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Coaxial cables guide

The correct selection of cable requires proper analysis of the electrical, physical and electromagnetic parameters of the system employing the cable to be selected.

To assist you in this analysis, our web site includes some reference data, enabling you to determine the characteristics of the cables presently available and also to evaluate how they may vary under physical operating conditions. First, review the Application Notes to determine all of the cable characteristics which must be considered. Then from the list of EC standard cables, select those which will best meet your requirements.


In choosing the appropriate cable construction for a particular application, the most important cable characteristics to be considered are the following :

  • Characteristic impedance [Z0]
  • VSWR
  • Capacitance [C]
  • Inductance [L]
  • Cw power rating
  • Max operating voltage
  • Attenuation
  • Velocity of propagation
  • Shielding effectiveness
  • Cut-off frequency
  • Flexibility

It should be noted that the stipulation of materials and dimensions does not guarantee that each lot of cable will have identical mechanical or electrical characteristics. Different types of manufacturing equipment or different manufacturing conditions can give substantially different performance characteristics.

FORMULAE COMMON TO ALL COAXIAL CABLE
Capacitance C = 6.28 ε / ln (D/d) (pF/ft)
Inductance L = 0.16 μ / ln (D/d) (mH/ft)
Characteristic impedance Z0 = ( 138 / √ εr ) log (D/d) (ohms)
Velocity of propagation (% di c)fc V = 100 / √ εr



where:

  1. d = outside diameter of inner conductor in inches
  2. D = inside diameter of outer conductor in inches
  3. c = velocity of light
  4. ε = ε0εr = the relative dielectric constant of the insulation of the cable
  5. μ = μ0μr = permeabilità magnetica dell'isolamento del cavo

Characteristic impedance

The most common values for coaxial cables are 50, 75 and 95 ohm. Other impedances from 35 to 185 ohm are available in the coaxial configuration. The VSWR (Voltage Standing Wave Ratio) of a particular length of cable is an indicator of the difference between the actual input impedance of the cable and its average characteristic impedance

Impedance (VSWR) uniformity

IThe VSWR of a cable assembly is the summation of reflections due to the connectors, the connector termination technique and the cable. The VSWR of the cable is the summation of random and periodic reflections within the cable, most commonly caused by variations within the processing equipment. The VSWR will vary with frequency. If required, cables can be procured in specified lenghts to a max VSWR requirement on a swept basis. If a very low VSWR is required, it may be necessary to procure complete assembly verified on a swept basis.

Capacitance

Capacitance values (shown below) for standard coaxial lines depend on cable geometrical parameters and dielectric material. Typical values for a few cables are the following :

Nominal capacitance Cables types
30.8 50 Ohm - Solid Polyethylene
29.4 50 Ohm - Solid PTFE
20.6 75 Ohm - Solid Polyethylene
19.5 75 Ohm - Solid PTFE

Velocity of propagation

The velocity of propagation of cable is determined primarly by dieletric constant of the insulating material between the conductors. This property is usually expressed as a % of the velocity of light in free space.

Average CW power rating

High ambient temperature and high altitude reduce the power rating of a cable by impeding the heat transfer out of the cable. VSWR reduces power rating by causing hot spots. To select the cable construction for a particular requirement, determine the average input power at the highest frequency from system requirements. Then determine the effective average input power as follows:

Effective Power = (Av. Power) x (VSWR fact.) / (Temp. fact.) x (Altit. fact.)

Temperature and altitude corrections are tabulated on available data sheet.

VSWR correction factor = ½ (VSWR+1/VSWR) + ½ K1(VSWR-1/VSWR)

Maximum operating voltage

A cable cannot operate continuously with corona which causes noise generation, dielectric damage and eventual breakdown. The maximum operating voltage must be less than the corona level of the cable. This should not be confused with the dielectric strength of the cabie, which is a test voltage which is applied for one minute only during manufacture.
The max permissible DC (rms) voltage level is conservatively 3 times the AC level.
To select a cable for a particular application, determine the actual rms or peak voltage from system requirements. Then apply : Effect. voltage = (actual voltage x Ö VSWR)

PROPERTIES OF WIRE AND CABLE INSULATING MATERIALS

Formule più comuni per cavi coassiali
Material Dielectric Costant Power Factor Normal operating Temperature limits (°C)
TFE 2.1 0.0003 -75 ÷ 250
Polyethylene 2.3 00003 -75 ÷ 80
Nylon 4.60 ÷ 3.5 0.04 ÷ 0.03 -60 ÷ 120
PTFE 1.5 0.0002 -75 ÷ 250

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