. This transition ot known type and another equal transition for the probe PR1, non-visible in the Figure, allow the probe connection with external coaxial connectors of standard impedance, e.g. 50 ohm, thus avoiding any impedance discontinuity. Each probe is tuned for the maximum power coupling by a short circuit and a screw. In the figure one can see the screw denoted by SC2. Fine-tuning screws are placed in the waveguide wall in a position diametrally opposite to the probes. During tuning, the screws allow small probe and short circuit tolerances to be compensated. The short circuit for probe PR1 is obtained by a circular disc TS, of diameter equal to the guide diameter, whilst for probe PR2 the short circuit is obtained by a metal plate LS, belonging to the same axial plane passing through probe PR2. Even this plate results so perpendicular to the other probe PR1 and presents a constant thickness equal to about 1/25 of free-space wavelength. The plate side facing probe PR2, placed close to the transducer input aperture, is rectilinear for the whole guide diameter and is parallel to the probe, while the opposite side facing probe PR1 is tapered towards the middle by two steps symmetrical with respect to the guide axis. The tapering allows a reduction equal to about 40% of interprobe distance with respect to a transducer using a non-tapered plate, the performances as to electrical isolation between coaxial ports remaining the same. Of course, the reduction of interprobe space allows an equal reduction in the orthomode transducer length to be obtained. The two probes PR1 and PR2 are mechanically equal and consist of various cylindrical sections of different diameter. A first section of diameter d1 lets the probe be supported by dielectric washer RT1 or RT2 and is such as to form a coaxial line having an impedance of about 50 ohm, by exploiting the hole of diameter D1 in the waveguide wall as external conductor. The impedance value is determined on the basis of the ratio D1/d1 and of the dielectric constant of the material the washer is made of. Analogously the section of diameter d2 forms a coaxial line with an impedance of about 50 ohm on the basis of the ratio with diameter D2 of the smaller section of the hole. A larger diameter section d3 follows, one of even larger diameter d4 and one of diameter equal to d3. Diameters d3 and d4 and penetration depth of probes inside the waveguide are optimized for the best power coupling. More particularly, the presence of the larger diameter section d4 allows good electrical performances to be attained on an operating band with a width at least equal to 10% of the mid-band frequency.

Claims: Orthomode transducer between a circular waveguide and a coaxial cable, consisting of a section of circular waveguide (WG), into which two probes (PR1, PR2) penetrate, which are placed along diameters belonging to orthogonal axial planes and which at the outside are connected to standard impedance coaxial connectors through constant impedance transitions (TR2), the probe (PR2) close to the input aperture of the waveguide being tuned by a screw and a metal plate (LS) situated in the same axial plane and the other probe (PR1) being tuned by a screw and a circular disc (TS) closing the waveguide, said orthomode transducer being characterized in that the side of said metal plate (LS) adjacent the probe (PR2) parallel to it is rectilinear and the opposite side facing the other probe (PR1) is tapered towards the middle, and in that said probes (PR1, PR2) consist of different cylindrical sections with different diameters, the first section (dl) of which allows the probe to be supported by a dielectric washer (RT1, RT2) inserted in a circular hole made in the wall of the waveguide and to form with said hole a standard impedance coaxial line, a second section (d2) of larger diameter, surrounded by a section (D2) of the hole of inferior diameter, continues the standard impedance coaxial line, and a third section (d3) of larger diameter, a fourth section (d4) of even larger diameter and a final section with a diameter equal to that of the third section (d3), as well as their lengths, are determined according to the optimum of power coupling between the waveguide and the coaxial lines over a wide operating band. Orthomode transducer as in claim 1, characterized in that the side of said metallic plate (LS) opposite to the side facing the probe (PR2) parallel to it is tapered towards the middle part by two steps symmetrical with respect to the guide axis. Orthomode transducer as in claim 1 or 2, characterized in that said metal plate (LS) presents a constant thickness equal to about 1/25 of the free-space wavelength.

Inventors: AICARDI ANTONELLO, MASSAGLIA PIERCARLO, AICARDI, ANTONELLO, MASSAGLIA, PIERCARLO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1991
Main classification: H01P1
IPC codes: H01P1, H01P5