INSIDE A COAXIAL cable the currents (Fig.4) on the centre conductor (I1) and the inside of the shield (I2) are equal and opposite (180° out of phase). The two conductors are very closely coupled along their entire length, so this condition of equal and opposite currents is very strongly enforced. Because of the skin effect, these RF currents flow only on the surfaces of the conductors, and in particular I2 flows only on the insideof the outer conductor. The electric field is completely confined inside the shield, and the external magnetic fields of the two conductors cancel because the currents are exactly equal and opposite. This is the main reason why coaxial cable is so successful in ignoring its external environment - everything happens on the inside.
Unfortunately this near-perfect screening doesn't apply at the open end of the cable. If the antenna currents on either side of the feed-point (Fig 5) are even slightly unequal, the difference (I3 = I1 - I2) will flow back down the outside of the cable. The skin effect makes the outer-surface current I3 quite separate from the current I2 on the inside surface of the shield. It is the outer-surface current I3 that causes all the problems.
Fig 4: Currents on the Inside of a coaxial cable (l1 and l2) are always equal and opposite. The skin effect allows a separate current l3 to flow on the outside surface.
Fig 5: If antenna currents on either side of the feed-point are unequal, the difference I3 =(I1 - I2) will flow down the outside surface of the cable.
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