Coax Cable Characteristic Impedance Explained
All feeders possess a characteristic impedance. For coaxial cable there are two main standards that have been adopted over the years. These are 75 Ω and 50 Ω.
The 50 Ω coax cable is used for professional and commercial applications whereas the 75 Ω coax cable is used almost exclusively for domestic TV and VHF FM applications.
The reason for the choice of these two impedance standards is largely historical but arises from the properties provided by the two impedance levels:
•75 ohm coax cable gives the minimum weight for a given loss
•50 ohm coax cable gives the minimum loss for a given weight.
Although these two standards are used for the vast majority of coax cable which is produced it is still possible to obtain other impedances for specialist applications.
Higher values are often used for computer installations, but other values including 25, 95 and 125 ohms are available. 25 ohm miniature RF cable is extensively used in magnetic core broadband transformers.
These values and more are available through specialist coax cable suppliers.
The impedance of the RF coax cable is chiefly governed by the diameters of the inner and outer conductors.
On top of this the dielectric constant of the material between the conductors of the RF coax cable has a bearing.
The relationship needed to calculate the impedance is given simply by the formula given in one of the images.
In the first he following parameters are used: Zo = Characteristic impedance in Ω εr = Relative permeability of the dielectric D = Inner diameter of the outer conductor d = Diameter of the inner conductor
The coax impedance is one of the major specifications associated with any piece of coax cable. As it will determine the matching within the system and hence the level of standing waves and power transfer, it is a crucial element. It is therefore necessary to ensure that the correct coax impedance is chosen for any system.