US3157846A - Card attenuator for microwave frequencies - Google Patents

Description

Nov. 17, 1964 B. o. WEINSCHEL 3,157,846

CARD ATTENUATOR FOR MICROWAVE FREQUENCIES Filed Aug. 23, 1962 2 Sheets-Sheet 1 FIG. 7.

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ZNVENTOR Bruno O. Weinschel ATTORNEY Nov. 17, 1964 B. o. WEINSCHEL 3,157,845

CARD ATTENUATOR FOR MICROWAVE FREQUENCIES Filed Aug. 25, 1962 2 Sheets-Sheet 2 INVENTOR Bruno O. Weinschel ATTORNEY United States Patent C) 3,157,846 CARD ATTENUATOR FOR MICROWAVE FREQUENCIES Bruno O. Weinsche'l, Bethesda, Md as'signor to Weinschel Engineering (10., End, Kensington, Md, a corporation of Delaware Filed Aug. 23, 1962., Ser. No. 219353 8 Claims. (Cl. 333-81) This invention relates to attenuators for microwave systems, and has for its primary object the provision of an attenuator which is substantially independent of frequency from direct current up into the high microwave frequency range.

The basic problem with which the present invention is concerned can be illustrated by considering the limitations of presently-used common forms of attenuators such, for example, as the T-pad attenuator and the line type attenuator. The T-pad attenuator consists of a series of fixed resistance elements in T-forrnation, wh ch gives a definite attenuation with a constant input and output impedance, depending upon the resistance values selected. However, the individual resistor elements will stay constant in value only below the frequencies where the wavelength is long compared to the physical dimensions of the resistor. When this condition fails, the value of the resistor, and therefore the attenuation of the unit, will change with frequency. In the line type attenuator, the inner conductor of a coaxial line is replaced with a resistive element, and therefore the field or wave traveling down the coaxial line is attenuated because part of the energy is dissipated in the resistive element. In this case, there is practically no high-frequency limit, but there is a low-frequency limit due to the length of the resistive element compared with the wavelength; in this case, the length of the element should belong compared with the wavelength, while in the T-p'ad, the resistive element should be short compared with the wavelength. Where the T-pad attenuator has a high-frequency limit, the line type attenuator has a lowfrequency limit.

A third type of attenuator, which does not have the above disadvantages, is the card-type attenuator, which is basically a flat insulating plate, usually of ceramic material, having a thin conductive or resistive coating on at least one surface thereof, which coating acts as the attenuatingelement. The card attenuator has no frequency limit as to size of the resistor or its dimensions, since it depends on a homogeneous resistance layer (which may be considered as a T-pad attenuator made up of infinitely small resistance elements). The attenuating layer should be sufiiciently thin so that skin or set can be neglected.

It is a major object of the invention to provide a card attenuator having a predetermined impedance which remains constant over all frequency values down to direct current.

A further object is to provide a card attenuator having terminal members for connection to a microwave circuit, which are designed to provide the characteristic impedance proper for the associated circuit, and which are shaped to provide essentially the same held of distribution over a wide range of input and output impedance values.

A further object is to provide a variable card attenuator having an inherently logarithmic characteristic to enable linear calibration in decibels.

A major advantage of the invention is that it makes possible an attenuator having a very low minimum insertion loss, typically in the order of 1 db, because the novel construction permts the two electrodes of the system to be placed very close to each other. A further advantage is that the card attenuator can be calibrated with direct current, which is a great advantage in the initial calibration.

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Typical prior art attenuators have inherently logarithmic characteristics and therefore can be calibrated only in the microwave range.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings, in which:

FIG. 1 is a simplified schematic diagram of a fixed card attenuator showing the principle of the invention;

FIG. 2 is a sectional view taken on line 2-2 of FIG. 1;

FIG. 3 is a diagrammatic drawing used in explaining the principles of a variable attenuator made according to the invention;

FIG. 4 is a sectional view of a practical variable card attenuator taken on line 44 of FIG. 5;

FIG. 5 is a sectional view taken on line 55 of FIG. 4;

FIG. 6 shows a practical fixed attenuator construction in cross section, taken on line 6-6 of FIG. 7;

FIG. 6a is a detail of the fixed card attenuator showing connection details;

FIG. 7 is a sectional view taken on line 7-7 of FIG. 6; and

FIG. 8 is a diagram similar to FIG. 1 of a modified form of the invention.

Referring to FIGS. 1 and 2, the card attenuator is shown incorporated in a coaxial line having an outer conductor 8 and an inner conductor 6. The card attenuator comprises a ceramic plate 1 having a very thin layer or coating 2 of carbon, metallic film, or similar known film attenuator material deposited on one or both sides thereof. The two long edges of the card are in contact with the outer conductor, and are therefore assumed to be at ground potential at all points along their length. In order to understand the principle of the present invention, it is necessary to consider briefly the theory of the field distribution upon which the present invention is based. It is assumed that the center conductor of the associated coaxial cable contacts the attenuator at points 3 and 4 respectively, so that the current flow along the center conductor is between points 3 and 4, except for the current which is shunted through the card attenuator to the outer conductor 8, by virtue of the contact of the long edges with the outer conductor. it will be apparent that the current flow distribution is essentially a two-dimensional field problem. It is desired to find a suitable size and shape for the electrodes 6 and 7, the length of the card, and resistance of the card in ohms per square so that the arrangement will function as a matched attenuator having predicted values of attenuation and input and output impedance. It is assumed that the flow is essentially that corresponding to D.C. current distribution, and this is valid for all high frequencies. As will be seen from the analysis given below, the Laplace equation can be applied in which the sum of the second derivatives of the potential along the two coordinate axes is equal to zero. Solving this equation, it is found that a field distribution exists, as can be seen intuitively, wherein the current will flow essentially as shown in FIG. 1 by the lines a. The logarithm of the voltage between the center line 34 and ground (long edges of the card) with reference to the input voltage is found to be proportional to the distance between points 3 and 4. This makes possible an attenuator having an attenuation in decibels (voltage or power) which is linearly related to the distance along the center line from point 4.

Assuming a resistive plane which has a certain uniform resistance per unit square, one takes a Cartesian coordinate system with the X axis vertical and the Z axis running horizontally to the right (FIG. 1). The resistivity is assumed to be p, and since there are no selfcontained sources of current, electric charges, or other arenas-e discontinuities, the current fiow must be continuous in the plane, and one can write del i=; that is Vi=0 (l) V V Since the resistivity is isotropic, one can say that i (the current) times p equals the field strength E, both i and E being vectorial and by substitution in Equation 1 one obtains V-E=0 (3) p Since p is not equal to zero, it follows that One can also say that divergent of i (current) must be equal to zero:

div i=0 It then follows that the divergent of E must be equal to zero. It further follows that a potential, vectorial E, must exist (the field strength) which equals the gradient of the potential E=grad.

The gradient is written similarly to the divergence in using the del operator V, and this being a scalar quantity one obtains Hence the divergence of the gradient of 3 must be equal to zero:

V V =O (8) This is essentially one form of the Laplace equation i his; an az (9) which is the Laplace equation expressed in Cartesian co ordinates.

Since this is a well-known equation, it can be solved relatively simply by reference to standard texts. One of the known functions which satisfies this equation is:

This can be shown to be a solution to Equation 9, since it can be readily demonstrated that the second derivative of this function will give a sum of zero in accordance with Equation 9.

In order to satisfy the boundary conditions existing in our problem, in which the potential is zero along the top and bottom grounded edges of the card where the value of x is a/2, and a/2, a being the width of the card, then it follows that 1 5 for these conditions is zero:

Consideration of energy conservation indicates that the field strength mustdiminish in the direction Z, which gives the sign of the exponential factor. In order to calculate the voltage which exists between any point on the center line or" the card and the upper or lower grounded edges, one must determine the value of the difference between at that point and at the edge, and since the edge voltage is Zero, the ditference between these two loci is therefore 5 the voltage along the center line, or

. l and 5 must be equal to the voltage V at the input of the card:

and

1E 1= V0 a 14 It can be seen that this voltage diminishes in such a manner that the logarithm of the ratio of two voltages at any point along the card will be directly proportional to Z, This can be shown as follows: Assuming two voltages at point Z and Z to be V and V respectively, then L V =V e and Jl V =V e (16) The ratio of these two last equations is ii: g( 1 2) 17 Taking the natural logarithm of this, then V 7: 1n Z1Z2 Z2Za 8 Reducing this to decibels:

20 0.434 ln x 20 log x (19) K KL 2 8.68 In -20 log 8.68 (Z Z db (20) This means that the unit can be calibrated directly in decibels on linear scale.

The above calculations indicate the field distribution on the basis that the center conductor contacts the card at points 3 and 4. In practice, of course, this cannot be attained, and in any event would be undesirable, because the resistance of the point would be infinitely large; even with a small point contact of practical size, the resistance in the immediate vicinity of the point would be very large and would comprise the major portion of the resistance of the attenuator element. In practice, it is desired not only that the attenuator should attenuate the signal, but also that the input and output impedances must be of a constant predetermined value, preferably matching the impedance of the coaxial line with which the device is associated. One can see intuitively that the input impedance, i.e., the ratio of the voltage developed between con ductors 6 and 3 across the attenuator (or, if we assume the battery Ill to be connected between 6 and 3, the current flow) will depend upon the shape of the electrode at the point 3. If the electrode is a point, the current density at the point 3 is high because there is no substantial cross section, and the current will be very low.

The design objective is to make a desired matching value of input impedance, e.g., ohms in the case of a 50 ohm coaxial line. The distribution of the equipotential lines b can be calculated according to the preceding discussion, and it will be apparent that any electrode which contacts the card surface in such manner that its edge coincides with one of the equipotential lines will not alter the current distribution, since all points along this edge of the electrode will also be at the same potential. The resistance or impedance value is determined essentially by the distance of the equipotential line from the assumed point of origin 3. It will be apparent that as this distance diminishes, the resistance increases and vice versa. Thus it becomes possible to select a value which matches the desired impedance while maintaining essentially the same current distribution as was calculated for the case of the point contact. At the same time, the power handling capabilities of the card attenuator are fully utilized, since the major portion of the eflficient radiating area of the resistor remains effective for a wide range of sizes of attenuators. The high frequency properties of the attenuator are also improved because of the distribution of resistance which decreases the inductive effects, while the planar distribution of the complete film serves to reduce the capacity between the different elements of the area.

It should be noted that the foregoing analysis assumes that the respective electrodes 3 and 4 do not affect each other. In practice, the electrodes are maintained sufliciently separated so that their effect upon each other can be neglected. If the electrodes 3 and 4 are too close to each other, they will affect each other and the calculated distribution will not occur. However, even though it is not possible to predict the attenuation value theoretically in this region, an attenuator having the electrodes close together can still be made, and its attenuation can be measured directly at D.-C., since the current distribution is essentially independent of the frequency.

It is important that the attenuating layer 2 be made uniform and homogeneous; this is possible with modern techniques which have been developed for microwave film attenuators, and in practice, it is possible to make a card attenuator according to the present invention which has a variation in attenuation of only 0.25 db for a fixed db attenuator, between D.-C. and 10 kmc., which is very good.

FIG. 3 shows the principle of a variable attenuator made in accordance with the principle of the invention. In this case, the card attenuator 21 is made in the form of the arc of a circle. If the attenuator card is made narrow relative to its length, the deviation from a rectangle will be negligible, and it can be considered that the same calculations as are applied to the case of the rectangle in FIG. 1 will hold in this case within practical limits. The microwave power is applied between electrodes 32 and ground and the periphery of contact between this electrode and the resistive material of the card is again made to coincide with the equipotential line distribution calculated in the preceding case. The respective arcuate edges 22a and 22!) are again grounded as before, that is, in the case of the coaxial conductor they are connected to the outer conductor of the cable. Electrodes 23 and 24 are connected respectively to the center line of the coaxial circuit. Electrode 23 is fixed in this case, and electrode 24 is a movable electrode in contact with the resistive surface 22 of the attenuator. For use with high microwave frequencies, this need not be a physical contact, but could be a capacitive contact through a thin insulating film applied over the resistive material, which would reduce wear on the insulating film due to motion of the electrode 24. Movable electrode 24 is carried on a conductive arm 25, which is pivoted at the center of the concentric arcuate paths 22a and 22b. This electrode 24 connects to the actual output connector. As arm 25 is rotated, electrode 24 therefore moves along essentially on the center line of the attenuator, where, as previously pointed out, the relationship between the distance and the attenuation is logarithmic. It is therefore possible to provide a pointer 31, which is shown directly attached to arm 24 (but which may be instead separately attached to its pivotal point for rotation therewith) for cooperation with a scale 32, which can now be marked off linearly in decibels. This linear relationship will, of course, not hold for the case where the two electrodes 23 and 24 are very close to each other, and therefore this portion of the scale should either be supressed or else empirically marked to designate the actual values, which can be determined by actual measurement.

It will be apparent that instead of using an arcuate construction and an angularly movable arm 24 as shown in FIG. 3, a linear construction similar to FIG. 1 can be used, in which one of the electrodes is made linearly movable along the center line. However, the mechanical construction for this is more complicated, and for most 6 applications the arcuate form is preferred. A preferred form of a practical variable card attenuator is shown in FIGS. 4 and 5.

FIGS. 6, 6a and 7 show a practical embodiment of the fixed card attenuator, that is, an attenuator having a fixed and definite value of attenuation. The unit is provided at its ends with conventional male and female coaxial cable connectors 51 and 52 respectively. These connectors are respectively screw-threadedly connected at 53 and 54 to the tubular center section 56 which is comprised of two halves 57 and 58 which are preferably square or rectangular externally as best seen in FIG. 7, in order to conveniently permit their being matched together by any suitable means such as screws 59 and 61, in such fashion as to clamp between them the thin sheet or card 62 of suitable insulating material, e.g., a high grade ceramic material. One surface of the card is coated at 63 with any suitable resistive material, which may be a thin layer of carbon particles, or a very thin layer of metal applied by methods well known in the art. The two side edges 64 and 66 of the conductive layer which are in contact with members 57 and '58 are preferably coated with a heavier layer of silver or other good conducting material, preferably extending continuously around the edge of the ceramic card 62 so as to make good contact with both of the halves 57 and 58 of tubular member 56. The ends 67 and 68 of the card 62 are likewise clamped between terminals 6% and 71 respectively by any suitable means, shown as screws 59 and 61 respectively. The contours of terminals 6? and 71, where they engage the conductive layer 63, are shaped to fall on equipotential lines in accordance with the design principles discussed above. The areas of contact of layer 63 with the input and output terminals are preferably also coated with silver or other conducting material overlying the resistive layer 63 in order to provide good electrical contact. It should be noted that the eifective perimeter of the terminals 59 and 61 is defined by the layer of highly conducting silver, and therefore the solid metallic portion of the terminal may occupy only a relatively small portion of this area, as shown in FIG. 6a, where the input terminal 69 may be of any suitable shape at its end for engaging the card 62. The silver layer 74, however, covers the resistive layer 63 over an area, the perimeter of which at 76 is made to lie along an equipotential line in accordance with the principles outlined above. This enables a standard center conductor 69 to be used, while the fixed impedance can be determined by the area and perimeter of the silver layer 74. In this case, the central conductor 69 together with the highly conductive layer 74 constitutes the input terminal.

The equipotential lines for a given value of attenuator can be determined either by calculation as indicated above, or can be determined experimentally with suflicient accuracy for many purposes by supplying D.-C. current to the card from essentially a point source corresponding to point 3 of FIG. 1 and plotting the equipotential lines by actual measurement with probes or other similar experimental technique. The attenuation value can then be given a final adjustment by painting on more silver to slightly change the area, or by trimming or scraping carbon from the back edge of the card, since this is a region of very low current density, where a slight change will make only a very small difference in the overall attenuation value.

FIGS. 4 and 5 show in cross section a practical construction for a variable attenuator utilizing the same principle as FIG. 3, except that the card attenuator is in this case curved so that its shape is essentially cylindrical rather than an arcuate section. The attenuator is mounted within a conductive housing 88 of highly conducting material such as brass, provided with two conventional coaxial cable connectors 83 and 94 for connection to conventional coaxial line circuits with which the device is to be used. The output connector has a central conductor tubular ceramic card 8'6.

82, corresponding to conductor 7 of FIG. 1, which is supported by insulating member 31 concentrically within outer shell 83. The rear end of connector 82 has a recess containing a spring 84 and a bullet-shaped member 85 which the spring presses into engagement with the cupped end of movable central conductor 89 terminating in a generally circular contact member 9%; this may be conveniently formed by splaying out the end of conductor 89 in the form of several spring fingers which resiliently press against and make contact with resistance layer 861) of the Inner conductor 39 is rigidly supported, as by suitable insulating members 89a and 8N) centrally within a T-shaped tubular outer member 87 which is suitably mounted for rotation within and relative to the outer casing 88, as by ball bearings 87a and 8711. A portion of the T-shaped tubular member extends through the casing 88 as shown at $70, and has fixed to its end a knob 87d whereby the assembly of tubular member 87 and inner conductor 39 may be rotated relative to the casing and to the card attenuator 86. Card attenuator as has a resistive coating 86!; on its inner surface, and is curved to conform to the circular outer casing 88, as best shown in FIG. 5. The ends of the card are spaced from each other to form a longitudinal gap as shown at 86a, and the central conductor 91 of the input coaxial connector 94 is connected to the resistive surface at 91a through this gap. This connection, of course, conforms to the principle described above, and input conductor 91 corresponds electrically to conductor 6 of FIG. 1.

It will be seen that the center conductor 89 will rotate when knob 87d is turned, while maintaining contact through bullet 85 with the output conductor 82.. Tubular member 8'7 is also grounded through the ball bearings 87a and 87b, as well as by direct conductive contact with the casing 38. As the assembly is rotated conductive contact is maintained over the generally circular area of contact provided by fingers 90 along the center line of the card attenuator, so that the principle of operation is the same as shown in FIG. 1.

Since it is desired that the attenuator used have a range of at least 130 db, proper shielding between the inner and outer conductor at all points is desirable, and this is provided by a spring finger 92, which may be a flexible strip of beryllium-copper 92, which extends from the outer casing 88 through the gap at 86.1 into sliding contact at 92a with the outer surface of a por tion ofythe tubular member 87. This grounding of the tubing 87 serves to shield the input conductor 89 from the output coaxial line. If finer manual control is desired, it will be apparent that any conventional vernier or gearing arrangement may be used between the manual control knob and the tube 370. In order to insure good electrical contact between the edges of the card and the casing, a heavy conductive coating, which may be silver paint applied by known techniques, is provided at edges 86c and 36d, and a soft solder connection may also be used to insure good contact along these edges. An indicator needle 87d may be attached to shaft 87c forrotation therewith, and the adjacent circular face of casing 88 may be provided with any suitable scale to indicate the setting of the device, which is generally marked in decibels. As previously explained, due to the logarithmic variation of the card attenuator, a decibel scale can be linearly marked off over most of the useful range of the device. At the low end of the scale, Where contact 9% is close to fixed contact 91a, linearity is not preserved, but this portion of the scale can be calibrated empirically if desired.

Although all of the previously described forms of the invention show the card attenuator as a symmetrical device, with the input and output terminals located on the center line of the card, it should be stated that because of this symmetry the analysis applies equally well to each half taken by itself, and therefore only one half of the card attenuatorcan be used, which in some cases may have certain advantages. This is illustrated in FIG. 8, which corresponds to FIG. 1, the same reference character with primes added being used to identify corresponding elements. In this case, the lower edge of the ceramic surface 5 is not coated with resistive material, and the terminals 3 and 4 may be located as close to the lower side of the card as insulation or other electrical design considerations permit. The lines of current flow will be the same as before, except that one of the two halves shown at FIG. 1 is suppressed, and the general design considerations will also be the same as before. Of course, the specific value of the electrical and physical parameters will be different, so that, for example, a lower ohms per square film can be used than with the design of FIG. 1, other values being equal. This may be of advantage, especially in a variable attenuator in order to obtain more stability under conditions of abrasion by the movable contact, or for a fixed attenuator may permit other design parameters to be more advantageously selected.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

I claim:

1. A microwave attenuator comprising an insulating member having at least one surface; a thin layer of resistive material on said surface; a coaxial connector having an outer conductive grounded member and an inner conductive current member coaxially spaced and insulated from said outer member, the periphery of the surface having two opposed and spaced linear sides and two opposed and spaced linear ends, said resistive material along at least one of said sides being in conductive contact along the side legnth with said outer conductive member, and said inner conductive member being in contact with the resistive material at a restricted area adjacent one of said ends and spaced from said sides.

2. The invention according to claim 1, and a second coaxial cable connector having an outer conductive grounded member electrically connected to said first outer conductive grounded member, and having a second inner conductive current member spaced and insulated from said second outer conductive member, said second inner conductive member being in contact with said resistive material at a second restricted area spaced from said first restricted area and also spaced from said sides.

3. The invention according to claim 2, said restricted areas lying substantially on a line equally spaced from both of said sides.

4. The invention according to claim 3, the area of contact between said second inner conductive member and said resistive layer being slidably movable along said line to vary the distance between said restricted areas.

5. The invention according to claim 4, said smooth surface being in the form of a cylinder having a longitudinal gap, the edges of the gap providing said opposed and spaced ends, and the curved ends of the cylinder providing said opposed and spaced sides; said inner conductive member lying along the central axis of said cylinder; and a contact arm rotatable about the central axis of the cylinder and in contact with said second inner conductor and with said resistive layer to provide the movable restricted area.

6. A microwave attenuator for use with a coaxial cable having a grounded outer conductor and a central inner conductor insulated from said grounded conductor, said attenuator comprising: a tubular outer conductor; a coaxial cable connector at one end of said outer conductor, said connector having an outer terminal memher and an inner terminal member; a thin sheet-like insulating member within said tubular conductor and supported thereby and having at least one substantially plane surface passing through substantially the longitudinal axis of s id tubular member; a thin layer of resistive material on said plane surface, said layer having two opposed sides and two ends respectively between said sides, said two opposed sides or" said layer being in electrical contact with said tubular member, said inner terminal of the connector being in contact with a restricted area of said layer at one of said ends, the perimeter of said restricted area lying on an equipotential line of voltage due to current flow from a point on the end at which said area is located.

7. The invention according to claim 6, said point lying substantially at the center of said end between said sides. 7

8. A microwave attenuator comprising an insulating member having a substantially smooth surface with two opposed sides and two opposed ends lying respectively between said sides; a thin layer of resistive material on said smooth surface; an input terminal at one of said ends of said surface in conductive contact With a restricted area of said resistive layer; an output terminal spaced from said input terminal and in contact with another restricted area of said layer, the perimeter at least one of said areas lying substantially on an equipotential line of voltage due to current such as would flow in the absence of the terminal from a single point on the corresponding end of said surface at which said terminal is located, the layer of resistive material on at least one of said sides being grounded.

References Cited by the Examiner UNITED STATES PATENTS 2,081,572 5/37 Bagno 3338'l 2,647,975 8/53 Jack et a1 338-164 XR 2,670,461 2/54 Learned 338lA 2,740,025 3/56 Polye et a1. 338-164 XR 2,842,748 7/58 Vallese 3338l 2,898,567 8/59 Drewitz et a1. 3389O HERMAN KARL SAALBACH, Primary Examiner.

Claims (1)

1. A MICROWAVE ATTENUATOR COMPRISING AN INSULATING MEMBER HAVING AT LEAST ONE SURFACE; A THIN LAYER OF RESISTIVE MATERIAL ON SAID SURFACE; A COAXIAL CONNECTOR HAVING AN OUTER CONDUCTIVE GROUNDED MEMBER AND AN INNER CONDUCTIVE CURRENT MEMBER COAXIALLY SPACED AND INSULATED FROM SAID OUTER MEMBER, THE PERIPHERY OF THE SURFACE HAVING TWO OPPOSED AND SPACED LINEAR SIDES AND TWO OPPOSED AND SPACED LINEAR ENDS, SAID RESISTIVE

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