US3464037A - Microwave attenuator and method of making same - Google Patents

Description

Aug. 26, 1969 J. o. BRAMICK ETAL 3,464,037

MICROWAVE ATTENUATOR AND METHOD OF MAKING SAME Filed Aug. a. 1966 INVENTORS JOHN O, BRAMICK CHARLES W. DFEYEF?` BY V/ f ATTORNEY.

United States Patent O 3,464,037 MlCROWAVE ATTENUATOR AND METHOD F MAKING SAME John 0. Bramick, Bernardsville, and Charles W. Dreyer,

Morris Plains, NJ., assignors to Miemlab, Livingston,

NJ., a corporation of New Jersey Filed Aug. 8, 1966, Ser. No. 570,850 Int. Cl. H01p 1/22 US. Cl. 333-81 11 Claims ABSTRACT OF THE DISCLOSURE A coaxial attenuator comprising a unitary central resistance element having a contact intermediate its ends which is electrically connected to the central resistance element. A radially extending disc is mounted on the central element and is provided with a second contact in overlying relationship to the first contact. The first and second contacts are soldered together to mechanically and electrically connect together the central resistance element and the disc. Contact means is provided on the central resistance element and the disc resistance element for connecting the attenuator in an electrical circuit with a transmission line.

A method for fabricating the attenuator also is disclosed.

This invention relates generally to a construction for a microwave attenuator and to a method of making the same and, more particularly, pertains to a microwave attenuator which has a relatively constant attenuation characteristic over an extremely wide frequency band.

Piand Tee-network attenuators are used in high frequency coaxial cables or microwave transmission lines to reduce the power flowing in the cable by a known ratio. A Tee-network attenuator, for example, includes two series resistors which are connected together by a screw arrangement to provide a central resistor assembly. A disc resistor receives the screw therethrough and is sandwiched between the two series resistors. The central resistor assembly is serially connected in the center conductor of the coaxial cable while the disc resistor is connected to the outer conductor of the coaxial cable by a pluralitycf spring clips. The disc is maintained in place between the two series resistors and in electrical contact therewith by the pressure of the series resistors which abut opposite surfaces of the disc. While this type of Tee-network attenuator construction is easy to assemble, it has a number of severe disadvantages associated with its use. i

More specifically, the mechanical screw connection between the two series resistors requires that the resistors be separated by an extremely large length as compared to the wavelength of the microwave frequencies under consideration. This separation represents an impedance discontinuity which requires appropriate compensation techniques. Such compensation becomes exceedingly difficult to obtain at high frequencies. Additionally, the fact that the disc is electrically connected to the series resistors by means of a contact pressure also contributes to the non-uniform operation of the above-described network. That is, the elements comprising the network expand and contract at different rates when the ambient temperature changes. The resultant pressure changes cause variations in the value of the network. Also contributing to the instability of networks of this type is the particular method which is commonly used to provide contacts on the resistors. Thus, the contacts are provided by electroplating small contact bands over resistive films which provide the resistive elements of the 3,464,037 Patented Aug. 26, 1969 network attenuators. These bands create discontinuities in the form of stray series and shunt capacitances. These capacitances have an adverse effect on the attenuation characteristics of the network.

The above-mentioned limitations severely limit the frequency range of attenuators of the type described. For example, Tee-network attenuators have been found to operate only as high as 5-8K rnc. while Pi-networks having a similar construction have an upper frequency limit of only 3-4K mc.

Accordingly, the desideratum of the present invention is to provide an attenuator construction capable of efciently operating up to at least 18K mc. without degradation of the electrical characteristics of the attentiator.

Another object of the present invention is to provide an attenuator construction wherein respective resistors comprising the attenuator are soldered to each other to eliminate variations in the value of the attenuator due to changes in contact pressure between the resistive elements.

A further object of the present invention is to provide an attenuator construction having reduced contact band surfaces to substantially Yeliminate abrupt changes in impedance between the elements comprising the attenuator.

Another object of the present invention is to provide an attenuator construction wherein the resistors comprising the attenuator are assembled in one homogeneous unit which includes its own housing to facilitate connection of the attenuator in a coaxial cable.

Still another object of the present invention is to provide a method for completely and easily assembling an attenuator for the attenuation of microwave frequency signals.

The attenuator of the present invention includes a rod having a resistive film thereon. yIf it is desired to provide a Tee-network attenuator, a contact is provided on the rod and it is approximately centrally located thereon. A disc resistance is placed on the contact and a preformed ring of solder is received between the contact and the disc. Two bushings are placed about the rod and disc combination so that their ends engage an outer contact on the disc resistance. A preformed solder ring is received between the ends of the bushings. Heat is applied to the assembly tomelt the solder rings thereby to firmly connect the disc resistance with the rod resistance and the vbushings with the disc resistance.

A feature of the present invention is to provide an attenuator construction which may utilize solder having a preformed shape to connect the elements comprising the attenuator together.

Other objects and advantages of the present invention will become more apparent from a consideration of the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. l is a sectional view of a Tee-network attenuator constructed in accordance with the present invention;

FIG. 2 is a vertical sectional View ofthe rod resistor portion of the attenuator shown in FIG. 1;

FIG. 3 is a vertical sectional view of the disc resistor shown in FIG. 1; and

FIG. 4 is an exploded View of an attenuator assembly constructed according to the present invention.

The attenuator construction of the present invention will be described in conjunction with the fabrication of a Tee-network attenuator. However, the present invention is not to be interpreted as being limited to this type of attenuation network since the invention described herein below is equally applicable to Pi-network attenuators. That is, people skilled in the art will readily recognize that the Tee-network attenuators described hereinbelow may easily be converted to a Pi-network attenuator.

A Tee-network attenuator assembly is designated generally by the numeral in FIG. 1 and comprises a rod 12, a disc 14 centrally mounted on the rod 12, and respective bushings 16 and 18 which are connected to the disc 14. The rod 12 provides the two series arms of the Teenetwork attenuator while the disc 14 provides the shunt arm therefor. Contacts 20 and 22 are provided on the ends of the rod 12 and are adapted to be connected to the central conductor of the coaxial cable (not shown). The outer conductor of the coaxial cable is adapted to be connected to the bushings `16 and 18.

The rod 12 is shown in detail in FIG. 2 and includes a central insulating core 24 having reduced diameter end portions 26 and 28. The core 24 may be comprised of any insulating material such as a ceramic or glass. Deposited on the core 24 is a carbon film 30. The carbon film 30 provides the central series arm resistors of the assembly 10 in the manner noted below. Plated on the carbon film 30 about the reduced diameter end portions 26 and 28 are respective nickel contacts 32 and 34. A nickel band 36 is plated on the carbon film 30 intermediate the ends of the rod 12. Then band 36 is centrally located on the rod. Plated to the respective nickel contacts 32 and 34 and the band 36 are respective silver contacts 20, 22 and 42.

The central contact bands 36 and 42 essentially divides the resistive film 30 into two serially connected resistive arms. Hence, by utilizing a construction of this type the former construction which required the use of a screw assembly to connect the series arms of the attenuation network is eliminated. Accordingly, the two series arms of the attenuation assembly 10 are separated by a distance which is substantially smaller than the distance between the series arms of the prior art construction. That is,`in the present invention, the two series arms are separated only by the Width of the central contacts 36, 42. Accordingly, this reduction in the spacing between the resistive elements allows the Tee-network attenuator of the present invention to be used at much higher frequencies than the attenuators used heretofore.

The disc resistor 14 as shown in detail in FIG. 3 and comprises an annular core 44 having a central aperture 46. Similarly to the core 24, the core 44 may be comprised of any insulating material such as a ceramic or glass. Deposited about the exterior surfaces of the core 44 is a carbon film 48 which forms the resistance portion of the core 44. Plated about the peripheral wall of the carbon film 48 is a circular nickel contact 50. Plated on the nickel contact 50 is a circular silver contact 52. Similarly, plated on the wall of the carbon film 48 surounding the wall defining the aperture 46 is a nickel contact 54. The silver contact 56 is plated on the nickel contact 54.

The assembly of the Tee-network attenuator 10 may best be understood with reference to FIG. 4. Thus, in fabricating the assembly 10 a pre-formed ring of solder 58 is received about the center contact 42 of the rod 12. The disc 14 is then slid over the contact 42 so that the solder ring 58 is in tight frictional engagement with the contact 42 on the rod 12 and the contact 56 on the ring 14.

The bushings V16 and 18 are provided with respective central through bores 60 and 62. Respective recesses 64 and 66 are provided adjacent the inner ends of the bushings 16 and 18 to define respective peripheral anges 68 and on the respective bushings. The recesses 64 and 66 are sized to receive the disc 14 therein with the peripheral flanges 68 and 70 in engagement with the outer contact 52 of the disc 14. A preformed ring of solder 72 is received between the ends of the peripheral flanges 68 and 70 of the respective bushings 16 and 18.

Heat is then applied to the assembly 10 suilcient to cause thesolder rings 58 and 72 to melt. Accordingly, the

solder ring 58 will connect the rod 12 with the disc 14' to provide a good electrical connection therebetween. Similarly, the ring 72 will melt to connect the outer conductor 52 of the disc 14 with the bushings 16 and 18 to provide a good electrical connection therebetween. It is to be noted that the bores 60 and 62 are sufficiently large so that the rod Y12 will be in spaced relation with respect thereto.

The central band 42 on the rod 12 is made substantially equal to the width of the disc 14. Hence, the two series resistors which are formed on the rod 12 terminate just at the point where the disc resistor begins. Accordingly, there are no abrupt changes in resistance between the rod and the disc and, additionally, the three lresistors are combined into one homogeneous assembly 10.

As noted hereinabove, a Pi-network attenuator may be provided utilizing a construction and method similar to that described above in connection with the Tee-network attenuator assembly 10. That is, instead of providing one essential band on the rod 12, two bands may be provided adjacent each end which receive discs thereon to provide the shunt resistance arms of the Pi-network.

Accordingly, an attenuation network has been described which substantially eliminates the effect of dimensional irregularities caused by wide spacings between the resistive elements comprising the attenuator network.

Additionally, the construction of the present invention substantially eliminates pressure contact resistance to improve the over-all mechanical stability of the attenuation network and to allow a reduction in the size of contact bands thereby to reduce the effects of stray capacitances.

While the preferred embodiment of the present invention has been shown and described herein, it will become obvious that numerous, omissions, changes and additions may be made in such embodiment without departing from the spirit and scope of the present invention. That is, multi-section Pior Tee-network attenuators may conveniently be realized by the present construction to obtain high attenuation values in those applications in which individual yresistor values become difficult to realize in practice. For example, the cascading of two Tee-network attenuators may be accomplished by providing a single elongated resistance rod having two contact bands adjacent to and spaced from each end of the rod to define three sections. Each outer section represents the series arm of separate Tee-networks and the resistance of the intermediate section is equal to the sum of the arms of the two Tee-networks connected in series. Disc resistors are mounted on the two contact bands to provide the shunt armv resistances for Tee-network. Accordingly, many such cascaded stages may be provided as desired simply by providing a sufficiently long rod having the desired characteristics thereby to provide an extremely compact assembly.

What is claimed is: 1. A coaxial attenuator comprising a unitary central resistance element,

at least a first contact intermediate the ends of and electrically connected to said central resistance element,

an outwardly extending resistance element,

a second contact on said outwardly extending resist* ance element,

said outwardly extending resistance element being mounted on said central resistance element with said second contact in overlying relation to said first contact,

connecting means for lixedly and electrically connecting said first and second contacts together whereby said central resistance element and said outwardly extending resistance element are fixedly connected together,

and contact means on said central resistance element and said outwardly extending resistance element for connecting said attenuator in electrical circuit with a transmission line.

2. A coaxial attenuator as in claim 1,

in which said first and second contacts are coextensive with each other.

3. A coaxial attenuator as in claim 1,

wherein said first contact is centrally located on said central resistance element,

said first contact dividing said central resistance element into respective series arms which terminate at said first contact.

4. A coaxial attenuator according to claim 3,

in which said central resistance element is in the shape of a rod,

and said first contact comprises a conducting band surrounding said rod.

5. A coaxial attenuator as in claim 1,

wherein said outwardly extending resistance element comprises a disc having a central aperture,

said second contact being positioned about the wall defining said aperture in said disc,

said connecting means comprising a solder connection Ibetween said first and second contacts.

6. A coaxial attenuator in accordance with claim 5,

in which said disc is provided with an outer contact about the periphery of said disc,

and a pair of opposed bushings,

each of said bushings having a recess to receive opposed surfaces of said disc therein,

and means for Iixedly and electrically connecting said pair of bushings with said outer contact. 7. A coaxial attenuator according to claim 6, wherein said means for fixedly and electrically connecting said pair of bushings and said disc comprises a solder connection.

-8. A coaxial attenuator as in claim 7, wherein each of said pair of bushings is provided with a central through bore,

said central through bores receiving different portions of said central yresistance element therein in spaced relation to the Walls dening said central through bores.

9. A method for making a coaxial attenuator com* prising providing a -rod resistance element having at least one contact thereon,

and a disc resistance element having an aperture defined by a contact surface,

mounting said disc on said rod with a pre-formed solder ring received between the contact on said rod resistance element and the control surface on said disc,

and heating said solder ring to cause said solder to melt.

10. The method of claim 9,

including the further step of cooling said solder to cause said solder to solidify whereby said rod and said disc are Iixedly and electrically connected to each other.

11. The method of claim 9,

including, before heating said solder ring,

the further steps of providing a pair of bushings to receive the opposed faces of said disc,

placing the bushings about said disc with a preformed solder ring received therebetween,

heating the solder ring received between said rod and disc resistance elements and said bushings to cause said solder rings to melt,

and cooling said solder after it has melted to solidify said solder whereby said rod and disc and said disc and bushings are xedly and electrically connected to each other.

References Cited UNITED STATES PATENTS 3,174,123 3/1965 Frederico 333--81 2,994,049 7/ 1961 Weinschel.

2,968,774 1/1961 Rodriguez.

2,884,606 4/ 1959 Stevens.

2,884,595 4/1959 Stevens 324-95 2,820,952 1/ 1958 Hancock et al. 333--81 HERMAN K. SAALBACH, Primary Examiner C. BARAFF, Assistant Examiner U.S. Cl. X.R.

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