AU614279B2

AU614279B2 - Wideband polarisation filter (duplexer)

Info

Publication number: AU614279B2
Application number: AU13399/88A
Authority: AU
Inventors: Eberhard Schuegraf
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG
Priority date: 1987-03-24
Filing date: 1988-03-23
Publication date: 1991-08-29
Anticipated expiration: 2008-03-23
Also published as: EP0285879A1; DE3881741D1; EP0285879B1; ATE90813T1; AU1339988A

Abstract

Measures are provided in two waveguides (7) carrying two orthogonal linear polarisations, e.g. the incorporation of an inner conductor (8) and/or of symmetrically arranged metal longitudinal webs, as a result of which its characteristic impedance can be approximated to (or, if necessary, matched to) the characteristic impedances of the two polarisation-selective rectangular waveguide arms (10, 11). In this case, two conditions must be met, namely, firstly matching the cross-section factors in the characteristic impedance equations of the waveguides to be matched to one another and secondly, matching the limit frequencies of the wave types which are to be merged. Residual reactances in the junction passages can then be matched, in a broadband manner, in a very short space without difficulties. The measures specified in the invention can be used in the case of broadband polarising junctions for satellite radio and directional radio antennas. <IMAGE>

Description

li s i i FORM 519 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: o o 0 0 ~o .4 00

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0o Class Int Class Complete Specification Lodged: Accepted: Published: Priority: Related Art: Name and Address of Applicant: Siemens Aktiengesellschaft Wittelsbacher Platz 2 8000 Muenchen FEDERAL REPUBLIC OF GERMANY Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Males, 2000, Australia Address for Service: 6re a U 0) 0 0 Complete Specification for the invention entitled: Wideband Polarisation Filter (Duplexer) The following statement is a full description of this invention., including the best method of performing it known to me/us 5845/3 WIDEBAND POLARISATION FILTER (DUPLEXER): The invention relates to a wideband polarisation filter (duplexer) for separating orthogonal, linearly polarised electromagnetic microwaves by means of a branching device which divides a waveguide carrying the two orthogonal polarisations into two rectangular waveguide arms which each carry only one of these TC polarisations.

Virtually all known polarisation filters lack the funda- S mental pre-requisite for genuine wideband matching, which is the 00 00 0 o° tO honmogenelty of the characteristic impedance along the two positions of such a filter; this is because, between each of the two polarisation-selective filter connections, which are always 0 00 a 0 constructed as rectangular waveguides having a ratio of 0. sides a 2b, and the waveguide with a circular or square Scross-section (a a jump in characteristic impedance Zround Sor Zsquare/Zrectangle 2 exists in accordance with the characteristic impedance equation: K Zo Z b/a V1 (Xo/k) 2 In the present equation, Z is the characteristic line impedance, 0° b K/a is the cross-sectional factor for the respective wave in the rectangular waveguide, Zo is the characteristic field impedance of a purely plane wave in free space, Xo is the wavelength in free space, and Ak is the respective critical waveleingth which is also called the critical wavelength of the type of wave considered. In principle, wideband matching of the jump in characteristic impedance between the lines is impossible, at least over wide band-widths of l o 1 octave and more.

i i 2 Attempts have been made to reduce the reflections of the characteristic impedance jump diagnosed at least over a narrow band (up to approximately by means of continuous transitions, with turned back and short-circuit plates, various front faces, with diaphragms, arrays of studs, and similar known measures.

It is an object of the present invention to substantially overcome, or ameliorate, the abovementioned problems through provision of an improved wideband polarisation filter/duplexer.

In accordance with the present invention there is provided a wideband polarisation filter/duplexer for separating orthogonal linearly polarised electromagnetic microwaves, said filter/duplexer comprising: a double branching device including a section of primary waveguide adapted to carry two orthogonal linearly polarised electromagnetic microwaves, said device dividing said primary waveguide into two waveguide arms each of substantially rectangular transverse cross-section 0000 0 °g and carrying only one of said polarisations; 000 matching means being provided in the primary waveguide in which its 0o o oo Co characteristic line impedance, originally approximately twice as large as 0 that of each of the waveguide arms, is substantially matched to each of o o 20 the mutually equal characteristic line impedances of the two waveguide arms, whereby each of the characteristic line impedances is determined by the product of a cross-sectional factor of the respective waveguide, tie characteristic field impedance of a purely plane wave in free space and the factor 0 25 1 1 (0 k) c c wherein k is the wavelength in free space and hk is the so called cut-off wavelength of a wave type to be transferred wherein the provision of said matching means involves determining the cross-sectional factor for the primary waveguide to provide equality of the characteristic impedances of the e' primary and both the rectangular waveguide arms independently of a. C frequency, and determining the inner dimensions of the primary waveguide so that it will have the same cut-off frequency of the wave type to be transferred in the two rectangular waveguide arms.

The present invention is based on the concept that the characteristic line impedances of the rectangular polarisation filter i waveguide arms with their ratio of sides a 2b are pre-determined, whereas the characteristic line impedance of the waveguide carrying the two orthogonal polarisations is not defined, and is therefore freely selectable. This opens up the hitherto unused possibility of lowering the characteristic line impedance of the waveguide carrying the two orthogonal polarisations, and thus at least approximating the oo00o characteristic line impedances of the rectangular waveguide arms, by 00 0 0000 00o" means of the measures specified. Ideal matching conditions exist o0 00 0 when the characteristic line impedances of the waveguide carrying the o0 00 0 0 0 two orthogonal polarisations match those of the rectangular waveguide 0 000090 arms over a wide band. A matching of the characteristic impedances over very wide bandwidths is achieved by satisfying the two 0 0O o0 0 conditions specified, namely, on the one hand, matching the 0 00 0 0 I 0 cross-sectional factors in the characteristic impedance equations of 00 0 0~ the waveguides to be matched to one another, for example the factor b K/a for the H10 wave in the rectangular waveguide in o 0 o, accordance with the characteristic impedance equation specified 0 0 o°°o 2 above, and matching the cut-off frequencies of the wave types to be transferred. Both transitions of such polarisation filters with O homogenized characteristic impedances do not then contain any further characteristic impedance jumps, but only reactances which, as is i known, can be matched over a very wide band, in contrast to jumps in i Characteristic impedances. This principle of homogenization of characteristic impedances can be applied to virtually all known polarisation filters. The result is invariably a low-reflection bandwidth which is considerably widened compared with previous constructions.

I

4 The invention will now be described with reference to the drawings, in which: Figure 1 shows, in cross-section, two alternative measures for lowering the characteristic impedance in the waveguide carrying the two orthogonal polarisations; Figure 2 shows a number of cross-sectional possibilities for use in a waveguide carrying the two orthogonal polarisations, in each case comprising an internal conductor with reduced characteristic i 'oe impedance and widened ranges of frequency selectability; 0 otO Figure 3 is a graph showing a quantitative relationship, eS obtained by measurement, between the characteristic impedance of a coaxial waveguide having a round internal conductor and its ratio of SonoP diameters between internal and external conductor diameter; SFigure 4 is a graph showing how the diameter of an imaginary circular waveguide having the same H1l cut-off frequency as the coaxial waveguide is determined for the ratio of diameters of a coaxial waveguide considered in each case; Figure 5 is a graph showing the ranges of frequency definability in coaxial waveguides as a function of their ratio of CO diameters; Figure 6 is a cross-section illustrating the E-field of the H31 interference wave within a representation of a coaxial waveguide; and Figure 7 is an exploded perspective view of the structure of a two-band polarisation filter having an internal conductor for reducing the jumps in characteristic impedance.

1.-1 I Y llll~ l~ llll~ IP~ lll~- l To lower the characteristic impedance as intended in a waveguide exhibiting an external conductor 1 of circular or square cross-section and carrying the two orthogonal polarisations, either a symmetrical arrangement of at least four metal fins 2,3,4 and 5 on the inside surface of the wall of the external conductor 1 is suitable, in accordance with the left-hand representation, and/or a concentrically-arranged internal conductor 6, as is shown in the I right-hand representation of Figure 1. In practice, the internal sa oo S conductor 6 is more easily produced than the conducting fins 2,3,4 0o O0 o~ O and 5 extending in the longitudinal direction of the waveguide. The 0 *o0 internal conductor 6 is arranged on the central longitudinal axis of the external conductor 1 and thus extends concentrically. The S* internal conductor 1 is preferably permanently conductively-connected to the contours of the external conductor in the bifurcation zone of S" the three polarisation filter waveguides. This attachment specially created for this purpose can be universally applied and can be utilised for compensation reflections of both polarisations.

The simplest shape of an internal conductor 6 is the circular cross-sectional shape shown in the right-hand representation S of Figure 1. Apart from the intended lowering of the characteristic impedance, this additionally achieves a considerable widening of the range of unambiguity in the coaxial waveguide and quantitative information relating to this will be given in the following description.

More advantageous cross-sectional shapes of the internal conductor 6, which are shown in detail in Figure 2, are possible for even wider ranges of unambiguity fkElifkHll, fkH31/fkHll and fkE21/fkHlO. According to this, the internal conductor 6 can be constructed, for example, to be cross-shaped, and combinations with a round or square external conductor 1, with or without conductive longitudinal ribs 2,3,4 and 5, are also possible.

According to the report by W. Baler: "Wavetypes in lines of rectangular cross-section" in the Journal "AEU", Volume 22(68), Issue 4, pages 184 etc., the internal conductor 6 causes very low additional losses and brings the following further advantages. The internal conductor 6 may be extended beyond the polarisation filter, 0 00 0 o r 0 00 0 o(oo o o0o 00 0 A 0 00 00 0 0 0 0 00 000 0 0 0 0 00 0 0 0 0 0 0 0 0 00 0 00 0 00 0a 00 a °oI 00 and is suitable for improving the characteristic of a load connected 0o0 to the polarisation filter, thus, for example, for improving the low-reflection bandwidth of a corrugated horn and its 0 cross-polarisaLion characteristics, compared with the onward feed by means of a pure waveguide without internal conductor. In this arrangement, the internal conductor 6 can end continuously or discontinuously, stepped in the neck of the horn, in the area of ao corrugations, or outside the horn aperture. Furthermore, space can be made in an internal conductor 6 of hollow construction for waves of equal or different type having the same or different frequency as the waves already existing outside the internal conductor 6. Fo: this purpose, the internal space of the internal conductor, in turn, can be suitably provided with conductive material OL with a dielectric. Purthermore, coupling devices for waves which are mr~opp--~-a-~.cr~;;rany~LI .uP-li^n-l-- coupled from the space outside the internal conductor into or out of its interior can be arranged in the interior space of the internal conductor 6 and/or near its surface.

Independently of its respective cross-sectional shape and that of the associated external conductor 1, the internal conductor 6 mainly increases the shunt capacitance in the equivalent eSt characteristic impedance circuit for H waves. Thus, the characteristic impedance of the H11 wave and of the H10 wave 0 4 4 drop as intended and the associated critical wavelengths rise.

0 0 0 For the simple case, which is of interest in practice, where the coaxial waveguide has a circular internal and external conductor, Figure 3 shows the quantitative relationship, obtained by 0 00 o measurement, between the characteristic impedance of this coaxial t 4 waveguide and the ratio of its diameters d/Dk, between the internal l* conductor diameter d and the external conductor diameter Dk. The measurements are made in such a manner, for coaxial waveguides having particular values of the diameter ratios (d/Dk)n, that in each case the rectangular waveguide with its ratio of sides (b/a)n is determined, which produces wideband matching at the discontinuous S transition between the respective coaxial waveguide and the rectangular waveguide. In this arrangement, the cut-off frequencies of the HI0 wave in the rectangular waveguide and of the H11 wave in the coaxial waveguide have first been made equal. For this purpose, the diameter Do of the imaginary circular waveguide having the same H11 cut-off frequency as the coaxial waveguide is determined for the diameter ratio d/Dk of the coaxial waveguide j I- 8 considered in each case, by reference to Figure 4 in the "Pocket Book of Radio Frequency Engineering", Second Edition, Springer-Verlag, page 309 by Meinke, Gundlach. Thus, the critical wavelength of the coaxial waveguide is first obtained as conductor kH1l 1.706 Do and from this the required wide-side a of the rectangular waveguide with the matched H10 critical wavelength 2a 1.706 Do.

oa.. In add:tion, the reactance remaining at the cross-sectional 0o 0 .0O discontinuity is compensated over a wide band in these measurements 00 0 So 0 by a suitable longitudinal offset of the start of the internal o o C om conductor compared with the point of discontinuity. Such o00. discontinuous transitions from rectangular waveguide to coaxial waveguide require virtually no constructional length. They achieve 0 O0 0"o0 low-reflection bandwidths of up to 1 octave and their reflection is 0 00 0o 0 less than 1% over 50% of the bandwidth. Thus, an important basic 0,"0 component of polarisation filters with homogeneous characteristic 0 4 impedance is available.

Firstly, for the rectangular waveguides with a 2b, given in cases, the coaxial waveguides matching these with respect to characteristic impedance and the H11 cut-off frequency can be RO determined with reference to the quantitative relationships of the diameter ratios d/Dk of coaxial waveguides with their characteristic impedances, shown in Figure 3, and their H11 cut-off frequencies, shown in Figure 4. In this connection, reference is made to the left-hand column of the table of calculations appearing at the end of the description. In addition, quite new conceptions are produced for impl, nenting polarisation filters having extremely wide 9 bandwidths, due to the introduction of lower rectangular waveguides having b a/2, and the matching coaxial waveguides having relatively thick internal conductors. For this purpose, the examples a 3b and a 4b are analysed in the table of calculations appearing at the end of the description. To assess the associated coaxial waveg .des, their theoretical ranges of unambiguity are then determined with a Vo« view to the Ell interference wave first occurring with aoo. symmetrical HI1 excitation. The cut-off frequency 0 0oI ,o ratios fkEll/fkHll are in quantitatively characteristic manner 0 0 00 0 oo 1 dependent on the diameter ratio d/Dk of the coaxial waveguides 0 according to Figure 5 from Meinke, Gundlach: "Pocket Book of Radio Frequency Engineering", Springer-Verlag, Second Edition, page 309.

oo0 For rectangular waveguides selected to be lower with b a/2 and the S o00 matching coaxial waveguides having relatively thick internal *OQ2 conductors, unambiguity ranges fkEll/fkHli are obtained which extremely rapidly widen with diameter ratio d/DK and their breadth ^for d/DK 1 goes towards c After that, the H31 interference wave following the Ell wave is also included in the consideration according to Figure 6.

SQ Despite symmetrical excitation, the H31 interference wave is excited in addition to the fundamental H11 wave because, according to Figure 6, for example, the E field strengths of the H31 wave at diametrically-opposite points on the periphery In the coaxial waveguide always have the same direction as the E fields of the Hll wave.

i, ~I I .x In addition to the wideband matching of the characteristic impedances, the introduction of the internal conductor also produces a widening of the range of unambiguity fkH31/fkHll. According to Figure 5, the coaxial waveguide with d/DK 0.37 matched to the rectangular waveguide with a 2b has a useful range of unambiguity fkH31/fkHll 2.73 (compared with the useful range of unambiguity 1 "s fkEll/fkHll 2.08 in the case of the circular waveguide without O0" internal conductor), Because of the necessary spacing between the o° operating frequencies and the cut-off frequencies, o0 o0 fkH31/EkHll 2,73 corresponds to a useful bandwidth of o 000000 o a fh/fn 2.5 maximum. According to Figure 5, the coaxial waveguide with d/DK 0.77 reaches a maximum of the width of the Oo 0 0 0 range of unambiguity at EkH31/fkHll 3.09, corresponding to a a 0 useful bandwidth of fh/fn 2.8 maximum.

SThese numerical values apply to coaxial waveguides having circular intenal and external conductors. Even greater useful bandwidths can be expected with coaxial waveguides having the cross-sectional variations outlined in Figure 2. The design of these cross-sections is intended for as high as possible a capacitive S loading of the H11 wave that is to say for a low Hll cut-off frequency with at the same time as low as possible a capacitive loading of the H31 wave and a subsequently high H31 cut-off frequency. Using these methods, it appears to be possible to control the combination of the two microwave ranges of 3.4 to 4.2 GHz and 10.7 to 11.7 GHz, which are of interest in practice, by means of a single polarisation filter, for example.

A wideband polarisation filter of a two-band antenna system for the microwave frequency ranges of 3.58 to 4.2 GHz and 6.425 to 7.125 GHz will now be described, as a practical application of the invention, referring to Figure 7. For this frequency quotient of fh/fn 1.99, the circular waveguide 7 is unsuitable both as a polarisation filter waveguide and as a horn waveguide, because of its "multi-wave nature" over frequency ranges which, including the Snecessary cut-off frequency spacings, reach fkEll/fkHll 2.08.

0 0 11 The unavoidable widening of the range of unambiguity is achieved with 0 0 00 00 S o o 0 0 Figure 7, a coaxial waveguide consisting of the internal conductor 8 0 o 0oooo0 and the external conductor 7 is connected to the double branch 9, 0 0 which is used for Ell and H21 interference-wave-free excitation 00o0 of both polarisations, known, for example, from the report by E.

0 D0 0 0QQ Schuegraf "Novel Microwave Filters for Two-band Antennae", in "NTZ", 0oo o Volume 38 (1985), Issue 8, pages 554 to 560. Its range of unambiguity can be widened to fkEll/fkHll 2.274, even with a relatively thin internal conductor 8 of d 7.3 mm in the external conductor 7, with DK 52.2 mm. Thus, fkHll 3.21 GHz (11% C) S below fn 3.58 GHz) and fkEll 7.3 GHz, that is to say above fh 7.125 GHz at which, accordingly, sufficient interference field attenuation is produced for the Eli wave. Thus, on the one hand, the Ell interference field of the double branch 9 Is sufficiently attenuated; and since the internal conductor 8 is extended up to the vicinity of the first corrugation of a connected corrugated horn the Ell useful excitation in the corrugation area is decoupled, as required, from the horn waveguide with the aperiodic Ell attenuation. Incidentally, the shape of the internal co,Aductor 8 has a quite decisive influence on the horn reflection and similarly on the cross-polarisation suppression, even with very small changes.

The rectangular wavegulde axes 10 and 11 of the polarisation filter shown in Figure 7 are constructed with a =2b =46 mm. Both 000:0 waveguide forks 12,12' and 13,13' and the double branch 9 are of 000 0 homogeneous characteristic Impedance and the matching coaxial aa00 a' 0 C wiavequi~ has DK 43 mmand d 16 mm, according to Figure 3.

a This dimensioning reptes~nts the prototype for a polarisation filter aao0 O0 with homogeneous characLrits'lc Imped1ance constructed according to the Invention.

.0.00, 0 0 In the arrangement shown In Figure 7, the remait,,ing 0 0 0 a06O0 characteristic Impedance discontinuity between the previously-determined coaxial waveguide (d =16 mm, DK 43 mm) and the one continuing to the horn, with DK 52,1 mm and d 7.3 mm, 0 is advantageously reduced to 1.6 and Is by-passed with low reflections by XH/4 transformation stages. The rotation of the ~O symmetrica'ly-constructed transformer offers many possible corrections, which can be carried out In a simple manner, and which olvays serve the same effect for both polarisations.

The polarisation filter represented In the Illustrated embodiment shown In Figure 7 has a very wile useful bandwidth. This to why it is particularly suitable for one frequency filter each for two or more microwave frequency ranges of different frequency, (directly) connected to Its rectanigul.ar wavegulde arms 10 and 11. In

-U,

.1 13 addition, the connection between the two rectangular waveguide arms 10 and 11 of the polarisation filter shown in Figure 7 and the two frequency filters can also be established by means of two long lines constructed, for example, as over-moded, flexiblL, rectangular waveguides provided with corresponding transitions, and which, due to all conceivable measures for expanding their unambiguous transmission frequency range, are suitable for transmitting in each case more than one microwave range of the same polarisation from the location of the frequency filters, for example at the foot of the antenna tower, with 16 00 oo 10 low attenuation, reflection and delay distortion to the wideband 0° polarisation filter arranged immediately at the antenna, that is to o000 C o say, for example, on the tower, and conversely.

It is also pointed out that the internal conductor c c S represented in Figure 7 of the report by E. Schuegraf in the t Journal "NTZ", Volume 38 (1985), Issue 8, already mentioned, is not a

S

Cc round internal conductor in the sense of the invention, by means of which a homogenisation of characteristic impedance is achieved along the two transitions of a polarisation filter, but a X/4 transformer.

The internal conductor shown in Figures 2a and 2b of German Patent SC Specification 28 42 576 is also a narrow-band X/4 transformer network with additional reactances, which is specially tailored for good matching in two narrow frequency ranges which are relatively far apart (not quite an octave) and is not comparable to an internal conductor dimensioned in accordance with the invention.

f i- According to the principles utilised in the invention, new polarisation filters can now be dimensiuned, the two rectangular waveguide arms each are equipped, for example, with the following ratios of sides (calculation table): a 2b2;b2/a 0.5 j a 3b3;b3/a 1/3 a 4b4;b4/a 0.25 For this purpose, the coaxial waveguide is determined in each case, with round external and internal conductors, exhibiting t the same H11 cut-off frequency and frequency-independently the same "too characteristic impedances as the rectangular waveguide arms.

e 0 o°O According to Figure 3, the diameter ratio of the coaxial waveguide with the same characteristic impedance follows from the b/a 0 .oo value of the rectangular waveguide arm as: d/DK 0.37 0.51 0.6 0From Figure 4, the diameter ratio (DK/Do) follows for 't the respective d/DK value of the coaxial waveguide, with Do as SC", diameter of the circular waveguide which has the same H11 cut-off frequency as the respective coaxial waveguide: DK/Do 0.80 I 0.71 1 0.67 From 2a 1.706 Do 1.706 DK/DK/Do, it follows that DK 2a/1.706 DK/Do: DK 0.938a 0.83a 0.803a According to Figure 5, the range of unambiguity fkEll/fkHll and fkH31/fkHll, respectively, is obtained from Me'nke, Gundlach "Pocket Book of Radio Frequency Engineering", Second Edition, page 309, for the respective coaxial waveguide with d/DK: fkEll/fkHll 3.54 5.01 6.56 fkH31/EkHll 2.73 2.91 3.006 The following complete dimensionings are specified as a phle,.dcal example for the 4 GHz rectangular waveguide with a 58.17 mm and the above side ratios.

a 58.17 b2 29.08 mm b3 19.39 nun b4 14.54 mm DK2 =54.56 mm DK3 =48.4 nun DK4 =46.71 mm d2 20.19 nun d3 24.7 nun d4 28.0 mm 0 00 oThe 1 -and the E wave rnodes are oftei terrind as o0 0 O00~ TE respectively THin wave mo~des, especially in Ehglish texts.

0 0 0m r o0 0 o0 00 0 000000 0a0 0 00 00 .10 0 00 00 0 0 0 00

Claims

1. A wideband polarisation filter/duplexer for separating Iorthogonal linearly polarised electromagnetic microwaves, said filter/duplexer comprising: a double branching device including a section of primary waveguide for carrying two orthogonal linearly polarised electromagnetic microwaves and being adapted to divide said primary waveguide into two waveguide arms each of substantially rectangular transverse cross-section wherein each carries only one of said polarisations; and matching means being provided in said section of primary waveguide by which its characteristic line impedance is substantially matched to the mutually equal characteristic line impedances of the two waveguide arms, whereby each of the characteristic line impedances is determined by the product of a cross-sectional factor of the respective waveguide, the characteristic field impedance of a purely plane wave in free space and the factor o oo o k 1 0 1 (0o k oo wherein o is the wavelength in free space and k is the so called o0o o k S 0 cut-off wavelength of a wave type to be transferred wherein the provision 0o 00 20 of said matching means involves ooooo' determining the cross-sectional factor for the primary waveguide o 0 to provide equality of the characteristic impedances of the primary and both the rect anguiar wavegulde arms independently of frequency, and determining the inner dimensions of the primary waveguide so ooooj 25 that it will have the same cut-off frequency of the wave type to be 0 C o o transferred in the two rectangular waveguide arms. 00 C

2. A filter/duplexer as claimed in claim 1 wherein any remaining o reactances in the waveguides are substantially reduced by transformation means having short constructional lengths.

3. A filter/duplexer as claimed in claims 1 or 2, wherein the 0"S characteristic impedance of the primary waveguide is exactly matched to 0 each of said mutually equal characteristic impedances of the two waveguide 0 4 arms.

4. A filter/duplexer as claimed in any oni of the preceding claims, wherein said double branching device comprises, for each orthogonal microwave, an electrically-symmetrical rectangular waveguide fork for IAD/10320 I ?rra~ -l 17 spatial-symmetrical excitation of the two linear polarisations, each said fork having fork arms adapted to join said section of primary waveguide, Seach said fork arm having a narrow dimension substantially equal to half the smaller dimension (height) of the waveguide arms, and with a larger (width) equal to that of the waveguide arms, the fork arms of each said Sfork joining to form a respective one of said waveguide arms.

A filter/duplexer as claimed in any one of the preceding claims, wherein said matching means includes at least four metal ribs symmetrically arranged in the longitudinal direction of the primary waveguide on its inside surface of the outside wall, said outside wall being of circular or square construction.

6. A filter/duplexer as claimed in one of the preceding claims wherein said matching means includes an internal conductor provided in the internal space of said section of primary waveguide, which is of circular 15 or square cross-section, said conductor being dimensioned and possibly 000 S'oo stepped cross-sectionally in such a manner that conditions and for Cao homogeneous approximation or homogenisation of characteristic impedances SOoO are satisfied. 0a

7. A filter/duplexer as claimed in claim 6, in which entrances to 0 0 S 20 the two rectangular waveguide arms are constructed with a height considerably reduced in comparison with the normal height b a/2, and that the characteristic line impedance of the primary waveguide is matched to the characteristic line impedance of these waveguide arm axes of reduced waveguide height by increased capacitive loading using a thicker internal S 25 conductor in the primary waveguide and/or by means of longitudinal metal ribs on the inside of its outer wall.

8. A filter/duplexer as claimed in claim 6 or claim 7 when dependent upon claim 4, wherein the internal conductor is attached in a zone of bifurcation of said primary waveguide to said fork arms.

9. A filter/duplexer as claimed in claim 8 wherein said internal conductor is attached in said double hranching device where it is permanently, conductively-connected to the waveguide contours.

A filter/duplexer as claimed in any one of claims 6 to 9, in which the internal conductor exhibits a circular cross-section.

11. A filter/duplexer as claimed in any one of claims 6 to 9, in which the internal conductor exhibits a cruciform cross-section. IAD/1032o 7- 1 i i 18

12. A filter/duplexer as claimed in any one of claims 6 to 9, in which the internal conductor exhibits a square cross-section.

13. A filter/duplexer as claimed in any one of claims 6 to 9, in which the internal conductor exhibits a circular cross-section with symmetrically-arranged longitudinal ribs.

14. A filter/duplexer as claimed in any one of claims 6 to 13, in which the internal conductor extends beyond the actual polarisation filter area in the direction of a connected load in an external conductor of circular or square cross-section.

15. A wideband polarisation filter as claimed in claim 12, in which the internal conductor ends continuously or discontinuously stepped in the load, for example in the horn radiator neck of a horn radiator, particularly a corrugated horn radiator, in the area of corrugations or outside the horn radiator aperture. 15

16. A filter/duplexer as claimed in claim 15 wherein said load is a 0000 o0 a horn radiator and said internal conductor ends in the radiator neck. o °o

17. A filter/duplexer as claimed in claim 16 wherein said horn 0,ooo radiator is corrugated and the internal conductor ends in an area of o 0 oo oo corrugations or outside the horn radiator aperture, 0 20

18. A filter/duplexer as claimed in any one of claims 6 to 15, ir oooo" which the internal conductor is of hollow construction so that waves of the same or different type having the same or different frequency as the waves already existing outside the internal conductor can be transmitted within 0 it.

19. A filter/duplexer as claimed in claim 18, in which the internal 00oo0 Co 0 space of the hollow internal conductor Is suitably provided with conductive and/or dielectric material. °e c

20. A filter/duplexer as claimed in any one of claims 15 to 18, in which coupling devices for waves which are coupled out of the space outside the internal conductor into its interior and conversely are arranged in the hollow internal space of the internal conductor and/or near to its cac surface.

21. A filter/duplexer as claimed in any preceding claim, in which a respective frequency filter is directly connected to each said waveguide arm. IAD/1032o I L~ 1 l r 19

22. A filter/duplexer as claimed in any one of claims 1 to 20, in which a respective frequency filter is connected to each said waveguide arm, in each cas. via a long line constructed as an over-moded waveguide having a substantially rectantular transverse cross-section provided with corresponding transitions.

23. A wideband polarisation filter/duplexer substantially as described herein with reference to any of the examples illustrated in Figs. 1 or 2, or, Fig. 6, or, Fig. 7 of the drawings. DATED this FOURTEENTH day of JUNE 1991 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON oo 0 0 0 00o0 0 00 oo o 00 0 00 400 0 0 0 00 o o o O 0g00 o t C IAD/10320