US 3896402 A
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United States Patent 1 1 1 3,896,402
Jackson July 22, 1975 DIELECTRIC WAVEGUIDE FILTERS OTHER PUBLICATIONS  Inventor: Lynden Ashbrooke Jackson, Suffolk, vzyatyshev et l gTh and Experiment, in
England viet Journal of Optical Technology, Vol. 35, No. 1, [73} Assignee: The Post Office, London, England 1968; PP- 73 77- Southworth,Principles and Applications of Wave-  Fled: 1974 guide Transmission, Van Nostrand, New York 1961; 1211 Appl. No.: 443,429 1 9- 97-100 9 i Primary Examiner-James W. Lawrence  Fore'gn Apphcatlon Pnomy Data Assistant Examiner-Marvin Nussbaum Feb. 19, 1973 United Kingdom 7937 73 Att r ey Agent r FirmHall & Houghton'  U.S. Cl 333/73 W; 333/10; 333/98 R [57 ABSTRACT V  Int. Cl. ..H01P 1/18; HOlP 1/20 i  Field of Search 333/10, 11, 73 w, 95 R, A f f mmersed a surrounding d1electr1c medlum permlts frequency se- 333/98 R, 84 R, 350/96 WG lect1ve energy transfer between the regions. The waveguide regions include parallel coupled sections. The
 References Cited length of the sections and the spacing therebetween UNITED STATES PATENTS determine the magnitude and frequencies of the en- 2.l29,7l2 9/l938 Southworth 333/95 X ergy coupled to and propagated the waveguide re- 2,794,959 6/1957 Fox 333 95 x giom 3,157,726 1l/l964 Hicks, Jr. et al 333/95 X 3,785,717 l/l974 Croset et a] 350/96 W6 13 Claims, 12 Drawing Figures ""I'I'U'LY'Z'I'UI'ITI'I'A 14 MM PORT , W amen-1.
PATENTEDJUL 22 ms SPEET IE \szmbbmmm m m PATENTED JUL 2 2 I975 SHEET Ems GEDEE mm IQNI PATENIEDJUL 2 2 ms SHEEI bamSmE QM Qm IGNI- PATENTEDJuL22 I975 3,896,402
SHEET 7 12 $2 a 59 Q EU I PATENTED JUL 2 2 I975 SHEET Q S5 653% mm am PATENTEDJUL22 I975 KQQSAOZ DIELECTRIC WAVEGUIDE FILTERS This invention relates to apparatus for separating the signals of each channel of a multi-carrier communications system.
Communications systems using hollow metal waveguides of circular cross-Section are well known. Such systems provide large available bandwidths which due to the nature of the traffic and the limitations of digital equipment must be split up into many signal channels. Means must therefore be provided at the receiver for separating single channels. Separation is normally provided by the use of resonant cavity filters.
Dielectric waveguides consisting of a body of arbitrary transverse cross-section of dielectric surrounded by an infinite medium of differing permittivity to that of the body, can be used at frequencies similar to those of hollow conducting waveguides. The body and surrounding medium need not possess uniformity of permittivity over the transverse plane, and the boundary between the two may be diffused. In most respects dielectric waveguides are similar to hollow waveguides. There are some cases, however, where the characteristics of dielectric waveguides make the construction of a particular waveguide component very difficult compared with its hollow guide counterpart. One such case is the broadband directional coupler where variation of the coupling coefficient with frequency between two dielectric waveguides is sufficiently large to prevent a useful component being constructed. This being so, new techniques are required to utilise the dielectric guide to its full advantage. It will be shown that whilst the coupling coefficient characteristic precludes certain components it allows for a totally new form of channel separation network.
It is the object of this invention to provide a dielectric waveguide filter which will operate over a wide band of frequencies.
According to the present invention a dielectric waveguide filter comprises a dielectric medium in which two dielectric waveguide regions are totally immersed, each waveguide region having a length over which the region is sufficiently close to a length of the other region to permit the regions to exchange electromagnetic energy, the lengths being referred to as the coupled lengths, the coupled lengths being such that, in use, energy transmitted, at a specified frequency, into one waveguide region from one end of the coupled length thereof is contained, at the other end of the coupled length, substantially within the other region, the dimension of each waveguide region transverse to its length being such that the ratio M/A lies in the range (both limits included) of from E l-I 0.05 0.95 V E;
same frequency as the electromagnetic energy, E, and.
E are, respectively, the dielectric constant of that part of the regions of maximum dielectric constant and the average dielectric constant of the medium, or are reand positioned adjacent to each other over a limited length herein referred to as the coupled length, the coupled length being the length over which the centroids of dielectric constant of the transverse sections of the waveguides are separated by a distance such that in use the anti-symmetric mode of the filter is propagatable, the transverse dimensions of the waveguides being such that the ratio lt /h is in the range (both limits included) of from 0.95 VET, +0.05 \/E,
0.05 via E /E to m where A, is the wavelength of the signal in the guides in those portions remote from the coupled length, k is the wavelength of a TEM wave of the same frequency in the free space, E is the dielectric constant of the material of the guides and E is the dielectric constant of the medium in which the guides are immersed.
The ends of the coupled lengths may be indicated by increasing the separation between the regions or the waveguides to a value at which energy transfer ceases or by interposing an electrically-conducting screen between the regions or waveguides.
The regions and waveguides and the dielectric medium may allbe isotropic.
According to a subsidiary feature of the invention the two dielectric waveguides may be mounted upon a conducting plane and the combination of the waveguides and the conducting plane totally immersed in a dielectric medium.
Instead of the single conducting plane, two conducting planes may be employed.
The dielectric medium between the two guides may have a dielectric constant of a value that is different from that of the rest of the surrounding medium. In this case, the dielectric constant E appearing in the range referred to above is that of the dielectric in which the waveguides and the material between them are immersed.
The invention is based on the fact that if two waveguides are uniformly coupled along their coupled lengths, then when one guide is excited at one end of the coupled length power transfer occurs between the guides. As the initial excitation progresses down the coupled length an increasing proportion of the power is carried by the second guide until at a distance determined by a quantity referred to herein as the coupling coefficient, the transfer is substantially total. Conditions then exist for the power to transfer back to the original guide in a similar distance. In this manner it may be said that the power cycles back and forth between the guides. The interval between points of substantially total energy transfer is solely determined by the quantity referred to herein as the coupling coefficient provided that the waveguides are identical and the coupling uniform along the coupled length. In dielectric guides uniform coupling can be obtained by maintaining parallelism between the guides over their coupled length and the coupling coefficient may be varied by adjusting the spacing of the guides.
The end of the coupled length can be made to coincide with one of the points where electromagnetic energy appears substantially totally in one of the guides, in which case the electromagnetic energy would leave on that guide in which it appeared at the end of the coupled length, which need not be the guide on which it entered. The coupled length may exceed the interval between points of substantially total transfer or power by many times but provided the ratio between the two lengths is an integer the power will appear substantially totally in one or other of the guides.
It has been found that, because the coupling coefficient is a function of frequency, the filter can be used to separate two carriers at different frequencies by arranging the coupled length and coupling coefficient so that at the end of the coupled length one carrier appears substantially totally in one guide and the other carrier appears substantially totally in the other guide. Signals derived by modulating the carriers would behave in a similar manner excepting that they consist of a band of frequencies and consequently it is not strictly true that the whole band will appear substantially totally in one guide. The condition that a carrier introduced on the input leaves either output port with virtually no attenuation can be stated as where C is the coupling coefficient, which varies with frequency,
L is the coupled length,
n is the positive integer.
Ifn is even the carrier leaves on the guide providing the input port. If n is odd the carrier leaves on the guide providing only an output port. The energy in a carrier not meeting this condition will not be substantially totally in either guide at the end of the coupled length, but the condition can generally be achieved for any carrier entering the filter by altering either the coupling coefficient or the coupled length, or both.
In addition, the filter can be used to combine carriers on to a single dielectric waveguide by so arranging the coupled length and the coupling coefficient that at the end of the coupled length the carriers appear in the same waveguide.
Further, by adopting, for each waveguide a dimension transverse to its length as defined above, the waveguides are able to carry a group of carriers and the filter will separate the band into a number of constituent carriers or channels. Alternatively, the filter can act in the reverse manner and combine carriers or channels.
A filter which has a single input port and two output ports is referred to as a coupled line filter. It provides a device in which a number of evenly spaced signal channels are input, and separation is performed so that alternate channels appear on each of the two output ports.
Embodiments of the invention can employ more than two waveguides. Electromagnetic coupling between waveguides is not restricted to a single plane, so that a particular waveguide can be coupled simultaneously to several waveguides, but the characteristics of an arrangement employing more than two waveguides can be related to the superposed characteristics of the waveguides taken as pairs, provided that all combinations of two waveguides are considered. Embodiments of the invention can employ three, four or more waveguides.
A channel separation filter employing the coupled line filter is constructed as a series of units in cascade, each being designed to take the output from one of the guides of the preceding unit and perform separation. Thus for a network required to separate eight spaced channels the first unit provides four on each of its output ports. The cecond stage of filtration consists of two units one being connected to each output port of the first unit. In this manner there will now be four output guides each carrying two of the original eight channels. The third stage performs likewise, utilising four units.
The functions of the input and output ports of the device can be interchanged to provide a device for combining a number of signal channels at the transmission end of a communication system.
A group of carriers introduced at the input port of a coupled line filter can be separated into two sub-groups according to whether n is even or odd for each carrier provided they can each be made to satisfy the condition that n is a positive integer for the same coupled length. This requires that the frequencies of the carriers bear a relationship with the variation of coupling coefficient with frequency. Those carriers satisfying the condition of n odd will transfer to the guide providing no input port while the carriers satisfying the condition of n even continue on the guide providing the input port. Each of the two sub-groups can be further separated by additional units acting in a similar manner to produce four smaller groups each containing not more than one quarter the number of carriers in the original group. This procedure can be repeated as many times as necessary to cause each carrier to appear alone on a particular waveguide, thus achieving complete separation of the original group into its composite carriers. It can be seen that the coupling described can exhibit the characteristics of a cyclic filter over a large bandwidth, and a series of such devices can be used as a channel separation network.
It has been found that the variation of the coupling coefficient with frequency can be made substantially linear, over a large bandwidth, by adjustment of the separation of the waveguides. In addition, it has been found that small variations in the spacing cause only minimal change in the rate of change of coupling coefficient with frequency but mainly cause a uniform variation over the bandwidth of the device. These relationships permit the construction of a channel separation network particularly useful for a multi-carrier communication system with evenly spaced signal channels, where the coupled length for all units at each stage is the same, and exactly half the coupled length for all the units at the immediately preceding stage. The separation between the waveguides of each unit need not be the same though.
A dielectric waveguide coupled line filter comprising isotropic waveguides immersed in an isotropic medium and a channel separation network constructed from a cascade of coupled line filters, the channel separation network being particularly useful for a multi-carrier communication system with evenly spaced signal channels, will now be described by way of example only and with reference to the accompanying drawings in which:
FIG. 1 respresents a dielectric waveguide coupled line filter,
FIG. 2 shows the transverse section s of the isotropic waveguides used in the embodiments,
FIG. 3 represents a channel separation network having eight output ports,
' FIG. 4 shows the construction ofa coupled line filter,
FIG. 5 shows the construction of a partial channel separation network,
FIG. 6 shows the variation of coupling coefficient with frequency,
FIGS. 7, 8 and 9 show the theoretical frequency characteristics of the partial channel separation network of FIG. 5,
FIGS. 10, ll, 12 show the measured outputs from various output ports of the network of FIG. 5, and confirm the theoretical results.
Referring to FIG. 1, a coupled region 3 is formed over a coupled length L where a dielectric waveguide 2 is parallel to a dielectric waveguide 5, the separation between the centroids of the transverse sections of the waveguides being d.
' The separation d between the centroids is such that the anti-symmetric mode is propagated along the waveguides.
An input 1 is provided by waveguide 2 and two outputs PORT 1, PORT 2 are provided by waveguides 2, 5 respectively. The coupled region is defined by an increasing separation between the waveguides. Exchange of electromagnetic energy is then virtually restricted to the coupled length.
FIG. 2 shows the transverse sections of the isotropic waveguides used in the embodiments. In the case of rectangular waveguides, as used, the separation d between the centroids of the transverse sections of the waveguides is related to the distance s between adjacent faces of the waveguides, so that parameters which are dependent on d will bear the same relationship to s. This will also hold for a number of symmetrical sections which can be used as transverse sections for dielectric waveguides. In FIG. 6 which shows the relationship between coupling coefficient and frequency, the distance .9 is used instead of separation d.
The dimensions a and b have values such that the ratio A /A- lies in the range (both limits included) 0.95 Va, +0.05 V5,
F I d 0.05 VE+095 3C1,
where E, and E are the dielectric constants respectively of the dielectric material from which the waveguides are constructed and that of the dielectric material in which the guides are wholly immersed, in this case air, and A and A are as defined above.
The operation of a channel separation network will now be described with reference to FIG. 3. Eight input signals occupying channels fl f8 are introduced at the input I of the network. This input is provided by the first stage coupled line filter made up of waveguides 2, 5. The signals enter the coupled region 3 and move towards the outputs PORT 1, PORT 2 of the first stage coupled line filter. Signals f2,f4,f6,f8 now appear on waveguide 5 while signals fl f3, f5, f7 remain on waveguide 2. Waveguide 5 now provides the input to a sec ond stage coupled line filter, and waveguide 7 provides the other waveguide. At the end of this coupled line filter, signals f2, f6 have transferred to waveguide 7 while signals f4, f8 continue on waveguide 5. Signals f2, f6 are now separated by the coupled line filter comprising waveguides 7 and 9, and f4, f8 are separated by the coupled line filter comprising waveguides 5 and 10. In the same way signals fl,f3,f5,f7 are separated by the coupled line filters comprising waveguides 2 and 8, waveguides 8 and l l and waveguides 2 and 12 to provide each signal on one waveguide. In this manner, fl
appears at PORT 6, f2 at PORT 13, f3 at PORT 7, f4-
at PORT l2,f5 at PORT 5,f6 at PORT l4,f7 at PORT 8 andf8 at PORT II.
The manner in which separation is achieved can be understood by reference to FIGS. 7, 8 and 9, which related to the frequency response characteristics of the coupled line filters. The frequency response curves of FIG. 7 can be provided by the coupled line filter represented by FIG. 1. If it is assumed that a coupled line filter suitable for use with frequencies lying between 30 GHz and 40 GI-Iz is employed, and that the signals applied at the input 1 of the coupled line filter are at frequencies showing 0 dB loss according to the curves of FIG. 7, then frequencies lying on the solid curve of FIG. 7 will appear at PORT l, and frequencies lying on the dotted curve of FIG. 7 will appear at PORT 2. This is the situation which exists at the end of the first stage coupled line filter of the channel separation network shown in FIG. 3. The action of the second stage coupled line filters is shown by FIG. 8, and the action of the third stage couplers by FIG. 9.
The frequency characteristics are made to differ from stage to stage by progressively halving the coupled length of successive stages of coupled line filters and changing the separations as required. Thus, in FIG. 3, the coupled length of waveguide 11 with waveguide 8 is half the coupled length of waveguide 2 with waveguide 8, and this in turn is half the coupled length of waveguide 2 with waveguide 5.
The coupled length of waveguide 7 and waveguide 5 is the same as the coupled length of waveguide 8 and waveguide 2. The coupled length of waveguide 9 and waveguide 7, the coupled length of waveguide 5 and waveguide 10, and the coupled length of waveguide 2 and waveguide 12 are all the same as the coupled length of waveguide 8 and waveguide 11.
The coupled length and separation between waveguides 2 and 5 are selected such that the energy at frequency fl transfers totally from one waveguide to the gther N times, where N/8 is an integer, and the energy at frequency f8 transfers N-7 times.
The separation between waveguide 8 and waveguide 2 is the same as that between waveguide 2 and waveguide 5. The separation between waveguide 12 and waveguide 2 is the same as the separation between waveguide 2 and waveguide 5. The separation between waveguide 11 and waveguide 8 differs slightly from the others to effect optimum tuning. The separation between waveguides 10 and 5 is the same as the separation between waveguides 7 and 5, but this separation differs slightly from that between waveguides 2 and 5. The separation between waveguides. 9 and 7 differs slightly from that between waveguides 7 and and is not related to any other separation.
Small changes in separation are required to tune the coupled line filter to the group of frequencies in use. Tuning is effected in the constructions shown in FIG. 4, and FIG. 5 by compressing the expanded polystyrene blocks in the appropriate direction to reduce the separation between the waveguides as required.
Waveguides of substantially circular, elliptical or rectangular transverse cross-section can be used in the construction of coupled line filters as described. The waveguides can be joined by a web of the same material instead of being physically separate. Alternatively, the waveguides could be supported by a tube, the rods being formed as internal ribs at the opposite ends of diameters of the tube.
FIGS. 4, 5 show constructional details of an embodiment of the invention. The arrangement shows a coupled line filter having expanded polystyrene members for supporting the dielectric waveguides.
Referring to FIG. 4, identical rectangular waveguides l, 2 are supported in such manner that they are parallel with the minor axes of the transverse sections coincident. Expanded polystyrene blocks 13 fill the space between the waveguides, and also contact the outer faces of the waveguides, totally enclosing the waveguides. The spacing between the waveguides is set by the expanded polystyrene blocks between them. This spacing can be altered by varying the transverse pressure on the assembly in the appropriate direction, to effect fine tuning of the filter. In a variation of this embodiment, the dielectric material filling the space between the waveguides may have a dielectric constant different from that of the material which contacts their outer faces.
FIG. 5 shows a three-stage filter network using the form of construction of FIG. 4. As before, the dielectric waveguides 2, 5, 8, 12 are embedded in expanded polystyrene 13, the waveguides being parallel and having the minor axes of their transverse cross-sections coincident. Electromagnetic energy is fed to and taken from the waveguides by conventional transducers 14 for providing a connection between rectangular waveguides and dielectric waveguides. Electromagnetic energy is introduced at the input 1 and travels along towards the output PORT 6 of waveguide 2, energy transferring to waveguide 5 appearing at PORT 2, energy transferring to waveguide 8 appearing at PORT 4 and energy transferring to waveguide 12 appearing at PORT 5.
Although the embodiments already described disclose coupled line filters having sharp transitions of dielectric constant across a transverse section, the invention is also applicable to structures in which these transitions are more gradual. For example, a coupled line filter can be constructed in which each transition is divided into several steps. Further, the assembly may be treated in such a way that these steps are smoothed into a gradual transition. For use at optical frequencies a multilayer drawn glass construction can be used.
A coupled line filter may be constituted from a body of dielectric material which is treated by ion implantation, selective polymerization or other methods so as to develop a variation of dielectric constant across the transverse section appropriate to the maintenance of the symmetric and the antisymmetric modes of the coupled line filter.
For example, a unitary dielectric body having a dielectric constant which varies in a continuous manner over the cross-section of the body can be produced by, for example, selective polymerization. Certain monomers and low order polymers polymerise on exposure to radiation for example ultra-violet light, the degree of polymerization achieved being a function of the intensity of the radiation and the duration of the exposure. If such radiation is brought to a focus within a body of monomeric or low order polymeric material then the degree of polymerisation produced within the body will be highest at the focus. By moving the body during irradiation an elongated region of increased polymerization can be produced. The rate at which the degree of polymerization varies with distance from the focus is a function of the aperture of the optical system used to focus the radiation. If the source is fixed then a sectorshaped region is produced in which the degree of polymerization is highest at the centre and falls off at the same rate along any radius within the sector, while outside the sector the degree of polymerization remains at its initial value. A broader sector can be produced by rotating or oscillating the radiation source and optical system at the focus. Preferably the source and optical systems are arranged to produce a line focus. The dielectric constant will be higher in the region of higher polymerization. Alternatively, a substantially parallel beam of radiation for example a laser beam can be used, selective polymerization being produced by rotating or oscillating the source about the point in the cross-section of the body at which the highest degree of polymerization is required.
In applying the formulae defining the limits of the ratio to the case filter in which the dielectric constant varies over the transverse section, the value of E to be taken is the maximum or minimum value, i.e. the value of the dielectric constant of that part of the waveguides of maximum or minimum dielectric constant. The required value of E is the value of the average dielectric constant of the medium.
It will be understood that although the waveguides will be identical they need not be uniform, in transverse cross-section over the coupled lengths. The separation over the coupled lengths may taper from the ends of the coupled lengths to the centre point thereof at which point the separation may be the maximum or the minimum or may vary progressively from one end of the coupled length to the other. The bandwidth of a filter embodying any of the configurations just described will be somewhat restricted as compared with the other forms of filter previously described.
1. A dielectric waveguide filter assembly comprising in combination a dielectric medium, a plurality of dielectric waveguide regions totally immersed in said medium, each of said regions having an energy input port and an energy output port, a first of said dielectric waveguide regions having a first length that is spaced from and parallel to a first length of a second of said dielectric waveguide regions, the first lengths and the spacing therebetween permitting a transfer from one region to the other of energy lying in a specified fre quency band, the said second of said dielectric waveguide regions having a second length spaced from and parallel to a first length of a third of said dielectric waveguide regions, the second length, the first length of said second dielectric waveguide region and the s pacing therebetween permitting a transfer between 2. A dielectric waveguide filter assembly comprising in combination a dielectric medium, a plurality of dielectric waveguide regions totally immersed in said medium, each of said regions having an energy input port and an energy output port. a first of said dielectric waveguide regions having a first length that is spaced from and parallel to a firstlength of a second of said di' electric waveguide regions, the first lengths and the spacing therebetween permitting a transfer from one region to the other of energy lying in a specified frequency band, the said first dielectric waveguide regions having a second length that is spaced from and parallel to a length of a third of said dielectric waveguide regions, said second length, said length of said second dielectric waveguide region and the spacing therebetween permitting the transfer between said first region and said third region of energy lying in another specified frequency band.
3. A dielectric waveguide filter assembly comprising in combination a dielectric medium, at least three dielectric waveguide regions totally immersed in said medium, two of said regions being coupled together over lengths thereof parallel to and spaced apart one from the other, the lengths and the spacing therebetween permitting the transfer between the coupled regions of energy lying in a specified frequency band, the third dielectric waveguide region being coupled to one of said two dielectric waveguide regions over further lengths parallel to and spaced from one another, the further lengths and spacing therebetween permitting the transfer between said third dielectric waveguide region and said one dielectric waveguide region of energy lying in another specified frequency band.
4. A dielectric waveguide filter assembly as claimed in claim 3 wherein the ends of the coupled lengths are defined by increasing the separation of the waveguide regions to a value at which energy transfer ceases.
5. A dielectric waveguide filter assembly as claimed in claim 3 wherein an electrically-conducting screen interposed between the waveguide regions terminates the coupled length.
6. A dielectric waveguide filter assembly as'claimed in claim 3 in which the waveguide regions and the dielectric medium are isotropic.
7. A dielectric waveguide filter assembly as claimed in claim 3 wherein the waveguide regions are supported by at least one conducting plane and the combination totally immersed in the dielectric medium.
8. A dielectric waveguide filter assembly as claimed in claim 7 wherein there are two conducting planes.
9. A dielectric waveguide filter assembly as claimed in claim 3 wherein the waveguide regions are of substantially rectangular transverse cross-section.
10. A dielectric waveguide filter assembly as claimed in claim 3 wherein the waveguide regions are of substantially circular transverse cross-section.
11. A dielectric waveguide filter assembly as claimed in claim 3 wherein the waveguide regions are of substantially elliptical transverse cross-section.
12. A dielectric waveguide filter assembly as claimed in claim 3 wherein the waveguide regions are joined by a web of material of the same dielectric constant as the waveguides.
13. A dielectric waveguide filter assembly as claimed in claim 9 wherein the waveguide regions are supported by a tube, the waveguide regions being formed as internal ribs at the opposite ends of diameters of the tube.