related pages at antennas  and transmission lines

Concentric Transmission Lines (Coaxial Cable)

When the shield is more than several skin depths thick coaxial cables, single-wire shielded cables, or concentric lines become the electrical equivalent of three wire concentric transmission lines. Skin depth isolates the inner and outer wall. 

Electrical rules require the center conductor and inside of the shield to always carry equal and opposite RF currents.

Notice in the drawing above the center and inside of the shield carry equal and opposite direction RF currents. This ALWAYS is the case when the shield is several skin depths thick.  We cannot force anything else to happen!

In the drawing above and below, the outside of the shield is isolated by skin effect. It behaves like a separate transmission outer conductor. Skin effect prevents any current, voltage, or field (even magnetic) from penetrating the shield when the shield is many skin depths thick. Only the breaks at the ends connect the inner and outer shield conduction layers.

The only egress or ingress points for voltage and current are where discontinuity in the shield might occur through an intentional or accidental break in the shield.

Radiation In or Out of Shielded Lines

To operate without radiation, coaxial cables require equal and opposite currents in the shield and the center conductor. The outside of the shield must not have an RF potential difference along it's length. We cannot have voltage gradients (electrical fields) along the length, or the outer conductor layer will conduct currents. This means both ends of the cable should have zero volts to earth or the environment the cable operates in. 

If we feed exactly equal and opposite current into the coax, currents have no choice but to flow INSIDE the cable shield! The outermost conductor can be treated as a single wire, since everything else is "hidden inside". There will be no "extra" current to flow over the outside of the shield. Because of this rule, we do not need to ground the shield to prevent radiation. The trick here is we need to have equal and opposite currents. When the shield has potential difference along the length we can greatly reduce outside currents by making the outside of the coax have a high impedance. We can do this by selecting a proper cable length, by adding sleeves of soft-iron magnetic materials or winding the cable in a coil with or without a core to form a choke. Skin depth prevents the inside of the cable from "seeing" what is done on the outside.

Most interesting in all of this is the claim or idea the "shield" of a shielded loop allows magnetic fields to pass while filtering electric fields. The same rules apply as with coaxial lines. The shield actually becomes the antenna, and it couples to the inside only at shield gaps. If the gaps aren't exactly opposite the grounded part of the loop and inner conductors don't exit exactly at that grounded point, the shield actually  UNbalances the loop!

Coax as a Shielded Ground Lead or Radial

Assume 1 ampere of RF current.

1A shows current in a single conductor.  In this case current is one ampere.

1B shows current when center conductor is insulated and fed with 1 ampere. In this case ALL currents are 1 ampere, but the effective resistance and reactance of  point A to B increases. This line radiates just as well as the line in 1A, but impedance and loss resistance increases.

1C shows current when a so-called "shielded ground" is used. With low reactance capacitors, if A to C is fed current from B to D inside the cable is negligible. The same is true if B to D is fed when looking at current flowing from B to D. All current is on the cable's outside wall. The RF impedance is the same as the shield alone. The line freely radiates with one ampere of unopposed current flowing over the outside wall.

1C if the capacitors are removed and the center conductor C to D is fed RF current, the inside of the shield and the conductor between B and D will have no RF current. The impedance is the same as the shield alone. The line once again freely radiates.

The same effects occur when the coax is used as a radial or as an antenna. It is electrically impossible to shield a ground lead.

since Dec 2004