Coax Tutorial Includes:
Coax feeder Coax specifications overview Coax impedance Coax loss / attenuation Coax power rating Coax velocity factor Coax environmental Coax installation tips Coax cable types Tips for choosing right coax cable Buying TV coax: points to note
Understanding the different specifications used to define the performance and mechanical characteristics of coaxial cable enables the right form of feeder to be chosen for the given situation.
There is a variety of the different coax cable specifications addressed below: some on this page and some are explained more fully on other pages.
Characteristic impedance specification
One of the most important specifications defining any type of coaxial cable 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. In order to ensure maximum power transfer it is necessary to match impedance of the signal source to the feeder, and then the feeder to the load.
The most common impedance figures are:
- 50/52 Ω: This cable is the form that is generally used for professional RF applications – everything from leads associated with test equipment, to equipment internal leads and then receivers and transmitters to antennas..
- 75 Ω: This impedance is more widely used in domestic applications for television and Hi-Fi RF signal leads, like those connecting the TV of VHF FM antenna to the television set or radio tuner..
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 coax cable characteristic impedance.
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 cable characteristic impedance & capacitance.
Loss / attenuation specification
Another major performance parameter for coax is the loss or attenuation. There is always a reduction in signal as the signal moves through a feeder. 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, normally 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 / attenuation.
Power rating specification
One specification that is particularly important for high power applications is the power rating. Normally the limiting factor arises from the heat loss within the cable. If the power in the RF cable is to be pulsed it could be maintained within the power limit as a result of heating. However it is also necessary to ensure that the maximum voltage is not exceeded. . . . . . Read more about coax cable characteristic power rating.
Velocity factor specification
The velocity factor of a coax cable can be important in a number of different applications. Effectively the velocity factor 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 many areas where the coax cable is being used for simply feeding signals it is not of major importance. However 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. . . . . . Read more about coax cable velocity factor.
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 power levels or as a result of lower power levels but a high standing wave ratio. 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.
Bend radius specification
One parameter that can be of importance in some instances is the bend radius.
It is important not to bend the coax cable too sharply otherwise it can damages the internal construction, changing its properties, presenting a discontinuity in the impedance, but more importantly destroying the construction in the region of the bend.
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