US3704434A - Skin effect rf bridge filter - Google Patents

Abstract

A filter in a bridge configuration having different types of iron coated conductors. The device excludes current from a load by utilizing the skin effect to attenuate RF energy. The bridge is unbalanced at low frequencies allowing low frequency energy and direct current to pass relatively unhindered.

Description

United States Patent Schlachter 5] Nov. 28, 1972 [54] SKIN EFFECT RF BRIDGE FILTER 3,191,132 6/1965 Mayer ..333/79 72 Inventor: Dohald A. Schlachter 222 Braehead 3,329,911 7/1967 Schlicke et a1. ..333/79 D i Fredericksburg, V3 22401 2,238,915 4/1941 Peters et 31 ..333/79 3,227,974 1/1966 Gray ..333/79 [22] Filed: April 20, 1971 [21] Appl. No.: 135,731 Primary Examiner,I-Ierman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. 52] US. Cl. ..333/75, 333/79, 333/74 s and wamn [51] Int. Cl. ..H03h 7/10, H03h 9/00 581 Field of Search ..333/74, 75, 79; 332/51 [57] ABSTRACT 1 A filter in a bridge configuration having different [56] v References C'ted types of iron coated conductors. The device excludes UNITED STATES PATENTS current from a load by utilizing the skin effect to attenuate RF energy. The bridge is unbalanced at low 3,425,004 1/ 1969 Warner ..333/79 frequencies allowing low frequency energy and direct 3,435,259 3/1969 Mette ..332/51 x current to pass relatively unhinderm 3,309,633 3/1967 Mayer ..333/79 3,125,733 3/1964 Holinbeck ..333/79 15 Claims, 3 Drawing Figures Pmmtnuovz m2 3.704.434

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ATTORNEY I SKIN EFFECT RF BRIDGE FILTER STATEMENT OF GOVERNMENT INTEREST BACKGROUND OF. THE INVENTION The present invention relates to filters and more particularly to filters for protecting electroexplosive devices from RF energy.

Most low pass filters use combinations of resistive,

capacitive and inductive elements in circuit configurations other than bridge circuits. Tee, el, and pi networks are most commonly used, as are lossy dielectrics in broadband applications. These filters are unacceptable for protecting electroexplosive devices from spurious RF energy because their performance depends upon their ability to cause impedance mismatch between generators and loads. Impedance mismatch cannot be assured with these filters because in protecting electroexplosive devices the generator impedance cannot be specified. The impedance of spurious electromagnetic generators may be any value from a few tens of ohms to megohms and will generally be complex.

Other known devices for protecting ordnance from RF radiation hazards employ 'an electrostatic shield around the load while providing a low resistance path to the firing circuit wires or use a transformer with a thin metal barrier to separate the primary and secondary windings and thus reduce the RF power transmission. While these devices are effective they suffer from the disadvantage of being bulky and more costly than the ordinary filters.

SUMMARY OF THE INVENTION The general purpose of this invention is to provide a much simplified approach to a device which excludes current from a load and thus can be used to protect electroexplosive devices from damage by spurious RF energy. The novel filter is very uncomplicated in construction and can be produced at a very low cost. The filter is in the form of a Wheatstone bridge or lattice network which uses resistive elements composed of a metal conductor coated with a second metal. At a predetermined frequency the filter, by utilizing the skin effect, becomes a balanced bridge and thus excludes current from a load. By suitable construction the invention can exclude either high or low frequency current from a load.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a filter which excludes current from a load at certain frequencies.

Another object of the present invention is to provide a filter which can be constructed to attenuate either high or low frequencies.

Still another object of the invention is to provide a low cost and uncomplicated filter which can protect electroexplosive devices from spurious RF energy.

Yet another object of the present invention is to provide a filter for protecting electroexplosive devices which can be made small enough to be an integral part of the firing mechanism.

Still a further object of the presentinvention is to provide a filter to reduce the detrimental effects of the rectifying action of RF arcs and glow discharges. 7 Other objects, advantages and novel features of the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates the skin effect RF bridge filter.

FIG. 2 shows a cross-section of the resistive elements used in the skin effect bridge filter. t

FIG. 3 illustrates the bridge elements disposed in a symmetrical fashion. t

DETAILED DESCRIPTION OF THE PREFERRED EMBODIM NT The invention is a device which is designed toutilize the well known skin effect to attenuate RF energy.

The skin effect is a property which is known in the art but is usually only incidental to a particular filter and not part of the design. When an alternating current flows in a conductor, the alternating magnetic flux within the conductor produces an emf which is greatest at the center of the conductor and decreases toward the outer skin. This induced emf causes the current density to be greater at the surface of the conductor and to decrease towards its center. As the frequency of the alternating current is increased, the current within the conductor becomes increasingly concentrated toward the outer skin. This phenomenon is known as the skin effect. The skin effect is responsible for the fact that the resistance of a conductor increases continuously with frequency while its internal inductance decreases continuously. The change in resistance and internal inductance becomes very pronounced for frequencies above a minimum value which depends on the conductors size, magnetic permeability, and conductivity. t

The skin depth is defined as the distance measured inwardly from the surface of the conductor in which the current in the conductor will decrease by l neper, i.e., the current density becomes l/e times the density at the surfaceof the conductor, where e is the natural logarithm base. The skin depth is given by the formula: 6= VZIwuE where w is equal to 21rf and is the angular frequency;

,1. is the permeability of the material, and

E is the conductivity of the material.

If a metal conductor is coated with a second metal, then for sufficiently high frequencies the current in the coated conductor will be confined primarily to the coating material due to the skin effect. When this is the case, there is a negligible difference in impedance between the coated conductor and a similar solid conductor made from the coating material. The elimination of hazards of RF energy to electroexplosive devices requires the protection of the electric firing circuits from randomly induced energies at all frequencies above a nominal frequency in the region of l0KI-Iz to IOOKI-lz FIG. 1 is a schematic of the resistance elements arranged in a Wheatstone bridge or lattice network configuration. Resistance elements R,, are composed of a ferromagnetic coating on a high resistance core. Resistance elements R, are composed of a ferromagnetic coating on a low resistance core. E is asource of electrical energy and L is an electroexplosive device or other load.

The bridge is unbalanced at DC and low frequencies due to the different low frequency resistance of R and R,,. As the frequency is increased a larger fraction of the total current is confined to the coating by the skin effect. At frequencies for which the thickness of the coating is approximately equal to the skin depth, the resistance as well as internal inductance of the elements R and R become equal. Given external symmetry the bridge is then balanced, and current through the load is cut off.

FIG. 2 shows a cross section of the resistance elements R, and R,,. In the preferred embodiment R has a high resistance core 12 of No. 36 Nichrome wire one inch long with an ironcoating l0. Nichrome is the trade name for a nickel-chromium alloy wire. R has a I low resistance core l4of No. 36 copper wire 1 inch long with an iron coating 10. The resistance elements could be constructed by threading a conductor within a ferromagnetic tube. The ratio of the radius of the resistance elements 16 to the skin depth 8 can vary over a wide range. For example a ratio of 5&1 is an acceptabl value.

The embodiment shown would have an efficiency of about 50 percent at low frequencies, and an attenuation of about 40db at high frequencies given that the effective resistances of the conductors at high frequencies are held within 2% of equality. The attenuation is, of course, proportional to the accuracy to which the four conductors are matched in effective resistance.

The ferromagnetic coating could also be nickel, cobalt or a ferromagnetic alloy. An alternate material for the high resistance core would be constantan or some other high resistance material. v

The advantage of using a ferromagnetic metal for the coating is that they have a high permeability and a skin depth shorter than paramagnetic and diamagnetic solid 52% nickel-iron alloy wire for the high resistance arms R The low resistance arms are formed from a similar nickel-iron alloy having a copper core occupying about one-third of the total wire diameter. All four bridge arms are twisted together in a spiral to form a single cable about 2 feet long. The twist is intended to give mechanical strength and flexibility as well as to balance the circuit to external magnetic fields. This embodiment is useful to connect the firing circuit contact on the aft end of a rocket motor to the electric squib in the forward end. r

Other alternative constructions for the bridge filter are possible by selecting suitable materials for the core and coating and by varying the coating thickness, bridge wire outer diameter, and the length of the bridge arms. The high resistance elements of the bridge could be iron tubing or iron coated dielectric filaments. lrori alloys to vary or optimize permeability, conductivity, and Curie point could be used, as well as conductive ferrites toadapt the filter to higher r'esistanceyloads. In the latter case, the bridge elements R would be ferrite tubes, the two low'resistance arms R having metal conductors through the center of the tube. Also, with larger conductors, lower cutoff frequencies and higher efficiencies would be feasible and the filter could be used in the form of an attenuating'cable or transmission line.

With similar constructions, high pass and notch filters are also possibleFor instance, two Nichrome wires and two iron wires with equal DC resistances would be balanced at DC and low frequencies, but as the skin effect occurred the bridge would become unbalanced, passing high frequencies. Also two or more filters could be connected in cascade to optimized attenuation in certain frequency bands or optimize power dissipation characteristics.

To enchance power dissipating capabilities, the

bridge elements could be embedded in a low melting.

point material with high heat of fusion. If this material is metallic, it could be arranged to short out when it melts, giving a fail safe property. A fusible series element would also accomplish this.

The filter could also be constructed to give a visible alarm indication by encasing the conductors in a glass envelope. For this embodiment the element heat to incandescence and the core materials should be selected so that at the Curie temperature of the magnetic coating the resistivities of the core materials are equal, maintaining balance even though the skin effect is reduced. This technique would be useful also to balance the bridge for DC and low frequencies at a critical RF level, so that the DC componentoof RF arcs would not present a hazard. Platinum and constantan should do this at about 900 Centigrade, as well as iron and Nichrome at some other temperature.

Thus there has been disclosed a skin effect bridge filter which has the advantages of simplicity of construction, low cost and is capable of providing reasonable DC efficiency while maintaining high RF attenuation. Obviously many modifications and variations are possible in the light of the above teachings.

What is claimed is:

l, A skin effect filter for excluding current from a load comprising: e

a load from which high frequency current is to be excluded; and

a plurality of resistive elements connected in the form of a Wheatstone bridge with said load connected to diagonally opposite junctions of said bridge;

each of said resistive elements comprising a central core of conductive material coated with a second conductive material;

the central core of a first two opposed arms of said bridge having a higher resistance than the centra core of a second two opposed arms;

whereby the bridge is unbalanced at low frequencies and, due to the skin effect, balanced at high frequencies to exclude current from said load.

2. The filter of claim 1 wherein the first two opposite resistive elements have a central core comprised of a nickel and chromium alloy; and

the second two opposite resistive elements have a central core of copper.

3. The filter of claim 2 wherein the thickness of the conductive coating is equal to the skin depth.

4. The filter of claim 3 wherein the conductive coating is a ferromagnetic material.

5. The filter of claim 3 wherein the ferromagnetic material is iron.

6. The filter of claim 3 wherein the ferromagnetic material coating is nickel.

7. The filter of claim 3 wherein the ferromagnetic material is cobalt.

8. The filter of claim 1 wherein the resistive elements of the first two opposite arms are comprised of ferrite tubes; and

the resistive elements of the second two opposite arms are comprised of ferrite tubes having a metal conductor through the center.

9. The filter of claim 1 wherein the resistive elements are constructed to have an equal resistance above a frequency of about lOKHz.

10. The filter of claim 8 wherein the load is an electroexplosive device whereby the filter provides protection from randomly induced RF energy.

'11. The filter of claim 1 wherein the first two op- 12. The filter of claim 1 wherein the resistive elements of the first two opposite arms are solid conductors; and

the resistive elements of the second two opposite arms have an outer portion of the same material as the first two opposite arms and core of a second conductive material.

13. The filter of claim 12 wherein the resistive elements of the first two opposite arms are comprised of a nickel-iron alloy.

14. The filter of claim 13 wherein the core of the second two opposite arms have a core comprised of copper.

15. The filter of claim 1 wherein the core materials for the respective arms are selected to maintain a balanced bridge at an increased temperature.

Claims (15)

1. A skin effect filter for excluding current from a load comprising: a load from which high frequency current is to be excluded; and a plurality of resistive elements connected in the form of a Wheatstone bridge with said load connected to diagonally opposite junctions of said bridge; each of said resistive elements comprising a central core of conductive material coated with a second conductive material; the central core of a first two opposed arms of said bridge having a higher resistance than the central core of a second two opposed arms; whereby the bridge is unbalanced at low frequencies and, due to the skin effect, balanced at high frequencies to exclude current from said load.

2. The filter of claim 1 wherein the first two opposite resistive elements have a central core comprised of a nickel and chromium alloy; and the second two opposite resistive elements have a central core of copper.

3. The filter of claim 2 wherein the thickness of the conductive coating is equal to the skin depth.

4. The filter of claim 3 wherein the conductive coating is a ferromagnetic material.

5. The filter of claim 3 wherein the ferromagnetic material is iron.

6. The filter of claim 3 wherein the ferromagnetic material coating is nickel.

7. The filter of claim 3 wherein the ferromagnetic material is cobalt.

8. The filter of claim 1 wherein the resistive elements of the first two opposite arms are comprised of ferrite tubes; and the resistive elements of the second two opposite arms are comprised of ferrite tubes having a metal conductor through the center.

9. The filter of claim 1 wherein the resistive elements are constructed to have an equal resistance above a frequency of about 10KHz.

10. The filter of claim 8 wherein the load is an electroexplosive device whereby the filter provides protection from randomly induced RF energy.

11. The filter of claim 1 wherein the first two opposite resistive elements are comprised of a nickel and chromium alloy wire; and the second two opposite resistive elements are comprised of iron wire having a DC resistance equal to the first two opposite resistive elements whereby the bridge filter is balanced at low frequencies and unbalanced at high frequencies.

12. The filter of claim 1 wherein the resistive elements of the first two opposite arms are solid conductors; and the resistive elements of the second two opposite arms have an outer portion of the same material as the first two opposite arms and core of a second conductive material.

13. The filter of claim 12 wherein the resistive elements of the first two opposite armS are comprised of a nickel-iron alloy.

14. The filter of claim 13 wherein the core of the second two opposite arms have a core comprised of copper.

15. The filter of claim 1 wherein the core materials for the respective arms are selected to maintain a balanced bridge at an increased temperature.

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