US3537041A

US3537041A - Resonant cavity having adjacent coupling elements to provide a rejection frequency

Info

Publication number: US3537041A
Application number: US667904A
Authority: US
Inventors: Richard S Kommrusch
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1967-09-15
Filing date: 1967-09-15
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27
Also published as: DE1791105B2; IL30691A; DE1791105A1; IL30691A0; GB1195653A

Description

7, 1970 R. s. KOMMRUSCH 3'5 RESONANT CAVITY HAVING ADJACENT COUPLING ELEMENTS To PROVIDE A REJECTIQN FREQUENCY' Filed Sept. 15, 1967 Z' SheetSY-Sfi'e et 1 PRIOR ART PRIOR ART TRANSMITTER RECEIVER TANsQ I REC.

FIG. 4 FIG. 9

lnvenfdr Richard S. Kom'mrusch MW. m rm ATTYS.

Oct. 27, 1970 R. SIKOMMRUSCH 4 RESONANT CAVITY HAVING ADJACENT COUPLING ELEMENTS TO PROVIDE A REJECTION FREQUENCY I Filed. Sept. 15, 1967 2 Sheets-Sibtifi I I I l I I I I I FIG. 8' FIG. I0

Inventor Richard S. Kornmrusc'h ATI'YS.

United States Patent O US. Cl. 333-73 8 Claims ABSTRACT OF THE DISCLOSURE The coupling elements of a resonant cavity are positioned adjacent to each other to provide a rejection frequency in addition to the resonant frequency of the cavity. The spacing of the coupling elements determines the location of the rejection frequency with respect to the resonant frequency of the cavity.

BACKGROUND OF THE INVENTION In communications systems it is often desirable to provide filtering for the receivers and transmitters so that intermodulation and splatter can be reduced, and also to permit the use of a single antenna for a transmitter and receiver in multiplex systems. Cavity resonators have been effectively used in such systems as filters since they are very high Q circuits which can be easily inserted in a line connecting a transmitter or receiver with the antenna.

However, the cavity resonators heretofore used did not provided sufficient attenuation of frequencies near the resonant frequency of the cavity. Thus, in order to effectively couple a transmitter and a receiver operating at different, but closely related frequencies, to the same antenna, a pair of cavity resonators have been required for the transmitter and another pair for the receiver. The use of two or more cavity resonators in series in this manner increases the insertion loss of the system due to the cavities. Thus, where one cavity would normally have an insertion loss of 0.5 db, a pair of cavities in series would have an insertion loss of 1 db.

SUMMARY It is, therefore, an object of this invention to provide an improved cavity resonator for use in a multiplex system.

Another object of this invention is to provide a cavity resonator for coupling a receiver and a transmitter to the same antenna and having a rejection frequency in addition to a resonant frequency.

Another object of this invention is to provide a cavity resonator in which the rejection frequency is separately variable without changing the resonant frequency of the cavity.

In practicing this invention a cavity resonator is provided having a pair of coupling elements within the cavity. The coupling elements may be probes or loops and are closely spaced so that their interaction develops a rejection frequency for the cavity different from the resonant frequency of the cavity. The cavity may be tuned in the normal manner and the rejection frequency may be varied by changing the spacing between the coupling elements. When the coupling elements used are probes the rejection frequency is lower than the resonant frequency of the cavity and when the coupling elements are loops the rejection frequency is higher than the resonant frequency of the cavity resonator.

The invention is illustrated in the drawings of which:

FIG. 1 is a cross-sectional view of a resonant cavity having adjacent coupling elements;

3,537,641 Patented Oct. 27, 1970 FIG. 2 is a plan view of a prior art cavity;

FIG. 3 is a curve showing the frequency response of the cavity of FIG. 2;

FIG. 4 is a block diagram showing the use of cavities similar to the cavity of FIG. 2;

FIG. 5 are curves showing the frequency response of the cavities of FIG. 4;

FIG. 6 is a plan view of the cavity of FIG. 1;

FIG. 7 is a side view of the cavity of FIG. 6 showing the use of a probe as a coupling element;

FIG. 8 is a curve showing the frequency response of the cavity of FIG. 6;

FIG. 9 is a block diagram showing the use of cavities similar to the cavity of FIG. 6;

FIG. 10 are curves showing the frequency response of the cavities of FIG. 9;

FIG. 11 is a view of the cavity of FIG. 1 showing the means for adjustably positioning the coupling element;

FIG. 12 is a cross-sectional view showing means for clamping a loop; and

FIG. 13 is a cross-sectional view showing means for clamping a probe.

DESCRIPTION OF THE INVENTION A cross-sectional view of the cavity of this invention is shown in FIG. 1. The cavity is illustrated as including an outer conductor in the form of a cylindrical can 10 having a bottom closure 11 and a top closure 13 with a central opening 14 therein. Secured in opening 14 is a tubular member 16 which extends concentrically into can a substantial portion of distance to bottom closure 11. Inside tubular member 16 is a plunger 17 which combines with tubular member 16 to provide a center conductor of adjustable length. Can 10 and center conductor 16 thereby form a resonant line which is shorted at the top by closure 13. Plunger 17 is supported by an actuating rod 19 which is adjustably mounted on bracket structure 20. Bracket 20 is supported on an annular top closure 22 which is secured to can 10 at its outer edge and having the inner diameter thereof positioned closely about tubular member 16. A pair of

nuts

23 and 25 threaded on rod 19 are positioned on either side of bracket 20 to permit rod 19 and accordingly plunger 17 to be adjustably positioned with respect to can 10. Bracket 20 includes upstanding portions 26 which are secured to top member 22. For making the portions 26 more rigid bracing members 28 may be provided. A bridged member 29 connects the two upstanding portions 26 and includes an opening 31 which receives rod 19.

As previously stated, plunger 17 is movable within the tubular member 16 and includes a portion 32 which extends beyond the tubular member. Tubular member 16 includes a reduced end 34 which engages plunger 17 in a sliding fit towards the plunger and makes electrical contact therewith. In the very high frequency range the cavity filter may be used as a quarter wave resonant line and by changing the position of plunger 17, by adjustment of the

nuts

23 and 25, the length of the line can be changed so that the filter can be made resonant at any desired frequency within a predetermined range. For use in a system operating in the frequency range from to 162 mega cycles, the overall size of the filter is slightly more than two feet.

The resonant cavity filter includes a pair of

coupling loops

36 and 37 which form the input and output connections to the filter. The loops are so formed that the characteristic impedance of the loop will be equal to that of the coaxial line to which it is connected. Coupling loop 36 is connected to a fitting 39 and coupling loop 37 is connected to a fitting 40.

Fittings

39 and 40 are adapted to receive coaxial cables, the fittings being arranged so that

loops

36 and 37 are connected between the center 3 connector and the shield of the coaxial cable.

Fittings

39 and 40 also connect the shields of the cable to can 10.

In FIG. 2 there is shown a prior art cavity resonator similar in construction to that shown in FIG. 1. Such a cavity is described in Pat. No. 2,637,782, issued May 5, 1953, to Henry Magnuski. The cavity resonator of FIG. 2 has an outer conductor 42 and an inner conductor 43 concentrically positioned with respect to outer conductor 42. A pair of

coupling elements

45 and 46 are positioned within the cavity of the cavity resonator. In the prior art structure of FIG. 2, the

coupling loops

45 and 46 are arranged opposite each other so that the angle between them is 180".

In FIG. 3 the frequency response curve of the cavity resonator of FIG. 2 is shown. The curve of FIG. 3 shows an insertion loss of 0.5 db at the resonant frequency f and a rejection of approximately 35 db at a second frequency I A single cavity resonator with the rejection curve shown in FIG. 3 does not provide suflicient rejection at frequencies close to the resonant frequency so that two or more cavity resonators are coupled in series as shown in FIG. 4. In FIG. 4

cavity resonators

48 and 49 couple transmitter 51 to antenna 52. Antenna 52 is coupled to receiver 57 by

cavity resonators

54 and 55. Conpling the cavity resonators in series as shown in FIG. 4 provides increased attenuation at frequencies close to the resonant frequency.

Assuming the transmitter 51 is transmitting on frequency f and receiver 57 is receiving on frequency fog, it can be seen from the curves of FIG. that the attenuation of

resonators

54 and 55 to the transmission at frequency f has increased to 70 db while the attenuation of

resonators

48 and 49 to the transmitter noise at the receiver frequency f has also been increased to 70 db. By this means the transmitter signal is prevented from reaching the receiver at sufiicient amplitude to cause receiver desensitization by overloading the RF. circuits. Also transmitter noise signals are prevented from reaching the receiver at suflicient amplitude to compete with the desired signals on frequency f However, the increase in the rejection achieved by the system of FIG. 4 is gained at the expense of an increase in the insertion loss, which now is 1 db, and also an increase in the number of cavity resonators used. This increases the cost and complexity of the system.

Referring again to FIG. 1, it can be seen that

coupling loops

36 and 37 are not positioned opposite each other but are adjacent. By changing the position of the coupling loops so that they are close together, a rejection frequency is developed in the cavity resonator without affecting the resonant frequency. In FIG. 6 the position of the coupling loops is illustrated. The cavity resonator is similar in construction to the cavity resonator shown in FIG. 2 and includes an outer conductor 56 and an inner conductor 60. Also included are coupling

loops

61 and 63. As shown in FIG. 6, lines connecting a center or axial position on the inner conductor 60 to the points at which the

coupling loops

61 and 63 pass through the outer conductor 56 form an angle 6 which defines the position of the coupling loops with respect to each other. In most of the systems in which this cavity resonator has been incorporated, it has been found that the angle 0 is equal to or less than 45. However, it has been found that useful results are obtained when the angle 0 is less than or equal to 90.

While the structure of FIG. 1 is shown as a coupling loop, it has been found that coupling probes are also useful with this cavity resonator. Such a coupling probe 64 is shown in FIG. 7. While only a single probe is shown to illustrate its use, the cavity resonator of FIG. 7 would include a second probe positioned in a manner similar to the

coupling loops

61 and 63 of FIG. 6. Again the angle 0 between the coupling probe is determined in the same manner as the angle 0 of FIG. 6 between the coupling loops and is definitive of the rejection frequency. As with the coupling loops the angles of 45 and 90 produce the same results when applied to the coupling probes.

The frequency response of the cavity resonator of FIG. 1 is shown in FIG. 8. Curve 66 illustrates the frequency response of a cavity resonator tuned to a frequency f and having a rejection frequency of fog. The insertion loss of the cavity is the same as with the prior art cavities 0.5 db. However at the rejection frequency f the attenuation is considerably increased. The rejection frequency can be changed from fog to f by decreasing the angle 0, that is, by moving the two loops closer together. This is shown in curve 67. Curve 66 represents the frequency response when using coupling loops. If coupling probes were substituted for the coupling loops, the curve would have a shape similar to that of FIG. 8 except that the rejection frequency would be less than the resonant frequency instead of higher than the resonant frequency.

In FIG. 9 the transmitter 69 is coupled to antenna 72 by cavity 70 and antenna 72 is coupled to receiver 75 by cavity 73. The system of FIG. 9 is similar to that of FIG. 4 except that cavities 70 and 73, having adjacent coupling elements, are used in place of the

prior art cavities

48, 49, 54 and 55 shown in FIG. 4.

FIG. 10 illustrates the result of using an adjacent coupling element in cavity resonators 70 and 73 of FIG. 9. Curve 77 illustrates the frequency response of cavity 70 which uses loops as coupling elements. It can be seen that, at the receiver frequency, the rejection of cavity 70 is very high, at least as high or higher than the rejection of the pair of

cavities

48 and 49 of FIG. 4. Curve 78 shows the frequency response of cavity resonator 73 which uses probes as coupling elements. In this instance the rejection frequency is lower than the cavity resonate frequency to provide a rejection frequency at frequency f the frequency of the transmitter 69 of FIG. 9. Only a single cavity is required in each of the lines connecting the transmitters and receivers to the antenna to achieve the same amount of rejection or a greater amount than was achieved by using two cavities in the prior art circuit of FIG. 4. The cavities having adjacent coupling elements introduce an insertion loss of 0.5 db which is approximately /2 that of the pair of the prior art cavity resonators.

In FIG. 11 a portion of the resonant cavity is shown consisting of an outer conductor 80 and an inner conductor 81.

Loops

83 and 84 are pivotally mounted on

fittings

86 and 87. The other end of the loops are clamped to outer conductor 80 by clamping

means

89 and 90 extending through slot 92 in outer conductor 80. By this means loop 83 can be moved to a new position 93 while loop 90 can be moved to a new position 95. This movement will have the effect of increasing the angle 0 thereby permitting the rejection frequency to be varied so that the rejection frequency can be tuned independently of the resonant frequency of the cavity.

In FIG. 12 there is illustrated means for clamping a loop 96 to the outer conductor wall 98. A screw 99 is inserted through the slot 101 in outer conductor 98. A nut 102 together with a pair of

washers

103 and 104 clamp loop 96 to the outer conductor to fix it into position.

A means for clamping a probe is shown in FIG. 13. The end of probe 106 is fastened to insulating material 107 by

screws

109 and 110. A stud 112 extends from insulating material 107 through slot 113 in outer conductor 115. The stud 112 is clamped in place by means of nut 116 and Washer 117.

An example of a cavity resonant at a frequency of mHz. and incorporating the features of the invention has the following dimensions:

Length of probes When the above cavity had its probes positioned approximately apart the rejection frequency was 200 kHz. from the resonant frequency. When the probe separation increased to 30 the frequency separation increased to 8 mHz.

What is claimed is:

1. A cavity resonator having a resonant frequency and a rejection frequency, including in combination, a cylindrical inner conductor having a center position thereon, an outer conductor surrounding said inner conductor and spaced apart therefrom, the spacing and dimensions of inner and outer conductors determining the resonant frequency, a first coupling probe located at a first point on said outer conductor and positioned within the cavity, said proble having a portion extending parallel to said inner and outer conductors for coupling a signal wave into the cavity, a second coupling proble located at a second point on said outer conductor and positioned within the cavity, said second probe having a portion extending parallel to said inner and outer conductors for coupling said signal wave from the cavity, said first and second points being so located on said outer conductor that lines joining said center position and said first and second points define an angle less than or equal to 90, the position of at least one of said first and second coupling probes being variable, whereby the magnitude of said angle can be changed to thereby change the rejection frequency.

2. The gravity resonator of claim 1 wherein said angle is no greater than 45 3. A cavity resonator having a resonant frequency and a rejection frequency, including in combination, a cylindrical inner conductor having a center position thereon, an outer conductor surrounding said inner conductor and spaced apart therefrom, the spacing and dimensions of said inner and outer conductors determining the resonant frequency, a first coupling element located at a first point on said outer conductor and positioned within the cavity for coupling a signal wave into the cavity, a second coupling element located at a second point on said outer conductor and positioned Within the cavity for coupling said signal wave from the cavity, said first and second points being so located on said outer conductor that lines joining said center position and said first and second points define an angle less than or equal to 90, the

magnitude of said angle determing the rejection frequency, and mounting means for at least one of said coupling elements for varying the position thereof with respect to the other one of said coupling elements whereby the magnitude of said angle can be changed to change the rejection frequency.

4. The cavity resonator of claim 5 wherein said angle is no greater than 5. The cavity resonator of claim 5 wherein, said first and second coupling elements are loops extending within the cavity, whereby the rejection frequency is higher than the resonant frequency.

6. The cavity resonator of claim 5 wherein, said first and second coupling elements are probes extending within the cavity, whereby the rejection frequency is lower than the resonant frequency.

7. The cavity resonator of claim 5 wherein, said outer conductor is in the form of a cylinder concentrically surrounding said inner conductor, the length of said inner conductor being adjustable whereby the resonant frequency of the cavity is changed by changing said length.

8. The cavity of claim 5 wherein, each of said first and second coupling elements has a first end pivotally fixed to said outer conductor and a second end, said outer conductor having a slot formed therein, separate adjustable fastening means mechanically connected to each of said second ends, said fastening means further being positioned in said slot and movable therein whereby the magnitude of said angle is adjustable to vary the rejection frequency.

References Cited UNITED STATES PATENTS 2,418,961 4/1947 Wehner 343830 2,637,782 5/ 1953 Magnuski.

FOREIGN PATENTS 32,749 1/1966 Germany. 337,886 6/ 1959 Switzerland.

HERMAN KARL SAALBACH, Primary Examiner W. H. PUNTER, Assistant Examiner US. Cl. X.R. 33382, 83