US3005967A - Frequency-compensated coaxial attenuator - Google Patents

Description

1961 B. o. WEINSCHEL ET AL 3,005,967

FREQUENCYCOMPENSATED COAXIAL ATTENUATOR Filed April 27, 1960 2 Sheets-Sheet 1 nvmvrozw Bruno O. Weinsche/ Jul/cm F. Coffre/l 06L 1961 B. o. WEINSCHEL ET AL 3,

FREQUENCYCOMPENSATED COAXIAL ATTENUATOR Filed April 2'7, 1960 2 Sheets-Sheet 2 Fig. 2

FREQUENCY SENSITIVITY OF INSERTION LOSS 2 0 Fig. 5A

O l 2 3 4 5 6 7 8 9 IO ll l2.4

Fig 5B O l 2 3KMg 5 6 7 B 9 IO ll l2.4

Bruno 0. Weinschel 'NVENTORS' Julian E Cofrrell BY M ATTORNEY United States Patent 3,005,967 FREQUENCY-COMPENSATED CGAXIAL ATTENUATOR Bruno 0. Weinschel, Bethesda, and Julian F. Cottrell, Silver Spring, Md., assignors, -by mesne assignments, toWeinschel Engineering Co., Inc.,' Kensington, Md, a corporation of Delaware Filed Apr. 27, 1960, Ser. No. 24,998 7 Claims. (Cl. 333-81) This invention relates to a lossy-line type of coaxial attenuator such as is used in high-frequency and microwave coaxial line circuits, and has for its primary object the provision of a lossy-line type of attenuator which has a constant attenuation over a much wider frequency range than is possible with presently-used types of attenuators. The usual lossy-l-ine attenuator is useful over a frequency range that is limited at the high end by problems due to the coaxial connectors which are necessarily used, or .by higher modes generated in the lines, that is, by factors outside the attenuator itself. At the low end, the attenuator tends to fall off rather rapidly below a certain fire. quency, depending on the type and size of coaxial cable used, etc. One reason for this is that the attenuator must have a certain minimum length, expressed in terms of wavelengths, to function properly as an eifectiveico-axial attenuator. If the resistive section is short compared to a wavelength, it is apparent that the attenuation will not be constant with respect to frequency; the major purpose of the present invention is to provide wide-band attenuation:

having substantially the same value of attenuation, expressed in decibels, for a wide band of frequencies.

A major object of the invention is to provide a lossyline coaxial attenuator of the type using a very thin layer of resistive material as one of the elements of a section of coaxial line, with a series condenser which is structurally a part of the lossy-line and which functions in a novel way to offset the usual droop at the low end of the insertion loss Vs'frequency curve. A further object is to provide such a construction without appreciably changing the bulk or physical configuration of the attenuator, and by means of a simple construction which adds little to the expense of fabrication or to the skill required to make the attenuator unit.

' Another object is to provide a wide-band high-frequency coaxial attenuator of simple and rugged construction and having high stability and resistance to physical and thermal stresses and aging.

Theinvention is particularly applicable to attenuators of the type shown in U.S. patent to Weber, No. 2,689,294, for Metal Film Attenuator, and also to attenuators of the type shown in the copending US patent application of'Bruno O. Weinschel, Serial No. 833,041, filed August 11, 1959, for Inside-Out Attenuator, wherein the resistive coatingis applied to the outer coaxial element of a coaxial section instead of to the inner coaxial element.

, According to the invention, a circumferential gap is axially for a suitable short distance on either side of the;

gap,- forming, in effect, a series condenser arrangement,

one plate of which is the conducting outer layer, the other plate (or plates) being formed by the portions of the re sistive layer under the coating of the conductive material. This functions partly as a. series condenser and partly as a' construction which shorts out a. portion of the resistive material at the higher frequencies and introduces more of the resistive material at the lower frequencies to level out plained in detail below.

ice

2 the attenuation-frequency characteristic as will be ex- 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 longitudinal cross-sectional view of one form of attenuator according to the invention; I

FIGS. 2, 3 and 4 show the central resistive element of the attenuator of FIG. 1,in successive stages of prepara-.

tion required to make'the finished products; and

FIGS. 5A and 5B show'a series of graphs used in explaining the characteristics of the invention in comparison.

with prior art devices.

The invention will: be explained in connection with an attenuator in which the inner coaxial element is the resistive element, similar tothat shown in the Weber patent previously referred to, but it will be understood that the.

same principle and construction are applicable to an attenuatorof the typeshown in the copending application of Bruno O. Weinschel, Serial No. 833,041, wherein the resistive coating is applied to the outer coaxial element.

FIG. 1 is a longitudinal sectional view of an attenuatorj made according to the invention, in the usual form of an element which can be inserted directly into a coaxial line,

and for (this purpose is provided with standard coaxialmale and female fittings 2 and 3 respectively. The outer coaxial conductor of the attenuator, 4, is of conventional construction. In this case, the actual attenuatorelement' is the inner conductor 6, which is essentially a rod of in; sulating material, usually of ceramic material, provided with bullet connectors 7 and 8 at the ends thereof mating; with the central terminal elements of the respective c 0 axial fittings 2 and 3. In a prior type of construction, such as shown in the copending application above referred.

' to end, and constitutes the resis'tive; attenuator element.

1 lower frequency range, the construction shownin the drawings is employed. For' this purpose-the central'con- In this usual construction, however, the attenuator ele ment typically has the characteristic shown inFIG. 5A, for a 3 db attenuator. It will benot ed that while at 3 db the insertion loss is reasonably constant over range from 112.4 kilomegacycles, at 10 'db, the insertion loss falls on very badly between 1 and 4 kmc. In making a 10 db attenu/ator, as shown in FIG. 513, it is necessary to use amuch higher resistance rod, and the ratio of resistance to wavelength goes up quiterapidly, and the attenuation correspondingly goes down for'the lower frequencies, as-

explained above. This can'be further understood by consideri-ng that if we assume alossless line, the characteristic impedance is determined by the distributed inductance and capacitance of th'e'line only, but in the case of an atbe transformed into a ratio of wavelengths to the physical parameters of the line.

In order to minimize'theidro'oping characteristic at the ductor is prepared in'the' usual'fashion, that is", the

ceramic rod 6 is coated with'the usualres'istive' layer 9 from end to end. A narrow circumferential-groove,-

typically '30-60 mils wide, is'then cut in-th'e resistive film.

. Patented Oct. 24, 1961.

This can be conveniently done by grinding, the purpose being to remove the resistive film, but as little of the ceramic as possible. The resulting construction is as shown in FIG. 2. A coat of insulating material, for example, black Glyptal, is now applied to the vicinity of the gap 11, as shown at 12. This may be done with a small camel-hair brush, or by any other convenient means, and should extend, in a typical case, at least onehalf inch on either side of the groove 11, as well as covering the groove. If Glyptal is used, the rod should then be baked at 150 C. for a minimum of eight hours, then a second coat of Glyptal should be applied and baked at the same temperature for an additional eight hours. A band of silver paint, 13, should now be applied directly over the groove, but not extending as far as the Glyptal, and baked for a minimum of four hours at 150 C. to provide, in effect, a tubular coating of conductive material over this portion of the rod. It will be understood that instead of silver, any other suitable conductive material may be similarly applied, and that band 13 may be of resistive material instead of highly conductive, depending on the characteristic required.

In order to determine the proper axial length of the conductive coating 13, the attenuator may then be assembled, and the attenuation measured at both the high and low ends of the frequency spectrum desired, say for example, from 0.5 kmc. to 11.0 kmc. The attenuation should not differ by more than +0.1 db to- 0.2 db at these two frequencies. If the attenuation is greater than this, the axial length of 13 should be increased, and vice versa.

As a final production step, the entire rod unit 6 may then be given an additional coat of black Glyptal" from end to end, and baked again for a minimum of eight hours at 150 C. As indicated by the above procedure, it is normally expected that an attenuator so made will have a substantially flat frequency-attenuation characteristic, within the limits noted, over the entire frequency band from 0.5 to at least 11 kmc. This is in sharp contrast to the characteristic of an ordinary attenuator as noted in FIG. 5.

It will be noted that the improved result is not due simply to the effect expected by placing a capacitor in series with a resistive line, since a pure capacitive element is not dissipative and does not produce the same effect as a lossy-line resistance type of attenuator. In the above construction, the groove 11 provides, together with the adjacent ends of the resistance material 9 and the conductive layer 13, a structure corresponding to two capacitors in series. However, this is obviously not a lumped capacity, but the capacity is also distributed along a portion of the line adjacent the groove. Due to this construction, the current in the resistive film at very high frequencies will mostly be bypassed right into the silver from the resistive films, tending to go directly from the resistive film to one end of the conductive band 13, and then to return into the resistive film at or very near the other end. In this case, only a small part of the capacity will be effective; furthermore, since there is a larger effective length of the outer silver coating acting in this case, there is correspondingly a shorter effective length of the resistive element being utilized. Conversely, at the lower frequencies more and more of the inner surface of the silver coating 13 is effective as an ordinary condenser, and more and more of the resistive material enters into the action. Therefore, at the lower frequencies, the attenuation does not fall ofi as shown in FIG. 5, but tends to remain substantially uniform throughout the entire range. It will be apparent from the foregoing that the distribution of current along the length of the capacitor-resistor section is different at different frequencies in a manner which tends to compensate for the usual low-frequency droop.

,While exact calculation of the effect at each frequency is diflicult, as previously noted, approximate calculations can be easily made with simplifying assumptions, which give sufliciently good results so that it is not diflicult to compute the approximate dimensions for any given set of practical conditions, using as a basis the capacitive reactance of a condenser of the dimensions which are determined by the size of coaxial cable and other parameters employed.

The above construction is of particular utility where a flat characteristic is essential, but adds so little to the cost of the attenuator, that it may be used in any situation where a general-purpose wide spectrum attenuator is required. It does not adversely affect the operation of the attenuator in any way, and the stability, aging, shock resistance, etc. are fully equal to those of an ordinary attenuator of this general type. The units are easily reproducible as to electrical characteristics in manufacture by the use of ordinary production control methods, and do not require more highly skilled labor than the ordinary attenuators.

While the band 13 has been shown as of conductive material, it will be understood that this could also be of resistive material, which may be desirable in some cases to produce a particular characteristic.

In FIG. 2, the gap 11 has been shown as being further from one end of the rod 6 than from the other. This is the preferred construction in the case of a unilateral attenuator, that is, one which is always to be fed from one end. In this case, it is desirable to put the discontinuity fairly close to the other end, so as to provide the maximum attenuation path for any slight reflections which may occur due to the unavoidable effects of the discontinuity. In the case of a bilateral attenuator, that is, one which is intended for use in both directions of transmission, then the gap 11 should obviously be placed at the center of the rod 6.

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 our invention as defined in the appended claims. For example, the resistive coating could be on the inner surface of the outer coaxial conductor, as in the copend' ing Weinschel application previously referred to, in which case the circumferential slit would be made as before, but on the inside surface of a hollow cylinder, and the respective bands of insulation and silver would be respectively nearer the central axis than the main resistive coating instead of being further away from the central axis, but otherwise the construction and principle of operation will be the same.

We claim:

1. A high-frequency coaxial attenuator having opposed conductive surfaces lying between coaxial terminals, one of said surfaces being constituted by a thin layer of resistive material adhered to the surface of an insulating member to constitute a lossy-atten-uator surface, and extending continuously between said coaxial terminals, except for a narrow circumferential gap; a thin layer of insulating material covering said layer of resistive material on both sides of said gap; and a further layer of conductive material covering said layer of insulating material for a portion of the axial extent of said resistive material on both sides of said gap, said further layer being electrically separated from said layer of resistive material by said layer of insulating material.

2. The invention according to claim 1, said opposed surfaces being concentric cylindrical surfaces.

3. The invention according to claim 2, said further layer of conductive material being a layer of highly conductive material of negligible resistance.

4. The invention according to claim 3, said highly conductive material being silver.

5. A wide-band high-frequency coaxial attenuator comprising an elongated inner insulator having a cylindrical outer surface, a very thin resistive layer of electrically conductive material adhered to the outer surface of said insulator and constituting a lossy-attenuator surface, a tubular outer conductor coaxially spaced from said resistive layer, there being a circumferential gap in the length of said resistive layer intermediate i-ts ends, a thin layer of insulating material covering said resistive layer in the vicinity of said gap, and a further layer of conductive material covering said layer of insulating material for a portion of the axial extent of said resistive material on both sides of said gap, said further layer being insulated from said layer of resistive material by said layer of insulating material.

5 conductive material being silver.

References Cited in the file of this patent UNITED STATES PATENTS Chatterton Sept. 12, 1950 2,892,984 Davis June 30, 1959

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