IMPROVED DUAL POLARISATION WAVEGUIDE PROBE SYSTEM

Abstract: An improved dual polarisation waveguide probe system is described which consists of a reflective twist plate (30) within the probe housing (14) and which has at least two signal reflecting edges (34a, 34b) so that at least two separate signal reflections are created. The multiple signal reflections enable the probe system to operate over a wider frequency range with minimal deterioration in signal output. In a preferred arrangement, this is achieved by making the reflecting twist plate stepped and by providing two steps (34a, 34b) spaced at different distances from the waveguide short circuit (32). The leading, reflecting, edges of the steps (34a, 34b) are orthogonal to the waveguide axis. Embodiments of the invention are described.

Description: IMPROVED DUAL POLARISATION WAVEGUIDE PROBE SYSTEM The present invention relates to a dual polarisation waveguide probe system for use with a satellite dish for receiving signals broadcast by a satellite which include two signals orthogonally polarised in the same frequency band. In particular, the invention relates to an improved waveguide for use with a low-noise block receiver into which two probes are disposed for coupling from the waveguide desired broadcast signals to external circuitry. In applicant's co-pending Published International Application W092/22938 there is disclosed a dual polarisation waveguide probe system in which a waveguide is incorporated into a low-noise block receiver in which two probes are located for receiving linearly polarised energy of both orthogonal senses. The probes are located in the same longitudinal plane on opposite sides of a single cylindrical bar reflector which reflects one sense of polarisation and passes the orthogonal signal with minimal insertion loss and then reflects the rotated orthogonal signal. The probes are spaced λ/4 from the reflector. A reflection rotator is also formed at one end of the waveguide using a thin plate which is oriented at 45° to the incident linear polarisation with a short circuit spaced approximately a quarter of a wavelength (λ/4) behind the leading edge of the plate. This plate splits the incident energy into two equal components in orthogonal planes, one component being reflected by the leading edge and the other component being reflected by the waveguide short circuit. The resultant 180° phase shift between the reflected components causes a 90° rotation in the plane of linear polarisation upon recombination so that the waveguide output signals are located in the same longitudinal plane . The above waveguide probe system has been found to perform well for the purpose for which it was designed; to provide significant signal isolation better than 40 dBs. across the current Astra satellite bandwidth being 10.7 - 11.8 GHz. and across other bandwidths such as 11.7 - 12.2 GHz. for DBS and 12.2 - 12.75 GHz. However, there has been a trend to increase the frequency range transmitted by new satellite systems. In fact, the frequency bandwidth is planned to increase from 10.7 - 11.8 GHz. to 10.7 - 12.75 GHz. on the Astra system in the near future. With the aforementioned design it has hitherto not been possible to use a single LNB or waveguide to cover this wider frequency range, the frequency range being covered by two or more LNBs which are tuned to cover part of the frequency range, for example 10.7 - 11.8 GHz. and 11.7 - 12.2 GHz. The existing LNB has been found to be frequency limited because of the bandwidth achieved by the reflection rotation of the existing design. It is an object of the present invention to provide an improved dual polarisation waveguide probe system which obviates or mitigates at least one of the aforementioned disadvantages. It is a further object of the invention to provide a dual polarisation waveguide probe system which covers all Astra satellite bandwidths in a single LNB. It is a further object of the present invention to provide an improved dual polarisation waveguide probe system with equivalent ease of manufacture to the existing waveguide probe system. This is achieved by providing a reflective twist plate within the probe housing which has at least two signal reflecting edges so that at least two separate signal reflections are created. The multiple signal reflections enable the probe system to operate over a wider frequency range with minimal deterioration in signal output. In a preferred arrangement, this is achieved by making the reflecting twist plate stepped and by providing two steps spaced at different distances from the waveguide short circuit. The leading, reflecting, edges of the steps are orthogonal to the waveguide axis. In an alternative arrangement, the reflecting twist plate may be replaced by a three step reflecting edge or by a castellated edge such that there are multiple spaced reflecting edges. This can be achieved by casting a probe system in which the waveguide has a two or three step reflecting twist plate. Alternatively, the single reflecting edge of an existing twist plate may be drilled to a predetermined depth into the twist plate to create separate reflecting edges. Alternatively, the reflecting edge may be provided by a continuous leading edge such as an oblique line or a curve or a series of curves. According to a first aspect of the present invention there is provided apparatus for receiving at least two signals which are orthogonally polarised, said apparatus comprising a waveguide into which at least two orthogonally polarised signals are received for transmission therealong, said waveguide having; a first probe extending from a wall of the waveguide into the interior of the waveguide, said first probe being adapted to receive said orthogonal signal travelling in the same longitudinal plane thereof, reflector means extending from the wall of the waveguide, said reflector means located downstream of said first probe and lying in said longitudinal plane for reflecting signals in said first orthogonal plane back to said first probe means and allowing said signal in said second orthogonal plane to pass along the waveguide, second probe means located downstream of said first reflector means and extending from said wall of said housing into the interior of said waveguide and lying in said longitudinal plane, signal reflecting and rotating means, including a short circuit at the end of the waveguide, located downstream of said second probe means for receiving, rotating and reflecting said second orthogonally polarised signal back along said waveguide such that said rotated and reflected signal is received by said second probe means, the first and second probes having respective first and second outputs located on the outside of the waveguide, the first and second outputs lying in substantially the same longitudinal plane characterised in that the reflecting and rotating means has a leading edge configured to provide at least two reflecting edge portions thereon, said edge portions being spaced at different distances from the short circuit at the end of the waveguide whereby said second orthogonally polarised signal is reflected from each of said reflecting edge portions for recombination with the signal reflected from the short circuit to provide a signal for detection by the second probe means. Preferably, said at least two reflecting edge portions are provided by spaced steps of equal width which are generally orthogonal to the waveguide axis of the waveguide. Alternatively, the reflected edge portions are provided by three spaced reflecting edges of equal length. The edges may be of different lengths. Conveniently, the reflecting edges are orthogonal to the waveguide axis and are spaced from the short circuit by a predetermined distance for minimising signal loss across the required bandwidth. In yet a further modification the reflecting edge may be provided by an edge which is not orthogonal to the waveguide axis, for example an oblique edge or a curved edge. According to another aspect of the present invention there is provided a method of receiving at least two orthogonally polarised signals in the frequency range 10.7 - 12.75 GHz. in a single waveguide and providing at least two outputs in a common longitudinal plane, said method comprising the steps of, providing a first probe in said waveguide to receive first orthogonally polarised signal, providing a reflector means in said waveguide parallel to and downstream from said first probe for reflecting said first orthogonally polarised signal and for allowing passage of said second orthogonally polarised signal, providing a second probe in said waveguide parallel to and downstream of said reflector means, said second probe being substantially orthogonal to said second polarised signal which passes the second probe without being received by said second probe, providing a rotating and reflector means at the end of the waveguide downstream of said second probe with a waveguide short circuit downstream of the reflector means, for receiving said second orthogonally polarised signal and for reflecting said second signal back along said waveguide towards said second probe, said rotating and reflecting means being oriented at an angle of 45° to said common longitudinal plane, said second signal also being rotated to lie in the same longitudinal plane as said second probe and to be received by said second probe, and taking outputs from the first and second probes on the outside of waveguide, the outputs being disposed in substantially the same longitudinal plane, characterised in that said method includes the steps of reflecting a portion of said second orthogonal signal from each of said reflecting edge portions and a portion from the short circuit at the end of the waveguide, the reflected signal portions being phase shifted so that they recombine to provide a resultant signal in the plane of said second probe means for detection thereby. These and other aspects of the invention will become apparent from the following description when taken in combination with the drawings in which: Fig. 1 is a partly broken-away view of a low-noise block receiver with a waveguide probe including a reflecting twist plate in accordance with a preferred embodiment of the present invention; Fig. 2 is a cross-sectional view of the waveguide taken on section 2-2 of Fig. 1; Figs. 3a, b and c are graphs comparing the responses of a twist plate with a single reflecting surface and with two reflecting surfaces where Fig. 3a is a graph of a transmission loss versus frequency, Fig. 3b is a graph of phase shift of the signal hitting the leading edge of the twist plate compared to the short circuit versus frequency, and Fig. 3c is a graph of signal return less in dB. versus frequency, and Figs. 4a to h are side views of reflecting twist plates with multiple reflecting surfaces in accordance with alternative embodiments of the invention. Reference is first made to Fig. 1 of the drawings in which a low-noise block receiver, generally indicated by reference numeral 10, is adapted to be mounted to a satellite receiving dish in a way which is well known in the art. As is also known, the low-noise block receiver 10 is arranged to receive high frequency radiation signals from the satellite dish and to process these signals to provide an output which is fed to a cable 12 which is, in turn, connected to a satellite receiver decoder unit (not shown in the interests of clarity) . The block receiver 10 includes a waveguide 14 which is shown partly broken away to depict the interior components. The waveguide is cylindrical and is made of metal. The waveguide has a front aperture 16 for facing a satellite dish for receiving electro-magnetic radiation from a feed horn 18, shown in broken outline, which is mounted on the front of the waveguide. The waveguide is substantially the same as that disclosed in applicant's co-pending Published International Application W092/22938. Thus, disposed within the waveguide in the same longitudinal plane is a first probe 20, a reflective post 22 and a second probe 24. In this embodiment, it will also be appreciated that the reflective post 22 does not extend the entire diameter of the interior of the waveguide for reasons disclosed in the aforementioned W092/22938 specification. The outputs of the probes 20 and 24 pass through the waveguide wall 26 along the same longitudinal plane generally indicated by reference numeral 28. The probes 20,24 are of the same length so that the outputs lie along the same longitudinal axis within the longitudinal plane 28. The distance between the probe 20 and reflective post 22 and probe 24 and reflective post 22 is nominally λ/4 where λ is the wavelength of the signals in the waveguide. At the downstream end of the waveguide which is the furthest end from the front aperture, there is disposed within the waveguide a reflecting and rotating or twist plate 30. As best seen in Fig. 2 the reflecting and rotating plate is oriented at an angle of 45° to the probes 20,24 and post 22. The furthest end of the plate terminates in wall 32 which acts as a short circuit which will be later explained in detail. It will be seen that the reflecting plate is thin and has a leading edge formed of two step edges 34a,b of equal length and about the same thickness. The step edges 34a,b are orthogonal to the waveguide axis. Step 34a is further from the short circuit 32 than step 34b. With this arrangement it will be appreciated that there are two reflective edges at the leading end of the reflecting plate spaced by different amounts from wall 32. In operation, signals from a satellite dish enter the waveguide 14 via the horn 18 and aperture 16 and in accordance with known principles are transmitted along the waveguide 14. The signals which are broadcast by the satellite include two sets of signals which are orthogonally polarised in the same frequency band and these are represented by vectors VI and V2 which are signals polarised in the vertical and horizontal planes respectively. As the signals travel along the waveguide the vertically polarised signal VI is received by the first probe 20 which, as it is spaced by λ/4 from the reflecting post 22, ensures the maximum field at the probe and hence optimum coupling to the probe. The probe 20 has no effect on the horizontally polarised signal V2 which continues to pass along the waveguide. As the reflecting post is vertically oriented the signal V2 is not reflected by the post and continues to pass along the waveguide 14 and also passes the second probe 24 for the same reason. As the horizontally polarised signal V2 passes along the waveguide it encounters step edge 34a,b, of the thin metal twist plate 30 which is about 1 - 1.5 mm. thick. When the horizontally polarised signal V2 encounters the plate 30, one component V2 p of the signal parallel to the plate encounters edges 34a,b; a first portion of the component is reflected by edge 34a and a second portion is reflected by edge 34b. The orthogonal component to V 2P , V 20 , is reflected by the short circuit 32 at the rear of the plate and is rotated by 180° shown as vector V20 R in broken outline in Fig. 2. The distance of step 34a from short circuit 32 corresponds to a quarter of a wavelength (λ-74) of a first frequency (f 2 ) near the lower end of the Astra frequency band and the distance of the step 34b from short circuit 32b corresponds to wavelength (λ 2 /4) of frequency f 2 at the upper end of the frequency band. The signals reflected from edges 34a,34b are out of phase and are represented by phase shifted vector V2 PRa , V2 PRb . The reflected signal (V 20R ) are recombined with the short circuit reflected signals to create a recombined vector V2 RC0MB , shown in broken outline, in the plane of probes 20,24. The reflected and recombined signal indicated by vector V 2RC0MB then travels towards probe 24 in the longitudinal plane which is received by probe 24 and conducted to the probe output. Probe 24 is spaced from post 22 by a quarter of a wavelength which ensures maximum field at the probe and hence optimum coupling. With this arrangement it will be understood that the total signal received at probe 24 consists of a combination of reflected and rotated signals and because the signal component from edges 34a,34b are not in-phase, the amplitudes on recombination may be less, in some cases, than the amplitude for a single straight reflecting edge as in the prior art. The reduction in signal amplitude is not significant. However, the isolation provided by this waveguide with the stepped reflecting twist plate is not substantially different to that disclosed in the applicant's aforementioned publication W092/22938. With this arrangement it will be appreciated that for different frequencies of transmitted signal the spacing between the various steps and short circuit corresponds more closely to particular wavelengths. Thus the waveguide is tunable by selecting the distance of step 34a at a distance λ/4 from the short circuit 32 where λ corresponds to a frequency at the lower end of the frequency range, for example 11.0 GHz. and step 34b is set at a distance to correspond to wavelengths at a higher frequency, for example 12.2 GHz. Such a bandwidth in a single waveguide was not possible with the aforementioned prior art waveguide and reflecting twist plate because of the single distance of the leading edge from the short circuit corresponding to a quarter wavelength at a single frequency. Thus, the stepped arrangement disclosed in Figs. 1 and 2 allows the low- noise block to be used to receive a wider range of frequencies; the bandwidth of the detector is substantially increased. There is, however, some loss in signal amplitude but in practice this has been found to be quite acceptable for this application. Reference is now made to Figs. 3a,b,c which compare the response of a waveguide with a single edge reflector as in the prior art with a waveguide having the two step reflector plate shown in Figs. 1 and 2. The two step plate is 18.5mm wide (the width of the waveguide 14) and the first step 34a is 15.1mm from the short circuit 32 and the second step 34b is 7mm from the short circuit. The length of each step is 9.25mm and the plate 30 is approximately 1mm thick. Fig. 3a shows transmission loss (dB.) with frequency with the graphs showing the limits of the new Astra band 10.7 and 12.75 GHz. respectively. It will be seen that the response of the single reflector falls off as it approaches the lower and, more particularly, the upper band limits. The loss of about 2 dB. at the high end is unacceptable. In contrast, it will be seen that the loss with the two step plate is much less than 1 dB. and there is also minimum transmission loss at the centre frequency. Similarly, Fig. 3b shows that the phase shift deviation from 180° for the two step plate above the mid- range is less than with the single step plate which means that more signal is recombined with the correct phase shift across the frequency range. Fig. 3c is a graph of signal return loss (dB.) versus frequency which shows that the minimal signal loss occurs at the single frequency with a single plate, that is, the frequency corresponding to the λ/4 distance of the edge from the short circuit. In contrast the response from the two step plate shows that minimal signals occur at a different frequency and that there is a broader band of frequency for minimal return loss which at the upper end of the frequency range shows at least a 5 dB. improvement over the single plate reflector. Reference is now made to Figs. 4a to h of the drawings which depict side views of alternative designs of reflector twist plates. It will be seen that a twist plate with three steps may be used as shown in Fig. 5a, or four steps as shown in Fig. 4b. In addition, it will be appreciated that variable reflecting edges may be created by machining out the twist plate to form an E- type profile as shown in Fig. 4c. This E-type profile may be modified by a deeper recess as shown in Fig. 4d. It will also be understood that reflecting surfaces need not be orthogonal to the waveguide axis. The leading edge may be provided by an oblique edge as shown in Fig. 4e or a curved edge as shown in Fig. 4f. The reflecting edges may be a combination of orthogonal or oblique edges or curves as shown in Figs. 4g and 4h. In another embodiment the reflective post can also extend across the entire waveguide; the waveguide operating satisfactorily with this structure. It will be appreciated that the principal advantage of the present invention is that the reflecting plate allows the LNB to be used across a much greater bandwidth than the aforementioned prior art LNB. Consequently, a single LNB may be used to detect signals across all of the presently useable satellite bandwidths between 10.7 and 12.75 GHz. A further advantage of this arrangement is that it can use existing manufacturing techniques and involves the selection of an appropriate plate for casting into the waveguide. The technique would be applicable to bandwidth improvement at other frequency ranges outside the Astra range.

Claims: CLAIMS 1. A waveguide into which at least two orthogonally polarised signals are received for transmission therealong, said waveguide having; a first probe extending from a wall of the waveguide into the interior of the waveguide, said first probe being adapted to receive said orthogonal signal travelling in the same longitudinal plane thereof, reflector means extending from the wall of the waveguide, said reflector means located downstream of said first probe and lying in said longitudinal plane for reflecting signals in said first orthogonal plane back to said first probe means and allowing said signal in said second orthogonal plane to pass along the waveguide, second probe means located downstream of said first reflector means and extending from said wall of said housing into the interior of said waveguide and lying in said longitudinal plane, signal reflecting and rotating means, including a short circuit at the end of the waveguide, located downstream of said second probe means for receiving, rotating and reflecting said second orthogonally polarised signal back along said waveguide such that said rotated and reflected signal is received by said second probe means, the first and second probes having respective first and second outputs located on the outside of the waveguide, the first and second outputs lying in substantially the same longitudinal plane characterised in that the reflecting and rotating means has a leading edge configured to provide at least two reflecting edge portions thereon, said edge portions being spaced at different distances from the short circuit at the end of the waveguide whereby said second orthogonally polarised signal is reflected from each of said reflecting edge portions for recombination with the signal reflected from the short circuit to provide a signal for detection by the second probe means. 2. A waveguide as claimed in claim 1 wherein said at least two reflecting edge portions are provided by spaced steps of equal width which are generally orthogonal to the waveguide axis of the waveguide. 3. A waveguide as claimed in claim 1 wherein the reflected edge portions are provided by three spaced reflecting edges of equal length. 4. A waveguide as claimed in any one of claims 1 to 3 wherein the edges are of different lengths. 5. A waveguide as claimed in any preceding claim wherein the reflecting edges are orthogonal to the waveguide axis and are spaced from the short circuit by a predetermined distance for minimising signal loss across the required bandwidth. 6. A waveguide as claimed in any one of claims 1 to 4 wherein the reflecting edge is provided by an edge which is not orthogonal to the waveguide axis. 7. A method of receiving at least two orthogonally polarised signals in the frequency range 10.7 - 12.75 GHz. in a single waveguide and providing at least two outputs in a common longitudinal plane, said method comprising the steps of, providing a first probe in said waveguide to receive first orthogonally polarised signal, providing a reflector means in said waveguide parallel to and downstream from said first probe for reflecting said first orthogonally polarised signal and for allowing passage of said second orthogonally polarised signal, providing a second probe in said waveguide parallel to and downstream of said reflector means, said second probe being substantially orthogonal to said second polarised signal which passes the second probe without being received by said second probe, providing a rotating and reflector means at the end of the waveguide downstream of said second probe with a waveguide short circuit downstream of the reflector means, for receiving said second orthogonally polarised signal and for reflecting said second signal back along said waveguide towards said second probe, said rotating and reflecting means being oriented at an angle of 45° to said common longitudinal plane, said second signal also being rotated to lie in the same longitudinal plane as said second probe and to be received by said second probe, and taking outputs from the first and second probes on the outside of waveguide, the outputs being disposed in substantially the same longitudinal plane, characterised in that said method includes the steps of reflecting a portion of said second orthogonal signal from each of said reflecting edge portions and a portion from the short circuit at the end of the waveguide, the reflected signal portions being phase shifted so that they recombine to provide a resultant signal in the plane of said second probe means for detection thereby.

Inventors: BAIRD ANDREW PATRICK, BAIRD, ANDREW, PATRICK
Applicants: BAIRD ANDREW PATRICK, CAMBRIDGE IND LTD, BAIRD, ANDREW, PATRICK, CAMBRIDGE INDUSTRIES LIMITED
Application year: 1996
Main classification: H01P1
IPC codes: H01P1