How does a Shield Affect Cable Capacitance?

By Douglas Krantz | Electronics

Shielding and Cable Capacitance

A capacitor is "two or more conductors separated by an insulator(s)". If there are two conductors in a cable, and each conductor has an insulator, that is one capacitor.

Adding a shield around the two conductors is adding another conductor around the two wires. Between each of the two inner conductors and the shield is an added capacitor. In other words, two more capacitors are added to the first capacitor.

Electrons and Capacitance

In any capacitor, the quantity of electrons does not determine the capacitance; the capacitance, combined with the voltage, determines the number of electrons and positive charges that are accumulated.

Using a drain wire does not "drain away" the capacitance. To some extent, the earth-grounded drain wire might drain away some of the electrons, but that draining is affected by the length of the wire as compared to the wavelength of the signal that the wire is carrying.

A Circuit is a Circle

Also, "draining away electrons" in not like pouring the electrons down the kitchen sink, never to be seen again. Electrons work in a circle; missing electrons have to be replaced. The only location that these "missing" electrons can be found by the circuit is inside the earth-ground. They come back up from the earth-ground and into the circuitry of the equipment at each end of the cable.

The draining of electrons affects the circuitry at each end of the cable. In other words, using an earth-grounded drain wire doesn't really reduce the effects of the capacitance in the cable.

Surface Area, Separation, and Insulator Dielectric Constant

There are three things that affect the capacitance of any capacitor, including the capacitance inside a cable.

Surface Area: In a cable, the surface area of a conductor is determined by its circumference times its length. If the length of the cable is 1000 feet (around 300 meters), the surface area of each of the inner conductors is their circumference times 1000 feet (about 300 meters). The greater the diameter, the greater the surface area; the smaller the diameter, the smaller the surface area.

When reducing the total capacitance of the cable, the diameter of the conductors inside the cable can be reduced, but there are limits. Both effects, the ability of the wire to carry the signal in the first place, and the resistance of the wire over its length, has to be taken into account.

Separation: The distance between each conductor affects the capacitance. The greater the distance between conductors, the smaller the capacitance; the lessor the distance between the conductors, the greater the capacitance.

When reducing the capacitance of the cable, increasing the thickness of the insulators around the inner conductors, and increasing the thickness of the insulating barrier between the inner conductors and the shield, will reduce the capacitance of the cable. But again, there are limits. The overall thickness of the cable increases with the thickness of the insulators, and the overall weight of the cable also increases with the greater mass of the insulators.

Insulator Dielectric Constant: The dielectric value is the ability of the insulator to conduct electrical forces. The lowest dielectric value (vacuum), when used as a capacitor's insulator, gives the capacitor the lowest capacitance; higher dielectric values (rubber, PVC, Vinal, paper, glass, etc.), when used as a capacitor's insulator, have higher capacitance.

Changing the type of insulator to a type that has a lower dielectric value will also reduce capacitance, but there are limits. Manufacturing techniques, material reliability, material availability, are just some of the limiting factors in the insulator material.

All Three

The wire size (affecting the surface area of the cable), the thickness of the insulation (affecting the overall diameter of the cable, and how far the conductors will be separated), and the material type of insulation (affecting the insulator's dielectric constant).

Adding a shield to the wire adds another two capacitors (the shield to one conductor and the shield to the other conductor).

Using Cable to Carry Signals

Read the specifications for the electronic equipment being used. If the manufacturer has a concern over cable capacitance, make sure the capacitance of the cable won't degrade or corrupt the signal being sent on the cable.