Common mode current and coaxial feed lines

An antenna feed line is intended to convey energy from the transmitter to the antenna, and usually without giving rise to radiation itself.

The term “common mode” comes from consideration of the currents on an open two wire line, and it refers to the netĀ  or unbalance current, ie the current that would give rise to external fields, to radiation.

This article looks at the equivalent common mode current in a coaxial transmission line.

Firstly, a review of the way in which coaxial cable operates.

Where coaxial cable has fully developed skin effect, and used in TEM mode (the usual mode below microwave frequencies for most practical cables), it has three conducting paths:

  • the outer surface of the inner conductor;
  • the inner surface of the outer conductor; and
  • the outer surface of the outer conductor.

These three paths are isolated from each other, except at an open end of the shield where the inner surface of the outer conductor and the outer surface of the outer conductor effectively join. It is skin effect that effectively isolates the inner surface of the outer conductor from the outer surface of the outer conductor, it is as if the shield was two concentric metal tubes with an insulating layer between them, and that at an open end of the shield, the two tubes are connected.

Another important property of such lines is that at any point along the line, the current on the inner surface of the outer conductor is equal to, but opposite in direction to current on the outer surface of the inner conductor.

Practical lines are not ideal, but for most practical lines, the departure from the above is small and it is a good model for understanding practical lines. The above might seem a bit complicated, but it is worth understanding as so many applications can be explained by this simple set of properties of coaxial line.

CoaxTEM

The diagram above shows the currents adjacent to the open end of the shield of a coaxial cable. Current I1 flowing rightwards on the outer surface of the inner conductor has an associated current I1 flowing leftwards on the inner surface of the outer conductor. A node is formed at the shield end where the inner surface of the outer conductor is connected to the outer surface, and so for I1 flowing left to exist on the inner surface, there must be a current I1 rightwards on the outer surface of the outer conductor.

Now to common mode current, the ONLY current that flows on the outer surface of the coax shield is COMMON MODE CURRENT (a consequence of the properties above). Just as the common mode current in an open two wire line gives rise to external fields, to radiation, so also does common mode current in a coaxial line.

CoaxTEMCM

The diagram above shows the case at the end of the shield where two conductors connect the antenna load to the coax (eg to each leg of a dipole). In an antenna system in the presence of ground, I1 need not equal I2, and it can be seen that when I1 is not equal to I2, there is a current I2-I1 flows on the outer surface of the outer conductor, this is the sum of the common mode components of the currents I1 and I2. Kirchoff’s Current Law is satisfied at the node denoted by the black dot, the sum of the currents into the node is zero.

Common mode current means that the outer surface of the coax is one of the radiating conductors on transmit, and on receive, currents induced in it by external fields (from distant AND nearby sources) contribute to the total power delivered via the coax inner conductor to the receiver.

An important effect to keep in mind in assessing the impact of common mode current, is that the power flux density can fall as the fourth power of distance in the near field area, that means doubling distance may reduce power flux density to 12.5%. That means that increasing the distance between your effective radiator conductors and receive noise sources (house wiring appliances etc) or things subject to interference when transmitting (amateur station itself, television, computers, Hi Fi etc) can achieve large reduction in the risk of interference. There are other benefits like preserving the radiation pattern of the intended radiator. One way of maximising the distance between the radiating conductors and these things is to ensure that your feed line is not a radiating conductor.

Common mode current is real, on transmit it is easily measured using simple equipment.

In summary, minimising common mode current improves station performance by:

  • reducing noise pickup from conductors near the feed line;
  • reducing induction into other conductors and eventually appliance of transmit energy, so improving Electro Magnetic Compatibility; and
  • preserving the radiation pattern of the intended radiator.

Some types of antennas are particularly prone to high common mode current, others less so, and preventative design can reduce the risk of problems (like interference with neighbour’s equipment) that can be much harder to resolve after the fact.