US3142028A

US3142028A - Waveguide stop-band filter utilizing hybrid circuit with lossy resonant cavities in branch arms

Info

Publication number: US3142028A
Application number: US198870A
Authority: US
Inventors: Robert D Wanselow
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1962-05-31
Filing date: 1962-05-31
Publication date: 1964-07-21
Anticipated expiration: 1981-07-21

Description

United States Patent 3,142,028 WAVEGUIDE STOP-BAND FILTER UTILIZING HYBRID CIRCUIT WITH LOSSY RESONANT CAVITIES IN BRANCH ARMS Robert D. Wanselow, Woodiand Hills, Califi, assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed May 31, 1962, Ser. No. 198,870

5 Claims. (Cl. 333-73) The present invention relates to waveguide components, and more particularly relates to a hybrid stop-band filter for introducing frequency sensitive attenuation into a waveguide.

In the prior art, frequency sensitive attenuation of microwave energy has been achieved by directional filter devices which attenuate a preselected band of microwave frequencies with minimum reflection at their input port. However, in such devices the reflection coefficient varies inversely with the presecribed attenuation, and in addition, passband attenuation is difiicult to control.

Accordingly, it is an object of the present invention to provide a novel waveguide stop-band filter which provides attenuation over a readily controllable range of frequencies.

It is a further object of the present invention to provide a waveguide stop-band filter for attenuating a specified amount of microwave energy within a predetermined frequency range with a minimum amount of reflection of microwave energy in the stop-band range, and in which the reflection coefiicient is independent of the amount of prescribed attenuation.

It is a still further object of the present invention to provide a device for introducing into a waveguide frequency sensitive loss which is smaller and lighter than prior art devices and capable of providing comparable attenuation over a comparable frequency range.

The objectives set forth above are achieved by the present invention by utilizing an attenuation-containing cavity resonator device in combination with a waveguide hybrid to provide frequency sensitive loss for electromagnetic waves propagating through the hybrid. The hybrid in-' cludes an input port, an output port, a first auxiliary port and a second auxiliary port, with electromagnetic waves of frequencies within a predetermined frequency range being propagated through the hybrid between the input port and the output port. A first cavity which is made resonant at a preselected frequency at which attenuation is to be introduced is coupled to the first auxiliary port, and an identical resonant cavity is coupled to the second auxiliary port of the hybrid. Lossy dielectric material is disposed in each resonant cavity to attenuate electromagnetic energy over a preselected frequency range.

Other and further objects, advantages, and characteristic features of the present invention will become readily apparent from the following detailed description of a preferred embodiment of the invention when taken in conjunction with the appended drawing in which:

FIG. 1 is a plan view in longitudinal section illustrating a waveguide stop-band filter constructed in accordance with the principles of the present invention; and

FIG. 2 is a longitudinal sectional view taken along 22 of FIG. 1.

Referring now to FIGS. 1 and 2 with more particularity, the waveguide stop-band filter of the present invention comprises a four-terminal hybrid network designated generally by the reference numeral and a cavity resonator circuit 12 electromagnetically coupled to two of the terminals of the hybrid 10. The hybrid 10 is preferably a conventional 3 db short slot forward wave directional coupler having an input port 14, an output port 16, and intermediate, or auxiliary, ports 18 and 20. Intermediate port 18 is aligned with input port 14 on one side of the hybrid 10, while intermediate port 20 is similarly aligned with output port 16 on the other side of the hybrid 10. The hybrid 10 is of less overall width in its central region than in its end portions adjacent the ports 14-16 and 18-20, respectively. Electrically

conductive plates

24 and 26 project into the hybrid 10 from opposite ends thereof in a plane parallel to the length of the hybrid to define a first Waveguiding passage 17 between ports 14 and 18, a second waveguiding passage 19 between ports 20 and 16, and a coupling aperture, slot, or iris, 28 between the regions 17 and 19.

A rectangular input waveguide 30 is disposed adjacent the input port 14 of the hybrid 10 to carry input microwave energy to the hybrid, and a rectangular output waveguide 32 is similarly disposed adjacent the output port 16 of the hybrid 10 to propagate output microwave energy from the hybrid. Inner walls 34 and 36 of the input and output waveguides 30 and 32, respectively, are contiguous to one another and lie in essentially the same plane as that which contains

plates

24 and 26.

Frequency sensitive attenuation is' introduced into the filter arrangement by means of the cavity resonator circuit 12. More specifically, the cavity resonator arrangement 12 comprises a block 15 of electrically conductive material which defines a pair of cylindrical cavities 38 and 40. Cavities 38 and 40 are coupled to the intermediate ports 18 and 20, respectively, of the hybrid 10 by means of respective rectangular waveguides 42 and 44. The waveguide 42 is aligned with and has the same cross-section as input waveguide 30, while the waveguide 44 is aligned with and is of the same crosssection as output waveguide 32. Inner walls 46 and 48 of waveguides 42 and 44, respectively, are disposed contiguous to one another in the plane of

plates

24 and 26. The cylindrical cavities 38 and 40 are disposed with their respective longitudinal axes parallel to a line defined by the intersection of the

plane containing plates

24 and 26 with the plane containing ports 18- and 20. Cylindrical cavity 38 is inductively coupled to waveguide 42 by means of coupling iris 50 in conductive block 15, and similarly, coupling iris 52 in block 15 inductively connects the cylindrical cavity 40 with the waveguide 44.

In order to provide the desired attenuation, cylindrical attenuator blocks 54 and 56 of lossy dielectric material are disposed in the cavities 38 and 40, respectively. The dimensions of the blocks 54 and 56 are essentially the same as those of the cavities 38 and 40 so that the cavities 38 and 40 are essentially completely filled with lossy dielectric material. An example of a material which may be used for the blocks 54 and 56 is a mixture of forsterite and silicon carbide, with the percentage of silicon carbide varying from essentially 3% to essentially 10%. Examples of other materials which could be used are silicon carbide and alumina, silicon carbide and talc, or other dielectric and lossy material combinations.

The cavities 38 and 40 are each of a radius R and are designed to resonate in the TM cylindrical cavity mode, with the radius R being determined by the desired resonant frequency. More specifically, for a given resonant frequency i (in kmc.) the cavity radius R (in inches) is given by the equation R=4.52l/(f where e is the relative dielectric constant of the lossy dielectric material filling the resonant cavity.

In the operation of the waveguide filter of the present invention, microwave energy of frequencies within the relatively broad frequency passband of the hybrid 10 will propagate through the hybrid in the following manner. Energy entering the input port 14 from waveguide 30 divides into two portions, with essentially half of the input energy traveling straight through region 17 of the hybrid to intermediate port 18, and the other half propagating through coupling iris 28 and region 19 to the intermediate port 20. The longer propagation path between ports 14 and 20 introduces a phase shift such that microwave energy arriving at intermediate port 20 lags the energy arriving at port 18 by 90.

Microwaves of frequencies other than the cavity resonant frequency f which pass through ports 18 and 20 travel along the respective waveguides 42 and 44, and are reflected by the walls of conductive block 15 back toward the intermediate ports 18 and 20, with the reflected energy arriving at port 20 lagging the reflected energy arriving at port 18 by 90. This reflected energy enters the hybrid through ports 18 and 21) and traverses the length of the hybrid it) in the opposite direction than before. Microwave energy from each of the auxiliary ports 18 and 20 divides essentially in half, with half propagating directly through the hybrid 10 to the aligned port at the opposite end of the hybrid and the other half crossing to the other side of hybrid via the iris 28. Since a 90 phase lag is experienced each time energy traverses the iris 28, energy arriving at input port 14 from auxiliary port 20 will be 180 out of phase with energy arriving there from auxiliary port 18. Hence, destructive interference will occur, and no energy will be directed out of port 14 into the input waveguide 30. On the other hand, energy from ports 20 and 18 will arrive at the output port 16 in phase, and the resultant sum of this energy will pass through the port 16 into the output waveguide 32.

Input energy in the vicinity of the resonant frequency i will travel through the hybrid 10 from input port 14 to the auxiliary ports 18 and 20 in the manner described above. However, after traversing the waveguides 42 and 44, this energy will not be reflected by the walls of the conductive block back toward the hybrid 10 but will be directed through the respective irises 50 and 52 into the resonant cavities 38 and 40 where it will be substantially absorbed by the respective lossy attenuators 54 and 56. Hence, a minimum amount of energy in the vicinity of f will be propagated back to the hybrid 10 and will reach the output port 16.

Thus, the filter of the present invention serves to essentially pass microwave energy of frequencies within a relatively broad band of frequencies while attenuating such energy in the vicinity of a preselected frequency, thereby introducing a stop-band about the selected frequency. The amount of loss introduced at the resonant frequency f can be readily controlled by varying the composition of the attenuators 54 and 56 or by changing the geometry of the coupling irises 50 and 52 between waveguides 42 and 44 and the resonant cavities 38 and 40, respectively. Moreover, as the composition of the attenuating material is changed the Q of the resonant circuit will be affected, i.e., as the percentage of lossy material relative to dielectric material is increased the Q will decrease. Thus, altering the attenuator composition may be used to adjust the width of the stop-band.

Although the present invention has been shown and described with reference to a particular embodiment, it is to be understood that changes or alterations obvious to one skilled in the art to which this invention pertains are, nevertheless, within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A waveguide filter for providing a stop-band of preselected width for electromagnetic energy in the vicinity of a preselected frequency comprising in combination: a waveguide hybrid device for propagating electromagnetic energy between an input port and an output port, means defining at least one cavity coupled to said hybrid device, said cavity being resonant at said preseletced frequency, and loss means disposed in said cavity for attenuating electromagnetic energy in the vicinity of said preselected frequency, said loss means comprising a mixture of dielectric and lossy materials in a preselected proportion to provide a preselected cavity Q and a corresponding preselected stop-band width.

2. The combination according to claim 1 wherein said cavity is of a cylindrical shape and is resonant in the TM mode.

3. The combination according to claim 1 wherein said loss means is a block having dimensions substantially the same as the dimensions of said cavity and comprising a mixture of essentially between and 97% forsterite and essentially between 3% and 10% silicon carbide.

4. A waveguide filter for providing a stop-band of preselected width for electromagnetic energy in the vicinity of a preselected frequency comprising in com bination: a waveguide hybrid device having an input port, an output port, a third port disposed opposite said input port and a fourth port disposed opposite said output port, for propagating electromagnetic energy between said input port and said output port, means defining a first cavity coupled to said third port and a second cavity coupled to said fourth port, said first and said second cavities being resonant at said preselected frequency, and loss means disposed in said first and said second cavities for attenuating electromagnetic energy in the vicinity of said preselected frequency, said loss means comprising a mixture of dielectric and lossy materials in a preselected proportion to provide a preselected cavity Q and a corresponding preselected stopband width.

5. A waveguide component for providing a stop-band of preselected width for microwave energy in the vicinity of a preselected frequency comprising: a coupling circuit having an input port, an output port, a first auxiliary port, and a second auxiliary port; said input port and said output port being symmetrically disposed at one end of said coupling circuit and said first and second aux iliary ports being symmetrically disposed at the opposite end of said coupling circuit; said coupling circuit further defining a first waveguiding passage of substantially rectangular cross-section between said input port and said first auxiliary port, a second waveguiding passage of substantially rectangular cross-section between said second auxiliary port said output port, and a coupling aperture between said first and second passages, whereby microwave energy of frequencies within a predetermined frequency range is propagated between said input port and said output port; a first waveguide of rectangular cross-section electromagnetically coupled to said first auxiliary port; a second waveguide of rectangular crosssection electromagnetically coupled to said second auxiliary port; a conductive member disposed at the ends of said first and second waveguides remote from said first and second auxiliary ports; said conductive member defining a first cylindrical cavity and a first coupling aperture electromagnetically coupling said first cylindrical cavity to said first waveguide; said conductive member further defining a second cylindrical cavity and a second coupling aperture electromagnetically coupling said second cylindrical cavity to said second waveguide; each said cylindrical cavity being resonant at said preselected frequency; and a cylindrical block of a mixture of dielectric and lossy materials in a preselected proportion to provide a preselected cavity Q and a corresponding preselected stop-band width substantially filling each said resonant cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,293,839 Linder Aug. 25, 1942 2,916,712 Artuso Dec. 8, 1959 2,951,216 Nelson et al. Aug. 30, 1960 3,034,076 Tomiyasu May 8, 1962 3,041,542 Bailey June 26, 1962

Claims

1. A WAVEGUIDE FILTER FOR PROVIDING A STOP-BAND OF PRESELECTED WIDTH FOR ELECTROMAGNETIC ENERGY IN THE VICINITY OF A PRESELECTED FREQUENCY COMPRISING IN COMBINATION: A WAVEGUIDE HYBRID DEVICE FOR PROPAGATING ELECTROMAGNETIC ENERGY BETWEEN AN INPUT PORT AND AN OUTPUT PORT, MEANS DEFINING AT LEAST ONE CAVITY COUPLED TO SAID HYBRID DEVICE, SAID CAVITY BEING RESONANT AT SAID PRESELECTED FREQUENCY, AND LOSS MEANS DISPOSED IN SAID CAVITY FOR ATTENUATING ELECTROMAGNETIC ENERGY IN THE VICINITY OF SAID PRESELECTED FREQUENCY, SAID LOSS MEANS COMPRISING A MIXTURE OF DIELECTRIC AND LOSSY MATERIALS IN A PRESELECTED PROPORTION TO PROVIDE A PRESELECTED CAVITY Q AND A CORRESPONDING PRESELECTED STOP-BAND WIDTH.