US 3600711 A
Description (OCR text may contain errors)
United States Patent  Inventor Richard Z. Gerlack Cupertino, Calif.  Appl. No. 849,715  Filed Aug. 13, I969  Patented Aug. 17, 1971  Assignee Varian Associates Palo Alto, Calif.
 COAXIAL FILTER HAVING HARMONIC REFLECTIVE AND ABSORPTIVE MEANS 6 Claims, 9 Drawing Figs.
52 us. 0 333173 c, 333/76, 333/81 A  Int. Cl ..H03h 13/00, H0 1p 1/22  Field of Search 333/73, 73 W, 76, 81
so References Cited UNITED STATES PATENTS 2,588,226 3/1952 Fox 333/73 FREQUENCY 2,785,381 3/1957 Brown 333/73 2,853,678 9/1958 Tomiyasu 333/73 X 2,853,682 9/1958 Epstein 333/73 X 2,869,085 1/1959 Pritchard et al. 333/73 X 2,961,619 11/1960 Breese etal. 333/73 X 3,078,423 2/1963 Lewis 333/73 X 3,353,123 11/1967 Met 333/73 3,437,959 4/1969 Webb 333/73 Primary ExaminerI-1erman Karl Saalbach Assistant Examiner-William H. Punter AttorneysStanley Z. Cole and Gerald L. Moore ABSTRACT: A coaxial filter is disclosed. The filter includes a coaxial line having resonant reflector means carried from the outer conductor for reflecting harmonic wave energy toward the source while permitting fundamental wave energy to pass through the coaxial line to a load. A harmonic resonant means is disposed between the harmonic reflectors and the source for picking up and absorbing the harmonic energy in a load disposed externally of the outer conductor.
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INVENTOR RICHARD Z. GERLACK %vubk If Am ATTORNEY COAXIAL FILTER HAVING HARMONIC REFLECTIVE AND ABSORPTIVE MEANS DESCRIPTION OF THE-PRIOR ART Heretofore, coaxial line harmonic reflective filters have been proposed. In one such filter, resonant second harmonic reflective antennas were carried from the inner conductor of a coaxial line for reflecting second harmonic wave energy toward the source while permitting fundamental wave energy to pass through the coaxial filter to the load. An antenna resonant at the reflected second harmonic wave energy was disposed on the center conductor and coupled to a load contained within the center conductor for absorbing the reflected second harmonic wave energy. The problem with this prior art filter was that it had limited power handling capability due to the fact that the second harmonic wave energy was absorbed within the center conductor and cooling of the center conductor became extremely difficult.
SUMMARY OF THE PRESENT INVENTION I The principal object of the present invention is the provision of an improved coaxial filter having harmonic reflective and absorptive means.
One feature of the present invention is the provision in a coaxial filter having harmonic reflective means of resonant pickup means disposed between the harmonic reflectors and the source, such pickup means being disposed on the outer conductor for coupling reflected harmonic wave energy out of the coaxial line to a load disposed externally thereof, whereby the power handling capability of the coaxial filter is enhanced.
Another feature of the present invention is the same as the preceding feature wherein the resonant pickup means includes antenna means disposed inside the outer conductor, such antenna means having a first pickup antenna tuned for resonance at the second harmonic and a second pickup antenna tuned for resonance at the third harmonic.
Another feature of the present invention is the same as the immediately preceding feature wherein the first and second pickup antennas are axially and circumferentially displaced from each other with respect to the longitudinal axis of the coaxial line.
Another feature of the present invention is the same as the first feature wherein the resonant pickup means includes a resonant pickup antenna affixed at one end to the outer conductor and curving to a free end in the circumferential direction in between the inner and outer conductors of the coaxial line, whereby the axial space of the coaxial line occupied by the antenna is reduced to a minimum.
Other features and advantages of the present invention will become apparent upon a perusal of the following specifications taken in connection with the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram, partly in block diagram form and partly broken away, depicting a coaxial filter incorporating features of the present invention,
FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows,
FIG. 3 is a sectional view of the structure of FIG. 1 taken along line 3-3 in the direction of the arrows, and depicting an alternative pickup antenna embodiment of the present inventron,
FIG. 4 is a schematic line diagram, similar to that of FIG. 1, and depicting an alternative embodiment of the present invention,
FIG. 5 is a plot of insertion loss in db. versus frequency depicting the performance characteristics for the filter of FIG. 1,
FIG. 6 is a plot of insertion loss in db. versus frequency depicting the performance characteristics of the filter of FIG. 4,
FIG. 7 is a detail view of a portion of the structure of FIG. 1 depicting an alternative reflective antenna embodiment,
FIG. 8 is a detail view of a portion of the structure of FIG. 1
delineated by line 8-8 and depicting an alternative antenna embodiment, and
FIG. 9 is a detail view of a portion of the structure of FIG. 1 delineated by line 9-9 and depicting an alternative reflective antenna embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2 there is shown a coaxial filter l incorporating features of the present invention. The coaxial filter l is connected at one end to a source of high frequency power 2, such as a UHF klystron, delivering power at a fundamental output frequency f as of 500 megahertz, such fundamental energy being accompanied by the harmonics of the fundamental which it is desired to absorb or to at least prevent from reaching a load 3 connected to the output terminal of the filter. In a typical example, the fundamental energy is pulsed having a peak power of 2 megawatts with 50 kilowatts average power and containing 100 watts second harmonic and 3050 watts third harmonic power.
The harmonic filter 1 includes a section of coaxial line 4 having an outer conductor 5, as of 3% inches OD, and an inner conductor 6, as of 1.3 to 1.5 inches OD. The coaxial line 4 can have any convenient length as of 24 inches. A plurality of reflective antennas 7 tuned for the second harmonic, are carried from the outer conductor 5 and are axially spaced along the length of the coaxial line 4. Successive ones of the reflective antennas 7 alternate from one side of the line to a diametrically opposed side of the line 4 in the axial direction. The axial spacing between successive second harmonic resonant reflective antennas 7 is approximately one-fourth of a wavelength at the second harmonic such that the second harmonic wave energy reflected from the antennas adds in the reflected direction toward the high frequency source 2. In a preferred embodiment, the reflective antennas 7 comprise quarter-wavelength sections of conductor affixed at one end to the outer conductor 5 and extending the axial direction between the inner and outer conductor and terminating at a free end.
A second set of wave reflective antennas 8, similar to the second harmonic antennas 7, are tuned to be resonant at the third harmonic. Successive third harmonic antennas 8 alternate from one side to the opposite side of the outer conductor 5 in a circumferential position relative to the first set of second harmonic antennas 7. As in the case of the second harmonic reflective antennas 7, the third harmonic reflective antennas 8 have an axial spacing between successive antennas of a quarter wavelength at the third harmonic frequency such that the wave energy reflected from successive reflective antennas 8 adds in the reflected direction taken toward the high frequency source 2.
A second harmonic resonant pickup antenna 9 is disposed in the longitudinal plane of the array of second harmonic reflective antennas 7 and is spaced from the reflective antennas 7 between the antennas 7 and source 2 to pick up the reflected energy at the second harmonic frequency. A conductor 11 is affixed to the pickup antenna 9 for tapping the wave energy therefrom to a load 12, such as a conventional 50-ohm coaxial load, for absorbing the reflected second harmonic wave energy picked up by antenna 9.
A similar third harmonic pickup antenna 13 is axially spaced from the reflective antennas 8 between the reflective antennas 8 and the high frequency source 2 to pick up the reflected third harmonic wave energy. A conductive tap l4 taps off the antenna 13 for coupling third harmonic wave energy picked up by antenna 13 to a coaxial load 15, for absorbing the reflected third harmonic wave energy. Loads at 12 and 14 may be conventional coaxial terminations or may comprise waterloads for handling higher power. In a typical example of the filter of FIG. 1, the second harmonic load 12 absorbs watts of second harmonic power and the third harmonic load 15 absorbs 30 to 50 watts of third harmonic power when the fundamental power is 50 kilowatts average at 500 megahertz.
and curving in the circumferential direction between the inner and outer conductors 6 and 5, respectively, to a terminating free end. The advantage of the pickup antenna 16 is that it requires substantially less length of the coaxial line 4 than that required for the axially directed pickup antennas 9 and l3.
Referring now to FIG. 4, there is shown an alternative embodiment of a coaxial filter l8 incorporating features of the present invention. The structure of FIG. 4 is substantially identical to that of FIG. 1 with the exception that the harmonic reflective structure 19 comprises a series of conductive discs 21 of various axial thicknesses and axially spacings carried upon a section of the inner conductor 6 of reduced thickness to provide a reflective harmonic structure that reflects the second through the sixth harmonic. Such a coaxial reflective filter structure is described in a text titled Microwave Filters, Impedance Matching Networks and Coupling Structures by Mattahei, G. L., L. Young and E. M. T. Jones, published by McGraw-Hill Company, Inc. of New York, N.Y., in l964.
i As in the embodiment of FIG. 1, pickup antennas 9 and 13 for the second and third harmonic, respectively, are spaced by the appropriate distance from the harmonic reflective structure 19 to pick up and absorb the second and third harmonic wave energy reflected from the reflector 19. If desired, additional harmonic pickup antennas for absorbing higher harmonic wave energy may be spaced at various circumferential locations about the center conductor 6 for picking up and absorbing the additional harmonic wave energy.
Referring now to FIG. 5, there is shown a plot of insertion loss in db. versus frequency for the filter 1 of FIG. 1. From the plot of FIG. 5 it is seen that the fundamental wave energy indicated at 22 passes substantially without attenuation to the load, whereas the second and third harmonics at 2}], and 3]}, are heavily attenuated.
Referring now to FIG. 6, there is showna plot of insertion loss in db. versus frequency for the filter embodiment of FIG. 4. In this embodiment, the fundamental wave energy 22 passes through the filter without attenuation, whereas the second through the sixth harmonics are substantially totally reflected and the second and third harmonics are substantially totally absorbed by the external loads 12 and 15, respectively.
Referring now to FIG. 7, there is' shown an alternative reflective antenna structure to that depicted in FIG. 1. In this embodiment, the reflective antenna is approximately half a wavelength long at the particular harmonic and is connected to the outer conductor 5 via a centrally disposed stub 23.
Referring now to FIG. 8, there is shown an alternative wave reflective antenna defined by a section of coaxial line 24 dimensioned to be approximately a half wavelength long at the respective harmonic frequency and being connected by a short length of coaxial line 25 to the outer conductor 5.
Referring now to FIG. 9, there is shown an alternative wave reflective structure 26 which comprises a quarter wavelength folded coaxial line section 27 disposed surrounding the outer conductor 5 and coupled to the wave energy within the coaxial line via an annular coupling hole 28.
The resonators of FIGS. 7 through 9 may also be employed for replacing the pickup resonators, in which case each of the respective resonators would be coupled to a suitable lossy material or load exposed externally of the outer conductor 5. In the case of the embodiment of FIG. 9, the lossy material may be disposed inside the quarter wavelength section of folded coaxial line 27.
Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
What I claim is: 1. In a coaxial filter, means forming a coaxial line having an inner conductor and outer conductor for transmission of fundamental mode energy from a source to a load, means forming a wave reflective structure disposed in said coaxial line for reflecting harmonics of said fundamental mode energy toward the source, means coupled to said coaxial line between the source and said harmonic reflective means for picking up and absorbing reflected harmonic wave energy, TI'IE IMPROVE- MENT WI-IEREIN, said harmonic pickup and absorbing means include, resonant pickup means disposed on said outer conductor of said coaxial line, such pickup means being resonant at least at a respective one of said harmonics, load means disposed externally of said outer conductor, and means for coupling harmonic wave energy from said resonant pickup means to said load means for absorbing reflected harmonic energy.
2. The apparatus of claim 1 wherein said resonant pickup means includes antenna means disposed inside said outer conductor and having a first pickup antenna tuned for resonance at the second harmonic, and a second pickup antenna tuned for resonance at the third harmonic.
3. The apparatus of claim 2 wherein said first and second pickup antenna are axially and circumferentially displaced from each other with respect to the longitudinal axis of said' coaxial line.
4. The apparatus of claim 3 wherein said harmonic reflective means includes a plurality of resonant reflective antennas extending into said coaxial line from said outer conductor, said antennas being axially spaced along said coaxial line and including a plurality of axially aligned reflective antennas resonant at the second harmonic and a plurality of aligned reflective antennas resonant at the third harmonic, said second harmonic reflective antennas being circumferentially displaced relative to said third harmonic reflective antennas.
5. The apparatus of claim 1 wherein said resonant pickup means includes an antenna member affixed at one end to said outer conductor and curving to a free end in the circumferential direction between said inner and outer conductors.
6. The apparatus of claim 1 wherein said pickup means includes a conductive antenna member affixed at one end to said outer conductor of said coaxial line and extending to a free end in the axial direction of said coaxial line in the space between said inner and outer conductors of said coaxial line.