U.S. patent number 3,886,506 [Application Number 05/338,335] was granted by the patent office on 1975-05-27 for magnetically enhanced coaxial cable with improved time delay characteristics.
This patent grant is currently assigned to Hilabs Company. Invention is credited to Irving Duboff, Harold Lorber.
United States Patent |
3,886,506 |
Lorber , et al. |
May 27, 1975 |
Magnetically enhanced coaxial cable with improved time delay
characteristics
Abstract
A magnetically enhanced coaxial cable of the type having a
center conductor, a dielectric spacer around the center conductor
and a conductive shield around the dielectric spacer, said magnetic
enhancement comprising a thin layer of circumferentially oriented
uniaxial anisotrophic magnetic material having high conductivity
and high permeability adapted to transmit wave energy and
conventional current without being switched or reoriented. The
magnetic enhancement provides an increase inductance and current
carrying capacity without corresponding increases in time delay and
other losses associated with magnetic loading.
Inventors: |
Lorber; Harold (Dresher,
PA), Duboff; Irving (West Chester, PA) |
Assignee: |
Hilabs Company (Wilmington,
DE)
|
Family
ID: |
23324392 |
Appl.
No.: |
05/338,335 |
Filed: |
March 5, 1973 |
Current U.S.
Class: |
333/243;
178/45 |
Current CPC
Class: |
H01B
11/1808 (20130101) |
Current International
Class: |
H01B
11/18 (20060101); H01p 003/06 (); H01b
011/14 () |
Field of
Search: |
;178/45
;333/96,31R,24.2,24.1 ;340/174PW |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
296,114 |
|
Aug 1928 |
|
GB |
|
529,981 |
|
Dec 1940 |
|
GB |
|
Other References
Kehler & Coren; J. Appl. Phys. 41 pp. 1346-1347, March 1970.
.
Thomas, H. "Magnetically Controllable Delay Line," IBM Tech.
Disclosure Bulletin, Vol. 8, No. 11, 4-1966, pp. 1592-1593. .
English et al., "Integrated Transmission Lines for Magnetic Thin
Film Memories," IEEE. Trans. on Magnetics, Vol. MAG-1, 12-1965, pp.
272-276. .
Ferguson, E. T., "Uniaxial Magnetic Anisotropy Induced in Fe-Ni
Alloys by Magnetic Anneal," Jr. of Applied Physics, 29, 3-1958, pp.
252-253. .
Hoffman et al., "High Speed Digital Storage Using Cylindrical
Magnetic Films," Jr. of British IRE, 1-1960, pp. 31-36..
|
Primary Examiner: Smith; Alfred E.
Assistant Examiner: Punter; Wm. H.
Claims
we claim:
1. In a communication system having means for generating high
frequency electro magnetic energy pulses representing intelligence,
a coaxial cable for transmitting said pulses, and means for
receiving said pulses, said cable comprising in combination:
an electrical center conductor,
a magnetizable material at the outside surface of said center
conductor,
said magnetizable material being magnetically oriented to provide a
magnetic field which is substantially circumferentially around said
center conductor.
electrically insulating dielectric material disposed to cover said
center conductor,
an electrical outer conductor formed to provide a shield around
said dielectric material,
said center conductor, said outer conductor, and said dielectric
material disposed together to form said coaxial cable,
said magnetizable magnetic material being magnetically oriented in
a direction to provide a sufficient magnetic field within said
coaxial cable to enhance electro magnetic energy pulse propagation
velocity,
whereby high frequency electro magnetic pulses from the generating
means will travel through said coaxial cable means to said
receiving means faster than would occur without said magnetic
field.
2. A coaxial cable as set forth in claim 1 wherein said
magnetizable magnetic material is magnetically oriented to provide
an easy axis of magnetization in a direction which is substantially
circumferential around the outside of said center conductor.
3. An improved flexible miniaturized coaxial cable of the type
employed in transmitting very high frequency pulses having magnetic
loading comprising:
a flexible center conductor having an outside diameter d,
said flexible center conductor comprising a conductive magnetic
material at the outside diameter d having a high permeability
creating an increased resultant permeability .mu..sub.r in the
cable,
said conductive magnetic material having a crystalline grain
structure and having an easy axis of magnetization at least
partially oriented to provide an internal magnetic field in a
circumferential direction,
flexible dielectric material spacer means surrounding said center
conductor having an outside diameter D.sub.r and a dielectric
constant .epsilon., and
a flexible conductive shield supported on and surrounding the
outside of said dielectric spacer means and having an inside
diameter D whereby the characteristic impedance of said improved
flexible cable Z = 1/2.pi..sqroot..mu..sub.R /.epsilon. log D.sub.r
/d is increased by an increase in .mu..sub.r through magnetic
loading and the time delay T.sub.r is increased by an amount less
than .sqroot..mu..sub.R /.epsilon. by providing said internal
magnetic field.
4. An improved flexible coaxial transmission cable of the type set
forth in claim 3 wherein said conductive magnetic material
comprises nickel and iron.
5. An improved flexible coaxial transmission cable of the type set
forth in claim 4 wherein said conductive magnetic material
comprises Permalloy having very low magnetostrictive
characteristics.
6. An improved flexible coaxial transmission cable of the type set
forth in claim 3 wherein said center conductor comprises a thin
layer of magnetic material greater than 500A and less than 15,000A
thick and is thin enough to maintain uniaxial anisotropy in a
circumferential direction.
7. An improved flexible coaxial transmission cable of the type set
forth in claim 3 wherein said conductive magnetic material
comprises a thin layer having a thickness which is greater than the
skin depth thickness at the frequency of operation.
8. An improved flexible miniaturized coaxial cable as set forth in
claim 3 further characterized in that said center conductor
comprises a very good conductive magnetic material center and an
outside layer of conductive magnetic material having a preferred
magnetic field oriented in a circumferential direction.
9. An improved flexible coaxial transmission cable for use in the
transmission of very high frequency pulses of the type having
magnetic loading for decreasing attenuation by increasing the
inductance L.sub.o and the characteristic impedance Z.sub.o,
comprising:
a flexible inner center conductor of low resistance material having
an outside diameter d,
a flexible outer shield of low resistance material surrounding said
center conductor and having an inside diameter D,
a flexible dielectric spacer disposed between said center conductor
and said shield, said elements forming a flexible coaxial cable
having a characteristic impedance Z.sub.o = .sqroot. L.sub.o /C and
a time delay T.sub.o = .sqroot. L.sub.o C, where C is the shunt
capacitance,
a layer of flexible circumferentially oriented high permeability
magnetic material formed on the outside of said center conductor,
said elements now forming a coaxial cable having a resultant
characteristic impedance Z.sub.r = .sqroot.L.sub.R /C where the
resultant inductance L.sub.r > L.sub.o, and the resultant time
delay T.sub.r of very high frequency pulses is substantially less
than .sqroot.L.sub.r C.
10. An improved flexible coaxial transmission cable as set forth in
claim 9 wherein said layer of high permeability magnetic material
is electrically conductive nickel-iron having a thickness between
500A to 15,000A and is uniaxial anisotropic.
11. An improved flexible coaxial transmission cable as set forth in
claim 10 wherein said layer of circumferential oriented magnetic
material and said center conductor form concentric conduction paths
for high frequency pulse current concentrated in a skin effect
pattern at high frequencies.
12. An improved flexible coaxial transmission cable as set forth in
claim 11 wherein the current flow in said conduction paths is
substantially proportional to the conductivity of the conduction
path.
13. An improved flexible coaxial transmission cable for use in the
transmission of very high frequency pulses comprising:
a flexible inner center conductor of low resistance material having
an outside diameter d,
a flexible outer shield of low resistance material surrounding said
center conductor and having an inside diameter D,
a flexible dielectric spacer between said conductor and said
shield, and
a layer of magnetic material formed at the outside of said center
conductor and having a high permeability and an easy axis of
magnetization oriented in a circumferential direction,
said center conductor and said layer of magnetic material formed to
provide a conduction path for conventional current,
said layer of magnetic material providing a path for the conduction
of very high frequency pulse energy, whereby said coaxial cable is
rendered capable of transmitting more energy at higher wave
propagation rates than conventional coaxial cables.
14. An improved flexible coaxial transmission cable as set forth in
claim 13 wherein said layer of magnetic material is a low
magnetostrictive Permalloy plated over a plated conductive base on
said center conductor.
15. An improved flexible coaxial transmission cable as set forth in
claim 13 wherein the usual D/d ratio for unloaded coaxial cable is
reduced by at least thirty percent without a change in the
characteristic impedance Z by the addition of said layer of
magnetic material.
16. An improved flexible coaxial transmission cable as set forth in
claim 15 wherein the D/d ratio is reduced by decreasing the inside
diameter D.sub.r of said outer shield and the thickness of said
wave-sustaining dielectric spacer.
Description
BACKGROUND OF THE INVENTION
This invention relates to coaxial transmission cables for use in
transmission of both conventional current and electromagnetic
waves. More particularly, this invention relates to a magnetically
loaded transmission cable which has greater current carrying
capabilities that previously known cables of equal size or
alternatively the novel magnetically loaded transmission cable is
capable of being made smaller in size and volume than previously
known cables without causing increases in losses or time delay
normally accompanying magnetic loading.
Heretofore, it was known that magnetically loading a coaxial
transmission cable would increase the inductance of the line. As
the inductance increases, characteristic impedance and the time
delay increases in conventional coaxial cables. Heretofore, it was
desirable to increase the inductance by magnetic loading to
decrease attenuation even though magnetic loading introduces
hysteresis and eddy current losses and causes time delays.
Magnetic loading materials spaced in the dielectric of coaxial
cables are described in U.S. Pat. Nos. 2,787,656; 2,929,034 and
2,727,945. These and other magnetic loading teaching are generally
concerned with high resistance and high permeability ferrities
which by their structural nature are large or relatively thick and
are not applicable for use in microminiature cables. Thin film
magnetically coated conductors have been employed in plated wire
memory planes such as those shown in U.S. Pat. No. 3,460,114,
however, such rigid planes are generally concerned with reducing
the characteristic impedance of an unshielded insulated memory
wire. Plated memory wires are not uniformly shielded and are
designed to switch from one state to another by coupling magnetic
fields.
There has long been a need to reduce the size and weight of
flexible coaxial cables. Our co-pending application Ser. No.
266,345 filed June 26, 1972 for "Improved Coaxial Cable" solves
many of these problems. These and prior art structures were
accompanied by increases in time delay. Coaxial cable for high
speed computers and communications equipment require minimized time
delays as well as miniaturization. Major computer manufacturers and
coaxial cable manufacturers have recognized this need but have not
miniaturized conventional coaxial cables by magnetic loading. Since
computer advances are often accomplished through faster operations
embodied in solid state devices, the need for faster propagation of
electrical pulse energy has become almost as important as
miniaturization of the circuitry.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a coaxial cable having increased
characteristic impedance achieved through magnetic loading which
increases the inductive reactance and decreases the attenuation,
however, the increase in inductive reactance is not accompanied by
the usual expected increase in time delay. The means employed to
achieve magnetic loading do not increase the eddy current and
hysteresis losses or other losses which would detract from the use
of the novel coaxial cable as a miniaturized high frequency coaxial
transmission cable.
A principle object of this invention is to reduce the size and
weight of a coaxial cable without changing its characteristic
impedance or increasing losses and time delay.
Another object of the present invention is to minimize time delay
of coaxial cables while increasing the inductive reactance.
Another object of the present invention is to provide a method for
producing a series of microminiature coaxial cables having new and
desirable attenuation and time delay characteristics.
Another object of the present invention is to provide means for
conducting larger amounts of conventional current without
increasing the overall size of a coaxial cable.
Accordingly, there is provided a conventional coaxial cable
structure comprising a center conductor and an outer shield
separated by a dielectric spacer. The center conductor is further
provided with a thin film of circumferentially oriented uniaxial
anisotropic magnetic material having a thickness much less than the
diameter of the center conductor but thicker than the skin depth
thickness at the desired highest frequency of the operation. Means
supplying electrical energy to the center conductor are preferably
limited to an operational level or direction which prevents
switching of magnetic orientation, thus permitting the minimum
delay time in wave propagation. These and other features of the
present invention will be set forth in greater detail in the
following description.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a transverse sectional view of a miniaturized coaxial
cable constructed in accordance with teachings of this
invention;
FIG. 2 is a longitudinal section view of a miniaturized coaxial
cable constructed in accordance with the teachings of this
invention;
FIG. 3 is a simplified equivalent network circuit of a conventional
coaxial cable;
FIG. 4 is a typical B vs. H curve for square loop high permeability
magnetic material;
FIG. 5 is a schematic curve showing current density vs. distance
from the center of a center conductor;
FIG. 6 is a simplified equivalent network circuit of the new
coaxial cable.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1 and 2 show a coaxial cable 10 constructed in accordance
with a preferred embodiment of this invention. The cable comprises
a center conductor 11 of conductive material such as copper,
copper-beryllium or other high strength conductive alloys. A thin
conductive coating of magnetic material 12 is conductively attached
to the center conductor 11 directly or to an extremely thin base
conductive layer (not shown) on the center conductor 11.
The thin layer of magnetic material 12 is preferably
circumferentially oriented with an easy magnetization axis
transverse to the direction of wave propagation. Conversely, the
hard magnetization axis is axially aligned with the direction of
wave propagation to assure the preferred mode of operation of the
invention. It has been found that uniaxial anisotropic Ni-Fe films
500A to approximately 15,000A thick provide a desirable oriented
film for the preferred embodiments hereinafter explained.
A preferred method of providing the desired uniaxial anisotropic
orientation is to plate the desired magnetic layer 12 onto the
center conductor 11 in the presence of a strong circumferentially
oriented magnetic field such as that which occurs when a strong
d.c. conduction current is passed through the center conductor 11.
The center conductor 11 may be annealed and then placed in the
presence of strong circumferential field while the magnetic layer
12 is cooled from above its recrystallization temperature.
Reheating and reorienting may be necessary when stress relieving a
plated center conductor 11 or when hot processes are employed to
attach the dielectric spacer 14.
A dielectric spacer 14 surrounds the magnetically coated center
conductor 11 and a continuous conductive shield 13 surrounds the
dielectric spacer 14. The shield 13 may be protected by an
insulating layer or jacket 16.
It was discovered that the novel coaxial transmission cable did not
operate in a manner to be expected by the addition of a magnetic
loading material. The operating characteristics of the coaxial
cable shown in FIGS. 1 and 2, both with and without magnetic
loading materials may be defined by equivalent circuit formulas. A
simplified equivalent network circuit for a conventional coaxial
cable is shown in FIG. 3.
The series inductance L of an equivalent tee section coaxial
transmission cable has been defined by ##EQU1## the shunt capacity
C by the characteristic impedance Z by the attenuation .alpha. by
and the time delay T by where .mu. is the permeability of the
dielectric, .epsilon. is the dielectric constant of the dielectric
spacer, D is the inside diameter of the outer shield, d is the
outside diameter of the center conductor, R is the series
resistance, C is the shunt capacitance and G is the shunt
conductance.
The values in a standard original coaxial cable may be designated
by sub-o notation and values of prior art magnetically loaded
coaxial cable may be designated by sub-r notations. Adding a very
thin layer of magnetic material to the center conductor will
increase the inductance L.sub.o to L.sub.r without measurably
changing the spacing of the elements, the dielectric constant
.epsilon. of the dielectric spacer 14 or the shunt capacitance
C.
Formula (1) indicates that the inductance L.sub.r increases with an
increase in resultant permeability .mu..sub.r. Formula (2)
indicates that the shunt capacity C is inversely proportional to
D/d and directly proportional to the dielectric constant .epsilon..
Formula (3) indicates that characteristic impedance Z.sub.r
increases with an increase in inductance L.sub.r. Formula (4)
indicates that attenuation .alpha..sub.r decreases with an increase
in inductance L.sub.r as the first term of formula (4) is large
compared to the second term. Formula (5) indicates that time delay
T.sub.r increases with an increase in inductance L.sub.r and
effective permeability .mu..sub.r.
A piece of standard construction coaxial cable having an impedance
of 17 ohms was found to have a time delay T.sub.o of approximately
1.5 nanoseconds per foot. An increase of the resultant .mu..sub.r
over .mu..sub.o by a factor of five would ordinarily increase both
Z.sub.r and T.sub.r by the square root of five. Applicants have
been able to so increase Z.sub.r without a corresponding increase
in T.sub.r.
In one series of tests the time delay of a seventeen ohm coaxial
cable appeared to be unchanged or reduced even though the
inductance L.sub.r and characteristic impedance Z.sub.r were
increased. A piece of the novel coaxial cable was altered to change
the circumferentially oriented easy axis of the uniaxial
anisotropic layer of thin magnetic material 12 to a partially
isotropic layer. The time delay T.sub.r of the altered sample
almost doubled. For comparison purposes, it has been established by
test and calculations that a seventeen ohm coaxial cable can be
increased to 50 ohms by magnetically loading the cable. Ordinarily
magnetic loading would increase the time delay from 1.5 nanoseconds
per foot, to over 4.0 nanoseconds per foot. The time delay of the
magnetically enhanced cable constructed according to the present
invention remained at approximately 1.5 nanoseconds per foot,
however, when the same coated center conductor was partially
annealed to partially destroy the desired magnetic orientation the
time delay increased to over 2.5 nanoseconds per foot.
It was discovered that 80% Ni - 20% Fe Permalloy plated on copper
beryllium center conductors to a thickness of 8,000A to 10,000A in
a manner which provides a circumferentially oriented magnetic easy
axis, when made into the novel coaxial cable, increased the
characteristic impedance Z.sub.r without an appreciable increase in
time delay T.sub.r over T.sub.o. Initial results indicate that the
time delay may be as low as or below that obtained for coaxial
cables without magnetic loading.
The properties of uniaxial anisotropic material may be enhanced by
first coating a flash layer of copper on the center conductor. It
is believed that the copper or other conductive undercoat aids in
maintaining uniform anisotropy and masks structural defects in the
center conductor. Thin films 500A to about 15,000A thick have been
made which have easy axis and hard axis orientation. It may be
desirable, as in plated wire memories, to restrict the
magnetostriction effects to a minimum as usually occurs in
Permalloy films having approximately 80% nickel. Films which have
the above described circumferential easy axis orientation may be
made by plating, vacuum deposition and sputtering in the presence
of a circumferentially oriented magnetic field. Other film
techniques such as cladding, rolling, evaporation, drawing,
chemical deposition and suspension deposition may require post
operative annealing in an induced magnetic field.
Nickel iron alloys and other conductive magnetic materials which
have crystalline grain structure may be coated on the center
conductor and then cold drawn so as to create a preferred desired
oriented magnetic field.
The plated thin films were deposited or treated in a magnetic field
so that the preferred orientation was circumferential and the film
displayed high anisotropy. The clad and drawn nickel iron wires,
known to have a crystalline grain structure, were oriented
mechanically through the process of drawing. Annealing the drawn
wire does not have to destroy the high anisotropy developed
mechanically, thus, drawn wire may be more stable for commercial
purposes. Solid magnetic drawn wires display the desirable
permeability increase but are not as conductive as clad wires.
While the special crystal orientation is not well understood, it is
presumed that an apparent effective internal magnetic field is
present as a result of interaction of grain crystals and the high
effective internal field. This magnetic field is present at the
wall of the center conductor which forms the wave guide. There
appears to be a faster phase change as a result of the oriented
magnetic field and the time delay is decreased (increase in phase
velocity).
Another feature of the present invention requires that the thin
layer of magnetic material 12 be relatively conductive so that the
radial electrical field E at the boundry of the center conductor be
continuous, as will be explained. Permalloy type thin films are
considered to be high permeability, low resistance films, whereas
ceramic ferrites are considered to be high resistance, high
permeability materials which do not conduct conventional
current.
The dielectric material 14 can be any low loss dielectric
preferably having a dielectric constant in the range of .epsilon. =
1.2 to 3.5. The dielectric constant for air is 1.0 and for foamed
plastics may be as low as 1.3, however, Teflon which is desirable
for many uses is 2.1. Materials known to be used in coaxial cables
such as Mylar, polyethylene, polypropylene and other polyolefins
are also useful in the present novel cable. The shield 13 is made
from a conductor which has low loss at operating frequencies.
Copper braid is sometimes employed because it is known to act as a
closed shield below 1000 megahertz. Solid copper in the form of
film, wire, foil or foil supported on insulating tapes may be
employed. It is desirable that the shield 13 be conductive and at
least twice the skin depth thickness at the upper frequency of
operation as is the case for coaxial cable outer shields.
The outer insulation layer 16 may be any typical electrical
insulation and may be applied as a tape either with or without the
shield 13.
High permeability materials usually have high eddy current and
hysteresis losses. Ferrites have been employed as magnetic loading
materials in coaxial cables because the B vs H loop is narrow
indicating that low hysteresis losses are to be expected. FIG. 4
illustrates an open B vs H loop of the type obtained with thin
films or Permalloy materials, where B is the flux density and H is
the magnetizing force intensity. The permeability .mu. of a
material is defined as .mu. = B/H. The slope of the curve is
substantially horizontal in the saturation region, and
substantially vertical in the unsaturated region. It can be seen
that the incremental permeability .mu..sub..delta. will increase if
the magnetizing force H switches the saturation region from
negative to positive saturation. The incremental permeability
.mu..sub..delta. of uniaxial anisotropic films may be held
substantially constant at very small values (about 5 gauss per
oersted, thus it is possible to transmit pulse power in the novel
coaxial transmission cable by remaining on a horizontal portion
.mu..sub..delta. of the B vs H curve, such as operating between
point 21 and 22 on the curve without incurring time delay increases
as would be incurred if operating at maximum permeability.
It has been found that 75 ohm novel transmission cable may be
operated in a conventional mode with very high frequency pulse
power surges up to three fourths of a watt without switching the
magnetic orientation or incurring hysteresis or eddy current
losses. The novel coaxial cables may be used within the usual power
ranges for standard coaxial cables without encountering undesirable
effects.
If the aforementioned formulas (1), (2) and (3) are combined it can
be shown that Z = .sqroot.L/C and ##EQU2##
As is well known, when the value of D/d approaches 3.59,
attenuation .alpha. approaches a minimum and Z at minimum
attenuation is approximately 77 ohms. Heretofore, it has been the
practice to make some small coaxial cable having 50, 75, and 100
ohm impedances and having center conductors of 0.008, 0.010 and
0.010 inches diameter respectively. Such coaxial cables employing a
Teflon dielectric require D/d ratios of 2.3, 3.4 and 5.3
respectively when no magnetic loading is employed. Prior attempts
to decrease the cable size without magnetic loading, increased the
characteristic impedance and attenuation. Prior attempts to
decrease attenuation were directed to an increase in inductance L
and characteristic impedance Z through magnetic loading without a
decrease in cable size.
Employing the present invention and maintaining the same center
conductor as employed in the above mentioned prior art 50, 75 and
100 ohm coaxial cables, the D/d ratio of the new novel 50, 75 and
100 ohm coaxial cables may be reduced as follows: for 50 ohm cable
D/d of 2.3 is reduced to 1.4, for 75 ohm cable D/d of 3.5 is
reduced to 2.2 and for 100 ohm cable, D/d of 5.3 is reduced to 3.0.
These figures are rounded representing a resultant or effective
permeability .mu..sub.r of 5 and a dielectric constant 68 for
Teflon of 2.1.
It will be understood that the values of .mu..sub.r and .epsilon.
may be changed and the desired impedance values of 50, 75 and 100
ohms maintained. As the D/d ratio is reduced and approaches unity
the maximum values of .mu..sub.r required to achieve the desired
impedance are obtained as follows. If .epsilon. is 1.0 as in air,
.mu..sub.r approaches 5.3 for 50 ohm cable; 12.2 for 75 ohm cable
and 28 for 100 ohm cable. If 68 is 1.6 as with foamed plastic,
.mu..sub.r approaches 8.3 for 50 ohm cable; 23.4 for 75 ohm cable
and 68.8 for 100 ohm cable. If .epsilon. is 2.1 as with Teflon
.mu..sub.r approaches 11.2 for 50 ohm cable; 37.6 for 75 ohm cable
and 126 for 100 ohm cable. Permalloy thin films which are oriented
by a magnetic field or by cold drawing of the type employed are
capable of achieving values of .mu..sub.r below the above five (the
value employed for a preferred embodiment). Other magnetic thin
film materials having the range of permeabilities described and
having the above-mentioned desire uniaxial circumferential
orientation may be employed.
Reduction of the ratio of D/d while maintaining the impedance Z
constant permits microminiature coaxial cable to be designed. The
inside diameter D of the shield can now be reduced without
increasing time delay or other losses which normally accompanied
magnetic loading impedance increases. Had the novel coaxial cable
been available heretofore it could have been employed in aircraft
and spacecraft electronics systems to decrease weight of the
coaxial cables by up to 50 percent and further decrease the size
and weight of the electronic hardware.
It is known that conventional current is displaced to the outside
annular area of a conductor as frequency increases. FIG. 5 shows
schematically how the current density increases exponentially
toward the outer diameter of the center conductor 11. At the center
23, where the radius r=0, the current density is substantially zero
at high frequency and rises to a maximum at the outer diameter 24
of the center conductor. Due to skin effect phenomenon the current
density is believed to fall sharply at the boundry and increase
exponentially again in the thin layer of magnetic material 12,
rising to a maximum at the outside 25 of the magnetic material and
then falling to zero. The skin depth thickness .delta. may be
defined by the equation: ##EQU3## where f is the frequency of
operation, .mu. the maximum permeability of the conductor, and
.sigma. is the conductivity. In a preferred embodiment, the skin
depth thickness for the magnetic material and the copper conductor
were estimated to be 0.62 microns and 4.6 microns thick
respectively at 350 megahertz. The actual thickness of the magnetic
layer was approximately 10,000 A or 1.0 micron. In the preferred
embodiment explained hereinbefore the thin layer of magnetic
material was thicker than the skin depth thickness. As frequencies
increase the skin depth thickness will decrease, and if the thin
layer of magnetic material is not reduced accordingly, attenuation
of the electric field will result.
The thin layer of magnetic material 12 is preferably conductive and
conductively attached to the center conductor 11 so that the radial
electric field is continuous. When the electric field is continuous
at the boundry, the conduction current is divided inversely
proportional to the skin depth thicknesses of the two materials. It
can be shown that highly conductive Permalloy having a small skin
depth thickness will conduct a much larger amount of conduction
current at high frequencies than the percentage increase in
diameter of the center conductor. Since this phenomenon is true for
one conductive layer of material having a different resistance, it
is true for multiple layers of material having different
resistances which are conductively connected or arranged in a
continuous electric field to independantly support conductive skin
layers.
Not only does the thin layer of magnetic material conduct a portion
of conventional current, but it enhances conduction of some form of
wave energy. Without changing the conductivity of the magnetic
material in a piece of novel cable, it was annealed partially
destroying the circumferential orientation of the easy axis. As a
result of the annealing the characteristic impedance Z.sub.R =
.sqroot. L.sub.r /C increased which is attributed to an increase in
aforementioned permeability .mu..sub..delta. and .mu..sub.r,
however, the time delay increased more than can be accounted for by
any possible increase in permeability. The propagation of the wave
energy is enhanced by the aforementioned circumferential
orientation of the uniaxial anisotropic magnetic material. It may
be possible that the partial annealing caused the shape of the B vs
H curve to change, thus the operation on a vertical slope region of
the B vs H curve causes hysteresis losses, flux changes and time
delays which did not occur when operating on a saturation portion
of the curve of square loop material described in FIG. 4.
Another explanation for the decrease in time delay T is made with
reference to FIG. 6 showing a schematic network representation of
the new coaxial cable. The shunt arm 26 of the equivalent Tee
network was found to have an inductive reactance 27 in series with
the shunt capacitance 28 which lowers the resonance point or cut
off point compared to coaxial cables without the thin layer of
magnetic material. At high frequencies above the cut off point,
usually above 20 megahertz where fast rise time of sub nanosecond
pulse power is operable, the inductive reactance 27 (X.sub.L) is
very large and tends to cancel out the capacitive reactance 28
(X.sub.C). Above the cut off frequency the shunt arm 26 appears
highly inductive so that the center conductor 11 is substantially
isolated from the shield-return 13. When isolation occurs the time
delay T is no longer equal to .sqroot.LC and the speed of
propagation begins to approach the speed of light as is achieved in
an open conductor, however, the skin effect losses and radiation
losses which accompany open conductors have been reduced to those
associated with standard coaxial cables.
Part of the current conducted in the layer of magnetic material may
be forced into a helical path which helps to explain the increase
in inductive reactance and the high magnetic field.
Another desirable feature of employing Permalloy Ni - Fe materials
is to achieve reduced phase distortion. At high frequencies
Permalloy acts as a magnetic damper providing a desirable roll off
characteristic. This characteristic substantially attenuates the
frequencies above the operating frequencies which would otherwise
provide phase distortion.
While this invention has been described with respect to particular
embodiments employing a particular high anisotropy substantially
circumferentially oriented magnetic material it is apparent that
other magnetic materials can be found which have the desirable
characteristic described hereinbefore. The coaxial transmission
cable and method for reducing time delay and attenuation have been
described as best available theoretical analysis will now permit.
Other embodiments for reducing the size of coaxial cables for high
frequency use without increasing time delay and attenuation will
become apparent to those skilled in this art.
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