US2964718A - Microwave circuits - Google Patents

Dec. 13, 1960 K. s. PACKARD 2,964,718

` MICROWAVE CIRCUITS Filed March 21, 1955 2 l FIG. l u

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ATTORNEYS Dec. 13, 1960 v K. s. PACKARD 2,964,718

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FREQUENCY m KMC INVENTOR KARLE s. PACKARD BY Gm, M, n W) @0mm MTW ATTORNEYS United States Patent O MICROWAVE CIRCUITS Karle S. Packard, Bellmore, N.Y., assiguor, by mesne assignments, to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Filed Mar. 21, 1955, Ser. No. 495,577

11 Claims. (Cl. 'S33-73) This invention relates to microwave frequency-selective circuits, and particularly to such circuits formed of and incorporated in symmetrical strip transmission lines of the substantially air-dielectric type.

. Application Serial No. 469,454, filed November 17, 1954, by Eugene G. Fubini and John N. Dyer for Microwave Circuits, describes |and claims such frequencyselective circuits employing half-Wave resonant elements or multiples thereof. The present invention employs resonant elements whire are a quarter-wavelength long or an odd multiple thereof.

-At microwave frequencies it is common to employ coaxial line or waveguide for the transmission of electromagnetic energy. In some cases, particularly at lower frequencies and for short distances, parallel wire transmission line is employed. It has also been suggested to employ an unsymmetrical transmission line consisting of a wire or conductive strip supported above a ground plane. An improved form of symmetrical strip transmission line having low losses is disclosed in application Serial No. 380,674, filed September 17, 1953, by Eugene G. Fubini for Strip Transmission Line.

In microwave circuits, as in the case of lower frequency circuits, frequency-selective circuits are often required. They find use as components of filters, in amplifiers and as the resonant portions of oscillators, etc. At microwave frequencies such frequency-selective circuits are usually short lengths of transmission line. Parallel wire, coaxial and waveguide transmission lines have been used in this' manner. Coaxial or waveguide line is inherently shielded, but parallel wire line is not. Thus, if energy from a parallel wire section is to be isolated from adjacent electrical circuits, aseparate shield is required. Unsymmetrical strip transmission line is not well suited to use in a frequency-selective network, since it radiates energy at discontinuities and thus introduces losses which would seriously impair its usefulness for this purpose.

The present invention is particularly directed to frequency-selective circuits or filters of the resonant type, employing tuned sections, as distinguished from thoseof the non-resonant type. Simple frequency-selective circuits of this type may consist of a single resonant section, or several resonant sections mutually coupled, through which the electromagnetic energy passes. Y

vIf filters of this type are constructed of coaxial line, each resonant section is enclosed within its outer conductor. Thus it is necessary to provide means for coupling Ainto and out of the resonant sections. For all but the very simplest filters, the construction becomes complicated and expensive.-

l A frequency-selective waveguide circuit is usually made in theform of three or more orthogonal pairs of conductive walls proportioned in modal relation to the wavelength'of the frequency to be selected. For best results the internal metallic surfaces should be smooth and highly conductiye. SuchA structures are expensive toI fabricate, particularly in all but the simplest filters, .andthe mechanical tolerances are tight.

ice

The difficulties in designing and constructing microwave filters are particularly acute in the case of band pass filters requiring several resonant circuits to obtain a suiiiciently broad band with adequate selectivity outside the pass band. Such filters should ordinarily have as low an insertion loss as possible within the pass band, and this requires the use of high Q circuits. If heretofore conventional methods are employed, such filters are usually bulky and expensive, and in many cases it is very difhcult to obtain the desired operating characteristics.

It is valso important in many applications to be able Y I to vary the tuning of a frequency-selective circuit over a desired frequency range. While in many cases this is possible with conventional constructions, the circuits are usually expensive and often unduly bulky. Also, impedance transforming is often required to meet the requirements of a given application.

In accordance with the present invention, frequencyselective circuits or filters of the resonant type are provided which employ so-called quarter-wavelength tuned sections. It is a primary object of the invention to provide such frequency selective networks or filters for use in the microwave region which can readily be fabricated at low cost, are efiiciently shielded and simple to excite, and have high electrical efficiency. It is a further object to provide a structure in which a plurality of filter sections are conveniently and simply coupled together to form band pass filters of highly desirable characteristics. In addition, it is an object of this invention to provide a filter structure in which tuning means and impedance transforming configurations may be conveniently and simply incorporated.

To this end, in accordance with the present invention, one or more filter sections, and advantageously input and output transmission line sections also, are formed of conductive strip supported midway between a pair of conductive surfaces, the pair of conductive surfaces being maintained at a common potential to form ground planes and the volume between the strip sections and the conductive surfaces being largely gaseous. Thus, the filter sections, and advantageously the input and output transmission line sections also, are made of portions of symmetrical strip transmission line of the substantially airdielectric type. Advantageously the construction described in application Serial No. 380,674, supra, may be employed.

The filter strip sections are resonant within the operating range of the filter, each strip section being elongated and having a length which is approximately equal to an odd integral multiple of quarter-wavelengths at the resonant frequency thereof, it being understood that the term multiple includes one.

It is particularly contemplated to employ `quarterwavelength sections which are short-circuited at one end thereof. However, it is possible to employ open-circuited quarter-wavelength sections, particularly in conjunction with short-circuited sections to obtain a desired overall characteristic. Short-circuited sections may be constructed by very simple means for :short-circuiting one end of the central conductor to the ground planes. The short-circuiting means may be stationary or movable, the latter providing a means for varying the active length of the filter strip section, thereby permitting tuning to a desired resonant frequency. In addition, tuning of a filter strip section may lalso be effected by varying its characteristic impedance, thereby varying its effective electric length. This may bereadily accomplished (in accordance with theory more fully described hereinafter) by` varying the effective spacing between conductive ground planes. I

It will be understood Athat afilter or frequency-selective circuit passes or accepts certain frequencies and rejects other frequencies, and in many cases the rejection band is as important as the acceptance band. Often, so-called trap circuits are employed to reject certain frequency bands and, when used, such rejection bands are considered in this specification to be .in the operating range of the filter or frequency-selective circuit. While various arrangements can be employed for supporting the conductive strips between the conductive ground planes, they can be conveniently supported on a thin insulating or dielectric sheet, and advantageously Iare formed in pairs on opposite sides of the insulating sheet, 4as described in the aforementioned application.

yCoupling between filter sections and from filter sections to input and output sections can be either conductive or non-conductive, as meets the requirements of a particular application. If conductive, the coupling can be altered by merely changing the position at which the coupling strip section contacts the elongated edge of the filter strip section. This change of position varies the impedance presented to the coupling strip and is conveniently used to effect changes in the coupling impedance. If non-conductive, the coupling may additionally be altered by changing the spacing between coupling and'filter strip sections.

When the conductive strips are carried by a thin dielectric sheet, it will be understood that techniques such as printing, etching, stamping, etc., can readily be used. Relatively complicated filter structures can be made by merely disposing the conductive strips on a plane surface in the proper configuration, and subsequently inserting the surface between a pair of spaced parallel conductive surfaces. The filter can readily be connected to a col axial line by means of very simple transitions.

In designing a filter for a particilar application, the designer may be guided by conventional filter theory applicable to tuned circuits in selecting the number of tuned circuits required, the degree of coupling between tuned,

circuits and to input and output circuits, loading, impedance matching, etc.

The invention will be more fully understood by reference to the drawings, which illustrate a number of specific embodiments thereof, taken in conjunction with the following description in which the advantageous features thereof will in part be pointed out and in part be obvious.

lIn the drawings:

Fig. 1 is a side view partially in cross-section, illus1 trating a simple form of filter arranged for connection in a coaxial line;

Fig. 2 is a plan view of the filter of Fig. 1 with most of the top plane removed;

Fig. 4 is a perspective View of a plurality of coupled short-circuited filter sections, using conductive coupling;

Fig. 4a is a modification of the filter arrangement of Fig. 4 in which the conductive coupling strips are positioned at an angle to the coupled filter sections;

Fig. 5 is a perspective view of coupled shortcircuited filter sections using non-conductive coupling and mounted in a different manner;

Fig. 6 is a perspective view illustrating a movable means for tuning a filter section;

Fig. 7 is a partial side View of Fig. 6 showing the open ends of the tuning members;

Fig. 8 is a plan view of a device for moving the tuning members illustrated in Fig. 6;

Fig. 9 is a side view of the device illustrated in Fig. 8;

Fig. 10 is a perspective view of another means for tuning a filter section;

Fig. l1 is a side view of a device for moving the tuning means illustrated in Fig. 10;

Fig. 12 shows details of the central strip configuration of 'a specific embodiment of the invention;

Since paired strips function like a single conducting strip, they are included in the term strip section as used herein.

Such a transmission line is commonly termed an airdielectric line, since dry air usually fills the space. However, for some applications it may be desirable to evacuate the line, or to use some other gaseous medium, and it will be understood that the term air-dielectrioy as used herein includes such cases.

Due tofthelstanding voltage and vcurrent waves along, the length ofV the4 quarter-Wavelength: element, variations of theSpositionrofitheinput and-output conductive strip. sections on the elongated sides of filter strip sections 18',- 18 ywill vary-the 'impedance L presented @to zthe f input .and outputsections.

Because of thevariaticn of impedance alonggthe ilterstrip section, the input.` and 'output sectionsmay be *po-l sitioned relative to one another along the oppositeV elon.y gated sides ofsse'ctionrl, 18 to effecten impedance transformation therebetween,A the junctions between theinput` andoutputsections andthe `filter section acting ineffect asia transformer which' may'either step up orstepdolwn the Vimpedance depending Aupon. the contact positions along` the sides of the filter section. g

As has been mentioned `lr'ereinbefore, symmetrical strip. line has very@ low losses, and therefore theQ of` a-:filten strip section formedl therefrom'is-veryvhigha Filtensec tion 18, 18, therefore, functions substantially-asa single; tuned resonant circuit with a .very high Q. Consequently, itis `possible to obtainavery narrowipass lbandforthe arrangement shown in Fig.` l. Thecouplingmay be Varied in the mannerindicated andlto a degree in accordance with 'well-known lilterijtheoryl to,meet -various requirements `of band `width and attenuation. y

Filter strip section18, 18! is preferably disposed p er. pendiculartofthe immediately adjacentk portions of the input and output conductive strip sections in orderto avoidy spuriousicouplings, although this is` not always` necessary and may be departed from if desiredfor va particular application.

Itis pertinentto note at this `point various relationships of strip line parameters that are helpful inthe design` of afilter formed from such units. Y l

The various constructional details illustrated in Figs. 1, 2 and 3 discussed above are omitted in Fig. 4 and the subsequent figures, but will be understood to apply throughout. For simplicity, only the arrangement of conductive strips and their functioning will be described.

As will be'understood by those skilled in -the art, the physical length of a resonant strip section such as 41 will not be exactly an odd multiple of a quarter-wavelength at the resonant frequency due to fringe effects and coupling to adjacent elements. Those skilled in the art will also realize that the lengths of the resonant strips may be made different so that they resonate at different frequencies, thereby altering the frequency characteristic to suit a particular purpose.

Fig. 5 illustrates a different construction and coupling technique within the scope of the invention.

As has been mentioned before, lateral coupling exists between adjacent filter strip sections if they are not sufficiently spaced from one another. This factor may be used to advantage if desired by spacing the adjacent filter sections closely together to obtain appropriate coupling without the necessity of using intervening coupling strip sections. It is also possible to obtain non-conductive coupling between adjacent filter strip sections by using an intervening coupling strip whose ends are closely spaced from the sides of the pair of resonant strip sections but insulated therefrom, thereby capacitively coupling the two. It is evident that any combination of the non-conductive and conductive coupling techniques described may be used for a particular application.

Particular filter configurations have been described to indicate the variety of filter circuits possible. It is evident that all of the described embodiments may be combined in whole or `in part to effect a desired filtering action.

Rather than move`the short circuit, it is sometimes more convenientk to change the effective electrical length of a resonant filter strip section by changing its characteristic impedance. If the characteristic impedance of the open end portion of the resonant strip is changed, the resonantfrequencyalso changes. As has been described before, Ithecharacteristic impedance of a strip section varies with theratio of its width to the spacing between ground planes. Changing the spacing between ground planes will therefore vary the characteristic impedance, other parameters remaining the same.V If this is effected at the open` end of the resonant strip, a change in resonant frequency results. l i f Fig.A lO'illustr'ates 'an embodiment of this feature of the invention whihsymrnetrically changes the spacing of 10 the ground planes near the open end of the shorted filter strip section shown.

Specific examples of filters embodying features already described, but designed for particular applications, together with a curve applicable to one embodiment will now be given. I

In order to tune the filter after construction, slight changes in the length of the -iilter sections may be required. This can be effected by trimming the filter strip sections to reduce their lengths slightly, or adding a little solder to increase the length.

The performance of the filter of Fig. 12 is shown in Fig. 13, where the response in terms of db is plotted against frequency in kilomegacy'cles.

Fig. 14 illustrates a filter arrangement which is used as a portion of the radio frequency circuit for a microwave receiver. The configuration of the center conductor of the circuit is shown, the constructional details being similar to those of Fig. l2. Ground planes (not shown) overlap both surfaces.

In all the preceding specific embodiments the resonant strip sections are straight, and the input and output lines are either straight or have right angle bends therein. This ordinarily facilitates construction. However, it is not essential. For example, the resonant strip elements can be curved, the input and output lines can be curved, or other than right angle bends can be employed.

For some applications, curvature of adjacent coupled resonant strip elements can be employed to obtain a desired variation in bandwidth with frequency in tunable circuits. As has been explained in connection with Fig. 4, if adjacent resonant strip elements are sufficiently close, some coupling exists between the strip elements themselves in addition to the coupling provided by the coupling strips therebetween. In Fig. 5 the entire coupling between the two resonant strip sections is obtained in this manner. Such coupling is a combination of capacitive and inductive coupling. Near the grounded ends of the resonant strip elements (e.g. 54, 55 in Fig. 5) the coupling is predominantly inductive. Near the open ends the coupling is predominantly capacitive. If such a filter is arranged to be tunable, as described in connection with Figs. 6 and 10, a certain relationship between bandwidth and frequency is obtained as the filter is tuned over a range of frequencies.

In some cases it is desired to obtain a different type of bandwidth variation with frequency from that obtainable with the coupling arrangements shown in Figs. 4 and 5, for example.

' Fig. 15b illustrates a differentcurved arrangement in which the grounded ends of the resonant' strips are far apart and the `open endsare close together. In this arrangement the coupling between the resonant strips is predominantly capacitive. If, then, the lter is tuned, as described hereinbefore, the percent bandwidth increases with frequency. While this arrangement is in general not as useful as that shown in Fig. 15a, it may be helpful in specific cases.

In both Figs. 15a and 15b additional coupling between the resonant strips may be employed to modify the bandwidth variation with frequency.

As pointed out hereinbefore, it is preferred to employ matching or paired strips on opposite sides of a thin sheet of insulating material for the central lconductor of the input and output section and lter sections, since very little loss and resultant high Qs are obtained. When desired, strips on only one side may be employed, and since the insulating sheet may be very thin the dielectric between conductive strip and ground planes is substantially air-dielectric.

There are now available foamed polystyrene (Polyfoam) and foamed vinyl chloride. In these materials, a very large percentage of the volume is air, and only a very small percentage is solid dielectric. Hence they may be termed essentially air-dielectric materials, and are usable to support the conductive strip sections between the ground planes. While localizing the support members is preferred, as illustrated in Fig. 5, for some purposes this is not necessary. Similar blown up, expanded or sponge-like materials whose volume is largely air may also be employed in some applications.

The frequency-selective circuits of the invention find particular usefulness at frequencies above 1000 megacycles, and become smaller and more compact as the frequency increases. vIn theregion from 5000 to 10,000 megacycles and upwards they are particularly valuable s ince ,comparable coaxial line and waveguide arrangements become very expensive even if practical to design. They may, however, be used at somewhat lower frequencies if theD physical size is not a drawback. It will be understood that the term microwave as used herein includes these frequency ranges.

The specific em-bodiments described herein are in the class known as four-terminal networks, having separate input and output pairs of terminals. When more than one resonant section is interposed between input and output sections, it will be understood that a given resonant section serves as a coupling between another resonant section and the input or output line. For some purposes a'two-terminal frequency-selective circuit suffices, and it will be understood that the arrangements of the present invention are usable in this manner.

A number of different embodiments of the invention have been described to illustrate the versatility thereof. Various combinations of the features described in different embodiments are, of course, possible, and may be selected to meet Vthe requirement of the particular application. Also, filters designed in accordance with the present invention can be combined with those described in application Serial No. 469,454, above mentioned.

I claim:

1. A microwave frequency-selective circuit which comprises a pair of conductive surfaces in spaced parallel planes maintained at a common potential to form ground planes, a circuit element resonant within the operating range of said frequency-selective circuit comprising an elongated conductive strip section supported midway between said conductive surfaces in parallel spaced rela- 14 tionship therewith and a shorting member adapted to .electrically short-circuit the elongated section to the conductive surfaces at a point along the length thereof, tuning means movable between at least a portion of said elongated section and said conductive surfaces to change ther characteristic impedance of said portion and thereby change the effective electrical length of said elongated section over a desired range, each effective electrical length within said range being substantially an odd integral multiple of quarter-wavelengths at theresonant frequency of the elongated section, and microwave transmission-line means coupled to said elongated section including a conductive strip section supported midway be` tween said conductive surfaces in parallel relationship therewith.

2. A microwave frequency-selective circuit which cornprises a pair of conductive surfaces in spaced parallel planes maintained at a common potential to form ground planes, a circuit element resonant within the operating range of said frequency-selective circuit comprising an elongated conductive strip section supported midway between said conductive surfaces in spaced parallel relationship therewith and a pair of movable conductive shorting members adapted to electrically short-circuit the elongated section to the conductive surfaces over a range of positions `from the open end thereof comprising the tuning range of said shorting members, said shorting members lbeing movable simultaneously between said elongated section and said conductive surfaces, respectively, and insulated therefrom, each shorting member comprising a pair of planar conductive sections in spaced relationship parallel to each other and parallel to the conductive surfaces and a conductive link connecting said planar sections together, said planar sections extending from said connecting link away from the open end of said elongated section, said elongated section having a length suicient to extend beneath substantially the entire length of the adjacent planar sections of said movable shorting members when the conductive links of said shorting members are positioned at their extreme tuning position from lthe open end of said elongated section, the distance from any position of the connecting links within the tuning range of the shorting members to the' open end of the elongated section being approximately an odd integral multiple of quarter-wavelengths at the resonant frequency of said elongated section, the planar sections of each shorting member extending from the connecting link to the open end thereof approximately an odd integral multiple of quarter-wavelengths at a selected resonant frequency within the frequency range of the elongated section, the active length of said elongated section and said conductive surfaces having substantially air-dielectric therebetween, and microwave transmission line means coupled to said elongated section including a conductive strip section supported midway between said conductive surfaces in parallel relationship therewith.

4. A microwave frequency-selective circuit which comprises a pair of conductive surfaces in spaced parallel planes maintained at a common potential to form ground planes, a circuit element resonant within the operating range of said frequency-selective circuit comprising an elongated conductive strip section supported midway between said conductive surfaces in spaced parallel relationship therewith and a shorting member adapted to electrically short-circuit the elongated section to the conductive surfaces at a point remote from the open end thereof, a tuning member having a pair of insert sections positioned between each conductive surface and said elongated section respectively and insulated from the elongated section, said insert sections being movable simultaneously between said elongated section and the conductive surfaces from the open end of the elongated section, the effective electrical length of said elongated section being substantially an odd integral multiple of quarter-wavelengths at the resonant frequency thereof, the portion of said elongated section between said insert sections and said shorting member being separated from said conductive sur-faces by substantially air dielectric, and a microwave transmission line means coupled to said elongated section including a conductive strip section supported midway between said conductive surfaces in parallel relationship therewith.

9. A microwave frequency-selective circuit which comprises a pair of conductive surfaces in spaced parallel planes maintained at a common potential to form ground planes, a circuit element resonant within the operating range of said frequency-selective circuit comprising an elongated conductive strip section supported midway between said conductive surfaces in parallel relationship therewith and a shorting member adapted to electrically short-circuit the elongated section to the conductive surfaces at a point remote from the open end thereof, a substantially U-shaped tuning member insulated from said elongated section comprising two substantially identical conductive planar insert sections positioned between each ground plane and said elongated section respectively and arranged parallel thereto, each planar section bearing against the respective conductive surface and in electrical contact therewith, and a link connecting said insert sections, said U-shaped tuning member being positioned with its opening directed towards the open end of the elongated section and being movable along the length of the elongated section so that the planar insert sections may be introduced `between the elongated section and the conductive surfaces, the active length of the elongated section being such that at any position of the tuning member within its tuning range the effective electrical length of the elongated section is equal vto an odd multiple of quarter-wavelengths at the resonant frequency thereof, the space between said elongated section and said conductive surfaces and Ushaped member being substantially air-dielectric, and microwave transmission line means coupled to said elongated section including a conductive strip section supported midway between said conductive surfaces in parallel relationship therewith.

10. A microwave frequency-selective circuit which comprises a pair of conductive surfaces in spaced parallel planes maintained at a common potential to form ground planes, a pair of coupled circuit elements resonant within the operating range of said frequencyselec tive circuit, each of said resonant elements comprising an elongated conductive strip section supported midway between said conductive surfaces in parallel relationship therewith and a shorting member adapted to electrically short-circuit the elongated section to the conductive surfaces at a point remote from the open end thereof, said elongated strip sections being arranged so that the separation therebetween at the grounded ends thereof is substantially different from the separation at the open ends thereof whereby one type of reactive coupling therebetween predominates, movable tuning means Ifor simultaneously changing the elective electrical length of said elongated sections over a desired range, each effective electrical length within said range being equal to an odd integral multiple of quarter-wavelengths at the resonant frequency of the elongated section, and microwave transmission line means coupled to said coupled elongated sections.`

11. A microwave lfrequency-selective circuit in accordance with claim l0 in which the separation between the grounded ends of the elongated strip sections is substantially less than the separation between the open ends thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,124 Conklin Apr. 16, 1940 2,513,761 Tyson July 4, 1950 2,530,089 Smith Nov. 14, 1950 2,572,672 Smith Oct. 23, 1951 2,594,037 Landon et al Apr. 22, 1953 2,774,044 Silvey et al Dec. l1, 1956 2,800,634 Grieg et al July 23, 1957 2,820,206 Arditi et al Jan. 14, 1958 FOREIGN PATENTS 601,514 v Great Britain May 7, 1948 513,257 Belgium Feb. 2, 1953 OTHER REFERENCES Electronics, September 1954, pages 148-150. Radio-Electronic Engineering, September 1951, pages 16, 3l.

Barrett: Electronics, .lune 1952, pages 114-118.