US3704434A - Skin effect rf bridge filter - Google Patents

Skin effect rf bridge filter Download PDF

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Publication number
US3704434A
US3704434A US135731A US3704434DA US3704434A US 3704434 A US3704434 A US 3704434A US 135731 A US135731 A US 135731A US 3704434D A US3704434D A US 3704434DA US 3704434 A US3704434 A US 3704434A
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Prior art keywords
filter
resistive elements
opposite
bridge
arms
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US135731A
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Donald A Schlachter
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H1/00—Constructional details of impedance networks whose electrical mode of operation is not specified or applicable to more than one type of network

Definitions

  • 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
  • FIG. 1 A first figure.
  • the present invention relates to filters and more particularly to filters for protecting electroexplosive devices from RF energy.
  • 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.
  • 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.
  • the invention can exclude either high or low frequency current from a load.
  • 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
  • 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.
  • FIG. 3 illustrates the bridge elements disposed in a symmetrical fashion.
  • 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.
  • 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.
  • 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.
  • 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.
  • E is the conductivity of the material.
  • 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,,.
  • 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.
  • 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
  • bridge filter 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.
  • 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.
  • 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.
  • bridge elements could be embedded in a low melting.
  • the filter could also be constructed to give a visible alarm indication by encasing the conductors in a glass envelope.
  • 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.
  • a skin effect filter for excluding current from a load comprising: e
  • each of said resistive elements comprising a central core of conductive material coated with a second conductive material
  • the second two opposite resistive elements have a central core of copper.
  • the resistive elements of the second two opposite arms are comprised of ferrite tubes having a metal conductor through the center.
  • the filter of claim 8 wherein the load is an electroexplosive device whereby the filter provides protection from randomly induced RF energy.
  • the filter of claim 1 wherein the first two op- 12.
  • 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.

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  • Coils Or Transformers For Communication (AREA)
  • Shielding Devices Or Components To Electric Or Magnetic Fields (AREA)
US135731A 1971-04-20 1971-04-20 Skin effect rf bridge filter Expired - Lifetime US3704434A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US13573171A 1971-04-20 1971-04-20

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US3704434A true US3704434A (en) 1972-11-28

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US (1) US3704434A (it)
JP (1) JPS5531644B1 (it)
AU (1) AU465741B2 (it)
CA (1) CA972814A (it)
DE (1) DE2218856A1 (it)
FR (1) FR2134987A5 (it)
GB (1) GB1341365A (it)
IT (1) IT951066B (it)
SE (1) SE376135B (it)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5065453A (en) * 1989-03-20 1991-11-12 General Electric Company Electrically-tunable bandpass filter
US5883565A (en) * 1997-10-01 1999-03-16 Harris Corporation Frequency dependent resistive element
US20160314876A1 (en) * 2015-04-23 2016-10-27 Corning Optical Communications LLC High-data-rate electrical interconnect cables

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2238915A (en) * 1937-10-13 1941-04-22 Titeflex Metal Hose Co Electric filter
US3125733A (en) * 1964-03-17 Transmission line having high attenuation for radiant
US3191132A (en) * 1961-12-04 1965-06-22 Mayer Ferdy Electric cable utilizing lossy material to absorb high frequency waves
US3227974A (en) * 1961-12-29 1966-01-04 Gray Reginald Irvan Radio-frquency interference guard in form of low-pass filter
US3309633A (en) * 1963-01-10 1967-03-14 Mayer Ferdy Anti-parasite electric cable
US3329911A (en) * 1963-02-25 1967-07-04 Allen Bradley Co Low transfer impedance capacitor with resistive electrode
US3425004A (en) * 1963-11-29 1969-01-28 Mc Donnell Douglas Corp Radio frequency energy attenuator
US3435259A (en) * 1965-05-13 1969-03-25 Us Army Filter circuit

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3125733A (en) * 1964-03-17 Transmission line having high attenuation for radiant
US2238915A (en) * 1937-10-13 1941-04-22 Titeflex Metal Hose Co Electric filter
US3191132A (en) * 1961-12-04 1965-06-22 Mayer Ferdy Electric cable utilizing lossy material to absorb high frequency waves
US3227974A (en) * 1961-12-29 1966-01-04 Gray Reginald Irvan Radio-frquency interference guard in form of low-pass filter
US3309633A (en) * 1963-01-10 1967-03-14 Mayer Ferdy Anti-parasite electric cable
US3329911A (en) * 1963-02-25 1967-07-04 Allen Bradley Co Low transfer impedance capacitor with resistive electrode
US3425004A (en) * 1963-11-29 1969-01-28 Mc Donnell Douglas Corp Radio frequency energy attenuator
US3435259A (en) * 1965-05-13 1969-03-25 Us Army Filter circuit

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5065453A (en) * 1989-03-20 1991-11-12 General Electric Company Electrically-tunable bandpass filter
US5883565A (en) * 1997-10-01 1999-03-16 Harris Corporation Frequency dependent resistive element
US20160314876A1 (en) * 2015-04-23 2016-10-27 Corning Optical Communications LLC High-data-rate electrical interconnect cables
US9983376B2 (en) * 2015-04-23 2018-05-29 Corning Optical Communications LLC High-data-rate electrical interconnect cables

Also Published As

Publication number Publication date
JPS5531644B1 (it) 1980-08-20
AU4096072A (en) 1973-10-18
CA972814A (en) 1975-08-12
FR2134987A5 (it) 1972-12-08
DE2218856A1 (de) 1972-11-09
IT951066B (it) 1973-06-30
AU465741B2 (en) 1973-10-18
SE376135B (it) 1975-05-05
GB1341365A (en) 1973-12-19

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