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RADIO FREQUENCY CABLE

An RF cable is a basic cable used primarily for carrying audio-visual signals. The

name comes from an abbreviation of "radio frequencies." The RF cable is a typeofcoaxial cable, which involves a series of casings to protect the signal from

interference.

The coaxial design used in an RF cable is designed to prevent the potential

problem of the wire carrying the signal also acting as an amplifier. This could

cause some of the signal to be lost in the form ofradio waves. To counteract this,

the coaxial cable uses four circular layers. From the inside to the outside they are:

the wire carrying the signal; an insulating material which is usually solid plastic; a

metal shield; and a plastic casing which protects the materials inside.

The introduction of 3G and xxCDMA wireless communication and

increasing raw material prices have driven the demand for high

performance Radio Frequency (RF) - coaxial cables from network

installers has increased. The performance demand for

attenuation, VOP (Velocity of Propagation) and VSWR (Voltage

Standing Wave Ratio) have increased for above applications. Also

of interest for the cable quality are PIM (Passive InterModulation),

TDR (Puls Return Loss) Phase Stability and power efficiency.

Together with this improvements, material substitution for outer

conductor and size substitution are an important economic effect

of this new developments.Attenuation properties are depending

on conductivity of inner and outer conductor and the dielectric

losses of the foamed insulation.An improved foaming process and

higher quality materials together with state of the art processing
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equipment enable cable manufacturer to improve their product

quality without compromising productivity.

The performance demand for attenuation, VOP (Velocity of

Propagation) and VSWR (Voltage Standing Wave Ratio) have increased

for above applications. Also of interest for the cable quality are PIM

(Passive InterModulation), TDR (Puls Return Loss) Phase Stability andpower efficiency. With these parameters in view equipment and

processes were developed to provide the RF cable manufacturer with

state of the art equipment. Continuous improvements on processes

and equipment guarantee state of the art manufacturing solutions.

Keywords: Low Attenuation RF cable manufacturing

equipment; gas injection system; nucleating agent; cell structure;

Attenuation; Velocity of prorogation; Water penetration; Gas

Solubility in polymer melts; physical foaming; viscosity; CO2;

Aluminum; Cable designs; Copper; Polymers; Process design;

Dielectric loss.

1. Introduction

2. Low Attenuation Cable

This cable type provides technical and economical advantages for the

network installer, service provider and cable manufacturer.With the

lower attenuation it is possible to reduce the cable size to handle the

same power reducing the initial investment. Transportation, installation

and cable cost per unit are significant parameters.

2.1 Cable Parameters


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For this new approach all cable relevant parameters are analyzed

and in accordance to existing standards, parameters to improve

Attenuation

Velocity of Propagation

have to be found, considering existing connector designs.

Fig. 1: Longitudinal cross section of a corrugated RF-cable

1.Inner Conductor

2.Dielectric (consisting of highly foamed PE and air under the

corrugated outer Conductor

3Outer Conductor

2.2 Attenuation

Attenuation of coax-cables is described as the attenuation of the

individual parts. Inner conductor, dielectric and outer conductor

attenuation form the overall attenuation of the cable according

equation [1].

tot = i + foam + o [1]

The individual components are described with equation [2], [3]

and [4].

Attenuation of inner conductor {1}

f i *

Z *d

36,1 * k

c e

= i [2]

1

2

3

International Wire & Cable Symposium 521 Proceedings of the 57th IWCS


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2,2

2,22

2,24

2,26

2,28

2,3

2,32

2,34

0 500 1000 1500 2000 2500 3000 3500

er

f [MHz]

Fig. 3 Dielectric constant over frequency

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

0 500 1000 1500 2000 2500 3000

f [ MHz ]

inner cond. dielect ric out er cond. total

Fig. 2 Attenuation

Loss Factor

0,00E+00

2,00E-05

4,00E-05

6,00E-05


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8,00E-05

1,00E-04

1,20E-04

1,40E-04

1,60E-04

1,80E-04

0 500 1000 1500 2000 2500 3000 3500

f [MHz]

t

a

n

Fig. 4 Dielectric loss factor over frequency

Attenuation of outer conductor {3}

f o *

Z * D

36,1 * k

c e

= o [3]Attenuation of the dielectric layer {2}

f foam r = 9,096 * * tan * [4]

i -attenuation inner conductor [dB/100m]

o -attenuation outer conductor [dB/100m]

foam -attenuation dielectric layer [dB/100m]

Zc -characteristic impedance [ohm]

f -frequency [MHz]

r -dielectric constant

ki -shape factor inner conductor

ko -shape factor outer conductor

de -electrical equivalent inner diameter

De -electrical equivalent outer diameter


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The electrical equivalent diameter considers the skin effect, which

occurs on high frequency signals where the current tends to flow

only in a very thin skin layer. The depth of penetration is given by

following formula.

= 15,9 / * f

-conducting layer [mm]

-conductivity [m/ mm2]

f -frequency [kHz]

With above relation

de = diCU 2*

De = DoCUinner + 2*

The skin effect influences also inductance on the coaxial cable and

thereby characteristic impedance and propagation velocity. More

details are in chapter impedance.

Taking a look at the diagram in Fig. 2 and to the equations [2] to [4]

it is obvious that increasing conductor diameters would reduce

attenuation, also reducing r of the dielectric results in reduced

attenuation.

According to formula [3], material purity (tan) and a low r are

contributing substantially to improvements. Where the dielectric

loss factor can only be influenced from material manufacturers, the

dielectric constant can be reduced by highly foaming the polymer.

Both parameters are frequency dependent. Fig. 3 and 4 show the

frequency relation for dielectric loss factor and dielectric constant

for a PE compound.

With the relations mentioned, several ways of improving attenuation

are given. Considering given standards for cable geometry and

required characteristic impedance Zc the improvements are limited.

The characteristic impedance is given according formula [5].

e

e


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r

c d

Z 60 * ln D

= [5]


With the restrictions mentioned above new cable designs with

very high foamed dielectric are required to achieve a substantial

attenuation reduction.

3. Foaming Technology

3.1 Line Concept

Manufacturing Low Attenuation cables requires a perfect

process setup. Starting with the best commercially available raw

materials and a manufacturing line configuration enabling

splitting of the process. Individual control over as many

parameters as possible is required to achieve highest product

quality together with high productivity.

As a precondition, wire transport has to be stable to avoid any

VSWR/SRL peaks on the final cable. A wire calibration andcleaning unit is used to get a round conductor with stable

diameter. Ultra-sonic cleaning produces a perfectly clean surface

for best adhesion of the dielectric.

3.2 Extruder Configuration

A cascaded extrusion group is preferred for processing. The group

consists of an 80mm melting extruder and a 100mm cooling

extruder. Base materials are blended online at the extruder to

achieve the required properties for the foam material. HDPE and

LDPE are blended with a minimum amount of an endoderm

nucleating agent to achieve required melt strength and viscosity

for bubble growth and even cell distribution. The melting

extruder has a smaller screw diameter. This provides good


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melting behavior and ensues all material is proper melted before

the gas injection point. A foaming agent is then injected into the

melt. Preferably CO2 is used as foaming gas due to its high

solubility in polymer melts and its inertness. After gas injection, a

mixing and homogenizing section follows. Typically the melting

extruder has an L/D ratio of 30. The cooling extruder with an L/D

ratio of 30 and its bigger screw diameter is used for homogenizing

and cooling of the polymer melt. The best cooling efficiency is

achieved with a screw cooling system using a thermo oil heat

exchanger.

3.3 Gas Injection

The gas injection system is designed to supply at constant pressure

the required amount of foaming gas. A precision flow meter

monitors the gas flow for highest process stability and repeatability.

The gas injector is designed as an adjustable needle type injector.

This allows product changes without injector change. Due to its

unique design clogging is prevented.

In comparison to a standard orifice the adjustable needle injector

features a much smaller gap. For example a 20 micron aperture

equals a 0,125 micron ring gap in cross section. Due to this

minimized opening and the design of the injector tip acc. Fig. 3 and

Fig. 4 no blocking of the opening by polymer melt is possible.

Additionally the injector can be closed completely and does not

have to be removed when running solid products. With this there is

no risk of damaging the barrel thread due to permanent removal and

reassembly of the unit.

Fig. 3: Orifice Type Adjustable Injector Cross section

ring gap injector


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Fig. 4 shows the basic design of the injector.

Fig. 5: Injector housing and needle

1.. Conductor Calibration and cleaning unit

2.. Caterpillar

3.. Preheater

4.. Adhesive resin extruder

5.. Cross head

6.. Bypass

7.. Gear pump


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8.. Cooling Extruder

9.. Temperature controlling device

10.. Melting Extruder

11.. Gas dosing unit

12/13 Diameter Gauge

14.. Telescopic Cooling trough

15/16..Capacitance Measuring System

3 4 5

6

7

8

10

11

2

1

12 14 15 16 13

9

8

10

D


Fig. 6

The cap works as micrometer dial with precision adjustment of the

needle in the injector. Under pressure, the adjustment system is

locked. For adjustments the gas pressure is released for a short time,

and the settings can be changed. In combination with the high

pressure gas pump working without pressure stabilisation delay (

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