ANTENNA DUPLEXER IN WAVEGUIDE, WITH NO TUNING BENDS

Abstract: Antenna duplexer including: an elongated hollow body (1) realized with two opposed half shells; a first opening (2), or antenna port; a second and a third opening (2), or ports of the transmitter and of the receiver; a filtering structure (5), including a plurality of metal inserts (6), available to uncouple the transmitter from the receiver. According to the invention the duplexer has at least an additional portion of waveguide (8) structured in order to keep the wave "under the cut-off frequency", interposed between at least second or third opening (2) and the relative end wall (3) of the elongated body (1). Such additional portion of waveguide including an additional metal insert (9), forming part of the above mentioned filtering structure (5), which determines an exponential attenuation of the signal, thus nullifying the negative effects deriving from the mechanical tolerance of the hollow body (1).

Description: ANTENNA DUPLEXER IN WAVE GUIDE, WITH NO TUNING BENDS" Field of the Invention The present invention relates to a particular "no tuning" filtering structure in rectangular wave guide with bends, for the connection of an antenna to a transmitter and a receiver In the specific technique, said filtering structure is identified by the term "antenna duplexer" and in particular it consist in a device enabling the simultaneous use of a same antenna both by the transmitting and by the receiving equipment In general, the operating frequency of the transmitting equipment differs from the operating frequency of the receiving equipment It is known that antenna duplexers for radio links, generally include a rectangular wave guide having three opening sides or ports A band pass filter is associated to each one of the two side ports, tuned on trasmitter and receiver operating frequency, while a "T" junction is associated to the central port (or antenna port), so that a side port and the antenna port are electrical matched for example on a first frequency band coinciding with the operating band of the transmitter and the other side port and the antenna port are matched on a second frequency band coinciding with the operating band of the receiver Therefore, the subject duplexer has the function to convey the received signals from the antenna towards the receiver and also to send the trasmitted signals to the antenna In particular, duplexers with bends are configured in such a way to have two mentioned side ports above on a side of the guide, thus enabling a considerably reduction of the overall dimensions of the whole transmission system Band-pass filters, in rectangular wave guide, are usually made by resonant cavities inside the guide, which are coupled among them through physical discontinuities obtained inside the guide itself A type discontinuities used to implement these band pass filters, is a series of thin traverse metal inserts (the relative filters are therefore identified by the name "metal insert" filters) that cross some portions of the cavity of the wave guide The filtering characteristics are determined by the length of these metal inserts and by the distance between one insert and the other one Therefore, it is clear that to obtain the best filtering characteristics, the accuracy in the realization and the positioning of the metal inserts inside the wave guide are of particular importance Background art This accuracy of realization and assembling is obtained by manufacturing the metal inserts in one piece only, with a rectangular frame inserted in the wave guide along the longitudinal midplane parallel to the wall where the opening sides are obtained To this purpose, the wave guide is subdivided into two half shells, between is placed the rectangular frame including a plurality of inserts and openings, which behave as resonant cavities placed in such a way to perform the desired filtering function It must be considered that the above mentioned rectangular frame can be manufactured through highly accurate mechanical techniques, such as for example electroerosion, and that these workings enable to obtain an accuracy in the range of microns, that is an accuracy higher than the allowed error margin, which is in the range of the hundreth of millimetre In known duplexers with bends, the ending parts of the internal cavities of the wave guide close on a short circuit, made of the end walls of the wave guide itself It must be considered that the bend is electrically matched if only the above mentioned short circuit is placed at a well defined distance from the axis of the relative side port, as it will result more clear hereafter, making reference to figure 2 However, it must be kept in mind that the end walls of the wave guide are made through traditional mechanical working, such as for example milling operations and even using high accuracy numeric control equipment it is not possible to obtain the accuracy required for the subject applications which, as said above, must be in the range of the hundreth of millimetre A recurring problem of the known type duplexers is therefore the difficulty to space the above mentioned end wall of the guide, with the due accuracy, from the axis of the relative port There is also the technical problem to compensate the negative effects deriving from the bending radius of the mill itself which, though small size mills are used, always cause a working radius between the internal walls of the cavity forming the wave guide itself These working tolerances negatively alter the filtering characteristics of the signal inside the wave guide, particularly in the case the portion of the cavities adjacent to an end wall is used as resonator In fact, in these conditions the working error of the mill highly influence both the tuning frequency of the bend (it is a problem for the electrical bend matching) and the frequency tuning of the above mentioned filter (when the last cavity of the filter actually coincides with the bend itself) In the solutions of the known type, tuning screws are generally employed, that are allowed to penetrate at least in part the wave guide and through adjustment of the penetration depth it is possible to compensate the negative effects deriving from the above mentioned working tolerance However, it must be kept in mind that the presence of these screws involves the introduction of a tuning step in production process and consequently the addition of a non negligible cost item Objects of the Invention It is an object of the invention to overcome the disadvantages of the prior art, that is to implement a duplexer whose filtering characteristics are not affected by the mechanical tolerance of the working of the internal cavities of the guide itself, so that it is possible to avoid the use of the mentioned tuning screws and realize a so-called "no tuning" duplexer Summary of the Invention The above object is solved by the combination of features of the main claim The appended claims disclose further advantageous embodiment of the invention Therefore, considering that in known type duplexers with bends the two end walls of the wave guide form a short circuit, the invention consists in closing the cavity on a inductive load instead on a short circuit In particular, said inductive load is realized through the insertion at least one end of the guide of an additional section of guide, "under the cut-off frequency", in which the size of the guide are such not to enable the signal propagation According to the invention these guide sections are realized through insertion in the guide of a metal insert, essentially identical in thickness and position to that used to make the filter mentioned above It results evident that if said additional metal insert is made during the manufacturing process of the metal inserts forming part of the filtering structure mentioned above, and in particular if a unique frame is realized, including both the metal inserts of the filtering structures and said additional metal inserts forming the above mentioned inductive load, it is possible to take advantage of the benefits deriving from the use of the electroerosion process to nullify the negative effects deriving from the mechanical tolerance related to the above mentioned milling operation Actually in these sections of guide ."under the cut-off frequency", the signal have an exponentially attenuation in its propagation from the relative side port towards the end walls or short-circuit Once the signal has reached (highly attenuated) the end walls, it is reflected and have an additional exponential attenuation during propagation in the opposite direction. The negative effects deriving from the above mentioned working tolerance are more attenuated as higher is the length of the two additional sections of guide "under the cut-off frequency". The new critical distance to be considered is therefore the one existing between the axis of the port and the above mentioned additional metal insert, which as said above, can be manufactured through electroerosion and therefore with an error rate neatly lower than the maximum accepted one. Therefore, the invention enables to avail of the advantages deriving from the electroerosion technology to electrically match both the above mentioned filter and the bend (no technological difference between filter and bend exists no more) Brief description of the figures The invention, together with further objects and advantages thereof, may be understood with reference to the following description, taken in conjunction with the accompanying drawings and in which: - figure 1 shows an axonometric view of the wave guide according to the present invention; and - figure 2 shows a cross view of the wave guide of figure 1. Detailed description of a preferred embodiment of the invention Making reference to figure 1 , we notice that the duplexer according to the present invention includes in a known way a hollow body 1 of conductive material having the shape of an elongated parallelepiped. A side wall of such hollow body includes three rectangular openings 2: a central one and two close to each end wall 3. Each opening 2 is preferably surrounded by a rectangular flange 4 used to fasten the antenna, the receiver and the transmitter (not shown in the figures). The hollow body 1 is also divided into two half shells 1 a and 1b along the longitudinal midplane parallel to the side wall on which the openings are obtained 2. In this way, during the assembling of the wave guide, it is possible to place between the two half shells 1a and 1 b a thin steel rectangular frame 5 that, according to the invention, has essentially the same length and the same height of the hollow body 1. Making now reference also to figure 2, we see that the frame 5 includes in a conventional way a plurality of traverse inserts 6 required to filter the signals transmitted through the wave guide. In particular, figure 2 shows ten inserts 6 gathered in two groups of four, which are placed in the two sections of the hollow body 1 comprised among the three openings 2. It is evident that in other embodiments of the duplexer the number and/or position of the inserts 6 can vary according to the filtering requested characteristics In the conventional type duplexers with bends, the wave guide ends coinciding with the dotted line identified with the reference number 7 in figure 2 In this figure, L1 identifies the distance between the axis of any of the two side ports 2 and the dotted line 7 Said distance L1 is particularly critical in the known type duplexers with bends for the reasons mentioned above The duplexer according to the invention on the contrary have two additional sections of wave guide "under the cut-off frequency" identified 8 and 8', respectively showing a length L2 Said additional sections of wave guide are divided by an additional metal insert 9 and 9' protruding from each end wall 3 towards the cavity of the hollow body 1 Said metal inserts 9 and 9' divide the ends of the hollow body 1 in two essentially equal parts, in such a way that the electromagnetic signals transmitted through the wave guide are exponentially attenuated when they reach end walls 3 and 3' In order that the presence of inserts 9 and 9' do not interfere with the signal transmitted through the opening sides 2 (placed close to the end walls 3), the end walls of the hollow body 1 are preferably moved away in such a way that the distance of the opening sides 2 from the closer end wall 3 results higher than the length of the metal insert 9 projecting from the end wall itself According to the presence of the metal foil projecting inside each end wall, the ending portion of the inside cavity of the wave guide closes with an inductive load and no more with a short circuit, as in the conventional wave guides Said inductive load makes the wave guide less sensitive to the working mechanical tolerance, since the electromagnetic field close to each end wall is exponentially attenuates Therefore, if the metal insert 9 or 9' is long enough, the electromagnetic field reflected by the short circuit formed by the end wall of the wave guide nullifies before returning into the adjacent cavity to the metal foil itself If the two metal insert 9 and 9' and the rectangular frame comprising the metal inserts 6 performing the filtering function mentioned above are made in one piece, it is possible to space with high accuracy each metal insert from the closer metal insert 6 and to avoid that assembling tolerances of the frame between the two half shells of the wave guide can negatively affect the filtering and transmission characteristics of It is evident that in other embodiments of the wave guide according to the present invention the shape of the hollow body, as well as the number of side openings and metal foils projecting from the contiguous enclousers can vary according to the type and number of devices connected to the wave guide itself. Therefore, while particular embodiments of the present invention have been shown and described, it should be understood that the present invention is not limited thereto since other embodiments may be made by those skilled in the art without departing from the scope thereof. It is thus contemplated that the present invention encompasses any and all such embodiments covered by the following claims.

Claims: CLAIMS 1. Antenna duplexer in wave guide with no tuning bends of the type including: • an elongated hollow body (1) realized with two opposed half shells and forming the above mentioned wave guide; • a first opening (2), or antenna port, obtained on a side wall of the elongated body adapted to be coupled to the antenna; • a second and a third opening (2), or ports of the transmitter and receiver, always obtained on said side wall of the elongated body (1), close to the end walls (3) of the elongated body (1) itself, adapted to be coupled to the transmitter and the receiver respectively; • a rectangular frame (5) including a plurality of metal inserts (6) of conductive material, which transversally cross the inside cavity of the hollow body (1 ) in such a way to realize a filtering structure adapted to uncouple the transmitter from the receiver, characterized in that it has at least an additional portion of wave guide(8) "under the cut-off frequency" placed between at least second or third opening (2) and the relative end wall (3) of the elongated body (1). 2. Antenna according to claim 1 characterized in that said additional portion of wave guide(8) "under the cut-off frequency" includes an insert (9) of conductive projecting material towards the inside cavity, which attenuates the electromagnetic signal transmitted through the wave guide in exponential way almost determining a nullification of the same coinciding with the relative end wall (3). 3. Antenna according to the previous claim, characterized in that said insert (9) of conductive material is essentially parallel to the side wall of the hollow body on which said openings are obtained (2). 4. Antenna according to claim 2, characterized in that said at least one insert(9) protruding from the inside of the relative ending wall (3) of the hollow body (1) is realized in one sole piece with said rectangular frame (5). 5. Antenna according to any of the previous claims, characterized in that the distance between the side opening (2) and the ending wall (3) close to the same is higher than the length of the insert (9) protruding from this end wall. 6. Antenna according to the previous claims, characterized in that it has an additional portion of wave guide (8) "under the cut-off frequency" at each end of the above mentioned elongated hollow body (1).

Inventors: COSTA MARIO, RAVANELLI ROBERTO, COSTA, MARIO, RAVANELLI, ROBERTO
Applicants: COSTA MARIO, ITALTEL SPA, RAVANELLI ROBERTO, COSTA, MARIO, ITALTEL S.P.A., RAVANELLI, ROBERTO
Application year: 1998
Main classification: H01P1
IPC codes: H01P1