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What makes a feed line balanced or unbalanced?

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6 Answers

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Balanced means that each of the conductors has the same impedance to ground. Generally, when you look at the physical makeup of the feedline, the two (or even more) conductors of balanced transmission line look the same, and you can't tell polarity if the wire went into a box and came out the other side. Coax cable is unbalanced, since one side IS ground, the other conductor (center) is above ground. Coax is an example of shielded transmission line, which is nice because what you do outside the cable has no effect on the RF. There is a type of balanced line you can buy that is also shielded, there are three conductors in it, two center wires and the shield, so whether it is shielded or not does not have anything to do with balanced or unbalanced.

Unlike one of the other answers, it is not balanced currents. Coax has balanced current, all transmission line does or you are in trouble, it becomes an antenna if the currents aren't balanced. I'm having a lot of trouble with the coax as 3 conductors theory, too. Not the right way to think about it. Skin effect is another different story, and has nothing to do with why you choke the "outside" of a coax. What is described above is because there is a 3rd conductor, but it has nothing to do with the coax, it is somewhere else, unbalancing the currents in the coax.

A good example of all this is power lines. Sometimes you see three wires going down poles or towers, sometimes you see two with one ground. 3 wires are balanced, this is 3-phase power which is really neat, a very efficient way of sending power. All three are at the same voltage to ground, but the AC is 120 degrees out of phase. But they take one phase of this and run it to a transformer for your house, which makes it unbalanced, but then the secondary has (usually) the center tap grounded with two 120V hot wires that are 180 degrees out of phase, balanced again. But then in your breaker box you use one side to run to outlets, unbalanced. Telephone lines are 100% balanced, mainly so you can get 600 pairs packed together cheaply in one big fat cable without cross-talk.

In general, balanced transmission line is cheaper and lower loss, but you can't put anything metal near it, and coax is more common because it's easier to deal with due to being self shielded with ground on the outside. Cable TV systems are 100% coax (75 ohm) and there is so much of this around it is now pretty cheap. If you're on a budget, visit the local CATV office and look in the dumpster. 100 feet is scrap for them, and they have everything from small RG-6 to very low loss 1" diameter solid aluminum, great for VHF. Most times you won't even notice the difference of 75 vs 50 ohms. But you'll have a hard time finding a connector for it.

-Dan, KW2T

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Dan, your answer has done a great job of explaining a technical subject in an easy to understand way. Best Regard K4RFE Larry – K4RFE Jul 30 at 23:58
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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.

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Here is a link to the feedline primer. There is a lot of really good information in this document. Certainly more than the question asks, but extra detail is never a bad thing.

http://www.qsl.net/kk6mc/FeedLinePrimer.txt

73, Jerry kd0bik

Host of The Practical Amateur Radio Podcast

http://myamateurradio.com

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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

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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
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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

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