What Are Balanced and Unbalanced Transmission Lines?

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Discussion Overview

The discussion revolves around the concepts of balanced and unbalanced transmission lines, focusing on their definitions, characteristics, and implications in signal transmission. Participants explore the role of reference points, the necessity of additional conductors, and the impact of these configurations on noise reduction and signal integrity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants explain that balanced transmission lines have two conductors carrying equal and opposite signals, while unbalanced lines typically use a shield as a reference point.
  • There is a discussion about whether balanced signaling requires three wires, with some arguing that it does, while others assert that it can function with just two wires.
  • One participant mentions that the purpose of a third wire in balanced systems is to reduce noise and interference, similar to differential signaling in operational amplifiers.
  • Another participant challenges the necessity of a third wire, citing examples of balanced systems that operate effectively without it, such as dipole antennas and certain audio connectors.
  • Some contributions highlight that the distinction between balanced and unbalanced lines relates to impedance and signal integrity rather than the number of conductors alone.
  • There is mention of specific types of cables, such as twinax, which are designed for balanced signals and include a shield for noise reduction.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of a third wire in balanced transmission lines, with no consensus reached. Some agree on the importance of impedance matching, while others dispute the definitions and requirements for balanced signaling.

Contextual Notes

Participants reference various definitions and practical examples, indicating that the understanding of balanced and unbalanced lines may depend on specific contexts and applications. The discussion reveals a range of interpretations regarding the role of reference points and the implications for signal transmission.

  • #31

The situation gets more complex at higher AC frequencies. Currents flow in the inner conductor (but more and more on the outside of the inner conductor as the frequency increases) and on the inner surface of the outer conductor. These two surfaces carry the forward and return paths of the signal.

I guess that is the so called skin effect...

Current induced on the outer side of the coaxial shield will return to the of the the transmitter case causing RF burns etc. This does not happen with the normal outgoing signal, both sides of which originate inside the transmitter case.

What do you mean by both side of the outgoing signal?

I know it probably sounds weird that the same conductor can have different voltages on different parts of it without a current flowing between them, but this is a consequence of the frequencies involved.

It is surely weired. I have read about the strange effects of transmission lines. Along a line there could be standing waves so the voltage at certain points can be different from other points. But voltage remains a relative concept so it must always be measured between two points...
When you mention that "the same conductor can have different voltages on different parts of it without a current flowing between them" are you referring to which conductor, the inner cylinder or the outer one?
 
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  • #32
I guess that is the so called skin effect...

Yes.

What do you mean by both side of the outgoing signal?

A signal is generated inside the transmitter. This may be coupled to the outside like this:

[PLAIN]http://dl.dropbox.com/u/4222062/RF%20output.PNG

A small coil is placed near the end of a larger coil and this picks up a signal for transmission.

Both ends of the coil must be taken to the transmission line, even if one side of the line is grounded as it passes through the case, as is usually the case with a coaxial connector.

When you mention that "the same conductor can have different voltages on different parts of it without a current flowing between them" are you referring to which conductor, the inner cylinder or the outer one?

Mainly the outer one. The shield may be only a few tenths of a millimeter thick, yet it can have totally different signals on the inside and outside of it.
 
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