US2890424A - Variable attenuators - Google Patents

Description

2 Sheets-Sheet l INVENTCRS MAUR/(- Ago/rl fak/ARQ A55-0l? ATTORNEY l M. ARDITI ETAL VARIABLE ATTENUATORS June 9, 1959 Flled May 2, 1955 June 9, 1959 Filed May 2, 1955 l l f r l I r l I l i 5 n I f r i l,

SGIVAL CURRENT SOURCE hJun M. ARDITI ErAL VARIABLE ATTENUAToRs 2 sheets-sheet 2 `INVENTORS MAUR/CE ARD/77 60W/D70 J: IVASSO "6MM, @am

ATTORNEY United Statesv n Patent C) Telegraph (olporzltion,l Nutley, N..L, a. corporation of Maryland.

Application May 2, 13955, Seril No. SUSJSS 8 Claims. (.Cl. S33- 31).

This invention relates to variable attenuator arr-ange,- ments inl microstrip structures for propagatingl microwaves, j including variablev attenuator: arrangements' adapted to be used-as modulators of microwaves propagated along such structures;

The use of; attenuators in transmission lines. for microwaves` is: known; In; adjusting the. amount of. attenuation introduced it has been customary' to vary the area= of the attenuation material inserted` in the: iield of the wave propagated along such transmission. means.. In` one type, suchl attenuation materiaL is. in the form. of: av card or pad In, waveguides arr opening is: provided; and the attenuation card is inserted' in the waveguide tol they desiredi amount to;v produce the desired. attenuation; In coaxial4 lines", the. card is: inserted between. thei central'v conf ductonand the outer conductor, the-area of the card'. inserted therebetween. being Varied; tovary' the attenuation; I`n= the type of line. in: which a; round- Wire is.V arranged above aY ground? plane conductor with an intervening air dielectric the attenuation card is-` either interposed between' the two conductors or brought adjacent the Wire conductor on thetop thereof and the'. position of the attenuation material brought intoy play is varied to vary the resutling attenuation.

In the type of transmission line now known as Microstrip?" ('(U.S. Patent' No.P 2,654,842 of F. Engelmann, issued October 6, 1953), there isdisclosedl a narrow strip conductor supported on aV sheet of" dielectric material above and in close proximity to a planar conductor` servingas a ground plane. Obviously, because ofthe solid dielectric it is impossible to insert an attenuation card between the strip conductor and the ground plane. Itv has, however, beenI proposedy with respect to the microstrip type of' line to place an attenuation card over the strip conductor so that it overlies andv extends beyond' thevside edges thereof' U.S`. application Ser. No; 4133805 of H. F. Engelmann, tiledv March 3, 1954-, entitled Attenuators). In this lastinstance also it has beenproposed to vary thei attenuation of the line by varying-theposition of the card` over the line whiley maintaining a constant pressurethereon; Position adjustment ofattenuation` is feasible,butV diiiicult. One of the reasons for this difficulty is` the peculiar nature of' the field distribution ina microstrip line, as will bey explained later, and the relatively minute dimensions of such lines where, for' example, the thickness ofthe strip conductor is of` the orderv of thousandths of an inch. Thus, slight changes inthe area of the attenuation material at critical points along said lines produce gross changes in the attenuation-l The position mode' of adjustingthe attenuationA of the microstrip` line does not lend itself' to continual variation of the attenuation such as is required when using4 a variable attenuatorv as a: modulator.

An object of the present invention is the provision of an improved variable attenuator arrangement for microstrip lines.r By the term var-iatale?" it. i'syalso intended to include adjustable attenuator arrangements, that is,

2` attenuators whose attenuation is varied; until. the desired setting is reached atwhich point no further adjustment is made.

Another object of the present invention is the: pro,.- visionof avariable attenuator arrangementfor microst'rip lines` whichis adaptedf for continual variation, particularly one whichserves asa modulator ofthe Waves'propagated along said lines.

In accordance with a major aspectoh the presentin;- vention it has been found that the: attenuation: produced by' anl attenuator materia-lassocatedf withf ai microstnip line may be substantially varied byvarying. thepressure on said material, particularly at certain criticali points: The attenuation mater-ial may he,I for example-Qinl the form of a pad or card. This variation of the attenuation by varying the pressure. om an: attenuationl member occurs only in 1:r1icrostr-iplinesA andy' is not found waveguides, coaxial lines, and round-wire-over-ground plane trans:- mission lines.

According to one. theory asf to: the` effect of. pressure in the present invention,.variation of attenuation. is be@ lieved to beV produced' by a minute distortion of the.I at tenuation member at critical p.oints especiallythe edge of. the stripcond'uctor, with-respectftothe microstrip line.

In( accordance with a further* feature. ol the: present invention, the pressure ont anV attenuaton card'. associated with a microstrip line'. is. varied'. in. accordance with; a modulating signal. to amplitude modulate. the microwave propagated along said. line.

Other and further objects; of the presentLinventi'on will become apparent, and the. fcmegoing will bebetter under'- stood with reference to the: following: description o embodiments thereof, reference: beingl had. to the drawings; in which:

Figs; 1A and 1B are theoretical diagrams used in. ex;- plaining the theory of the present invention;

Fig; 2l is a plan; view of an adjustable:att'enuator microstrip arrangement;

Fig. 3:t is a cross-sectional Viewl the line 3-3 of. Fig. 2;.

Fig. 4 is: a cross+sectional1view off a; modied? embodiment of the present'invention': showing a variables attenu atorimicrostrip arrangement;

Fig. 5 is a schematic crosssectional view o ai further modification ofv they present invention showing; an@ ampli'- tude. modulator' systemy using a continually variable 'at tenuation microstrip arrangement;l and Fig. 6 is a curve illustrating the eiect: of thevvariation of pressure on the attenuation producedf inf one example off the presentV invention.

In connection. with the. subsequent discussion oitl the gures, it is important to note that thesey figures are grossly exaggerated, this being necessary in( vorder to bringA out the details' ofthe present' invention; better idea of the relative magnitudes involved will be' obtained from the dimensionsof the various-portions of the microstrip pointed out hereinafter.

According to the present invention, it' has been found that' by varying the pressure applied to* critical points of an: attenuation padI which overlies thel strip conductor of a rn-icrostripv line the attenuation produced isi likewise varied over a substantial range. rFhis is illustrated in Fig. 1A in which the microstripline consists of aA very thin stripconductor-10? supportedon' a thin dielectric 11 over a ground plane' or planar conductor 12'; Thev` dis# tance between the strip conductor andy ground plane is electrically of the order 0150.12 or less of" the midfiequency of the wave propagated along the line. Microstrip lines and the theory underlying them have been described invmany articles as, for example, in the Prof ceedings ofthe I2R`.E., December 1952, pages 1644`1650`l off Fig.. Z taken along and reference is made to such articles forrfurther dev An attenuator card 13 tails with respect to such lines.- is placed over the strip conductor and extends beyond the side edges thereof.Y Variable pressure is applied at points' P1 and'Pz, which lie outside theV side edges of strip conductor 10, as indicated by the arrows, and depending upon. changes in the magnitude of this pressure the attenuation of the line is changed. p

This effect does not occur with, for example, a roundwire-above-ground plane line as is illustrated, for example, in Fig. 1B where a round wire 14 is shown above a ground plane 15 separated by, for example, an air dielectric 16. If an attenuation card 17 is placed overlying the wire 14 and pressure is applied at points P3 arid P4 in an attempted analogy to the arrangement of Fig. lA, no appreciable variation in attenuation is produced when the card is distorted by the same minute quantity as in the microstrip case when varying the pressure at points P3 and P4.

The reason for the foregoing difference is believed to be due to the nature of the eld distribution in microstrip lines as opposed to other lines. It is believed that the pressure produces a minute distortion in the configuration of the attenuation pad. In doing this a portion of the attenuation pad is moved very slightly from a region in which the field is relatively weak into a region where the eld is much stronger. For the same order of magnitude of distortion of the attenuator pad the round-wireabove-ground system does not produce an appreciable effect. Examining first the field around the wire-aboveground plane as shown in Fig. 1B, it will be seen that there is no strong concentration of the field around the wire 14. Thus, any small bending of the attenuation pad 17 produces no substantial change in attenuation. In the microstrip arrangement of Fig. lA the field is highly concentrated near the edge of the strip. The main field appears directly below the strip conductor 10 in the area marked a. A highly concentrated fringing eld is found at the side of the strip conductor 10, and a very weak field y even goes to the top of the conductor 10 towards the ends thereof. The latter part of the eld above the strip conductor 10 is extremely weak, and no substantial attenuation is produced by the attenuation pad in said area y. It will be seen that by applying pressure at points P1 and P2 the minute distortion of the attenuator pad will move portions thereof towards the side edges of the conductor 10 into the field where it would produce a greater attenuation than if no pressure were applied to the attenuator pad, and it lay ilat on top of the strip conductor 10. It is thus seen how the foregoing theory explains the effect of the variation of attenuation with pressure on selected points of the attenuation pad on the microstrip line and that this effect does not occur to an appreciable extent where the eld is not concentrated as would be the case in a round wire-conductor-above-ground plane.

`Turning now to Figs. 2 and 3 in which a practical embodiment of the invention is disclosed, there is shown a microstrip line 18 having a strip conductor 19 supported on a solid dielectric 20 above a ground plane 21 in the form of a planar conductor. An attenuation card 22 is symmetrically disposed over the strip conductor 19 so that the edges of the card extend beyond the side edges of the strip conductor. The attenuator card is planar and has a slight amount of flexibility. It may consist, for example, of a Bakelite, or a similar plastic base, coated on the under side with a thin lm of graphite or other attenuating material and have a thickness of the order of .02". For the purpose of applying pressure to the attenuator card we provide a pressure plate 23 consisting preferably of a preferably lossy dielectric material. We prefer lossy material to absorb any residual field from extending to the device applying pressure to the pressure plate. To control the pressure of the pressure plate on the attenuator card 22 we i provide -a set of thumb screws 24 which are positioned --Y-beyond the edges of the attenuator pad and engage female threads preformed in the dielectric, or for simplicity of construction,.may pass through the dielectric and ground plane and engage nuts below the ground plate bearing thereagainst. For even distribution of the pressure there is preferably a series of pairs of such thumb screws longitudinally spaced along the microstrip line. Depending upon the pressure exerted by the thumb screws the attenuation of the Wave propagated along said line is varied. In order to minimize reection of the Wave propagated along the line, it is preferred to taper the forward and rear edge of the attenuator card, and since the greater part of the attenuation is produced just outside the side edges of the strip conductor 19 we taper the attenuation material at each of the sides resulting in a W-formation of the forward end of the attenuator with the base points of the W each lying to the side of the edges of the strip conductor 19.

In the cross-sectional view of Fig. 3 of the arrangement which has just been described, an extremely exaggerated picture is shown of the distortion of the attenuation pad due to pressure. The pressure pad 2 is also minutely distorted.- In actual practice, because of the minute dimensions involved, such distortion cannot be seen with the naked eye. The strip conductor in a typical case has a thickness of the order of thousandths of an inch and a width, for example, of 7/32 of an inch. The solid dielectric may have a thickness of the order of l/f; of an inch while the planar ground conductor is at least three or four times as wide as the strip conductor. Such a line is adapted for the propagation of microwaves having a frequency of the order of kilomegacycles.

While the arrangement of Figs. 2 and 3 provides for an adjustment of attenuation, the arrangements shown in Figs. .4 and 5 are adapted for continual variation of attenuation, and include a modification in the actual arrangement of the attenuator pad. Referring now to Fig. 4, thearrangement there disclosed may be used as a pressure gauge in which, in response to variations in pressure, there is produced variations in the attenuation of a microwave, which variations in the wave can be easily detected by any conventional detector means and applied to a suitable indicator or meter in a manner obvious to those versed in the art. In the arrangement of Fig. 4 the attenuator card 22 is supported above the strip conductor 19 on two parallel lengths of dielectric material 25, said lengths being spaced apart a substantial distance greater than the width of strip conductor 19, the dielectric lengths 25 extending parallel to the strip. The edges of the dielectric card rest on said lengths 25 and the card itself is at a slight distance above the strip conductor 19. Pressure is exerted on the area of the card intermediate said side edges by a pressure plate 26 of relatively lossy plastic which is driven downwardly by shaft 27 connected to some source for applying pressure (not shown). A pinion 28 on the shaft 27 may be used to drive a pinion gear 29 to which is mechanically attached the indicating needle 30 of an indicator 31 whose dial 32 may be calibrated either in attenuation decibels or pounds of pressure.

In Fig. 6 a curve is shown of pressure in pounds versus attenuation in decibels of an arrangement, such as shown in Fig. 4. In this arrangement the following dimensions are of interest: the strip conductor 19 was 0.2" wide, the width of the attenuation card 22 between the dielectric lengths 25 was about l", the pressure plate 26 was approximately 3%" wide, the length of the attenuation card was between 2" and 2.35" and said attenuation card had a resistance of ohms per square inch. It will be noted from the curve 33 that there was a variation in attenuation of approximately ll decibels and that there was an extremely linear portion 34 between 7 and 13 decibels. If linearity of modulation is required, it can be seen how satisfactory the present arrangement would be 8 for this purpose. By changing the value of the resistance card used, that is, increasing the resistance per square inch, a smaller attenuation may be achieved. Greater flexibility in the resistance card would also enable the use of much lower pressures.

The present invention may be satisfactorily employed for the direct modulation of microwaves as depicted in Fig. 5. In the arrangement of Fig. the attenuator card 22 is supported on dielectric lengths 25 of lossy plastic, as described with respect to Fig. 4, at a distance above the strip conductor 19. instead of employing a pressure plate as described in Fig. 4, there is arranged on top of the attenuator card 22 a plate 35 of magnetic material, such as, for example, soft iron. A source of magnetism such as, for example, an electromagnet 36, is used to control the pressure on the plate in accordance with a modulating current from a source 37, which current passes through the electromagnet and varies the magnetomotive force. This, in turn, varies the pull on the magnetic plate 3S which in turn varies the pressure on the attenuation card 22 and amplitude modulates the wave propagated along microstrip line 18 in accordance with the modulating signal.

To prevent dust and dirt from settling on the microstrip line, it may be encased in a metallic housing 38, but in order to prevent waves from bouncing around the Walls of the housing 38 there is provided an absorptive attenuator cloth 39 below the top of the housing, said cloth 39 consisting of a textile or plastic material having a lossy material such as graphite dispersed in rubber therein.

While there has been described specific embodiments of the present invention, it is obvious that numerous modifications may be made therein. For example, a modulation may be made using a piezoelectric crystal in place of the electromagnet and electromagnetic material of Fig. 5 which would be responsive to signals of higher frequency than would the arrangement described in Fig. 5. Accordingly, while we have described our invention above with reference to specific embodiments, it is to be understood that the invention is to be interpreted according to the state of the prior art and the appended claims.

We claim:

1. An attenuator arrangement for high frequency waves comprising a microstrip line including a first planar conductor, a planar layer of solid dielectric material on said first conductor and a second planar conductor on said dielectric layer, said first conductor being wider than said second conductor, the main field produced by a wave propagated along said arrangement lying between said iirst and second conductors underneath said second conductor, and a fringe field extending from the side edges of said second conductor towards said first conductor, an attenuator element overlying said second conductor with portions thereof extending beyond the side edges of said second conductor and in a plane substantially parallel to the planes of said first and second conductors, means for applying variable pressure to said element to ex said portions towards and into the plane in which said second conductor lies and adjacent the side edges of said conductor, and means for controlling said pressure exerting means to vary the attenuation produced.

2. An attenuator arrangement for high frequency waves comprising a microstrip line including a first planar conductor, a planar layer of solid dielectric material on said first conductor and a second planar conductor on said dielectric layer, said first conductor being wider than said second conductor, the main field produced by a wave propagated along said arrangement lying between said first and second conductors underneath said second conductor, and a fringe field extending from the side edges of said second conductor towards said first conductor, an attenuator element overlying said second conductor With portions thereof extending beyond the side edges of said second conductor, a flexible pressure plate mounted on said element and extending beyond it for exerting pressure on said portions to distort the element and urge said portions into said fringe field, and means for controlling the pressure exerted to control the attenuation produced.

3. An attenuator arrangement for high frequency waves comprising a microstrip line including a first planar conductor, a thin planar layer of solid dielectric material on said first conductor and a second thin planar conductor on said dielectric layer, said first conductor being Wider than said second conductor, an attenuator card overlying said second conductor with portions thereof extending beyond the side edges of said second conductor, means for exerting variable pressure on said portions to distort said card and urge said portions towards the plane in which said second conductor lies adjacent the side edges of said second conductor, and means for controlling said pressure to vary the attenuation produced.

4. An attenuator arrangement according to claim 3, in which said attenuation card abuts and is supported on said second conductor.

5. An attenuator arrangement according to claim 3, in which said pressure exerting means comprises a plate of lossy material lying on said attenuation card.

6. An attenuator arrangement according to claim 3, further including means for supporting said card at points beyond the sides of said second conductor, and wherein said pressure exerting means includes means for exerting pressure on said card in the area between said points.

7. An attenuator arrangement according to claim 6, in which said pressure exerting means is a member of magnetic material abutting said card and said means for controlling said pressure includes means for establishing a magnetic field affecting said magnetic member, and means for varying said magnetic field.

8. An amplitude modulator arrangement for high frequency waves comprising a microstrip line including a first planar conductor, a planar layer of solid dielectric material on said first conductor, and a second planar conductor on said dielectric layer, said first conductor being wider than said second conductor, the main field produced by a wave propagated along said arrangement lying between said first and second conductors underneath said second conductor, and a fringe field extending from the side edges of said second conductor towards said first conductor, an attenuator member overlying said second conductor with portions thereof extending beyond the side edges of said second conductor, means for supporting said attenuation member at spaced points beyond the side edges of said second conductor, a member of magnetic material lying against said attenuation member between said points, means for producing a magnetic field exercising a substantial force on said magnetic member to flex it and said attenuation member towards and into the plane of said second conductor, and signal controlled means for varying the strength of said magnetic field.

References Cited in the file of this patent UNITED STATES PATENTS 2,725,535 Grieg et al. Nov. 29, 1955 OTHER REFERENCES Microstrip Wiring Applied to Microwave Receivers, Federal Telecommunication Laboratories, copyright 1953, page 5. (Copy S33-84M.)

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