Balanced vs unbalanced feed lines

The brief answer is that balanced lines have balanced flow of current on each conductor - think of them as being symmetrical in terms of current flow. Unbalanced lines (coax) have, due to their physical design, an unbalanced flow of current.

In addition, a coax cable really has 3 conduction paths even though it has only two physical conductors. These are 1) inner conductor, 2) the inner surface of the shield, and 3) the outer surface of the shield. That is, the shield acts as two conductors due to the skin effect of RF signals. The currents flowing on the outside of the shield are typically different from those flowing on the inside of the shield. This is why coax run to a vertical antenna typically has a "choke" to neutralize the outer shield currents. By the way, a vertical antenna is also considered to be unbalanced and is typically fed with unbalanced coax. A dipole antenna is a balanced design and is best fed with balanced line. A "balun" is used to join a BALanced line with an UNbalanced line either at the dipole itself, or somewhere along the way.

Unbalanced feed lines are typically coax and have a center conductor (the hot so to speak) and the outer shield or braid that is grounded. I.E. a hot and a ground.

A balanced line is typically ladder or window line consists of two parallel conductors at equal but opposite potential.

To transition between the two you use a balun, short for balanced-to-unbalanced.

73, Rick - KK4GV

To what Dan said, I would ask and answer the following questions:

Is coaxial cable inherently unbalanced?

What would happen if you used coaxial cable without grounding its shield conductor? Would it or could it then be a balanced line?

If not, why not?

The answer is that coaxial cable is never suitable for building a balanced transmission line. The physical construction of coax cable guarantees that even if you do not ground a coax cable's shield, this conductor will still exhibit a different impedance to ground than the inner conductor.

This is because the outside conductor, even if not grounded, is exposed to free space, while the inner conductor is not. Regardless of the distance from ground, or from a grounded object, any exposed conductor exhibits capacitance to ground. (In free space, capacitance to ground from any conductor is called isotropic capacitance.)

• Eamon VE2EGn

This is one of the most interesting questions in radio. It is hoped that all see the simple truth that all well performing lines have current symetry. It would be hard to understand how they work otherwise.

The reference to unbalanced here means grounded versus un-grounded transmission lines. The use of a balun (in all it's forms) isn't because coax has different currents in the conductors. It has to do with where those currents might flow. If the coax allows a path back toward the transmitter then the current in the antenna will be naturally unbalanced. This would have the line as a radiating element. That's not good in most cases.

Baluns can stop this by phase delaying (as in a half wave coax balun) or by causing a high impedance path to current such as in a sleeve balun or ferrite beads on the outside of the coax. Also some baluns work on flux coupling that in a sense use transformers to form a isolated ungrounded source. At any rate the currents in the coax conductor were equal and the balun keeps them from distributing in an unequal manner.

R Bibby K5BO

Balanced and unbalanced describes a certain circuit behavior in a Signal Source, Feedline or Load and most RF Communication – Electronics applications deal with 'two-terminal' Signal Sources, Feedlines, and Loads. This includes coaxial cable, two-wire ribbon, two-wire ladder line, two-wire open line, and systems working against earth ground or a ground plane (counterpoise) as the ”second conductor”.

Any two-terminal item (Signal Source, Feedline and Load) operating in ‘ideal’ fashion will have exactly equal and opposite currents flowing through each terminal. Current flowing out one terminal will be matched or duplicated by exactly equal current flowing in the other terminal at any instant of time.

1. Two-Terminal Items:

a. Signal Sources

 i.      The Transmitter stage of a Transceiver during transmission.
 ii.        An antenna during reception.

b. Feedlines

 i.      Coaxial Cable 50 Ohm Characteristic Impedance (Zo).
 ii.        Coaxial Cable 75 Ohm Characteristic Impedance (Zo).
 iii.      Two-Wire Ribbon 300 Ohm Characteristic Impedance (Zo).
 iv.        Two-Wire Ladder Line 450 Ohm Characteristic Impedance (Zo).
 v.      Two-Wire Open Line 600 Ohm Characteristic Impedance (Zo).

c. Loads

 i.      An antenna during transmission.
 ii.        The Receiver stage of a Transceiver during reception.

If the current is not equal and opposite on each conductor, the Feedline will radiate and receive unwanted signals. This is true no matter how good a shield is, or how many layers of shielding a cable has. Even the grounded shield of a coaxial cable has the same current as the center conductor (in a perfect system).

The difference between ideally operating unbalanced and balanced lines lies in system voltages, rather than currents. *Balance is referenced to voltage, not current, in an ideal system.*

Coaxial cable (50 Ohm & 75 Ohm) are 'unbalanced lines' and has significantly different voltage from each conductor to ground. In a "perfectly" working unbalanced (coaxial) line, the amount of voltage unbalance is infinite. One terminal (the shield) has zero voltage to the outside world, even while currents are equal and opposite. Current on the center conductor is balanced by an equal but opposite flowing current on the INSIDE of the innermost shield. No matter how we feed or connect a coaxial line, all current on the center conductor is always matched by an equal and opposite current on the inside of the innermost shield. If the two terminals of the load or source do not carry equal currents, some current will flow in a loop through the ground or along the OUTSIDE of the shield. The outside of the shield or shields is isolated by skin effect in the conductor wall. At Radio Frequencies (RF) the outside of the shield can be treated as an “independent conductor” connected to the “inside shield” at the ends of the coax.

Two-Wire Ribbon (300 Ohm), Two-Wire Ladder Line (450 Ohm) and Two-Wire Open Wire (600 Ohm) are 'balanced lines' and if "prefectly" operating they have equal and opposite voltages, as well as equal and opposite currents, all along the length of the line. Any difference in opposing voltages along the line can cause the line to radiate, since that often means currents will become unbalanced. All operating balanced lines are surrounded by an external magnetic and electric fields. This effect is caused by the necessary separation of conductors in the line. “To minimize radiation, balanced lines should be twisted or transposed at fractional wavelength intervals.” If you look at older open-wire telephone or signaling lines, they are periodically transposed.

The portion of current not equal in amount and opposite in phase on any transmission line is called the “Common-Mode Current”. The amount of opposing phase current is the “Differential-Mode Current”. Differential mode operation, is the normal desired method of operating a transmission line, and has impedance. This is the characteristic impedance we talk about when we say a line is 50 Ohm, 75 Ohm, 300 Ohm, 450 Ohm, or 600 Ohm characteristic impedance. This impedance is different than the common mode impedance.

Most transmission lines fall somewhere short of perfect examples, but the closer to perfect the less energy lost as unwanted radiation. Perfection also means that the feedline does not pick up unwanted signals and noise, and Radio Frequency Interference (RFI) will not appear on equipment near the transmitter unless it is from antenna or equipment radiation. In short, your antenna becomes the point of most signal reception and radiation. Most of us want the antenna to be an antenna, and the feedline, which often runs near computers, radios, TV sets, and noise sources to NOT be an antenna!

Best Regards Larry K4RFE (Retired RF Engineer)

I have been asked by an RF engineer for the company I work for to to build an RF/coax cable that is phase balanced. From reading this it appears that RF cable is always unbalanced.

Any feedback? Thanks Stacie

I'm just as confused as before reading the above "explanations" a few of which directly contradict each other.