US2610250A - Electromagnetic-wave energyabsorbing material - Google Patents

Description

BP 9, 1952 H. A. WHEELER 2,610,250

ELECTROMAGNETIC-WAVE ENERGY-ABSORBING MATERIAL Filed Nov. 5, 1946 INVENTOR, HAROLD A. WHEELER,

ORNE

Patented Sept. 9, 1952 ELECTROMAGNETIC-WAVE ENERGY- ABSORBING MATERIAL Harold A. Wheeler, Great Neck, N. Y., assignor to I Hazeltineltesearch, Inc., Chicago, 111., a corporation of Illinois Application November 5, 1946, Serial No. 707,787

purely resistive wave impedance yet is capable of dissipating substantial amounts ofwave energy. In such cases, it may be desirable to absorb all or part of the wave energy. In the former category may i be classified wave-energy terminating loads and in the latter wave-energy attenuators.

Dissipation of either the electric-field or magnetic-field energy of a translated wave, however, causes a reactive component to appear in the otherwise purely resistive wave impedances of the wave-propagation path. The amount of the reactive component is proportional to the rate of attenuation per Wave length of the translated wave.- 1

In all such wave energy-absorbing arrangements, it has heretofore proven-exceedingly difficult to effect the desired wave-energy dissipation without at the same time causing at least some, and frequently a substantialamount, of waveenergy reflection by virtue of the presence of the energy-absorbing materialutilized. Such reflection: occurs if the wave impedance of the medium is notrpurely resistive but has a reactive component.

Oneprior proposed arrangement for dissipating the energy of an electromagnetic wave utilizes the so-called -lossy coaxial transmission line. This line includes at least a conductor having substantial resistance or a dielectric material having substantial dissipation regarded as shunt conductance. To avoidreflection of Wave energy atthe input terminals of the line, the line is caused to have a substantially purely resistive input impedance by making its rate of attenuation per .wave-length so small that the resulting reactive-component of its waveimpedance is negligible. .Thatis, such. lineshave heretofore been designed :for a small rate of attenuation so as to reduce the reactive component below :a permissible tolerance. No attempt has been made to balance thefltwo kinds of reactive components 5 Claims. (Cl. 17844) caused by the electric-field and magnetic-field energy dissipations. It is therefore necessary to employ a great length of ossy line where it is essential that a substantial part or all of the wave energy applied thereto is to be dissipated in the line. The resulting bulk and expense of such a long line is a serious disadvantage in many applications.

When the frequency of a wave signal becomes so high that transmission lines may no longer be conveniently used for wave propagation, a hollow conductor having parameters selected with relation to the wave length of the wave is conveniently used for this purpose. Energy-absorbing arrangements heretofore proposed for wave guides are in the, nature of strips of dielectric material or bodies of sand, either of which is coated or impregnated with a resistive material such as collodial graphite. The material is conveniently molded or otherwise shaped into a configuration so selected as to minimize reflection of wave energy caused by the presence of the material in the wave-propagation path.- This-result is accomplished by providing a rate of attenuation per wave length which is tapered from a low value to a high value; It is very difilcult to avoid such reflection in limited space, however, regardless-of the selected shape of the material; and this ,fact has heretofore necessitated the use: of some additional means for compensating or avoiding the efiect of the reflected Wave energy. Substantial values of energy dissipation in limited'space necessitate large values of energy attenuation per wave length of the translated wave, so large in fact that dissipation of either'the electric-field energy or the magnetic-field energy by itself would cause adetrimental amount of reactive component'in the wave impedance presented to the translated wave. It may be mentioned that reflected wave energy, unless compensated as mentioned, is very undesirable in many applications for the reason that it impairs the operation of wave-signal apparatus located along the wavepropagation path at points preceding the point of wave-energy reflection. Additionally, it is apparent that wave-energy reflection impairs the efficiency of those arrangements which are intended to absorb all of the wave energy supplied thereto.

It is an object of the present invention, therefore, to provide a new and improved electromagnetic-wave" nergy-absorbing material which avoids one or more of the disadvantages and limitations of prior such materials. 7

2 It is a further object of the invention to provide a new and improved electromagnetic-wave energy-absorbing material which is capable of dissipative wave-propagation path having a predetermined wave impedance, which is capable of absorbing substantial amounts of wave energy yet has a substantially purely resistive war/aim pedance equal to that of the non-dissipative wave- "propagation path." i In accordance with a particular form of the invention, an electromagnetic-wave energy-absorbing material, adapted to be interposed in a wave-propagation path of which at leasta por- I tion has a predetermined ratio of dielectric c'on-' stant to magnetic permeability for a predetermined mode ofv wave propagation therethrough comprises a quantity of'dielectric material anda quantityof high-conductivity material consist ing of particles having at least one-dimension which is small in relation to the wave length of an electromagnetic wave to be translated thereby. The relative quantities of the 'aforesaiddielectric and high-conductivity materials are soproportioned with relation to their dielectric constants and magnetic permeabilities as to provide, for the above+mentioned predetermined mode of Wave ropagation ztherethroirgna and :along the efere said-portion of saidfp'ath, a ratio of effective dielectric constant to effective magnetic permeability for; the energy-absorbing material 'substantially equal toythe above-mentioned predetermined'ratio. "The energy-absorbing material inchides a, substantial quantity of high-resistance materialdisp'ersed in .at least a portion ofthe .aforesaidzdielectrie material consisting "ofparticles having at least. one dimensionsma'll in m lation tov the wave-length of the translated vv'ave and..-efiective. to cause substantial dissipation of li'heielfitlic energy of the translated'wave.

*For a betterunderstanding of the present 'inv'ention, together with other and further objects thereof, eferenc is had. to the following description 'takenin-eonnection with the accompanying drawing; and. itsl'scopewillzibe pointed out in the appe ded c a ms 2* r "Referring now'to the drawing, Fig; '1 illustrates an electromagnetic-wave energy-absorbingmaterialie'mbodying the present invention and 11121- lized inia. particularapplication; Figs. 2 and-3 i1 lustratemodified forms. of :the invention; and Fig. 4 illustrates the use of: the material embodyingnthe. invention. in anotherapplication; Referring noware more particularly tjoIFigL 1 there iisxillustratcd a, high-frequency electroma'g neti'crewave energy-absorbingmaterial embodying thepresentinvention inaparticular form. En' ergy absorbing material to embodying the presentinventionis here shown by way of example as utilized in .a coaxialctransmission line which in-'- cludes anouter conductor and an inner con ductor l2, the latter being supported by the material, 1H3 'incoaxialrelation with the conductor H11 Thegtransmission line 11,- F2 is essentially a wave 'signa-l propagating device: adapted to effect propagation of highefrequencya electromagnetic posed in thewave-propagation pathfland com T material It.

in, the former conductive particles, for example combined iron or any comminuted conductive material such as copper, brass, silver, 7 j aluminum, etc. Iron','of 'course, is recognized not --'-only'as a conductive material but also as a mag- 4 prises three materials; namely, dielectric material, conductive material and resistive material.

The conductive material is dispersed in at least a portion of the dielectric material and is of such configuration and conductivity as to cause dissipation of wave energy in substantial amount in a givenimanner with reference to"thejdensity of the magnetic energy of a'n'electr'omagnetic wave translated through the energy-absorbing This conductive material may be neticinaterial'inthat it has a coefficient of magneticpermeability greater than that of free space i Copper, silver, or the like are conductors not referred to as magnetic materials, but particles thereof cause dissipation in accordance with the density of. magnetic energy inia waver; Usually the given manner. of energy, dissipation to which reference: was last made'is a proportional one. 'I'h'atis; the; dissipation of the wave energy'bythe conductive material is usually proportional to the density of the magneticenergy of the translated electromagnetic wave. v

- Theiparticles of the conductive material of the energy absorbing material I!) are individually insulated and may be held together ,in a compact mass by a suitable dielectric binder.

The resistive material of, the energy-absorbing material lli'also is dispersed in atlea'st. a portion of. the dielectric material previouslymentioned andvis of such configuration and resistivityasto cause. dissipation of. I wave energy in substantial amount and in the aforementioned given manner, with'reference torthe'density of the'electric field of that wave translated through the energy-absorbingniaterial l0. This resistive material: also may be: in. the form of 'individuallyinsulated: particles, fol-example =powderedcarbon or graphite, one preferred form being colloidal graphite.v The relative concentrations of. the conductive and re sistive materials areso proportioned as to -cause substantially. equal amounts of. the magneticfield-energy 'idissi-pationniand the, electric-field:- energy dissipation of the translated electromagnetic ..wave.- This causes the wave impedance; of

the:energy absorbingmaterial If]. to be substantially purely resistive, because; the two, kinds of.

dissipation tend "to. cause opposite reactive com;- ponentslinthe wave impedance.

The dielectric, ;conductive and resistivematerialsma'y be mixeditogetherin the-proper quane titles :into an isotropic ;-'mixture and are then molded,fcast'orcompresseddnto1a. rigid mass of configuration suitable for the use intended. 'lhus in a coaxial transmissionline of-the type shown in: -Eig-.: l; the energy absorbing;material: l 0 may;, after proper mixing of its ingredient materials,

be molded'or. compressedinto apertured cylindrii- I cal; blocks Y or discssuitable for. insertion between the linejconductors :l I Hi.v Alternatively, the material l0; may be extruded. upon the-line con,- ductor l;2 after which ;the ;line conductor 'HJmay be applied. overthe material 1:0 as a woven braid or. tube. t When the energy-absorbing material 1.0a

isrforme'd as described, theiparticle's of conductive and resistive: materials. are hleld in solid'zsusp'ene sionaf in, "and "dispersed throughout, the: dielectric. materialso that the latter provides individual-insulation-ior thep'articles. I a .The; particle si'zeand p'article spacingfor an isotropicmedium must be much less than :airadian length. of the translated wave and, of course, the density must be substantially uniform throughout the medium. Particles having a radius of the order of one radian length in the material of the particle are perhaps somewhere near the optimum. In the case of metals and graphite, one radian length is equal to the depth of penetration in the material. Where the material is not isotropic, as a laminated form ofthe material presently to be described, the particles of conductiveand resistive materials should nevertheless have somedimension much less than a wave length of the translated wave signal. For magnetic material, the dimension along the vector of the electrical field of the wave should be small; for resistive material, the. dimension along the vector of the magnetic field should be small. The term particle as used in the present specification and. claims is thus intended to mean pieces of material having sizes small in relation to the wave length of the translated wavesignal. Forvery long wave lengths, the magnetic material might thus .be scrap iron, sheets of con-. ductive material, and the like.

A Inthe operation of a transmission line of the type illustrated, electromagnetic wave energy whenapplied to one end of the line is propagated between the line conductors and travels along the line. The magnetic field of the electromagnetic wave sets up eddy currents in the conductive particles of the material and these currents dissipate some of the magnetic-field energy of the translated wave. The wave thus experiences what may be considered equivalent to a series resistance to its propagation through the material I0. The particles of resistive material included in the energy-absorbing material I0 cause, on the other hand, dissipation of some of the. electric-field energy of the translated wave so that the resistivev material may be considered equivalent to a shunt conductance between the line conductors ll, 12.

In this type of transmission line, it maybe desirable to use such quantities of the dielectric, conductive and resistive materials as to provide a desired ratio of effective inductance to eifective capacitance for the line, in which case the invention contemplates a relative concentration of conductive and resistive particles to provide a substantially equal ratio of equivalent series resistance to equivalent shunt conductance. When this is done, the transmission line has a wave impedance which is substantially purely resistive. This result is obtained over a wide range of electromagnetic-wave frequencies if the particles are sufiiciently small and closely packed. Thus if the particles are so small that the radius of each particle is less than one radian length in the material of the particle, the resulting attenuation caused by the particle is proportional to the square of the frequency. If both kinds of attenuation are obtained by particles small enough to meet this requirement, it follows thatboth kinds of attenuation are proportional to the square of the frequency and, therefore, the equality of the electric field and magnetic field dissipations is maintained over a large frequency range. It fails only to the extent that other causes of attenuation become more important at lower frequencies, such as the resistance of the conductor in a transmission line. the power factor of the dielectric binder, etc. When'the material .ln has apurely resistive wave impedance, it is capable of dissipating a large part of the electromagnetic wave energy in a small distance without causing reflection.

. It frequently is desirable that the energy-ab sorbing material of the present invention be interposed in a wave propagation path of which at least a portion has a predetermined ratio of dielectric constant to magnetic permeability which determines its wave impedance. Such a path may be non-dissipative and so have a purely resistive wave impedance. This need may occur, for example, whenf a transmission line of the type shown in Fig. 1 is coupled to a transmission line which utilizes an air dielectric; between its conductors. An air-dielectric line has a unity ratio of its'dielectric constant to magnetic permeability,'if both are expressed relative to free space. If this ratio ofdielectric constant to magnetic permeability is not maintained along the entire length ofthe wave-propagation path, reflections of wave energy occur at those points where there is experienced achange of the ratio mentioned. This is undesirable in many applications for wellknown reasons.

Fig. 2 illustrates a form of transmission line of the balanced type having spaced'conductors H, I2. The energy-absorption material I0 used between the line conductors has such composition that most desired ratios of dielectric constant to magnetic permeability may be readily obtained. The method here shown of forming the material 1.0 for, use in a balanced'line may also be utilized forracoaxial line of the Fig. 1 type. The ma terial I0." here includes a layer of dielectric material :3 having a lamination thickness a1, a dielectric constant 101, and a'coemcient of magnetic permeability n. It also includes a contiguous layer of conductive material I 4 in the form of individually insulated conductive particles held together' by a suitable'dielectric binder. This layer of conductive material 14 has a lamination thickness (12, a constant lea-and a magnetic permeability ,u2. An electromagnetic wave has its electric and magnetic fields normal to each other. A wave of this type is propagated through the material ID with such polarization that the magnetic field H is parallel to the layers of the materials I3, l4 and the electric field E is normal to the layers, as indicated by the arrows H and 'E in Fig. 2.

' "The laminated construction here utilized is disclosed and claimed in applicants copendingapplications Serial No. 563,716, filed November 16, 1944, now Patent No. 2,508,479, issued May 23, 1950, entitled -High-Frenquency Electromagnetic-Wave Translating Arrangement}? and Serial No. 563,715, filed November 16, 1944.,now Patent 2,511,610, issued June 13, 1950, entitled High-Frequency Electromagnetic-Wave Translating" Element, both assigned to the same assignee as the present application. As there explained, the thickness in of the layer of dielectric material [3 and the thickness (12 of the layer of conductive material M are selected in relationto the dielectric constants I01 and kg and magnetic permeabilities 1 and ,u2 as to provide for the material I 0' a'ratio of effective dielectric constant to effective magnetic permeability of the desired value. For reasons explained in applicants aforementioned copending applications, the maximum lamination thickness of the dielectric material l3 and of the conductive material I 4 should be much less than one radian length of the translated electromagnetic wave. As in the arrangement of Fig. 1, resistive material in the form of particles is dispersed in either or both of the dielectric mate'r'i'al l3 and conductive material l4. Also as in Fig. 1, the quantity of the resistive material is such as to cause an amount of electric-fleld-energy dissipation equal .to the amount of magnetic- :field-energy dissipation caused by the .magnetic conductive material duringthe propagation of the electromagnetic wave through the material Fig; 3 illustrates an energy-absorbing material 1.0" which isessentially similar to that of Fig. 2,

similar elements .and materials being designated by :similar reference numerals, except that the material 10" is formed of alternating laminations of dielectric material Island conductive material zl 4having quantities relatively proportioned dielectric constant to effective magnetic permeability. As in Fig. :2, the maximum lamination thickness of the dielectric and conductive materials should be much less than one radia length of the translated wave signal.

.When the energy-absorbing material of the present invention is interposed in a wave-propagation .path and provides unity ratio of the effective dielectric constant to effective magnetic permeability as is'characteristic of free space, the wave-propagation velocity through the material is less than that of free space." Fig. 4 represents aneapplication of the present invention in which it'zis desired that the wave-propagation path through the. portion thereof occupied by the energy-absorbingmaterialhave an effective electrical. length greater than its physical length by virtue of the reduced propagation velocity mentioned In this arrangement, the energy-absorbingtm'aterial IO is of cylindrical cross section and is positioned on'thexinner conductor l2 of the transmission line I I, I2. Assuming theline to' be air .filled, uniformity 'of impedance along *the transmission line is maintained when the radial thickness of the material IO has a value given by the relation:

where rthe eficctive magnetic permeability of the Tmaterial of element 1' kzthe effective dielectriccons'tant of the materialof element 40. v

The length of the material is selected, of course, to providethe desired increase of electrical length of the transmission line. As before, the relative concentrations of the conductive and resistive materialsprovided in the energy-absorbing material IO' are'such as to cause substantially equal amounts of the magnetic-field-energydissipation and electro-field-energy dissipationso that the wave impedance-0f the transmission line is substantiallypurely resistive.

From the above description of the invention, it will be'apparent that an elebtromagnetic-wave energy-absorbing material embodying the present' invention is adapted to provide, in a limited space, substantial-absorption of electromagneticwavezenergy yet is one whic'h'has a substantially purely resistive wave impedance and thus does not cause reflection of wave-signal energy. The

I magnitude of attenuation of which the material yet maintaining the wave impedance of: the ma-' asinr ig. 2 to 'attaina desired ratio of effective 7 B te'r ial substantially purely resistive. For example, it'is easily possible to attain with the material of the present invention an attenuation of about one napier per .radian'length or decibels per wave length While-maintaining the net reactive component of wave impedance of the material to within a few vper cent. of the resistive component;

with-only one kind of dissipatiomthat is dissipa- .tion of only the electric-field energy or the magnetie-field-energy of the translated wave signal,

While there have been described whatfare tat presentzconsidered to :be the preferred, embodiments of this inventiomit will be jobviouszto those skilled in the art that various changesandmodifications may be made therein without departing from the invention, and it is, therefore, aimed tin the appended claimsto cover all :such changes and modifications as fall within the true spirit andiscope of the invention. l e What is claimed'is:

g 1. An electromagnetic-wave energy-absorbing material, adapted to beinterposed in a "wavepropagation pathof which at least a portionhas a predeterminedratio of dielectric constant to magnetic permeability fora predetermined-mode of wavepropagation therethrough comprising: a quantity-ofdielectrical material; a quantity of high-conductivity material consisting of particles having at least one dimension which is small in relation to'the wave length ofanelectromagnetic wave to betranslated thereby; the relative quantities of said'materials being so proportioned with relation to their dielectric constants, and magnetic permeabilities as to provide, for

material adapted'to be interposed in a wavepropagation path ofwhich. at least a portion has a 1 predetermined ratio ;of dielectric constant to magnetic permeability and a -substantially purely resistive wave impedance for a predete'r mined'mode of wave propagation therethrough comprising: a ciutntity of dielectric material} a substantial quantity of high-conductivity mate-- rial consisting of individually insulated particles having maximum dimensions which are small in relation-tethe wave length of'an electromagnetic wave-to be translated thereby and-effective to cause substantial'dissipation or the magnetic energyor said translated wave the relative quan- -tities of said materials being so proportioned with relation to their dielectric constants and magnetic permeabilities as to provide, for said predetermined mode of wave propagation therethrough and along said portion of said path, a ratio of efiective dielectric constant to effective magnetic permeability for said energy-absorbing material substantially equal to said predetermined ratio; and a substantial quantity of high-resistance material dispersed in at least a portion of said dielectric material consisting of individually insulated particles having maximum dimensions small in relation to the wave length of said translated wave and effective to cause substantial dissipation of the electric energy of said translated wave.

3. An electromagnetic-wave energy-absorbing material, adapted to be interposed in a wavepropagation path of which at least a portion has a predetermined ratio of dielectric constant to magnetic permeability and a substantially purely resistive wave impedance for a predetermined mode of wave propagation therethrough comprising: a quantity of dielectric material; a quantity of high-conductivity material consisting of individually insulated particles having maximum dimensions of the order of one radian length of an electromagnetic wave to be translated thereby; the relative quantities of said materials being so proportioned with relation to their dielectric constants and magnetic permeabilities as to provide, for said predetermined mode of wave propagation therethrough and along said portion of said path, a ratio of effective dielectric constant to effective magnetic permeability for said energyabsorbing material substantially equal to said predetermined ratio; and a substantial quantity of high-resistance material dispersed in at least a portion of said dielectric material consisting of individually insulated particles having maximum dimensions of the order of one radian length of said translated wave and efiective to cause substantial dissipation of the electric energy of said translated wave.

4. An electromagnetic-wave energy-absorbin material adapted to be interposed in a wave-propagation path of which at least a portion has a predetermined ratio of dielectric constant to magnetic permeability for a predetermined mode of wave propagation therethrough comprising: at least one layer of a quantity of dielectric material; at least one layer of a quantity of highconductivity material contiguous to said firstmentioned layer and comprising individually insulated particles having maximum dimensions which are small in relation to the wave length of an electromagnetic wave to be translated thereby; the relative quantities of said materials being so proportioned with relation to their dielectric constants and magnetic permeabilities as to provide, for said predetermined mode of wave propagation therethrough and along said portion of said path,

a ratio of effective dielectric constant to efiective magnetic permeability for said energy-absorbing material substantially equal to said predetermined ratio; and a substantial quantity of highresistance material dispersed in at least a portion of said dielectric material consisting of individually insulated particles having maximum dimensions small in relation to the wave length of said translated wave and effective to cause substantial dissipation of the electric energy of said translated wave. 7

5. An electromagnetic-wave energy-absorbing material, adapted to be interposed in a wavepropagation path of which at least a portion has a predetermined ratio of dielectric constant to magnetic permeability for a predetermined mode of wave propagation therethrough comprising: at least one layer of a quantity of dielectric material having a maximum thickness much less than one radian length of an electromagnetic wave to be translated thereby; at least one layer of a quantity of high-conductivity material, contiguous to said first-mentioned layer, comprising individually insulated particles having maximum dimensions which are small in relation to the wave length of said translated wave and having a maximum thickness much less than one radian length of said translated wave; the relative quantities of said materials being so proportioned with relation to their dielectric constants and magnetic permeabilities as to provide, for said predetermined mode of wave propagation therethrough and along said portion of said Path, a ratio of effective dielectric constant to effective magnetic permeability for said energy-absorbing material substantially equal to said predetermined ratio; and a substantial quantity of high-resistance material dispersed in at least a portion of said dielectric material consisting of individually insulated particles having maximum dimensions small in relation to the wave length of said translated wave and eifective to cause substantial dissipation of the electric energy of said translated wave.

HAROLD A. WHEELER.

REFERENCES CITED The following references are of record in the file of this patent:

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