How is the transfer impedance of cable explained easily?

Cable braiding is a technique used to reduce electromagnetic interference (EMI) in cables by wrapping them in a braid of conductive material, such as copper or aluminum. The braid acts as a shield, blocking EMI from entering or leaving the cable. The transfer impedance, also known as the common-mode impedance, of a cable braid is a measure of its effectiveness as a shield. It is defined as the ratio of the voltage at the outer surface of the braid to the common-mode current flowing through the braid, and is typically measured in ohms.

The transfer impedance of a cable braid is a frequency dependent parameter, which means that it will change with the frequency of the signal. In general, the transfer impedance of a cable braid will decrease as the frequency of the signal increases. This is because at high frequencies, the skin effect causes the current to flow mainly on the surface of the conductors, rather than throughout the entire cross-section of the conductor. As a result, the current flowing through the braid will be concentrated on the outer surface of the braid, where it can be effectively shielded by the braid.

The transfer impedance of a cable braid is also affected by various other factors, such as the type of braid material, the braid coverage of the cable, the braid tightness, the cable insulation, and the frequency of the signal. For example, using a braid made of a highly conductive material, such as copper, will result in a lower transfer impedance than using a braid made of a less conductive material, such as aluminum. Similarly, increasing the braid coverage of the cable will result in a lower transfer impedance than using less braid coverage.

It is also known that the transfer impedance of a cable braid can be modeled using the following equation:

Zt = (2π * a / ln(b / a)) * (E / I)

where Zt is the transfer impedance, E is the electric field at the outer surface of the braid, I is the current flowing through the braid, a is the radius of the cable, and b is the radius of the braid. The term (2π * a / ln(b / a)) is a geometric factor that accounts for the shape of the braid.

In summary, the cable braid transfer impedance is a measure of the effectiveness of the braid as a shield against EMI. It is a frequency dependent parameter that decreases as the frequency of the signal increases, and is affected by various factors such as the type of braid material, the braid coverage of the cable, the braid tightness, the cable insulation, and the frequency of the signal.

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