US3034076A - Microwave diplexer - Google Patents

Description

May 8, 1962 KlYO TOMIYASU MICROWAVE DIPLEXER 2 Sheets-Sheet 1 June 8, 1963 Filed INPUT FREQUENCY/2 I/0LTA6EAT ONE OUTPUTARM VOLT/IGEATOTl/ER OUTPUT ARM FREQUENCY INVENTOR /(/v0 ToM May 8, 1962 KlYO TOMIYASU MICROWAVE DIPLEXER 2 Sheets-Sheet 2 June 8, 1953 Filed IIII 4 I I I I IIIIIIIIIIII 4 W'MIMWF/ a Am i I I I V IIIIIIIIIIIIIId nI/IW/I I IIIIIIIIIIIIIf NNK lNVEblTOR hn o 70M msu BY %M C/ ATFORNEY This invention relates to diplexer systems, and more particularly, is concerned with apparatus for coupling two microwave signals of slightly different frequencies to a single antenna or for separating two microwave signals of slightly different frequencies received by the antenna.

Diplexing systems have been used heretofore for transmitting and receiving two signals of different frequencies utilizing a single antenna. Such systems, however, have been used at lower frequencies in open-line or coaxial transmission lines. Well-known filter techniques have generally been employed to separate the two frequencies. At microwave frequencies where wave guides must 'be used for transmission, because of the narrow band transmission properties of the wave guide, the diplexing frequencies may be very close together in the frequency spectrum. Filters for separating frequencies so close together in the frequency spectrum must be designed with a very sharp cut-off to effect separation. Although high Q cavity resonators may be designed to have the necessary frequency discrimination to provide isolation between the two narrowly separated diplexing frequencies, any diplexing system using cavity resonators is generally unsuited for high power transmission, since high Q resonators are inherently low power transmission devices, being susceptible to breakdown due to high voltage and/ or current concentrations associated with the phenomenon of resonance. Furthermore, resonators give rise to a high VSWR if the frequency shifts only slightly oif the design frequency.

It is the general object of the present invention to avoid the foregoing and other difiiculties of and objections to the prior art practices by the provision of an improved diplexing system incorporating hollow wave guide transmission lines operating at microwave frequencies.

It is another object of the present invention to provide a diplexing system which achieves. substantial isolation between two signals separated by aminimum frequency dif ferential of the order of 5% or less.

Another object of this invention is the provision of a microwave diplexer capable of operating at the high peak powers encountered in a radar transmission system.

Another object of this invention is to provide a diplexer which may be tuned to diplex a large number of frequencies within the operating band of the system.

Another object of this invention is the provision of a diplexing system which is well matched to the source over a broad band of frequencies, so that drift-off frequency does not result in mismatch.

Another object is to provide tuning means for the diplexer in the form of a line stretcher which introduces large phase shifts with: minimum adjustment.

These and other objects of the invention which will become apparent as the description proceeds are achieved by providing a microwave diplexer whichcomprises a pair of four-terminal hybrid junctions which are preferably in the form of directional couplers that effect an even power division of energy coupled in at either of the terminals at one end thereof, the energy being coupled out of the two terminals at the other end. The two terminals at one end of one junction are coupled to the terminals at one end of the other junction by respective interconnect- 3,534,976 Patented May 8, 1962 ice of one of the intercoupling line sections includes a third hybrid junction with two terminals at one end connected in series into the line, the two terminals at the other end of the third hybrid junction having adjustably positioned short-circuiting terminations therein, movement of the position of the shorting terminations varying the length of the line. I

For a better understanding of the invention, reference should be had to the accompanying drawings, wherein:

FIG. 1 is a schematic view useful in explaining the operation of the diplexer of the'present invention;

FIG. 2 is a schematic showing of a suitable phase shifter or line stretcher used in tuning the diplexer;

FIG. 3 is a plan view partly in section showing a preferred form of the invention;

FIG. 4 is an enlarged cross-sectional view taken on the line 44 of FIG. 3; and

FIG. 5 is a graphical showing of the variation of voltage at the two output arms of the diplexer with changes in frequency of either of the two input signals.

Referring to the schematic showing of FIG. 1, the numerals l0 and 12 indicatea pair of rectangular wave guides having a common wall 14 therebetween. The two Wave guides 16 and 12 are directionally coupled together at two separate points by suitable coupling slots in the common wall 14, the coupling slots being indicated at 16 and 18. Thus, the wave guides l0 and 12 and the coupling slots 16 and 18 define two directional couplers joined in tandem. The path length between the directional couplers along the wave guide 12 is made longer antenna. A suitable non-reflecting energy absorbing termination 34 is provided in the end 35 of the wave guide It It is essential to the operation of the present invention as a diplexer that the directional couplers be of a type which functions as a hybrid junction, that is, which can beso dimensioned that power that is incident upon a terminal at one end is evenly divided and appears in equal quantities at both terminals at the other end of the directional coupler. A preferred directional coupler having the properties of a hybrid junction is described in the Proceedings of the I.R.E., February 1952, page 1 80. This type of directional coupler couples energy in the forward direction, and in addition, is electrically symmetrical in that a wave being coupled encounters the same electrical conditions on either side of the coupler. One of the properties of such a directional coupler is that the phase of the coupled wavelags by the phase of the direct wave at the output terminals of the coupler, so that a quadrature phase shift is effected between the two output signals derived from the directional coupler. In addition, there is a 45 phase lag of the direct wave due to the coupling slot.

Operation of the diplexer in its schematic form as above described will now be considered. The polar form of the electric vector of the two input signals F and F at various points along the wave guides is shown in FIG. 1 to indicate the changes in phase and amplitude which take place. In the following analysis, the 45 phase lag of the direct wave due to the coupling slot in the hybrid coupler is neglected since it produces no net effect in the operation of the diplexer. The phase angles indicated are mere- The output end 30 of the wave guide 12 is coupled (3 7 1y assigned values which indicate the relative phases of the various waves -at a common'transverse plane;

The signal F from'the source 28, indicated by the cross hatched line in FIG. 1, is coupled to the wave guide it and on reaching the coupling slot 16 is :divided equally between the Waveguide 10 and the wave guide'lli "By virtue of the power split at the coupling slot 16, the-direct-wave from the sourceZS at the point X in thewave guidc lll hasan electric vectorhaving a reducedjrnagnitude :3...707 at an assigned phase of zero. {It should benoted .that only an approximate figure of 0.7 is used to indicate the-magnitude in the drawings.) The energy from the source 28 coupled into the wave guide 12, however, is reduced :in magnitude and shifted in phase by the con- 7 plin g slot, so that the electrical vector at the point X in f the wave guide 10 is .707 with a phase angle of 90 relative to the direct'wave, due to the phase quadrature shift effected by the directional coupler as described above.

If it is assumed that the additional'path length in the Wave guide 12 introduced by the folded section isequal to an integral number of guide wavelengths at the frequency E the phase relationship between the direct Wave in ithe Wave guide 10 and the coupled wave in the waveguide 12; will be the sarneat point Y as it is at point X. The energy of the direct Wave is evenly split at :the coupling slot 18 so that thee'lectric'vector of the wave continuing on down'the'wave guide'itt'is again reduced in magnitude to .5' at the same assigned phase angle of zero. The-electric vector of the wave coupledinto wave guide 12 atthe'slot Isis changed injbothrnagnitude aoeaeve V 4 the wave guide it} have the same magnitude but they are 180 out of'phase, so that they cancel each other.

The other portion of the coupled wave which is coupled back into the wave guide 12 has a-magnitude-of .5 but is shifted in phase by anadditional 90, giving it a phase angle 180-in the Wave guide 12. Thus, the direct and coupled Waves from the source 24 reinforce each other ill i116 Wave guide 12 and continue on to the load 32.

andphase-by the coupling slot 15, so that it has a'rnagni- ,tude of .5 and a'relativephase angle of 90, again as theresult of the quadrature 'phase'shift introduced by the directional coupler.

'Similarly, thecoupled wave from the source 2% in the to the .load 32 being-I reduced in magnitude with an as-.

signed phase angle c590. The portion of the coupled wave in the wave guide 12 from the source 23 which is coupled back into thewave guide ill is both reduced in magnitude and shifted in relative phase by 90, so that it is equal in magnitude to the direct wave from the source 28 but 180 out of phase by virtue of the' two 96 phase shifts at each ot'the-coupling slots 16 and'lta. Thus, the 1 two waves of energy cancel each other in the region of .the termination 34-, while the two waves reinforce each other in'the region of the load 32.

It will be seen from the above discussion that under certain circumstances, namely, where the difierence in path length between the points X andY via the wave guides lll and 12 is equal to an even number of guide half wavelengths at the frequency'F and equal to an odd number of half wavelengths at the frequency F all the energy from both sources is transmitted to the load 32 with none-oi the energy reaching the termination 34. I

These conditions, necessary to effect diplexing in the manner above described, may beexpressed mathematically as V l=mligl (l .and 2 where l is the difference in path length between the points X andfY via the two" wave guides 1d and 12, 'ltgl and 'AgZ are the'guide wavelengths at the two frequencies F and F and m and 11 are integers. For instance, if )tgl,

'the guide wavelength of, energy from: the source 28, is

fixed, l may 'be :any one of a number at discrete values which provide complete transfer of energy at frequency 'F to the load 32. With Agl fixed, there are a number wave guide 12 is evenly split at the coupling slot 18, the energy-continuing directly onkdown the wave guide 12 oi discretewavelengths Ag2zof energy from the source 24 for each of the discrete values'of l whichproduce complete transfer ofenergy at the frequency F to' the load 32, that is, there are a number of discrete values of F for each of the discrete values'of l which result in 'tion 20 in the wave guide 12 of FIG. '1.

- diplexing.

From the-above discussion it will be apparent that ,a large number of frequencies F and F can be madeto diplex by changing the value of l in Equations 1 and 2. A preferred means'of varying the value of l is the phase shifter shown schematically in FIG. 2 and includes a hybrid coupler with two sliding short circuit terminations. The hybrid coupler is substituted for the folded linepor- The hybrid coupler includes a pair of wave guide sections 36 and 33 In a similar manner, thesignal F from the source 24 is coupiedl tothe input end 22 ofthe waveguide 12. The coupling slot I16 eiiects an even energy split of the signal wave in the wave guide 12 has a magnitude of .757 at "zero phase angle'while the coupled wave in the wave guide :10 has a magnitude of .707 at a phase angle of '90 relativeto the direct wave.

Noivassuniing that the foldedportionztl. of the Wave the points X and Y of an odd number of half wavelengths 1 :at theirequency F the relative phaseof the direct wave F ,'so that atthe point X the electric vector of the direct a Qguide '12 introduces an additional path-length between V at the point Y'will be shifted by a half wavelength or .180", so thatthe electric vector'at the point Y has a mag- -nitude o..707 at a phase angle of +180". This direct .wave is. split bythe coupling slot 18-, the direct portion continuing on to the loadiwithwa magnitude of .5 at an assigned phase angle of +180", The wave coupled into the wave guide It? is reduced-in magnitude to .5 but with i 1 the phase angle shifted190 by virtue of the quadrature 'phaseshift of the, directional coupler. I

i The coupled wave from the sourceZd in the wave guide 10 is alsoisplit at the coupling slot 18, the direct portion .continuingltowards the termination 34- with a reduced magnitude .of .5 with an assigned phaseangle of -19( Thus',,the two portions of the energy at frequency E in having a common narrow wall All therebetwecn.

A coupling slot 42 in the common wall 40' directionally couplesenergy between the Wave guide sections 36 and 38 in 'the manner of the hybrid couplers described above an amplitude 1 is split at the coupling. slot l2.-and rcflected by the shorts 46 and 348. Each of the reflected waves in turn isa'gainsplit .by the coupling slot 42, the two wavesrefiected back along the wave guide 36 in the direction of the incident energy being 180 out of phase with each otherso as to effect cancellation, while thetwo waves reflected along the outputportion of the wave guide section 38 again combine to have an amplitude of 1.

Thus, there is no resultant phase shift introduced by the phase shifter due to intercoupling, the only phase shift being due to the path length as determined by the posinon of the shorts 44 and 46. Since the additional path length introduced in the wave guide 12 .is twice .theincremental movement of the shorts 44 and 46, it will be seen that the hybrid coupler with two sliding shorts is capable of introducing large values of phase shift.

For a given difference in path length, by varying either M51 or M 2, or both, the proportion of each appearing at the output arms varies. This is illustrated graphicaliy in FIG. 5. Thus, the apparatus as defined may be used as a power divider, the division of power being a function of wavelength or frequency. At discrete frequencies diplexing occurs, that is, all the power from two sources of different frequency combine in one output. However, at intermediate frequencies there is a division of power between the two output arms depending on the frequency.

While a 59% power split of the energy at the directional couplers is essential to get diplexing, where it is merely desired to get a particular power division, the coupling slots may be proportioned to give other ratios of power division. This provides a useful design parameter where a particular ratio of power division is desired at selected frequencies.

F165. 3 and 4 illustrate a practical embodiment of the diplexer of the present invention. The numerals 56, 52, and 54 indicate generally three identical hybrid couplers, each of which is cast as a single unit including a hollow rectangular pipe section 56 with septums 58 and 6%} extending between the broadwalls to form a coupling slot '62 and four wave guide terminals indicated at 64, 65, 66, and 6 7. Spherical segments 68 and 69 project into the interiors of the hybrid couplers and are symmetrically positioned at the centers of the coupling slots 62. These 1 spherical segments, together with'the reduced cross sec tion provided by thethickened wall portions 76 and 72 of the pipes 56, improve the isolation and bandwidth characteristics of the couplers. Flanges 74 are provided 'at each end of each of the hybrid couplers.

Connected to one end of the hybrid coupler 50, by means of the flanges 74 and 76, are the input wave guide sections 73 and 8d, the latter being in the form of an H-plane bend. Flanges 82 and 84 provide convenient means of connecting the input wave guide sections '78 and 3d of the diplexer to any desired wave guide transmission system. i

The hybrid couplers 5t and 54 are directly connected by a section of rectangular wave guide 86, which is preferably cast as an integral unit with two H plane wave guide bends 88 and 9% that couple the hybrid couplers 5t} and 54- to the third hybrid coupler 52. Suitable flanges, indicated at 92, are provided for joining the various wave guide sections to the respective hybrid couplers. Suitable means, such as bolts $4, may be used to secure the flanges together.

Output from the hybrid coupler 54 is provided by means of wave guide sections 96 and 98, the latter being an' H-plane bend. The output wave guide sections are secured to the hybrid coupler 54 by means of a flange 99 secured to the flange 74 of the coupler. A non-reflecting termination for the wave guide 96, indicated generally at 100, includes a section of wave guide 102 secured to the wave guide section 96 by means of coupling flanges 1M and 105. Energy absorbing material, such as Polyiron or the like indicated at 105, is inserted in the wave guide 162 to provide a non-reflecting energy absorbingtermination.

The adjustable short circuit terminations for the phase shifter include a pair of rectangular wave guide sections 11d and 112 coupled at one of their ends to the hybrid coupler 52, the other end of the wave guide sections 114) and 112 being terminated in a cap 114. Short circuits are provided by plungers or pistons within the wave guides 11% and 112, the plungers being indicated at 116 and 118 respectively. The plungers are separated from the walls of the wave guides to form a wave trap type of short circuit, such as described in Patent No. 2,503,256, issued to E. L. Ginzton et al.

The plungers are secured to the lower ends of parallel rods 129 and 122, which extend through holes in the capv 114. The upper end of the rod 120 is secured to a yoke 123 which is slidably supported in slots provided in the guide member 124. The upper end of the rod 122 is adjustably secured to the yoke 123 by means of a micrometer type of adjusting element, indicated at 126, which provides for relative adjustment of the two plungers 116 and 118.

The yoke 123 is positioned along the guide member 124 for varying the position of the short-circuiting plungers 116 and 118 by means of a micrometer screw 128 which threadedly engages the yoke 123. A calibrated dial 130 on the end of the micrometer screw 128 together with a linear scale 132. provide means for positioning the shortcircuiting plungers 116 and 118 at any predetermined positions Within the wave guide 116} and 112 respectively.

From the above description itwill be seen that the variousobjects of the invention have been achieved by the provision of a diplexing system suitable for operation at microwave frequencies using hollow wave guide trans mission lines. In theory, a large number of discrete frequencies may be diplexed by varying the path length of one or both of the wave guide sections between the hybrid couplers. The longer the folded Section of of wave guide is made, i.e., the greater the difference in path length of the two wave guide sections,-the closer together in frequency may be the signals which willdiplex. Practical considerations, however, limit the distance over which the shorts may be adjusted and the extent to which the wave guide may be lengthened by movement of the shorts, the principal limiting factor being the frequency sensitivity of the system which is greatly increased as the difference in path length is increased. I

It was seen that, in theory, for a given frequency setting of one of the signals, only discrete values of frequency of the other signal would diplex; 'In practice, however, it has been found that diplexing occurs for bands of finite width. These finite frequency bands are determined by the tolerable insertion loss, which for design purposes has been set at 0.5db. It'has been found that'a phase shift of at the output of the second hybrid coupler ispermissible without exceeding the 0.5 db insertion loss figure. The allowable change in frequency (and hence the bandwidth) at the input of the first hybrid coupler, which limits this phase shift to :25 is a function of the path length in terms of guide wavelengths between the two hybrid couplers. Hence, the larger the difference in path lengths between the points X and Y of FIG. 1, the greater is the phase sensitivity between the two lines with consequent sharper rise in insertion loss. For this reason the phase shift. introduced by the shorted hybrid coupler section is limited to a maximum of 8 or 9 wavelengths.

Although a drift away from the discrete diplexing frequencies causes greater insertion loss because there is not complete cancellation in the one output arm, the match is not affected. If the hybrid couplers are well matched, and the source and load are well matched, the shift in frequency does not afiect the impedance: of the diplexer, "as in the case of ordinary filter-type systems;

It should be noted that while the diplexer has been described in terms of two input signals of different frequency being coupled to a single antenna load, the diplexer is bilateral in operation, so that it may be used, for example, to receive two signals of different frequencies at a single antenna and separate them to provide two output signals.

Thus, the diplexer may be used to receive as well as transmit two signals of different carrier frequencies.

Also, it should be noted that while the diplexer has been described as having a phase shifter in one of the wave guide lines, a second phase shifter may be inserted in the other wave guide line to provide an additional adjustment parameter.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended vthat all matter con- 'tained in the above description or shown in the accompanyingdrawingsoshall be interpreted as illustrative and ,not in a limiting sense.

What is claimedis: 7 p 1. ,-A diplexer transmission system for; isolating :two microwave signals of dififerent frequencies at one point and combining them' at another, said diplexer' comprising a au t e first hollow rectangular wave guide, a second hollow j rectangular :wave guide ending in a non-reflecting energy absorbing termination, the first and second guides havuing-a cornmon narrow wall portion at two separated posi-' itions along the guides, each of the common narrow wall .portionshaving an elongated opening therein, the opening having a width extending the, width of the common narrow wall and having a length such as to couple half the reflecting means is longitudinalymovablewith respect to I incident power, and frequency-sensitive phase shifting means positioned in one of the wave guides intermediate said common wall portions, said means including first and second hollow rectangular wave guide sections having a common narrow wall portion therebetwcen with an elongated opening therein, the opening having a width extenda jvposit e theishortingmeans being connected into said one of the wave guides, whereby changes in position or the,

shorting means varies the electrical length of said one of act the' wave guides into the other wave guide, and tirequency-sensitive phase'shifting means positioned in. one

7 of thewaveguides intermediate said common wall portions, said meansincludng first and second hollow 'rec l tangular waveguide sections having a common narrow Wall portion therebetween, means in said common wall :portion for directionally coupling the wave guide sections,

said means coupling substantially half the energy propagat'ed' in one directionin vone of the sections into the other section, and shorting means adjustably positioned in reach of the wave guide sections, the ends of the wave -gu-idesections opposite the shorting means being connected into said one of the wave guides, whereby changes in position of the shorting means varies the electrical Iengthof said one ofthe wave guides.

3. A microwave diplexercomprising three hybrid junc- 7 focus, each junction including apair of wave guide sections with acommon :wall betweenrand means for directionally coupling said sections together, said means couplingsubstantially 'half the, energy propagated in-one of tsaidsections into the other ,of said sections, wave guide ,means, ,forcoupling one end of one wave guide section of thetfirst ofthe hybrid junctions re one wave guide 7 section'of the second of the hybrid junctions, wave guide ;means;for coupling one end of the other wave guide sec- 1 tionjof the first hybridjunction to one wave guide section of the third of the hybrid junctions, means for coupling ftogether the remainingwave guide sections of the second aandthird hybrid junctions, and short-circuiting means ad- .justably positioned in the 'twowave guide sections of one of the hybrid junctions at the ends of said sections remote from said wave guide means.

4. A microwave diplexer comprising three powerdi ziding directional couplers, each coupler including two .adjacent inte rcoupled transmission-line sections, means along theguides, means in each of the common wall portor coupling one end of one linese'ction of the first of said couplers to one line section of the; second of said couplers, means for coupling one end of the other of the line sections of the first of said couplers to one line section of the third of said couplers, means for coupling together the remaining line sections of the second and third of said couplers,.and energy reflecting'means terminating the ends of the line sections of the first coupler opposite the ends coupled to the second and third couplers.

5. Apparatus as set forth in claim 4, wherein the energy the line section of the .firstlcoupler. V

6. A microwave diplexer comprising at least two hybrid junctions, each junction including .a. pair of'wave guide sections with a common wall between and means for di-' I rectionally coupling said sections together, said means coupling substantiallyhalf the energy-propagated in one of said sections into the. other oflsaid sections, wave guide means'for coupling one wave guidersection of one of the hybrid junctions at one end thereof directly to one wave guide section of the other hybrid junction, and variable length wave guide means for coupling one end of the other wave guidesection ofsaid one hybrid junction to one end of the other wave guide section of said other hybrid junction. 7 q i 7. 'A microwavediplexercomprising three hybrid junctions each having four wave guide terminals, wave guide means for coupling. one wave guide terminal of one of the. hybrid fjunctions directly to one terminal of one of the other. hybrid junctionsand'ano-ther of the waveguide terminals of said one of the hybrid junctions directly to one terminal of the remaining hybrid junction, means for.

junction, and adjustable lengths oftshort-circuited wave guide sections coupled respectively to the remaining two terminals of one of thehybrid junctions. V

8. inlcornbination, a source of first .and second signals of different frequency and a wave guide channeling system for differently directing said first and second signals of different frequency, said channeling system comprising first, and second wave guide directional couplers, each of said. directional couplers having. two input waveguides and two output wave guides, the characteristics of said directional couplers being such that energy introduced into either input wave guide divides equally between the two output waveguides with the energy inone output wave guide shifted 90: intphasc with respect to energy in the other output waveguide, afirst wave guide section coupling an output wave guide of said firstdirectional coupler V to an input wave guide .of said second directional coupler, I and asecond wave guide section'coupling .the other out- ,put wave guideof said first directional coupler to the otherinput waveguide of said second.directionalcoupler, said second wave guide section having an electrical length which is an evenintegral number of half wavelengths 'longers than that ofsaid first wave guide sectionat the frequency of said first signal and an odd integral number of half wavelengthsilonger than the electrical length of said first'waveguide section at the frequency of said second ,slgnal,'

9, ,A," dipleiter transmission system for isolating two microwave signals of dilferent frequencies at one point and.

combining them at another, said diplexercomprising a first hollowrectangular wave guide, a second hollow rectangular' Wave guide, a source of microwave signals at afirstfrequency coupled to said first wave guide, a source of microwave signals at a second frequency coupled to said second wave guide, the first and second wave guides having a cornon wall portion at two separated positions shifting means positioned in one of the wave guides intermediate said common Wall portions, said phase shifting means introducing a difference in the path lengths in said wave guides between said common wall portions of substantially an even number of half Wavelengths at one of said frequencies and of substantially an odd number of half wavelengths at the other of said frequencies.

Tyrrell July 27, 1948 Lewis Nov. 28, 1950 Lewis July 17, 1951 Purcell Aug. 14, 1951 Gutton et a1 Oct. 30, 1951 Riblet Feb. 12, 1952 Van Hofweegen Apr. 8, 1952 Dicke Apr. 15, 1952 Zaslavsky Oct. 28, 1952 Riblet Mar. 24, 1953 Korman et a1 May 25, 1954 Sunstein Feb. 15, 1955 Tomiyasu Sept. 2, 1958

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