IL30691A - Cavity resonator - Google Patents

Description

CAVITY RESONATOR This invention relates to a cavity resonator, and mo'r·e specifically to a cavity resonator having a resonant frequency and a rejection frequency.

In communications systems it is often desirable to provide filtering for the receivers and transmitters so that inter-modulation 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 provide sufficient attenuation of frequencies near the resonant frequency of the cavity. Thes, 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.

The present invention provides a cavity resonator having a resonant frequency and a rejection frequency, including 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 determinin the resonant fre uenc , a first coupling element locat 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 determining 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.

The cavity resonator of the present invention is particularly well-suited for use in a multiplex system.

This cavity resonator can couple a receiver and a transmitter to the same antenna and has a rejection frequency in addition to a resonant frequency, and the rejection frequency is separately variable without changing the resonant frequency of the cavity.

In practicing the various preferred aspects 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 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 resonate cavity having adjacent coupling elements; 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 o£ 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.

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 10 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 extendi 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 hi h fre uenc ran e the cavit filter ma be used as a uarter by adjustment of the nuts 23 and 25 , the length of the line can be changed so that the f ilter can be made resonant at any desired frequency within a predetermined range . For use in a system operating in the frequency range from 150 to 162 megacycles , the overall size of the f ilter 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 f ittings being arranged so that loops 36 and 37 are connected between the center 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 . The cavity resonator of Fig . 2 has an outer conductor 42 and in 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 F i 2 is shown The curve of Fi 3 shows an insertion loss of 0.5 db at the resonant frequency FQ1 and a rejection of approximately 35 db at a second frequency f 02· A single cavity resonator with the rejection curve shown in Fig. 3 does not provide sufficient 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. Coupling 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 fQ and receiver 57 is receiving on frequency fQ2» it can be seen from the curves of Fig. 5 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 fQ2 has also been increased to 70 db. By this means the transmitter signal is prevented jfrom reaching the receiver at sufficient amplitude to cause receiver desensitization by overloading the R.F. circuits. Also transmitter noise signals are prevented from reaching the receiver at sufficient amplitude to compete with the desired signals on frequency fg2« 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 Referring again to Pig. 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 58 and an inner conductor 60. Also included are coupling loops 61 and 63. As shown in Fig. 6, lines connecting a center position on the inner conductor 60 to the points at which the coupling loops 61 and 63 pass through the outer conductor 58 form an angle -Θ- 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 - - is equal to or less than 45°. However, it has been found that useful results are obtained when the angle -Θ· 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 -G- between the 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 Pig. 1 is shown in Fig. 8. Curve 66 illustrates the frequency response of a cavity resonator tuned to a frequency fQ1 and having a rejection frequency of fQ2 . 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 fQ2 to f03 decreasing the angle «6-, 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. Θ except that the rejection frequency would be l&ss 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. , he system of Fig. 9 is similar to that of Fig. 4 except that ©canities 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. 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 fQ^, 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 ½ 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 thereby permitting the rejection frequency to be varied so that the rejection frequency can be tuned independently of the resonant frequency of the cavity. 2 here s illustrated means for clam in 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 ilO. 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 150 MHz and incorporating the features of the invention has the following dimensions: i.d. of outer conductor 10½ in. length of outer conductor 22 in. o.d. of inner conductor 3½ in. length of inner conductor 19 in. width of probes ½ in. length of probes 4 in .

When the above cavity had its probes positioned approximately 10° apart the rejection frequency was 200 KHz from the resonant frequency. When the probe separation was increased to 30° the frequency separation increased to 8 MHz

Claims (6)

1. A cavity resonator having a resonant frequency and a rejection frequency, including 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 determining 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.

2. The cavity resonator of claim 1, wherein said angle is no greater than 45°.

3. The cavity resonator of claim 1 or 2, wherein, said first and second coupling elements are loops extending within the cavity, whereby the rejection frequency is higher than the resonant frequency. U

4. The cavity resonator of claim .1 or 2, wherein said first and second coupling elements are probes extending within the cavity, whereby the rejection frequency is lower than the resonant frequency.

5. The cavity resonator of claim .1 or 2, 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.

6. The cavity resonator of claim 1 or 2, wherein each of said first and second coupling elements has a first end pivotally fixed to said outer conductor and a second end, said oute 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. AGENTS FOR APPLICANTS