Coax Cable Specifications & Parameters

- definitions and explanations of the variety of specifications and parameters used to define the performance of a type of coax cable.

When choosing a type of coax cable to be used, it is necessary to understand its performance.

Coax cable specifications define the performance so that decision can be made about which type to use for a given application.

In order to understand the performance of the coaxial cable it is necessary to understand the specifications for the different parameters.

Characteristic impedance specification

Possibly one of the most defining coax cable specifications is its characteristic impedance. This is the impedance seen looking into an infinitely long length of cable by a signal source. The dimensions of the cable along with the dielectric used determine the overall impedance. This specification is measured in ohms and is resistive.

The most common impedance figures are:

  • 50/52 ohms :   This cable is the form that is generally used for professional RF applications.
  • 75 ohms:   This impedance is more widely used in domestic applications for television and hi-fi RF signal leads.
  • 93 ohms:   Coax with this impedance specification was used in many early computers, linking the computers themselves and also monitors. It was used because of its low capacitance level.

Other values of impedance are available although they are considerably less widely used. Some searching may be required to locate coaxial cable with an unusual impedance level.

Read more about characteristic impedance

Loss / attenuation specification

Another major parameter for coaxial cable is its loss or attenuation. It is found that there is a degree of loss as a signal travels along a coax cable. This arises from a number of factors and is present on all cables. It is also proportional to the length.

The coax loss or attenuation parameter is specified in terms of a loss over a given length. It is generally specified in terms of a loss measured in decibels over a given length, e.g. 0.5dB / 10 metres.

Unfortunately not all manufacturers define the loss over the same length and therefore comparisons take a little more calculation to determine.

Read more about coax cable loss

Power rating specification

Although for low level signal applications the power rating is unlikely to be important, where higher power levels are being carried, this specification can be an issue. Normally the limiting factor arises from the heat loss within the cable. If the power in the RF cable is to be pulsed, then it is necessary to check that the operating voltage is not exceeded.

Read more about power rating

Velocity factor specification

The velocity factor specifications of a coaxial cable is the speed at which the signal travels within the cable compared to the speed of the signal (i.e. speed of light) in a vacuum.

In some instances, the velocity factor specification for the coax cable may be of importance. For many areas where the coax is simply being used for feeding signals from one point to another, it will not be important.

For applications where the phase of the signal is of importance, the velocity factor needs to be known.

The velocity factor specification is quoted as a figure which is less than "1". It cannot go above unity otherwise signals would be travelling faster than the speed of light.

It is found that cables have very similar velocity factor figures. This is because the dielectric between the two conductors governs the velocity factor. Cables using a solid polyethylene dielectric will have a velocity factor around 0.66, and those using foam polyethylene will have velocity factor figures ranging from about 0.80 to 0.88.

Read more about velocity factor

Capacitance specification

For some applications the capacitance specification of the coax cable will be important. As can be imagined, there is a capacitance between the inner and outer conductors of the cable, and this is proportional to the length of cable used as well as the dielectric constant and the inner and outer conductor diameters.

Read more about coax capacitance

Maximum voltage

In some applications the voltage may rise to high levels. At some voltage it is possible the cable may break down, causing damage to the cable itself.

Voltages can arise as a result of high levels of standing waves and high power levels. Checks should be made, before selecting a particular type of coax, that it will be able to withstand the level of voltage anticipated.

Coax mechanical dimensions specification

The mechanical dimensions specification of the coax is important for a variety of reasons. The dimensions of different coax cables are obviously often different. Larger diameter coax cables often tend to have lower loss levels and higher power ratings.

As cable size may be important to ensure that it fits apertures etc this may be an issue. However one of the major reasons to know the size is to ensure that correct terminating connectors can be used. As connectors need to have the correct dimensions to ensure the cable will fit with the connector correctly, it is necessary to know the dimensions of cable. Often connectors will be made specifically for a popular size of cable.

By Ian Poole

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