US2540734A - High-frequency filter - Google Patents

Description

Fd@ 6. Jf W. H. HUGGHNS ETAL QMQVM HIGH-FREQUENCY FILTER Filed Jan. 7, 1946 F|G.| FIGB Fia@ INSERTION LOSS cib Patented Feb. 6, 1951 William H. Huggins and Paul I. Richards, Cambridge, Mass., assignors to the United States of America as represented by the Secretary of War Application January 7, 1946, Serial No. 639,651

` ployed at lower frequencies.

. It is an obiect of the present invention, therefore, to provide a novel coaxial L -section bandpass filter. It is a further object to design this filter in such a manner that the center frequency of the pass band may be varied over a desired range of frequencies. It is another object to design said filter so as to have a narrow bandwidth, asharp cutoif,`and high off-band attenuation.

' Other objects, features and advantages of this invention will suggest themselves to those skilled in the art and Will become apparent from the following description of the invention taken in connection with the accompanying drawing in which:

j Fig. 1 is a cross-sectiona1 diagram of a coaxial lter embodying the principlesof this invention:

Fig. 2 is a schematic diagram of the lter of Fig. 1;

Fig. 3 is a diagram of the equivalent circuitl formed by the coaxial lines included in the filter of Figs. 1 and 2; and

Fig. 4 is a curve showing a typical frequency response for a filter as contemplated by the invention.- A Reference is now made more particularly to the filter shown in Fig. 1 which includes an input section of transmission line composed of an outer conductor Il] and a tapered inner conductor l2, and anoutput section of transmission line composed of an outer conductor I4 and a similarly tapered inner conductor 'I'6. AThese sections of line communicate with a lateral section which. ingludesboaxial conducting cylindrical sleeves IS'andZIJ, conducting plungers'22 and 24 and central conductor 28. Plunger 22 has a section of small diameter which fits between cylin ders I8 and 2D and a section of larger diameter which extends beyond cylinder I8 and is threaded on its inner surface. These threads engage similar threads on a collar 26 which is part of the cylindrical sleeve 20. Plunger24 fits between 2 cylindrical sleeve 20 and central conductor 28, making contact with both by means of sliding contacts 30. vThe body of plunger 24 is threadedy on its outer surface, these threads engaging ay threaded collar 3| mounted on the inside 'off sleeve 23. Conductor I2 is connected to central conductor 28, while conductor I6 is connected to cylindrical sleeve 20. Conductors IIJ and I4 are joined to the outermostcylindrical sleeve I8.

Fig. 2 shows schematically theI coaxial lines] formed by the device of Fig. 1. The input sec tion 32 of Fig. 2 corresponds to the transmission line formed by conductors I0 and I2 of Fig. -1 While the output section 34 of Fig. 2 corresponds to the line formed by conductors I4 and I6 of Fig. 1. series section 3 5 of Fig. 2 corresponds to the line formed by conductors'ZU and 28 of Fig. 1, and the shunt section 38 of Fig. 2 corre` sponds to the line formed by conductors I8 and 2l) `of Fig. 1. As indicated in Fig. 2, line 33 should' be one-half wave length long and line 38 should be one-fourth wave length at the Acenter frequency of the pass band. These lengths are adjusted vby plungers 24 and 22. respectively.

The principle of operation of the filter may be clearly seen from the conventional Lsection' filter of Fig. 3. Line section 315 of Fig. 2 is a shortcircuited line connected in series with the input and output sections 32 and 34. It may therefore be represented by the series tuned circuit ofl Fig, 3 composed of inductance 42 and capacitor 44, since its operation over the frequency band is analogous to that of a series resonant circuit. At frequencies below the center frequency of the pass band, this line section is between one-fourth wave length and onefhalf wave length in elec-1 trical length, and hence presents a capacitivr-i` reactance. VAtthe center frequency it is exactly one-half wave length and acts as a short circuit, While at higherfrequencies it is betweenV one-half and three-fourths wave length and presents an inductive reactance. Similarly, line section 38 of Fig. 2 may be represented by the parallel tuned circuit of Fig. 3 composed of in-fV ductance and capacitor 48 and connected in' shunt with the inputand output lines. At fre-v quencies `below the center frequency it presents; an inductive short-circuited line of lessl than one-fourth Wave length, while at the center frequency it presents a very high impedance and at higher frequencies a capacitive impedance.

It can be seen that at the center frequency the series circuit will offer little opposition to the passage of R.E.j signals and the parallel circuit will bypass only a 'small amount. so that a maximum signal is passed. At other frequencies the series circuit will offer greater opposition to signals and the parallel circuit will bypass a larger percentage, so that the resultant output will be decreased. To obtain a narrow bandwidth the sections of line making up the lter are so constituted that the characteristic impedance of line section 36 (Fig. 2) is high while that of section 38l is low. To allow the filter to be usedA in a standard transmission line the total input characteristic impedance must be low, hence the f center conductors of the input and output sections have been tapered, thus avoiding the production of undesirable reflections duetoa sudden change of impedance at thezpoints.ofconnection..

The mid-frequency of the pass band may be varied by simultaneously varying the lengths of the component line sections. Referring to Fig. 1, this would be accomplished by rotating plungers 22 and 24 simultaneously. If desired, these plungers may be connectedby a mechanism which` willl enable them to move longitudinally with respectv to. each other while rotating in unison. 'Iheplungers are threaded in a. ratio of 22.1 so that the proper ratio is maintained between the lengths of the line sections being adjusted. Tapered conductors l2, and I 5 may be terminatedr by standard coaxial transmissionline.connectors, so that the assembly may bereadily inserted. ina standard4 transmission. line.. filter as described'has: a. very sharp cutoff. and high. off-band attenuation,l as may beseen by reference to' the typical curve of Fig. 4. There are no undesirable pass. bands at freqi-ienciesy lower than 2% times the mid-frequency of the-y pass band.

While there has been. described what is at present considered to be the preferred embodiment of this invention, it will be. obvious to those skilled in the art that. various changes and modificationsV may be made therein without departing from the scope of the invention.

lThe invention. claimed is 1. A coaxial high. frequency band-passA filter including an. input section of. coaxial transmission line having a tapered inner conductor; an output section ofv coaxial transmission line havingV a tapered inner conductor; a rst cylindrical conductor connecting with thev outer conductors of said input andl output lines; a second cylindrical conductor connecting with. one of said. tapered inner conductors, said second. conductor being coaxial with saidrst cylindrical conductor and fitting wthinsaidv firstA cylindrical` conductor for a. portion of. its length', two threaded collars respectively disposed .on the inner and outer sur-v. facesof said second conductor; a central conductor within and coaxial with said first and second conducting cylinders and connecting with.

the other of saidtapered inner conductors;v am

first conducting plunger. having a cylindrical portion. fitting between said rst and second cylindrical conductors and a threaded portion engaging the outer of said. threaded collars; and a second conducting plunger. fitting. between said secondconducting. cylinder and said centralconductor and making electrical contact withv both,` said`VA second .plunger being.A threaded onl its outer The band-pass surface so as to engage the inner of said threaded collars, both of said plungers being rotatable so as to vary their positions relative to said first and second cylindrical conductors and said central conductor.

2. A coaxial high frequency band-pass filter comprising an input section and an output section of coaxial transmission line, said sections having tapered inner conductors', a one-fourth wave length shorted transmission line connected inlshunt with said input and output sections, and a. one-half wave length shorted transmission line connected in series with said input and output sections, the characteristic impedance of said onehalf wave length line being larger than that of said one-fourthwave length line, said two shorted transmission lines being adjustable in length so as to vary the mid-frequency of the pass band of saidzlter.

3. A narrow high frequency band pass filter of the coaxial line. type comprising a main. coaxial transmission line, a first coaxial transmssionline, sectionforming effectively a series. tuned' circuitr connected in series inV the inner conductor' of said main line, said inner conductor of saidmai-n line being tapered to a larger diameter at its pointsof connection to said rst line section than at its ends remote from saidpoints of connection, anda second` coaxial transmissionl line sectionforming e'ectively a. parallel tuned circuitconnected.v in shunt to said main. line, the. charac,-l teristic impedance of said first line section being larger than that of sai'dseeond line section..

4.5 A- narrow high frequency band pass filter of the coaxial line type comprising a main coaxial transmission. li-ne, a half. wavelength coaxial: transmission line sectionV connected in series in the inner conductor of said.V main line,4 a first plunger shorting said half wavelength section,. said inner conductor of said main line being, tapered toa larger diameter at its points of con-- nection to said half wavelength section than.. ati its. ends remote from said points of connection, a. quarter wavelength coaxial. transmission.. line,

section connected in shunt across said main linefr REFERENCES CITED The following references are of recordA iria the file of this patent;

UNITED STATES. PATENTS" Number Name Dater 2,214,041 Brown Sept.. 10, 1940 224065372 Hansen i;rAug. 27;,1946'" FOREIGN PATENTS Numlier Country Date 860,224 France .v.. Jan. 9.. 1941.

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