Abstract:
A coaxial characteristic-impedance transformer for dividing RF power on a first terminal onto n second terminals situated in the same radial plane by multi-stage serial transformation via λ/4 lines is provided with a short overall size, where the λ/4 lines between the first connection and the second connections are at least partly disposed to surround each other concentrically.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. §119 to Application No. DE 102005061671.2 filed on Dec. 22, 2005, entitled “Coaxial Characteristic-Impedance Transformer,” the entire contents of which are hereby incorporated by reference.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to a coaxial characteristic-impedance transformer for dividing RF power on a first terminal onto n, where (n≧2), second terminals situated in the same radial plane by multi-stage serial transformation by means of λ/4 lines.  
       BACKGROUND  
       [0003]     Known characteristic-impedance transformers are used for evenly dividing in a matched and thus reflection-free manner RF energy supplied via an incoming coaxial line among two or more outgoing coaxial lines having the same characteristic-impedance as the incoming coaxial line, which as a rule is 50Ω. Such characteristic-impedance transformers are also known as distributors or splitters. They usually comprise several transformation stages, each of which includes a coaxial line section having a mechanical length of approximately λ/4 (λ is the wavelength of the operating or center frequency). A software known as APLAC which is available on the market can be used for calculating the precise length and the diameter of an inside conductor and an outside conductor of the line sections. For reasons of brevity, the individual line sections will therefore be referred to below and in the claims as λ/4 lines.  
         [0004]     In principle, a characteristic-impedance transformer should be as free as possible from reflections, i.e., it should have a low VSWR (voltage standing wave ratio), especially at the first terminal. However, VSWR values that are acceptable at adequate bandwidth require at least three transformation stages, and four or more stages when large bandwidths are required simultaneously. Since the transforming line sections are disposed in series, not only electrically but also mechanically, known characteristic-impedance transformers are constructed to be of a large length. Their (theoretical) length is at a minimum equal to n·λ/4, i.e., proportional to the number n of the transformation stages.  
       SUMMARY  
       [0005]     The described device relates to a coaxial characteristic-impedance transformer for dividing RF power on a first terminal onto n second terminals situated in the same radial plane (n≧2) by multi-stage serial transformation via λ/4 lines. The described device is a characteristic-impedance transformer with a substantially short length without impaired electrical characteristics.  
         [0006]     The λ/4 lines between the first terminal and the second terminals are at least partly disposed to surround each other concentrically, which allows for shorter lengths of the characteristic-impedance transformer. According to the invention, the outside conductor of the first λ/4 line at least along a part of its length is used as the inside conductor of the second λ/4 line, and the outside conductor thereof in turn is used as the inside conductor of the third λ/4 line, etc. This makes possible embodiments of the characteristic-impedance transformer with a short overall length.  
         [0007]     In particular, the λ/4 lines can be arranged concentrically with respect to each other such that the open end of one λ/4 line forms the beginning of the subsequent, i.e., successive, λ/4 line.  
         [0008]     When the λ/4 lines are arranged concentrically with respect to each other in such a way that an electromagnetic wave propagates from one λ/4 line to another λ/4 line in an opposite direction, the (theoretical) length of the characteristic-impedance transformer thus will not be substantially larger than λ/4 (irrespective of the number of stages), as long as supplementary compensation, to increase the bandwidth, is avoided.  
         [0009]     An increase of the number of stages without substantial enlargement of the diameter of the characteristic-impedance transformer can be achieved when at least one of the λ/4 lines is folded such that a part of its length concentrically surrounds the remaining part of its length. In this embodiment, an electromagnetic wave therefore propagates in at least one of the transformation stages, i.e., a corresponding line section having a length of approximately λ/4, within a first volume in one direction, and in an opposite direction within a second volume surrounding the first volume.  
         [0010]     A compact four-stage characteristic-impedance transformer which has an overall length only slightly longer than, for example, a three-stage embodiment, but which can have the same diameter, will be obtained provided that an inside conductor of a first stage has a first diameter and forms, together with an outside conductor of the first stage, a first λ/4 line; that an extension of this inside conductor having a second, larger diameter forms, together with an inner jacket surface of the same outside conductor, a first section of a second stage, a second section of which consists of an outer jacket surface of the outside conductor, having a first outside diameter, of the first stage as a second inside conductor, together with an inside jacket surface of a surrounding hollow cylinder as a second outside conductor; that a section of the outside conductor having a second, larger outside diameter is contiguous to this second stage as an inside conductor, which together with an inside jacket surface of the surrounding hollow cylinder, forms a first section of a third stage, a second section of which includes an outside jacket surface of the surrounding hollow cylinder, having a first outside diameter, as a third inside conductor, together with an inside jacket surface of a hollow-cylindrical housing, to which a fourth stage is contiguous that includes a second section of the surrounding hollow cylinder, having a second, larger outer diameter, as a fourth inside conductor, together with the inner jacket surface of the hollow cylindrical housing as an outside conductor, the surrounding hollow cylinder being connected to inside conductors of the second terminals. A folding of the second and third stage, as effected in this manner, thereby avoids the necessity of enlarging the diameter of the housing in order to accommodate the fourth stage, which would lead to a lowering of the limiting frequency.  
         [0011]     A larger bandwidth and a more uniform variation of the reflectance factor in dependence upon the frequency can be achieved when the inside conductor of the first terminal comprises an inside conductor that is designed to be a compensating λ/4 open-circuit line, and is accommodated concentrically and in an insulated condition within the inside conductor of the first λ/4 line.  
         [0012]     Another improvement for the same purpose is achieved when the inside conductor of a compensating λ/4 short-circuit line is connected to a junction of the inside conductors of the second terminals.  
         [0013]     The above and still further features and advantages of the coaxial characteristic-impedance transformer will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof, wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the device, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The characteristic-impedance transformer in accordance with the invention is explained below with reference to the drawings which relate to schematically simplified examples of embodiment and supplementary diagrams, wherein:  
         [0015]      FIG. 1  shows the principle, as known per se, of a coaxial characteristic-impedance transformer;  
         [0016]      FIG. 2  shows a longitudinal section of a four-stage implementation of the characteristic-impedance transformer according to an exemplary embodiment of the invention;  
         [0017]      FIG. 3  shows a cross-section corresponding to the line III-III in  FIG. 2 ;  
         [0018]      FIG. 4  shows a longitudinal section of a three-stage embodiment of the invention;  
         [0019]      FIG. 5  shows a longitudinal section of another four-stage embodiment of the invention;  
         [0020]      FIG. 6  shows the frequency-dependent course of the reflectance factor of the four-stage characteristic-impedance transformer according to  FIG. 5 ; and  
         [0021]      FIG. 7  shows the frequency-dependent course of the reflectance factor of the three-stage characteristic-impedance transformer according to  FIG. 4 .  
     
    
     DETAILED DESCRIPTION  
       [0022]     In the following paragraphs, exemplary embodiments of the device are described in connection with the figures.  
         [0023]      FIG. 1  shows the known principle of a four-stage characteristic-impedance transformer for transforming or matching a low characteristic-impedance Z(L 5 ) to a higher characteristic-impedance Z(L 0 ) via four successive line sections L 1  to L 4  of approximately λ/4 length with characteristic-impedances Z(L 1 ) to Z(L 4 ) that decrease stage-by-stage. For increasing the bandwidth and for smoothing the course of the reflectance factor in dependence upon frequency, a λ/4 open-circuit line LL is additionally incorporated into the first stage L 1 , and a λ/4 short-circuit line KL is connected to the end of the fourth stage L 4 . The characteristic-impedance Z(L 5 ) which is lower in comparison with Z(L 0 ) arises in the case of a power distributor or splitter owing to coaxial lines (not shown) which are connected in parallel to the last transformation stage L 4 , and which are, for example, the feed lines of a corresponding number of antennas.  
         [0024]      FIGS. 2 and 3  show a longitudinal section and a cross section along the line III-III in  FIG. 2  of a four-stage characteristic-impedance transformer for uniformly distributing RF power supplied via a coaxial line to a first terminal K 1  to three second terminals K 2  to K 4 . An inside conductor IL 1  and an outside conductor Al 1  jointly form a first transformation stage L 1  having a characteristic-impedance Z(L 1 ) and a length of approximately λ/4. The outside diameter of IL 1  and the inside diameter of AL 1 , and also the precise length, can be calculated, as can be also the respective sizes of the subsequent transformation stages, via the aforementioned software APLAC. The inside diameter IL 1  on its part concentrically accommodates an inside conductor IL 0  which in combination with the inner jacket surface of the inside conductor IL 1  and a dielectric D forms an open circuit line LL that is slightly shorter than λ/4 and serves, as in the case of  FIG. 1 , for frequency response compensation. A second stage L 2  having the characteristic-impedance Z(L 2 ) is contiguous to this first stage L 1 . While their outside conductors AL 2  and AL 1  have the same inside diameters, the inside conductor IL 2  has a larger outer diameter than IL 1  in order to achieve a Z(L 2 ) that is comparatively smaller than Z(L 1 ).  
         [0025]     The open end of the outside conductor AL 2  of stage L 2  is likewise the beginning of the stage L 3  with the even lower characteristic-impedance Z(L 3 ). This stage L 3  uses as an inside conductor IL 3 , in other words, the outer jacket surface of the outside conductor AL 2 , and as an outside conductor the inner jacket surface of a cup-shaped hollow cylinder H which surrounds the stage L 2 . The open end of the cylinder H forms the end of stage L 3  in analogy to the configuration of stage L 2 , and the beginning of the stage L 4  with the even lower characteristic-impedance Z(L 4 ). The RF power accordingly changes its direction of propagation at the open end of the outside conductor AL 2  and at the open end of the hollow cylinder H. An outer jacket surface of the hollow cylinder H forms an inside conductor IL 4  of the stage L 4 , and an inner jacket surface of a housing G of the characteristic-impedance transformer forms its outside conductor AL 4 . At the end of stage L 4 , the RF power is distributed uniformly onto the second terminals K 2  to K 4 , the inside conductors of which contact a floor B which seals off one end of the hollow cylinder H.  
         [0026]     For further frequency response compensation, the housing G is extended beyond the region of the terminals K 2  to K 4  and forms, jointly with a coaxial extension of the inside conductor IL 2  through the floor B of the hollow cylinder H, a short-circuit line KL which has a length of approximately λ/4, again in analogy with the corresponding short-circuit line in the schematic diagram of  FIG. 1 .  
         [0027]     In the case of lower demands made on the bandwidth, it is possible to omit the short-circuit line KL and/or the open-circuit line LL. When the short-circuit line KL can be omitted within this sense, the characteristic-impedance transformer has an even considerably shorter overall size.  
         [0028]      FIG. 4  shows a three-stage embodiment of the characteristic-impedance transformer. The same reference numerals as used in  FIG. 2  apply. The housing G has the same diameter as the housing G in  FIG. 2 , so that the limiting wavelength is the same for both embodiments (undesirable wave modes of higher order occur in coaxial systems beyond the limiting wavelength determined approximately by the inside diameter of the housing). The three-stage embodiment according to  FIG. 4  differs from the four-stage embodiment according to  FIG. 2  in principle only in that, owing to the omission of the fourth stage, sufficient space is available for also accommodating the first stage L 1  including the open-circuit line LL in the housing G. As a result, not only all stages L 1  to L 3  and therewith the λ/4 lines forming them, but also the compensating line LL are concentrically nested within each other.  
         [0029]      FIG. 5  shows an embodiment similar to  FIG. 4  with the same or corresponding reference numerals, but with four transformation stages L 1  to L 4 . In order to enable the four stages L 1  to L 4  to be accommodated within a housing G 1  which has the same inside diameter as the housing G in  FIG. 4 , not only are the stages L 1  to L 4  nested concentrically within each other in this embodiment, but the stages L 2  and L 3  are additionally folded. The stage L 2  thus has a first inside conductor section IL 2 ′ which has a larger outside diameter than the inside conductor IL 1  of the first stage L 1 . The second inside conductor section IL 2 ″ comprises the outer jacket surface of the (extended) outside conductor Al 1  of the first stage L 1 . This jacket surface has a larger outside diameter at the beginning of the third stage L 3  than in the region of IL 2 ′, and thus forms the first section IL 3 ′ of the third stage L 3 . The second section IL 3 ″ forms the outer jacket surface of the hollow cylinder H having a first diameter. Contiguous to this is the stage L 4  which is configured like the stage L 4  in the embodiment according to  FIG. 2 .  
         [0030]     The diagram in  FIG. 6  shows the frequency-dependent course of the reflectance factor of the characteristic-impedance transformer in the embodiment according to  FIG. 5 .  
         [0031]     The diagram in  FIG. 7  shows the frequency-dependent course of the reflectance factor for the three-stage characteristic-impedance transformer according to  FIG. 4 . A comparison of the two diagrams shows that the three-stage characteristic-impedance transformer has a large bandwidth of approximately 370 to 2,560 MHz in which the reflectance factor remains below 0.06, but that in the case of a four-stage configuration this bandwidth further increases to 280 to 2,700 MHz.  
         [0032]     While the coaxial characteristic-impedance transformer has been described in detail with reference to specific embodiments thereof, it will be apparent to one of ordinary skill in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that the described device covers the modifications and variations of this coaxial characteristic-impedance transformer provided they come within the scope of the appended claims and their equivalents.