Description
HIGHLY FOAMED COAXIAL CABLE
Technical Field
[1] The present invention relates to coaxial cables, more particularly, highly foamed
coaxial cable having stably increased foaming rate using macro cell, and by reason of
the increased foaming rate, dielectric constant of the insulator can be decreased and
signal transmission speed could be increased.
Background Art
[2] Recently, in wireless communication environment, coaxial cable has been used to
transmit an ultra high frequency signals, because of coaxial cable's low signal at-
tenuation characteristic.
[3] More particularly, the coaxial cable has good characteristi; such as stabilized
impedance, low signal attenuation characteristic; and ejcellent shielding property in
high frequency band.
[4] By reason of said high frequency characteristic; coaxial cable is suitable for high
frequency communication cable which is used in base station of cellular phone using
high frequency signal of microwave band.
[5] In general, polymer material has been used as insulating material for surrounding
copper wire.
[6] In this case, if dielectric constant of insulating material decreases, then signal at-
tenuation of cable decreases.
[7] Therefore, developments and applications for materials of low dielectric constant
have been continued in these days.
[8] Currently, polyethylene resin has been applied for insulator, and especially,
HDPE(High Density Polyethylene Resin) has been mostly used among various
polyethylene resin.
[9] On the other hand, FEP(Fluorinated Ethylene Propylene Resin) or etc. has been used
for insulator among various fluororescein.
[10] Among said materials, polyethylene resin is the best choice to make insulator of
coaxial cable, because it has several advantages such as gocxi physical properties, easy
processing, and low cost.
[11] But, by just using said materials, it is hard to transmit the signal to long distance with
high speed and low signal loss rate.
[12] Therefore, a chemical foaming method which foaming the mixture of chemical
foaming agent and said materials, and a gas foaming method which foaming said
materials with injected gas has been used to increase the signal transmission speed by
decreasing the dielectric constant.
[13] Here, due to the limitation of foaming skill, raising foaming rate of small size porous
cell has been used to make insulator with low foaming density.
[14] Referentially, to gain low dielectric constant, the technique of foaming a small cells
which have diameter of llOfm at most, and average diameter of the cell is from 90 to
130;Mn with reduced density of foaming materials less than 0.22g/cirf, especially from
0.17 to 0.19g/cirf, is described in US6037545.
[15] But, if traditional size of ceU is used with extremely high foaming rate to decrease
the foaming density, cell collapse or unbalanced external appearance can be oaairred.
[16] In this case, coaxial cable which produced by traditional foaming method is not
suitable for long distance transmission with low signal loss, because of increased
return loss of it.
Disclosure of Invention
Technical Problem
[17] The purpose of the present invention is to solve above-described problems, and is to
provide highly foamed coaxial cable with stably decreased foaming density, by using
macro cell diameter of which is at least 300/ffli in foaming process.
[18] Another purpose of the present invention is to provide highly foamed coaxial cable
with low signal loss for transmitting the signal to long distance, by improved foaming
rate and reduced dielectric constant of insulator.
[ 19] Another purpose of the present invention is to provide highly foamed coaxial cable
by which it is possible to avoid undesired uniformity and return loss of the cable.
Technical Solution
[20] To achieve said objects, A highly foamed coaxial cable aoaording to present
invention comprising, an inner conductor disposed in the cable, and a foamed insulator
comprising porous cells and surrounding the inner conductor, and an outer conductor
surrounding said insulator, and a sheath surrounding said outer conductor.
[21] And, in said insulator, the total area of macro cell which has a diamter of at least
300;um is larger than the total area of micro cell which has a diameter smaller than
300;Mii at cross section of cable.
[22] Preferably, the ratio of total area of mxro cell to total cross sectional area of cable is
from 63.6% to 92.0%.

[23] And, the density of the insulator is from O.Q5g/ciif to 0.20g/cnf.
[24] And, the foaming rate of said insulator is firom 78.9% to 94.7%.
[25] And, a relative dielectric constant of the insulator is from 1.085 to 1.291.
[26] On the other hand, the ratio of cable signal transmission speed in the cable to signal
transmission speed in the air is from 88% to 96%.
[27] Preferably, said insulator is composed by gas foaming the mixture of 50 ~ 90wt% of
HDPE(High Density Poly Ethylene) and 10 ~ 50wt% of LDPE(Low Density Poly
Ethylene) and 0.1 ~ 3wt% of nicleating agent.
[28] And, an outer skin layer is further comprised to surround the insulator.
Advantageous Effects
[29] By the present invention, it is possible to decrease foaming density stably by foaming
macro cell with diameter of 300/fln at least in foaming process of insulator.
[30] And, it is possible to transmit the signal to long distance with low signal loss, by
improved foaming rate and rediced dielectric constant of insulator.
[31] And, it is possible to avoid undesired uniformity and return loss of cable caused by
collapse of the cell which oaairs in high foaming rate with micro cells that have
diameter less than 300/flii.
Brief Description of the Drawings
[32] The drawings attached illustrating the preferable embodiment of the present
invention only helps further understanding of the idea of the present invention along
with the detailed description of the present invention described in the below, and thus
the present invention is not Umitedly interpreted to the matters shown in the drawings.
[33] Hg. 1 is perspective view of highly foamed coaxial cable according to present
invention.
[34] Fig.2 is cross sectional view of highly foamed coaxial cable aocording to present
invention.
[35] Rg.3 is fragmentary enlarged view of insulator of conventional coaxial cable.
[36] Hg.4 is fragmentary enlarged view of insulator of highly foamed coaxial cable
ax»rding to present invention.
[37] Hg.5 shows cross sectional view in which of an inner skin layer and an outer skin
layer are inserted into highly foamed coaxial cable aocording to present invention.
[38] Rg-6 is schematic view of a extruder producing highly foamed coaxial cable
according to present invention.
[39]
4-
[40] * Explanation of indicating marks of figure
[41] 21: inner conductor 23 : insulator
[42] 25 : outer oondu;;tor 27 : sheath
[43] 30 : cell 31 : macro cell
[44] 32 : mfcro cell 41 : inner skin layer
[45] 43 : outer skin layer
[46]
IVfode for the Invention
[47] Hereinafter, the present invention is described in detail with reference to the attached
drawings.
[48] Before the detailed description, it should be noted that the terms used in the present
specification and the claims are not to be limited to their lexical meanings, but are to
be interpreted to conform with the technical idea of the present invention under the
principle that the inventor can properly define the terms for the best description of the
invention made by the inventor.
[49] Therefore, the embodiments and the constitution illustrated in the attached drawings
are merely preferable embodiments aaoording to the present invention, and thus they
do not express all of the technical idea of the present invention, so that it should be
understood that various equivalents and modifications can exist which can replax the
embodiments described in the time of the application.
[50]
[51] Rg. 1 is perspective view of highly foamed coaxial cable aaoording to present
invention, and Rg.2 is cross sectional view of highly foamed coaxial cable axording
to present invention.
[52] Referring to Rg. 1 and Rg.2, highly foamed coaxial cable acrording to present
invention comprises, an inner oonductor(21) disposed at center of the cable, and a
foamed insulator(23) having porous cells and surrounding the inner oonductor(21), and
an outer condictor(25) surrounding the insulator, and a sheath(27) surrounding the
outer oonductor(25) and foaming external shape of the cable.
[53] Said inner oonductor(21) made of oondicting material such as metal transmits the
signals, and is disposed at the center of the cable.
[54] Here, said inner conductor(21) can be foaming in various size, and it can have
hollow stncture at the center for improvement of flexibility and low cost of
procudtion.
[55] And, said inner oondu;tor(21) can be made of various metals sich as copper or
5"
AT
aluminum, especially, it can be made of copper or copper alloy whfch has good
corrosion resistance and oondu;ting property.
[56] Besides, said outer oonductor(25) prevents leakage of signal from the insulator(23),
and shields the inner condu;tor(21) from alien crosstalk su:h as outer electronic
waves.
[57] By this reason, said outer oondiEtor(25) can be made of conducting materials such as
metal which has good shielding property.
[58] " Here, said outer condu:tor(25) can be made of various metals such as copper or
aluminum, especially, it can be made of copper or copper alloy which has good
corrosion resistance and condicting property.
[59] And, said outer condiEtor(25) can be formed in cylindrical pipe appearance spaced
away from the inner oonductor(21) at regular interval, and it can have corrugate
surface with regular pitch for flexibility ot the cable.
[60] Said insulator(23) is made from a insulating polymer, and it is disposed between said
inner oondiEtor(21) and outer condu:tor(25) for insulating and making distance
between them.
[61] Therefore, characteristb impedance is derived from dielectric constant of the
insulator(23), and signal transmission speed is determined by said characteristic
impedance.
[62] [Fbimula 1]
[63] 1
signal transmission speed oc =^
[64] Here, transmission speed of signal transmitted by cable is inverse proportional to root
of dielectric constant, as seen in Formula 1, so, if dielectric constant of insulator(23)
decrease, transmission speed of signal increases.
[65] Referring to Hg.2, highly foamed coaxial cable aowrding to present invention has
insulator(23) made of foamed material whbh has many porous cell.
[66] To improve the signal transmission speed, dielectric constant of insulator(23) has to
be decreased, and low dielectric constant can be achieved by low foaming density of
insulator(23) caused by high foaming rate.
[67] Here, said foaming rate is ratio of air volume in unit volume of foamed material to
unit volume of that.
[68] Therefore, said foaming rate can be expressed as follows.
[69] [Formula 2]
[70]
I
[71] Here, it is hard to measure the volume of air inside the foamed material, so, this
formula can be converted to function of density.
[72] So, it can be expressed as follows.
[73] [Fbimula 3]
[74]
--------------- ^ _, _. .^3--------------------------—
[75] In Formula 3, the density of foamed material can be measured fi-om mass of foamed
material and volume of foamed material.
[76] Besides, volume of foamed material can be measured by sinking the foamed material
in water.
[77] Here, density of water is 1, so, increment of water volume is same as increment of
water mass caused by sinking the foamed material.
[78] Therefore, said density of foamed material can be expressed as follows.
[79] [Formula 4]
[80]
[81] Apply Formula 4 to Formula 3, we can have following formula.
[82] [Formulas]
[83]
[84] Highly foamed coaxial cable according to present invention comprising the
insulator(23) formed to have foaming rate from 78.9% to 94.7%.
[85] Here, if foaming rate of insulator(23) is lower than 78.9%, like as seen in following
embodiment which has foaming rate of 78.0%, improvement of signal transmission
speed is weak.
[86] And, if foaming rate of the insulator(23) is larger than 94.7%, insulator(23) can be
mechankally weakened by high forming rate, and it cannot fix the relative position
supporting the inner a)ndictor(21) and outer condu2tor(25), so the space between the
inner oondu;tor(21) and the outer oonductor(25) cannot be maintained.
[87]
[88] On the other hand, Rg.3 is fragmentary enlarged view in insulator of oanventional
coaxial cable, and Rg.4 is fragmentary enlarged view in insulator(23) of highly
foamed coaxial cable according to present invention.
[89] Comparing Fig.3 and Fig.4, insulator(23) of highly foamed coaxial cable aooording
to present invention has more macro cells(31) than insulator of conventional coaxial
cable.
[90] In present invention, while foaming masro cells(31) with foaming rate which is at
least 78.9%, so it avoid undesired uniformity by cell collapse, and lessen return loss
which can be arurred in conventional that comprises micro cells.
[91] Here, what we call mxro cell is the cell which has the largest diameter at least
300/fln, and what we call micro cell is the cell which has the largest diameter smaller
than 300//m.
[92] As seen in Rg.3 and Rg.4, we can figure it out the mxro cells(31) by micro scope,
but it is hard to describe macro cells(31) by numerical values in the concrete.
[93] So, in present invention, we describe the ratio of total area of macro cell(31) to area
of cross section of cable.
[94] And, it is possible to measure the ratio of macro cell(31)'s area through
SEM(Scanning Electron Microscope) or microscope.
[95] Here, in cross section of cable, if total area of macro cell(31) is smaller than area of
micro cells(33), problem of undesired uniformity or return loss cannot be improved so
much.
[96] And, if ratio of macro cell(3 l)'s area is set to extremely high value, cells merge by
cell collapse and it can consequentially weaken durability of the insulator(23).
[97] Therefore, the ratio of mxro cell(31 )'s area in cross section of cable to total cross
sectional area of the same is preferably 63.6% to 92.0%.
[98] And, if the macro cells which has the largest diameter larger than lOOO/ffli take the
greater part of insulator, it is hard to hold the figure of cable, so, ratio of macro cells
which has the largest diameter larger than lOOOiM should be smaller than 10%.
[99] Besides, because said insulator(23) is composed with foamed material having 78.9%
~ 94.7% of foaming rate, density of said insulator(23) is formed in the range of
¦O.Q5g/cnf ~ 0.2g/ad.
[100] And, by said range of density, relative dielectric constant of said insulator(23) is
formed in the range of 1.085 -1.291.
[ 101 ] At this point, highly foamed coaxial cable according to present invention which has
8
said range of relative dielectric constant can transmit the signal fern 88% to 96%
compared with aerial signal transmission speed.
[102] This means speed improvement more than 1% compared with comparative example
which has less than 87% of signal transmission speed.
[103] In present invention, we use relative dielectric constant compared to dielectric
constant of air 1, and express signal transmission speed as a ratio to aerial signal
transmission speed (3x108 m/sec).
[104] On the other hand, said insulator(23) can be composed by gas foaming the mixture of
HDPE(ffigh Density Poly Ethylene) and LDPE(Low Density Poly Ethylene) and
nucleating agent.
[105] Here, nitrogen gas(N ) and carbon dioxide((D ) gas can be used for gas foaming,
2 2
and especially, carbon dioxide gas is preferable for gas foaming by high foaming rate.
[ 106] Because carbon dioxide gas has good solubility property and it is easy to compose
the foamed material which has high foaming rate by using the same.
[107] Besides, if only HDPE is used for composing said insulator(23), transmission
property of cable is excellent, such as less signal loss, but it is hard to increase the
foaming rate more than 80%.
[ 108] And, if only LDPE is used for composing insulator(23), it is easy to increase the
foaming rate more than 80%, but transmission property is bad.
[109] Therefore, through mixing of fee HDPE and the LDPE, it is possible to control die
density and foaming rate of insulator(23).
[110] On the other hand, said nicleating agent is an additive which promote the crys-
tallization of mixed polymer of HDPE and LDPE, and it enhances the mechanical
properties of insulator(23), and makes polymer crystal in minute size,
[ill] In other words, the crystallization speed and can be controlled by nucleating agent,
and by this way, the cell(30) size can be controlled by nicleating agent, because said
cell(30) is composed by polymer crystallization.
[112] Inorganic additive sich as talc; silica, kaolin and organfc additive such as carboxylic
acid and mono or polymer carboxylic acid can be act as said nucleating agent.
[113] Here, said insulator(23) is composed by carbon dioxide gas foaming the mixture of
50 - 90wt% HDPE(High Density Poly Ethylene) and 10 ~ 50wt% LDPE(Low Density
Poly Ethylene) and 0.1 ~ 3wt% nucleating agent with 78.9% of foaming rate.
[114]
[115] Besides, Hg.5 shows cross sectional view of which an inner skin layer(41) and an
outer skin layer(43) are inserted into highly foamed coaxial cable aaording to present
invention.
[116] Referring to Fig.5, inner skin layer(41) can be positioned between inner
(X)ndvctor(21) and insulator(23), and an outer skin layer(43) can be inserted between
insulator(23) and outer condu:tor(25).
[117] Here, said inner skin layer(41) is a thin coating layer which increase the interface
adhesion between inner condiEtor(21) and insulator(23), and it can be composed by
polymer resin which is similar to materials of said insulator(23).
[118] In the present invention, if said insulator(23) is composed by a polyethylene resin,
preferably, the inner skin layer(41) should be composed by poljolefin resin which has
good compatibility to serve interfacing characteristic without influencing the dielectric
characteristic of the insulator(23).
[119] Here, said polyethylene resin can be one of HDPE, MDPE(Medium Density Poly
Ethylene), LDPE, and LLDPE(Linear Low Density Poly Ethylene), or polymerized
resin ftx)m at least one of HDPE, MDPE, LDPE, LLDPE.
[120] And, said poljolefin resin can be polymerized resin comprising polyethylene or
polypropylene or poljiosbutylene.
[121] At this time, if the thickness of said thin coating layer is smaller than O.Olnmi, it is
hard to make uniform coating on said inner condu:tor(21).
[122] And if thickness of said thin coating layer is larger than Intmi, dielectric constant of
cable can be larger, so signal transmission speed can be decreased.
[123] By this reason, thickness of said thin coating layer should be preferably from
0.01mm to 1mm, more preferably, it should be from 0.05 to 0.5mm.
[124] Besides, said outer skin layer(43) is positioned between insulator(23) and outer
condu;tor(25), and it prevents excessive foaming of insulator(23) and collapse of
foamed cells in the insulator(23).
[125] And, said outer skin layer(43) can be composed by polymer resin whfch is similar to
materials of said insulator(23), and if said insulator(23) is composed by a polyethylene
resin, said polymer resin can be polyethylene, polypropylene, and
PET(polyethyleneterephthalate) or polymerized resin from at least one of
polyethylene, polypropylene, and PET.
[126] Here, during prodiction process of cable, outer skin layer(43) cool down faster than
insulator(23) to suppress excessive foaming of insulator(23).
[127] If thickness of outer skin layer(43) is smaller than 0.01mm, cell collapse can be
ooairred,
[128] And, if thickness of outer skin layer(43) is over than 0.5mm, dielectric constant of
ID
XT
cable can be larger, so, signal transmission speed can be decreased.
[129] Therefore, thickness of said outer skin layer(43) should be preferably from 0.01mm
to 0.5mm, more preferably, it should be from 0.05 to 0.3mm.
[130]
[131] When composing insulator(23), to increase foaming rate stably with masro cell, said
materials of insulator have to be mixed by said mixing ratio, and materials for in-
termediate step for production the cable separate cooling process.
[132] After extruding process of inner skin layer(41) on inner conductor(21), heat can be
still remained in inner skin layer(41) of low thermal conductivity.
[133] If foamed insulator(23) is laminated on the heat of inner skin layer(41) which is not
cooled down enough, foamed cell of insulator(43) can collapse.
[ 134] Therefore, said inner oondiEtor(21) and inner skin layer(41) should be cooled down
enough to prevent cell collapse of insulator(23) in a broad cooling zone with sufflaent
cooling time.
[ 135] Here, said cooling zone can be processing equipment to cool down the incoming
materials gradually or rapidly using water cooling system or air cooling system for
high cooling efficiency.
[136] This cooling zone can be prepared at following processing stage after extruding
process of inner skin layer(41) and outer skin layer(43).
[137] And, inside pressure of die nipple of extruder which extrudes insulator(23) on the
inner skin layer should be decreased gently, because of high foaming rate of
insulator(23) and for stable composing of macro cell
[ 138] Preferably, structure of said die nipple cann be cylindrical form of which cross
section diameter decreases gradually toward processing direction.
[139] In the meanwhile, to increase the foaming rate of insulator(23) with macro cells, a
pressure difference between inside and outside of extruder should be increased.
[140] Therefore, said insulator(23) should stay at the cross head die of extruder for a long
time, and amount of foaming gas and nucleating agent should be increased.
[141]
[142] Rg.6 is schematic view of a extruder for producing highly foamed coaxial cable
according to present invention, and process for production highly foamed coaxial cable
aoDording to present invention is as follows, but not limited to this.
[143] Referring to Hg.6, after inner oonductor(21) passes through the first extruder(70), it
becomes the first linear member(21') on which inner skin layer(41) is positioned, and
after the first linear member(2r) passes through the second extruder(80), the first
linear member(2r) becomes the second linear member(21").
[144] Here, said the first extruder(70) and the second extruder(80) can be strew type
extruder or non-skrew type extruder, preferably, those can be a single shaft strew type
extruder, but not limited to this.
[145] First of all, said inner oondiEtor(21) is made of copper, and it can have cylindrical
form which has hollow stricture in the center.
[146] And, this inner condu:tor(21) is progressed to the proceeding direction(90) with ap-
propriate speed, and it enter the first extruder(70) which has the first resin supplier(71).
[147] Here, poljolefin resin can be supplied to said the first resin supplier(71).
[148] The inner condiEtor(21) is supplied to the first extruder(70) for being laminated by
inner skin layer(41) and extruded to the second to the second extruder(80).
[149] At the first extruder(70), inner conductor(21) is coated by thin poljolefin resin film
to make the first linear member(2r).
[ 150] Before the first linear member(21') enter the second extruder(80), the first linear
member(2r) can pass through the cooling zone which is next to the first
extruder(70)(not figured).
[151] Said the first linear member(21') is cooled down by water oooling or air spray to
avoid insulator(23)'s cell coUapse at the second extruder(80).
[152] And, if water oooling methcxl is used at the cooling zone, the first linear member(21')
should have enough drying time to get rid of moisture whfch can exist at the surface of
the first linear member(2r) for preventing inferior production.
[153] Next, at the second extruder(80), the first linear member(2r) is laminated with
insulator(23) and outer skin layer(43) through the extruding process.
[154] The second extruder(80) has the second resin supplying part(81) and the diird resin
supplying part(82).
[ 155] Here, HDPE and LDPE can be supplied to the second resin supplying part(81), and
polymer resin for outer skin layer(43) can be supplied to the third resin supplying
part(82).
[ 156] And, insulator(23) and outer skin layer(43) are laminated on the inner oondictor(21)
by the double extruding process sequencely in the second extruder(70).
[157] In other words, after the first linear member(21') is covered with foamed
polyethylene, then polymer resin film in covered on the outside of foamed
polyethylene resin in melted condition and it becomes the second linear member(21").
[158] Here, said foaming can be achieved by mixed gas which is injected to melted state of
polyethylene resin until the gas reaihed to supersaturation condition.
12^
[ 159] In embodiment of present invention, said outer skin layer(43) is cooled down repidly
while passing through the nozzIe(83), to suppress the excessive foaming and for
uniform composing of insulator(23)'s foamed cell.
[160] Here, water cooling can be prepared for cooling method.
[161] And, in the embodiment of present invention, inside of the second extruder(80)
maintained above the temperature of 140°C and pressure of lOObar, and passing
velocity of said first linear member(2r) through the second extruder(80) is about
lOm/min.
[162] After this, outer oondu;tor(25) and sheath(27) are laminated on the second linear
member(21") to make coaxial cable, but this technique is universally known, therefore
detail explanation can be omitted.
[163]
[ 164] Hereinafter, referring to Chart 1, comparison of embodiments of highly foamed
coaxial cable aoxirding to present invention and conventional coaxial cable wUl be
described.
[165]
[166]
[167]
[168]
[169] The signal transmission speed of Chart 1 is ratio of coaxial cable's signal
transmission speed to speed of light.
[170] And, comparative example 1 and comparative example 2 are composed by con-
ventional coaxial cable production method, and while composing insulator, micro cells
1^
of whth the largest diameter is smaller than 300;im were used.
[171] Besides, insulator of all the coaxial cables listed in chart 1 were composed by gas
foaming the mixture of HDPE(High Density Poly Ethylene) and LDPE(Low Density
Poly Ethylene) and nucleating agent, using carbon dioxide gas.
[172] And, diameter of inner conductor of ail the coaxial cables listed in chart 1 is 9.4mm
and made of copper, and outer diameter of insulator is 23.5mm, and outer diameter of
corrugated outer conductor is 25.2mm, and thickness of inner skin layer is 0.15mm,
and thickness of outer skin layer is 0.1mm.
[173] Meanwhile, we can see that if foaming rate increases, then density and relative
dielectric constant decreases, oonsequendy, signal transmission speed increases.
[174]
[175] Example 1
[176] The insulator(23) of the first examplary highly foamed coaxial cable axording to
present invention has 85% of macro cell in the unit area of cable's cross section, 82%
of foaming rate, O.lllg/asf of density, and 1.262 of relative dielectric constant.
[ 177] Because macro cell is widely distributed in insulator by 85%, even though foaming
process was maintained in condition of high foaming rate of 82%, external appearance
of cable could be kept uniform.
[178] By these characteristics, example 1 has 89% of signal transmission speed and 30dB
of return loss, and this property is good for communication cable.
[ 179] But, coaxial cable according to comparative example 1 has 82% of foaming rate,
0.17g/ciif of density, 1.26 of relative dielectric constant, and 89% of signal transmission
speed which is similar to example 1.
[180] But, external appearance of coaxial cable aooording to comparative example 1 was
unbalanced, and by this reason, it had 12dB of return loss.
[181]
[182] Example 2
[183] The insulator(23) of second examplary highly foamed coaxial cable according to
present invention has 92% of macro cell in the unit area of cable's cross section, 94.7%
of foaming rate, O.CBg/cnf of density, and 1.085 of relative dielectric constant
[184] Because macro cell is widely distributed in insulator by 92%, even though foaming
process was maintained in condition of high foaming rate of 94.7%, external
appearance of cable could be kept uniform.
[185] In this condition, example 2 had 96% of signal transmission speed and 32dB of return
loss, and this property is essellent for communkation cable.
1^
[186]
[187] Example 3
[188] The msulator(23) of third examplary highly foamed coaxial cable aax)rding to
present invention has 63.6% of macro cell in the unit area of cable's cross section,
78.9% of foaming rate, 0.2g/ciif of density, and 1.291 of relative dielectric constant.
[189] Because macro cell is widely distributed in insulator by 63.6%, even though foaming
process was maintained in condition of high foaming rate of 78.9%, external
appearance of cable could be kept uniform.
[190] Besides, coaxial cable aax)rding to comparative example 2 has 78% of foaming rate,
0.212g/ciif of density, 1.321 of relative dielectric constant, and it's properties is similar
to example 3.
[191] Comparing example 3 and comparative example 2, return loss of two cables are
similarly 30dB and 3 IdB, but signal transmission speed of example 3 is 88% that is
faster than signal transmission speed of comparative example 2.
[192] And comparative example 1 and comparative example 2, as seen in comparative
example 2, we can see that if foaming rate is low, external appearance of cable and
return loss is not so bad.
[193] But, as seen in comparative example 1, if foaming rate is increased to gain high
signal transmission speed when the insulator is composed by conventional produang
method, undesired uniformity of cable and return loss can be a signi&ant matter.
[194] Consequently, insulator should be foamed in the foaming rate larger than 78.9% to
obtain high signal transmission speed and low return loss, and ratio of macro cell in the
unit area of cable's cross section should be increased to increase the foaming rate
stably.
[195]
[196] Although the present invention has been described with reference to the specified
examples in the above, but the idea of the present invention is not limited to the above
described matters and various changes and modifications can be made within the
equivalent scope of the present invention and the following claims by the ordinary-
skiUed person of the art.
[1] A highly foamed coaxial cable comprising,
an inner oondirtor disposed in the cable;
a foamed insulator comprising porous cells and surrounding the inner conductor;
an outer conductor surrounding said insulator;
a sheath surrounding said outer conductor;
and in said insulator, the total area of macro cell which has a diamter of at least
300/im is larger than the total ai-ea of micro cell which has a diameter smaller
than 300jMni at cross section of cable.
[2] The highly foamed coaxial cable according toclaim 1, wherein,
the ratio of total area of macro cell to total cross sectional area of cable is from
63.6% to 92.0%.
[3] The highly foamed coaxial cable aacording tcclaim 2, wherein,
the density of the insulator is from O.QSg/cnf to 0.20g/ciif.
[4] The highly foamed coaxial cable aaoording toclaim 3, wherein,
the foaming rate of said insulator is from 78.9% to 94.7%.
[5] The highly foamed coaxial cable aaoording toclaim 4, wherein,
a relative dielectric constant of the insulator is from 1.085 to 1.291.
[6] The highly foamed coaxial cable aax>rding toclaim 5, wherein,
the ratio of signal transmission speed in the cable to signal ti-ansmission speed in
the air is from 88% to 96%.
[7] The highly foamed coaxial cable according toclaim 1 or 6, wherein,
said insulator is composed by gas foaming the mixture of 50 ~ 90wt% of
HDPE(High Density Poly Ethylene) and 10 ~ 50wt% of LDPE(Low Density
Poly Ethylene) and 0,1 ~ 3wt% of nucleating agent,
[8] The highly foamed coaxial cable aaoording toclaim 3, wherein,
an outer skin layer is further oomDrised to surround the insulator.

A highly foamed coaxial cable comprising, an inner conductor disposed in the cable, and a
foamed insulator comprising porous cells and surrounding the inner conductor, and an
outer conductor surrounding said insulator, and a sheath surrounding said outer conductor,
and in said insulator, the total area of macro cell which has a diameter of at least 300/M is
larger than the total area of micro cell which has a diameter smaller than 300/M at cross
section of cable.