Coaxial Cables

 

Field theory

Coax-to-microstrip adapters

    End launchers:

        Emerson Network Power: SMA end launch connectors

 

[Features of the Coaxial Cable]

- Wide bandwidth

- Small size

- Low power handling capability

- No dispersion

- High loss

- Lines can be bended

- Imperfect noise shielding

 

* Applications of Coaxial Cable

- Signal routing over a short distance

 

[Coaxial Cable Structure]

Quad-braid

Triple-braid

 

 

Dual-braid

Single-braid

 

1) Conductors

AL      Tin-plated alumuinum

CU      Copper

CW     Copper plated steel (Copperweld) = Copper-clad steel (CCS)

SPC     Silver-plated copper

SCW       Silver-plated copperweld

SSC     Stranded silver-plated copper

TC       Tinned copper

-S        Strand

 

2) Dielectrics

Acronym

Full name

Velocity

factor (%)

¥år

Loss tangent

ASP

Air space polyethylene

84-88

 

 

AST

Air space Teflon

85-90

 

 

FE

Foam polyethylene

80.0

 

 

FEP

Fluorinated ethylene polypropylene

78.6

1.62

 

FS

Foam polystyrene

91.0

 

 

FTPTFE

Foamed-tape PTFE

 

 

 

FP

Fluoropolyme, cellular

84.0

1.416

 

PE

Polyethylene, solid (PE-S)

65.9

2.30

 

Polyethylene, semi-solid (PE-SS)

82.6

1.466

 

Polyethylene, cellular (PS-C)

83.6

1.430

 

PTFE

Poly-tetra-fluor-ethylene (Teflon¢ç)

69.4

2.10

 

 

3) Shielding materials (wires and foils)

Flexfoil

CCS    Copper-clad steel

CU      Copper

AL      Aluminum

TC       Tinned copper

 

 

 

[Why 50 ¥Ø for Coaxial Cables?]

In most cases we use Z0 = 50¥Ø. In TV applications we use Z0 = 50¥Ø. In an air-filled coaxial cable, Z0 = 75¥Ø offers minimum attenuation while Z0 = 35¥Ø gives maximum power handling. Z0 = 50¥Ø is a compromise between these two values.

 

[Power Handling Capability of a Coaxial Cable]

- Peak power handling is limited by the dielectric breakdown.

   

- In the production test, 35% of the theoretical maximum power level is used. Usually a DC test voltage is applied [1].

- Average power handling is limited by the temperature rise on the inner conductor. Typical industry conditions have been to allow the inner conductor to reach a temperature of 100¡ÆC with an ambient temperature of 60¡ÆC

   

    :  average power (W) for 60¡ÆC rise of the inner conductor temperature from the ambient temperature.

    :  heat transfer coefficient from the inner conductor to the outer conductor and the ambient environment (W/in2). The following table is from [1].

    b :  radius of the outer conductor (inch)

    :  correction factor for attenuation ralative to 20¡ÆC

       

    :  temperature coefficient of the resistance (copper)

    :  temperature, current and reference

    :  attenuation constant (dB/100ft) at 20¡ÆC

 

6. Upper Usable Frequency(UUF)

- Cutoff frequency of the 1st higher-order mode (TE11 mode)

   

    (usable frequency range)

- An example: RG-142, a = 0.89mm, b = 2.95mm,

- Graph of kc (cutoff wavenumber)

 

7. Attenuation in a Coaxial Cable

- Conductor loss is proportional to the square root of the frequency, while the dielectric loss is proportional to the frequency.

       

       

       

- Attenuation of the real-world cable:

1) Theoretical attenuation deviates heavily from the measured values because the theoretical model assumes a conductor of infinite extent (thickness or diameter in coaxial cable case) in computing the conductor loss.

2) Attenuation of the real-world coaxial cable is always greater than the theoretical attenuation of the

 ideal coaxial cable due to various imperfections.

 

Skin Effect

- The resistance and the inductance of both the inner and outer conductors will vary with frequency because of skin effect. There are no simple general relations which can be used at all frequencies.

- Inner conductor: skin effect at low frequencies [Race & Larrick]:

   

8. Standard Specifications of Coaxial Cables

- MIL-HDBK-216 (1962)(obsolete): RG-number, RG-number/U

- MIL-C-17 (current military standard for coaxial cables): M17/75-RG214

- RG-series designations are still in use.

 

[Coaxial Cable Engineering Data]

RG coaxial cable specifications

Antenna feeder cable specifications (LS Cable)

Coaxial cable calculation by RFS World

Coaxial cable attenuation table

RF cable selection guide by Times Microwave Systems

Advanced coaxial cable capaibilities, Times Microwave Systems

 

[High-Voltage Cables]

* High-voltage pulse cables: RG-17A/U (11kV), RG-18A/U (11kV), RG-19A/U (14kV), RG-20A/U (14kV), RG-27A/U (15kV), RG-28B/U (15kV), RG-36 (13kV), RG-157(15kV), RG-158(15kV), RG-177(11kV), RG-190(15kV), RG-191/U (15kV), RG-192/U (15kV), RG-193/U (30kV), RG-194/U (30kV), RG-218(11kV), RG-219(11kV), RG-220(14kV), RG-221(14kV), RG-230(15kV), RG-328/U (20 kV), RG-329(15kV)

- MIL C-17 specification for HV coax cable

- Times AA-6778: RG-220 version with an AL foil tape layer between the poly and the outer braid. This foil layer helps reduce the air pockets that cause many cable failures. 80kV rating. [Harris(1991), SLAC-PUB-5489]

- 10kV 10¥ìs rise-time pulse cable design for the modulator in the superconducting linear accelerator TESLA (120kV 15MW 1.7ms pulse width with PRF of 5Hz, avg. power 127.5kW): Eckoldt(2000)

* High-peak power cables:

- RG-288 (440kW), RG-321(320kW), RG-322(320kW), RG-367(830kW)

- HRL875 8-3/16" HRLine (1800kW)

- HFC42D 1-5/8" (17.2mm/42.5mm foamed PE) 11kV 50-ohm, 315kW peak

- Times Microwave Systems and Dielectric Sciences Type 2158 cable: 65kV

* Experiments with high-voltage cables, a paper collection

 

[Various Topics]

Coaxial cable optimum characteristic impedance

Coaxial cable assembly

Coaxial cable passive intermodulation (PIM)

Coaxial cable lightning protector by PolyPhase

Coaxial cable average and peak power consideration by Andrew

High voltage/power RF cables by Times Microwave Systems

Coaxial cable shielding effectiveness

Coaxial cable corona discharge

MIL-C-17G General specification for cables, radio frequency, flexible and semirigid

Semirigid cable

 

[Glossary]

Annealed copper: Copper which is softened by heat treatment to improve its flex life and conductivity

Braid: An interwoven covering used as the second conductor in coaxial cables and as a flexible screen.

Braid coverage: The percentage of surface area covered by the braid.

Cellular: Foam. Air is introduced into the insulation to reduce the material dielectric

Conductor types used in the coaxial cable

  Solid: Single strand conductor (1/1.78 = 2.5 mm2)

Stranded: A number of smaller single strand conductors twisted together to reduce rigidity for the same cross sectional area (7/0.67 = 2.5 mm2)

Flexible: A large number of smaller wires to further increase conductor flexibility (50/0.25 = 2.5 mm2)

Corona: A luminous discharge due to the ionization of the gas surrounding a conductor caused by a                 voltage gradient exceeding a certain critical value.  It does not greatly heat the conductor, and it is limited to the region surrounding the conductor.  While corona is a low energy process, over long periods of time, it can degrade insulators, causing a system to fail due to dielectric breakdown.

Corona extinction voltage: The highest voltage at which continuous corona of specified pulse amplitude no longer occurs as the applied voltage is gradually decreased from above the corona inception value.  

Corona inception voltage: The lowest voltage at which continuous corona of specified pulse amplitude occurs as the applied voltage is gradually increased.

Creepage: Current flowing between two conductors along a surface that is in contact with both conductors. Generally this can be neglected up until the voltage where corona or flashover occurs.  

Creepage distance: The shortest distance separating two conductors as measured along the surface touching both conductors.  Along the surface of most materials, flashover can occur at distances much shorter than the flashover distance in air.  Therefore, it is extremely important in high voltage designs to look for places where creepage can occur. 

Derating factor: A factor applied to a current rating which takes into account environmental and installation conditions which can effect the performance of the cable.

Dielectric: An insulating material between two conductors.

Dielectric withstanding voltage (DWV): Max. voltage before the dielectric breakdown

Flashover: A disruptive discharge of electricity through an insulator, normally characterized by a voltage drop. Also, a discharge around or over a liquid or solid material.  This can be a single event, intermittent or continuous.  Same as Arc.

Flex life: The ability of a cable to resist failure under repeated flexing.

Foam: Same as cellular

Hygroscopic: Moisture-absorbing

Jacket: The outer protective plastic material of a cable

Nylon jacket: An abrasion resistant thermoplastic with good chemical resistance. Used as a termite barrier.

Polyethylene: A thermoplastic material with superior electrical characteristics (breakdown voltage, dielectric constant)

Polystyrene: Similar to PE but harder, stiffer and a higher softening point.

RG/U: Military designations for coaxial cables.

Screen: Same as shield

Shield: Same as screen. a barrier to prevent signal leakage or interference. Aluminum foil with drain wire, woven copper braid or combination of both. 

  Overall screen: cable is screened overall

  Individual screen: pairs are individually screened

  Individual and overall screen: pairs are individually screened, laid up and overall screened

Tinned copper: Tinning provides a barrier between materials, resists corrosion, improves solderability and contact properties.

 

REFERENCES

REFERENCES

[1] M. D. Fanton, "Transmission line for broadcast applications," Electronics Research Inc.

[2] www.rfcafe.com/references/electrical/coax_chart.htm

[3] http://en.wikipedia.org/wiki/Coaxial_Cable