Coax Impedance / Coaxial Cable Impedance

- details of the calculation, application and determination of coax impedance - coax cable impedance.

Coax cable tutorial information includes:

    •  Coaxial feeder overview
    •  Coax impedance
    •  Loss or attenuation
    •  Coax power rating
    •  Coax velocity factor
    •  Coax environmental factors
    •  Coax data & specifications
    •  Coaxial installation tips

All forms of feeder including coax cable have a characteristic impedance. The coax impedance is one of the main parameters in its specification, one that governs which type of coax cable is obtained.

Coax impedance / characteristic impedance

All feeders posses a characteristic impedance. For RF coax cable there are two main standards that have been adopted over the years, namely 75 and 50 ohms.

75 ohm coax cable is used almost exclusively for domestic TV and VHF FM applications. However for commercial, amateur and CB applications 50 ohms coax cable has been taken as the standard. The reason for the choice of these two standards is largely historical but arises from the fact that 75 ohm coax cable gives the minimum weight for a given loss, while 50 ohm coax cable gives the minimum loss for a given weight.

These two standards are used for the vast majority of coax cable which is produced but 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.

Coax impedance determination

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:

D = Inner diameter of the outer conductor
d = Diameter of the inner conductor

Capacitance and inductance

The capacitance of a line varies with the spacing of the conductors, the dielectric constant, and as a result the impedance of the line. The lower the impedance, the higher the capacitance for a given length because the conductor spacing is decreased. The capacitance also increases with increasing dielectric constant, as in the case of an ordinary capacitor.

It is also often necessary to know the inductance of a line as well.

Importance of coax impedance

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.

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