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Publication numberUS3899759 A
Publication typeGrant
Publication date12 Aug 1975
Filing date8 Apr 1974
Priority date8 Apr 1974
Also published asDE2510854A1
Publication numberUS 3899759 A, US 3899759A, US-A-3899759, US3899759 A, US3899759A
InventorsBenz Konrad K, Hines Marion E
Original AssigneeMicrowave Ass
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electric wave resonators
US 3899759 A
Abstract
In a resonant cavity which has multiple modes of resonance within a desired frequency band, certain undesired modes are selectively detuned out of this frequency band, leaving a single desired mode within this band. This is accomplished by the insertion of elongated conductive members into the cavity, so placed and oriented that they cause strong field distortion and detuning of the undesired modes, but cause little effect on the field pattern of the desired mode with little detuning. In the case of a cylindrical cavity operating in a TE011 mode, coupling can be accomplished through an end wall of the cavity substantially without mode interference effects, permitting more economical methods of construction. The principle has been applied to single cavity and multiple cavity resonators and wave filters.
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Hines et al.

i l Aug. 12, 1975 ELECTRIC WAVE RESONATORS Primary ExaminerPaul L. Gensler [751 Inventors: Marion E. Hines, Weston; Konrad f f Agent or F'rmAlfred Rose; Frank K. Benz, Winchester, both of Mass. Stemhllper [73] Assignee: Microwave Associates, Inc.,

Burlington, Mass. [57] ABSTRACT Filedl P 1974 In a resonant cavity which has multiple modes of resol 4 4 nance within a desired frequency band, certain unde- [211 App No 58 56 sired modes are selectively detuned out of this frequency band, leaving a single desired mode within this CL 333/73 /83 A, 333/98 M band. This is accomplished by the insertion of elon- V HOIP U HUIP gated conductive members into the cavity, so placed Field 05 Search." 333/73 73 33 83 v and oriented that they cause strong field distortion 333/98 M and detuning of the undesired modes, but cause little effect on the field pattern of the desired mode with litl56] R f r n s Cit d tle detuning. in the case of a cylindrical cavity operat- UNITED STATES PATENTS ing in a T5 mode, coupling can be accomplished 2 593 4/1952 Kinzer 333/33 A through an end wall of the cavity substantially without 2H955 Langem 333/33 A mode interference effects, permitting more economi- 2,749 523 6/1956 DiShar 333 73 w cal methods of construction. The principle has been 2954.536 9/1960 Mullerm. 333/73 W applied to single cavity and multiple cavity resonators 3.559.043 l/l97l Hyde 333/83 A X and wave filters. 3.758.880 9/l973 Morz 333/98 M X 18 Claims, 6 Drawing Figures 72 90 74 9/ E 1 E m 63 E 7 I W 64 52 U I Q i a 7 i t n r 6 5 4/ @I V/ //l .K///@ J V////A W///A 4V -4/ C 6 47 62 M 05 5 DETUNING PIN 53 54 54 55 T J n L f I j I +9 AI 7/ 92 73 89 93 PATENTEB RUB 1 21975 .IzET

ELECTRIC WAVE RESONATORS INTRODUCTION This invention relates to microwave cavity resonators and to frequency-sensitive and selective networks such as wave filters, frequency meters, etc., which use such resonators. It particularly applies to those resonators in which more than one mode of resonance is present within the frequency band of interest, for example, those cylindrical cavities using the TE mode. In this case the TM modes are particularly troublesome in that they have essentially the same resonant frequencies and may interfere with the desired frequencysensitive characteristics.

Cavity resonators have many applications in microwave systems, because of their frequency-selectivity characteristics. It is well known that enclosed cavities with electrically-conductive walls act as frequencyselective resonators such that, when coupled to electric waves from the outside, electromagnetic wave energy is more strongly excited in the cavity at particular resonant frequencies than at other frequencies. When coupled to an input waveguide and to an output waveguide such a cavity will act as a frequency-selective wave filter and pass energy with small loss from one waveguide to the other at those resonant frequencies which are coupled, and will substantially attenuate waves of other frequencies. For other applications, a cavity may have only one coupling means to a single waveguide. Such singly coupled resonators selectively absorb energy from an incident wave at the resonant frequencies but strongly reflect wave energy at other frequencies.

The various resonant frequencies have distinctive patterns of electrical and magnetic field excitation within the cavity and each such frequency and its associated pattern is designated as a mode of the cavity. The field patterns are different for each mode, and many modes are possible in all such cavities which are substantially enclosed by electrically-conductive walls. In a given band of frequencies, normally only one mode is desired, and this situation is often obtained by dimensioning the cavity according to well known theory such that the desired mode is the one with the lowest resonant frequency. Then the next higher frequency mode may be substantially higher in frequency, out of the band of interest. However, for those applications requiring a very high degree of frequency selectivity, one of the higher-order modes may be preferred, because it may have a larger O, which means that energy losses are lower for a given amount of energy stored in the internal electromagnetic fields. One of the most useful of these higher-order modes of resonance is that which is commonly designated as the TE mode which is found in a cylindrical cavity. Unfortunately, this high-Q low-loss mode is not unique within a given band of frequencies. In a simple cavity designed for this mode, it is found that there are two modes commonly designated as TM modes which resonate at the same frequency at the TE niode and there are other modes as well, at nearby frequencies. It is found that the fre quency selective characteristics of such a cavity or filter are distorted by the presence of more than one mode at the same frequency or at nearby frequencies. Prior art for this problem has involved a restriction in that the coupling irises have been placed only on the cylindrical side wall to couple only to the internal magnetic field in the axial direction H, and avoid coupling to the circumferential component of the magnetic field H9 or the radial component H,.. Because TM modes have no axial magnetic field component, coupling to the TM modes can be suppressed in this way, theoretically. However, in practice, manufacturing imperfections are present and a slight degree of coupling to the TM modes commonly occurs. Another practice in the prior art is to use an internal disk-shaped end plate in the cylindrical cavity which does not contact the cylindrical wall and to place lossy material in the region hidden behind this plate. The TE mode does not couple well into this region, causing little loss for that mode. The TM modes, however, couple strongly into this zone and thereby suffer a loss, which reduces their deleterious effects. Use of these techniques results in a high cost of manufacture and is not always fully effective.

For reasons of manufacturing economy, it is sometimes more desirable to couple into a cylindrical cavity through an end wall, coupling in this case to the radial magnetic field. In this case, both TE and TM type modes may be directly coupled with very undesirable effects. It is a basic feature of the present invention that these deleterious effects are eliminated by detuning the undesired TM modes, shifting their resonant frequencies out of the frequency band of interest. When this is done, coupling through the end wall is practical and feasible and the desired TE, mode is the only one present within a substantial frequency band.

SUMMARY OF THE INVENTION In the present invention, wave energy is coupled into and from a resonant cavity, particularly a high-Q cylindrical cavity, through one or both end walls. Undesired modes are selectively detuned out of the desired frequency band by the insertion into the cavity of elongated conductive members, such as pins, that are so placed and oriented as to cause field distortion and detuning of the undesired mode or modes, but which have little effect on the field pattern of the desired mode. Filters and resonators employing such cylindrical cavities with end-wall coupling but substantially without mode interference effects allow more economical methods of construction that is found in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a longitudinal section through a single cavity resonator or filter illustrating the general principles of the invention;

FIG. 1A is a section along line IA-IA of FIG. 1;

FIG. 2 is a longitudinal section of a two-cavity filter;

FIG. 3 is a section on line 3-3 of FIG. 2;

FIG. 4 is a section on line 4-4 of FIG. 2; and

FIG. 5 is a longitudinal section through another filter illustrating the invention with a series of cylindrical cavities joined alternate to opposite sides of a common coupling plate.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 shows a resonant cavity 1 of cylindrical shape designed for operation in the TE mode of resonance. The cavity is the air space within the cylindrical side wall between the planar end walls 4 and 5. Iris openings 8 in each end wall are used to couple the cavity to two waveguides 6 and 7, respectively. Iris tuning screws 9 are used to adjust the degree of coupling according to common practice. A larger tuning screw I0 in the side wall is used to adjust the resonant frequency of the cavity, also a common practice.

FIG. I shows four elongated mode-detuning pins 2 extending into the cavity from each wall 4, in the axial direction. These pins are spaced at 90 separations in angular position on the locus of a circle centered on the cylinder axis and having a radius such that the pins are about mid-way between the side wall and the cylinder axis. Also shown are four elongated modedetuning pins 3 extending into the cavity from the central zone of the cylindrical side wall 1. According to well established theory, as for example in Simon Ramo and John R. Whinnery, Fields and Waves in Modern Radio", published by John Wiley and Sons, copyright I944, the TE mode has an electric field pattern whose lines of force are circles about the cylinder axis, lying in planes perpendicular that axis. These electric field lines are perpendicular to the long dimension of the pins 2, 3, and therefore are only slightly perturbed by their presence. However, the theory also shows that the TM, mode has a strong axially-oriented electric field at the end wall at a radius intermediate between the axis and the side wall. The pins 2, therefore, being located about mid-way between the axis and the side wall, strongly perturb this field because they lie along or parallel to it. The effect is to reduce the resonance frequency of the cavity for the TM modes. The TM modes also have a strong radially oriented electric field component in the axially-central region of the cavity at the side wall, and the pins 3 being located in that region and oriented parallel to that component have a similar effect there. The effect of these mode-detuning pins is to add capacitance to the equivalent circuit of the resonator insofar as it concerns certain modes, and to reduce the resonance frequency of the cavity for any mode which has a high-frequency electric field aligned with the pins at their positions in the cavity. In this way, one is able, selectively to reduce substantially the resonance frequency of the cavity for such modes, thereby shifting the resonance frequencies for those modes outside of the frequency band of interest. In the case of the cavity shown in FIG. I, the high-Q resonance is retained for the TE mode in the frequency band of interest, and the mode-detuning pins 2, 3 serve effectively to shift the TM modes from that band.

Not all of these mode-shift pins 2, 3 are necessary in a given apparatus. A single pin 2 or 3 in a single position is sufficient to detune the cavity for one of the two TM modes. A second pin, at an angular displacement of perhaps 90 from the first allow detuning for both TM modes. Three equally spaced pins 2 or 3 are also sufficient. We have obtained satisfactory results in most cases by using four pins 2 at 90 separation on one end wall only. Five or more might also be used. FIG. I shows more mode-shift (detuning) pins than are necessary, to illustrate various options that are available to the designer.

FIG. I shows two coupling irises 8, 8 and two coupled waveguides 6, 7, one on each end wall, 4, 5, respectively. The cavity is also useful if only one iris and one waveguide are present. In this case the frequency selective characteristics of the reflected waves from the single waveguide are used. With two waveguides, the device can serve as a band-pass filter which allows lowloss transmission from one waveguide to the other for frequencies near the resonance, while strongly attenuating other, non-resonant or shifted-resonance frequencies.

FIG. 1 also shows the two waveguides on opposite end walls. The device, however, behaves guite similarly if both irises 8, 8 and both waveguides 6, 7 are on a single end wall, placed at different angular positions about the cylinder axis. The technique of coupling wave energy into and out of a cavity through two irises in a single end wall is used in the embodiment illustrated in FIG. 5.

Filters of this type are commonly constructed of metal, but any strong hard material can be used provided the inner surfaces of the cavities and irises are coated with a high-conductivity material.

The structure of FIG. 1 is a single-resonator filter. For many applications, multiple-resonator filters are needed to obtain a higher degree of frequency selectivity. It is a well-known practice to design such filters as a tandem array of cavity resonators with coupling means to a waveguide or other transmission line from each end cavity and coupling means between each pair of adjacent cavities in the tandem array. The resonators are commonly tuned to approximately the same frequency and the coupling means are designed according to well-known theoretical principles to give a bandpass" frequency selective characteristic. The application of this invention to multiple cavity filters will next be described.

In FIGS. 2, 3 and 4 two cylindrical cavities l1 and 12 are assembled with a common interstage coupling plate 13 between them. FIG. 3 shows an end view of this filter. FIG. 4 is a cross-section view taken through the coupling plate 13. The plate 13 has mode-shift pins I4 passing through it. These pins are located in position favorable for detuning the TM modes. An iris opening 15 in the coupling plate serves to couple the cavities I I, 12 together. The cavities are operated in the TE cylindrical mode. The iris 15 is located off-center, preferably at the radius for maximum H, (magnetic field in the radial direction). The mode shift-pins 14 are perpendicular to the E field of the TE mode and cause little detuning for that mode. However, for the two TM modes which are "degenerate" (at the same frequency) these pins are parallel to the E field, and are preferably located a radial distance from the cylinder axis where the E field of the unwanted TM mode ap' preaching the relevant end wall is maximum. The pins l4 effectively add capacitance with respect to such TM modes thereby reducing the resonant frequency of the cavity for those modes. In that sense, the pins cause significant detuning of the cavity or cavities in which they are installed, relative to the TM modes, especially to the TM modes, dropping the resonant frequency of those modes below the operating frequency band.

The iris 15 can be adjusted in the degree of coupling with a screw 16 extending through the body of the plate 13 into the iris form outside the plate where an adjusting arrangement 17 is provided. Tuning screws 18, I9 for the respective cavities 11, 12 are provided in the side wall of each cavity to tune the cavity resonance.

Plates 2], 22 are provided at the extreme ends of the cavities ll, 12, respectively, with irises for coupling energy into and out of the filter. The input plate 2] has a coupling iris 25 similar to the interstage coupling iris l5, and having similar tuning means 26, 27. The outer surface 28 of the input plate is arranged for coupling to an input waveguide 31, the end of which couples to the input iris 25. Mode-shift pins 24 are located on the inner surface 23, and these appear similar to mode-shift pins 14 for wave energy propagating in the filter. The output coupling plate 22 is functionally the mirror image of the input coupling plate 21, having mode-shift pins 34 within the second cavity 12, an iris opening 35 for coupling that cavity with a waveguide 36 and an iris tuning pin 37. As in the interstage coupling plate 13, each and coupling plate has four mode-shift pins; there are four mode-shift pins in each end wall of each cavity. Again, more mode-shift pins are shown than are absolutely necessary.

To extend the principle of FIG. 2, 3, 4 to three or more cavities, it is only necessary to add more cylindrical members like 11 and 12 with additional coupling plates like plate 13 between each pair of cylindrical member. According to well known principles, the cavities should be carefully tuned and the degree of coupling at each iris should be properly adjusted to give the desired band-pass characteristic.

A drawback to structures like those shown in FIGS. 1, 2, 3, and 4, and multiple-cavity extensions of them is that the tuning screws 10, 18, 19, on the cylinder side walls can provide only a limited degree of tuning range without introducing significant loss and significant distortion of the internal field patterns. The cylindrical members 1, ll, 12 must be quite precisely cut to length before assembly as determined by the desired frequency. Tuning screws are more desirably placed on one of the flat end walls. It is also desirable to permit an ajdustment of the length ofthe cylindrical cavity as measured along its axis. In this way, a single filter structure can be tuned over a much wider band of frequencies. This drawback is overcome in this way in the structure shown in FIG. 5, where the end plates 71, 72, 73, 74, and 75 are axially adjustable in position, and include tuning screws 89, 90, 91, 92, 93, for fine trimming adjustment.

In FIG. 5, a series of cavities 51, 52, 53, 54 and 55 are assembled to a common coupling plate 41. This plate contains all the coupling irises 42, 43, 44, 45, 46 and 47 and their tuning screws 42 -47', respectively, for the filter. For convenience in describing this figure, the two sides of the coupling plate will be referred to as the first side 61 and the second side 62. An input waveguide 63 is coupled to the first iris 42 at the first side. The first cavity 51 is attached to the plate 41 at the second side, located for coupling in the TE mode with the first and second irises 42, 43. The second cavity 52 is attached to the plate at the first side, located for coupling in the TE mode with the second iris 43 and the third iris 44. Likewise, the third cavity 53 is attached to the second side 62 located for coupling in the TE mode with the third and fourth irises 44, 45; the fourth cavity 54 is affixed to the first side 61 so as to couple in the same mode with the fourth and fifth irises 45, 46; and the fifth cavity 55 is affixed to the second side 62 so as to couple with the fifth and sixth irises 46 and 4'7, respectively. An output waveguide 64 is coupled to the last iris 47 at the first side 61.

In the embodiment of FIG. 5, each cavity has its input and output irises located in the same end wall; however, each iris has the same properties as a coupling iris in FIG. 2 it is located off-center, at the radius for maximum H, in the TE mode. The coupling plate 41 provides one end wall for each of the cavities through which energy is coupled. The opposite end of each cav ity is closed by an individual end plate 71, 72, 73, 74, 75, respectively. Each of these end closure plates has a set of four mode-shift pins and a tuning screw for the cavity in which it is installed. These details are identitied at one such plate 73, where two mode-shift pins 84 and a tuning screw 89 are shown. It will be understood that each closure plate has four mode-shift pins 84, however. Likewise, the coupling end plate 41 may have a set of four mode-shift pins in each cavity; two pins 84', 84' of one such set are illustrated in the intermediate cavity 53. In this embodiment of the invention, the cavities are tuned through end walls, and there is no need to provide any structure through the side walls.

It may be noted that while FIG. 5 shows a five-cavity design, the principle of assembly is applicable to any number of cavities in tandem. For a single cavity design, for example, the cylindrical members 52, 53, 54, and 55 may be eliminated, together with their separate end plates, and the plate 41 may be cut off between members 51 and 53. The waveguide 64 is then attached to plate 41 over the iris hole 43. For two cavities, one would retain cavity member 51 and 52 and attach the waveguide 64 over iris 44 on the bottom side of plate 41.

We claim:

1. An electric-wave cavity resonator with a plurality of modes or resonance in a given frequency band intended for operation in a single one of said modes in said band of frequencies, comprising electricallyconductive surrounding walls to define said cavity, coupling means on one or more of said walls, and mode detuning means comprising one or more elongated conductive pins having no cross-sectional dimension that is greater than a minor fraction of the length dimension placed within the cavity in positions and orientations so selected that the length dimension of each lies substantially perpendicular to the high frequency electric field of said single mode of resonance and substantially parallel to the high frequency electric field of others of said modes of resonance for tuning the resonance frequency of said cavity for said other modes to frequencies outside of said frequency band with substantially minimum addition of surface upon which electric currents must flow.

2. Cavity resonator according to claim I wherei said conductive walls comprise a cylindrical side wall and two planar end walls.

3. Cavity resonator according to claim 2 whe' ein said mode detuning means comprise one or more of said detuning pins extending into the cavity from at least one of said end walls in a direction parallel to the cylindrical axis and placed at a radial distance from the axis intermediate between said axis and said side wall.

4. Cavity resonator according to claim 2 in which said mode detuning means comprise one or more of said detuning pins extending into the cavity in a radial direction from the cylindrical wall at a position inter mediate between said end walls.

5. An electric wave band pass filter comprised of one or more resonant cavities as in claim 2, each of said cavities having two of said coupling means, said cavities being coupled together via said coupling means.

6. An electric wave band-pass filter according to claim 5 wherein two or more of said cavities are coupled together in tandem, with each pair of adjacent cavities sharing a common end wall and common cou pling means in said common wall between said adjacent cavities.

7. An electric wave band-pass filter according to claim wherein said mode detuning means comprise one or more pins protruding into the cavity from at least one of the said end walls in a direction parallel to the axis of the cavity and placed at positions intermediate between the axis and the side wall.

8. An electric wave band-pass filter according to claim 5 wherein said mode detuning means comprise one or more pins protruding into the cavity from the cylindrical wall in 'a radial direction from positions intermediate between the end walls.

9. An electric wave band pass-filter comprised of a sequential plurality in excess of two of said resonant cavities as in claim I, each of said cavities having two of said coupling means, said cavities being coupled together via said coupling means.

10. An electric wave band-pass filter according to claim 9 wherein two of said cavities are coupled together in tandem, with each pair of adjacent cavities sharing a common wall and common coupling means in said common wall between said adjacent cavities.

1]. An electric-wave band-pass filter according to claim 10 wherein the two coupling means of each cavity are located side-by-side in the same wall of the cavity, and said cavities are all attached to a common electrically-conductive plate which serves as a wall for each of said cavities, said cavities being aligned sequentially thereon but placed alternately on opposite sides of said plate with the intercoupled cavities in said tandem sequence being on opposite sides in each case, and with coupling means sequentially arrayed in said common plate between said cavities, an intermediate cavity in said sequential plurality being coupled at a first of its two coupling means to the immediately preceding cavity via one of the means in said plate, and at the second of its two coupling means to the immediately following cavity via the sequentially-next one of the coupling means in said plate.

12. An electric-wave band-pass filter according to claim 11, said cavities having cylindrical shapes with axes aligned perpendicular to said common plate which provides an end wall for each cavity, the two coupling means of each cavity being an adjacent pair of said sequentially arrayed coupling means in said plate, the axis of said cavity meeting said plate at a point that is between the members of said pair.

13. An electricwave band-pass filter comprised of a plurality in excess of two of resonant cavities coupled together in tandem sequence, the first and the last of said cavities in sequence having external coupling means, with each adjacent pair of cavities in said sequence having a mutual coupling means, said cavities being mounted on a common electrically-conductive plate which serves as a wall for each cavity, said cavities being alternately placed on opposite sides of said common plate in said sequence, said mutual coupling means connecting the members of said pairs of cavities through said common plate there being two coupling means for each cavity located side-by-side in the cavity wall that is provided by said plate, an intermediate one in said sequence of cavities being mutually coupled through one of its said coupling means with the immediately preceding cavity in said sequence, and through the other of its said coupling means with the immediately following cavity in said sequence.

14. An electric wave cavity resonator for use at microwave frequencies dimensioned to resonate to wave energy in a TE mode in a frequency band centered at a specified frequency, said cavity having the property of resonance to other wave modes therein, end wall means for said cavity disposed transverse to the cylinder axis and having coupling means therein lo cated off-center at the radial distance from said axis which favors coupling to the radial component of the magnetic field H for the TE, mode, and mode-shift means each comprising an electrically conductive elon gate pin having no cross-sectional dimension that is greater than a minor fraction of its length dimension extending into said cavity from at least one end wall, said mode-shift means being located on said end wall with its length dimension extending into said cavity in an axial direction so as to detune the cavity for wave energy in said other modes to effectively shift the resonance frequency of the cavity for such other modes out of said frequency band with substantially minimum addition of surface upon which electric currents must flow.

15. A filter comprising at least two of said cylindrical cavities according to claim 14, and between them coupling means in the form of an electrically-conductive common endwall member having an iris opening through it located to favor H, wave energy in said TE mode between said cavities.

16. A filter according to claim 15 having a plurality of said cavities in tandem, and a set of said pins on each end wall of each cavity.

17. A resonator according to claim [4 in which said mode-shift means comprises a set of pins extending into said cavity from the locus of a circle at a radial distance from said cylinder axis where the E-field of at least a substantial component of said other modes approaching said end wall is substantially maximum.

18. A resonator according to claim 17 in which each end wall has a set of said pins.

* i i 8 l

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Classifications
U.S. Classification333/212, 333/228
International ClassificationH01P1/208, H01P7/06, H01P1/20, H01P1/16, H01P1/212, H01P7/00, H01P1/161
Cooperative ClassificationH01P7/06, H01P1/16, H01P1/208
European ClassificationH01P1/16, H01P7/06, H01P1/208