Talk:Characteristic impedance

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Does it depend on the length of the cable?

Does the Characteristic Impedance depend (like resistance) on the length of the transmission line, or is it (like resistivity) independent of the length? Thank you.CountMacula (talk) 11:06, 22 September 2012 (UTC)

It is independent of length. But unlike resistivity, will change if the cross-section of the line is changed (or any other physical characteristic of the line is changed). In future, please go to WP:RDS to ask questions unrelated to improving the article. SpinningSpark 12:17, 22 September 2012 (UTC)

Thank you for that explanation. In future, please think rather than assume that a question is unrelated to improving the article. And in future, when a worthy improvement is pointed out with sublety, please do the actual improvement by editing the article rather than by responding only in the talk page. The article should blurt out right away that characteristic impedance is a property of the type of cable and is independent of length. Thanks again.CountMacula (talk) 00:54, 23 September 2012 (UTC)

I have made some improvements. -—Kvng 14:34, 25 September 2012 (UTC)

You wrote: "Characteristic impedance is determined by the geometry and materials of the transmission line and is not dependent on its length." Well said---perfectly clear right off the bat. Thank you.CountMacula (talk) 00:03, 27 September 2012 (UTC)

I try to give people the benefit of the doubt, as far as applicability to improving the article. If they have to ask a question that the article should answer, then it helps the article. Gah4 (talk) 23:13, 28 September 2021 (UTC)

Flow of reactive power

The "Surge impedance loading" section claims that the line supplies or absorbs reactive power depending on whether the load is less than or greater than the SIL. I'm not sure what this is driving at, but it is incorrect as written. The flow of reactive power reverses every quarter cycle at any loading (except perfect match). It is not possible for the line to continuously supply or absorb power; the first case is free power forever, and the second will quickly melt the line. Perhaps what is meant is that, in the case of low loading, the overvoltage causes the source to supply more power than predicted by the nominal line voltage, and vice versa for high loading. But this is not a flow of reactive power, it is a flow of regular real power. SpinningSpark 09:00, 17 November 2021 (UTC)

I am not sure this helps, but it reminds me of the impedance of some loads, often antennas, which are inductive or capacitive depending on the characteristics. For a dipole antenna, if you are a little off one way or the other from the ideal match, then it looks like an inductor or capacitor in parallel (usually) with the desired resistive load. An ideal antenna radiates all, and so has no reactive power. A less than ideal antenna will have reactive power, and that will depend on it looking like a capacitor or inductor. That is, the sign of reactive power changes for inductor vs. capacitor. Also, what is SIL? Gah4 (talk) 17:44, 31 May 2022 (UTC)

You have me reaching way back to my undergraduate days. My first course in network theory was taught by a professor who had worked in the power industry. We were talking about leading, lagging, and power factor. He discussed how the power utility company would charge a premium if you let you power factor get too low. Conversely, if you could maintain a leading power factor then you could negotiate a rate reduction. He referred to as buying and selling kvars. From his point of view, a factory with a leading power factor was producing kvars and the a factory with a lagging power factor was consuming kvars. I wonder if that is the meaning of producing and absorbing reactive power in the SIL discussion? Anyway, if the article isn’t clear enough that the three of us cannot figure it out, maybe the dubious content should be removed. Constant314 (talk) 00:49, 1 June 2022 (UTC)

Yes, I've seen metering in factories of kVArh as well as kWh for just that purpose. Also, (but a long time ago so I may have misremembered) I believe peak kVAr was recorded, for which there was a penalty for going over a certain figure. SpinningSpark 07:27, 1 June 2022 (UTC)

I think that I know what this means now. If you have a power transmission line that is electrically short (which is almost always the case at 50/60 Hz) and load it resistively with an impedance that is greater than the surge impedance, then the impedance presented by the transmission line and its load, at the driven end, has an capacitive reactance which would cause a leading power factor at the driven end. If you load the transmission line with a resistance that is less than the surge impedance, then the transmission line presents a lagging power factor at the driven end. Where the article text refers to the system, I presume that means the system at the driven end of the transmission line.

Given that, however, the load is almost never a pure resistance. When the line is short, as seen from the driven end, the line does not modify the load impedance very much. If the load is an ordinary lagging load, the impedance seen at the driven end is a lagging load regardless of whether the power draw is less than or greater than the SIL. So, I think that the article text is correct given a resistive load, but is practically unimportant. I am inclined to remove the text from the article. Constant314 (talk) 02:08, 3 July 2022 (UTC)

I don't think this effect has anything to do with reactive loads. But I agree on removal and have edited out all talk of flow of reactive power. SpinningSpark 07:42, 3 July 2022 (UTC)

It's nothing to do with reactive loads; additionally the fact is not practically unimportant but of considerable importance in electrical transmission. Any transmission line can be viewed as having shunt admittance (to ground), which generates lagging vars; and series reactance, which absorbs them. The vars generated by the shunt admittance are proportional to the square of the (rms) voltage, and so for practical purposes are largely constant. The vars absorbed by the series reactance is proportional to the (rms) current, which is a measure of line loading. At some load point, the vars generated by the shunt admittance become equal to those absorbed by the series reactance; at that point the line is said to be operating at its natural loading. Because buried cables have far greater shunt admittance than their overhead line counterparts, a buried cable is always a source of lagging vars and therefore tends to increase system voltage. This is of vital importance in large cities largely supplied by cables, as the voltage gain at times of light load can exceed operators' abilities to contain it. —BillC ^talk 13:43, 9 July 2022 (UTC)

As interesting as that is, I think that it is an obscure fact for an article on characteristic impedance. Constant314 (talk) 14:29, 9 July 2022 (UTC)

I agree with BillC, this is clearly important in power engineering. It only seems obscure to you and me because it is not our field. For a medical researcher specializing in modelling the human ear, all this talk of telegraph transmission probably looks even more obscure. SpinningSpark 07:11, 10 July 2022 (UTC)

Sounds like a complicated way to say that underground transmission lines have a lot of capacitance and that can be a problem for system operators. Although I think BillC got leading and lagging backward. An unloaded or lightly loaded transmission line looks like a capacitor which would cause leading current which would raise voltage in a system that was dominated by lagging loads. Constant314 (talk) 08:47, 10 July 2022 (UTC)

I suspect this isn't quite as obscure as it looks. I believe it also occurs in some radio antenna designs, especially those that resemble transmission lines. An open end line has a positive reflection at the far and, such that the voltage is twice the source voltage. Radio people use VSWR, standing wave ratio, to describe it. At 60Hz, the line would have to be pretty long, but some lines are long. Gah4 (talk) 23:33, 5 December 2022 (UTC)

Archiving size

User:Spinningspark wants a smaller archive size 17500 bytes. I think that the maximum size 17500 Byte of one archive is very small. Sawol (talk) 07:18, 26 March 2022 (UTC)

Let's not have this discussion repeated on multiple pages. See Talk:Fractal antenna#Archiving size. SpinningSpark 09:09, 27 March 2022 (UTC)

{{about}}

I notice that the {{about}} has acoustic impedance but not mechanical impedance. I wonder if it should have both? Gah4 (talk) 00:28, 1 June 2022 (UTC)

The mechanical impedance article does not cover characteristic impedance so unless something is added, it should not be in the hatnote. However, a better question is why are we limiting this article to the electrical domain? The concept of characteristic impedance originated on telegraph lines, but has been extended to, and is fundamentally the same concept, in all energy domains. We would be better off with a section that explained that with links to articles that had in-depth information on particular energy domains. SpinningSpark 07:40, 1 June 2022 (UTC)

In the early days of electrical systems, especially RLC circuits, but I suspect including transmission lines, it was usual to use the mechanical analogy. Now, it is more usual to use the electrical analogy for mechanical systems. Characteristic impedance is most useful when it is (close to) constant, and so more characteristic. Mechanical systems are often less ideal, but often enough have a characteristic impedance. Waves on a violin string are a very close analogy to waves in a coaxial cable. (There should be overlap between acoustic impedance and mechanical impedance, but I haven't figured out that yet.) In any case, yes, this article should generalize, but also the other articles should explain it, too. Reminds me, in my undergraduate physics class there was a lecture demonstration showing the analogy between voltage on a coax cable, and pressure wave in a tube, with open end and closed end configuration. During the demonstration, it was figured out that the analogy was wrong. Next lecture, the demonstration was back, but with current probe on the oscilloscope. Gah4 (talk) 11:50, 1 June 2022 (UTC)

OK, the way I got here was from turbofan which is a jet engine more closely impedance matched to subsonic flight. That is, the force (thrust)/velocity is better, and so more efficient, and so should be a mechanical impedance. I suspect that article needs some work, but the first problem was that it says harmonic motion. (Not especially useful for an airplane.) As above, you can get an impedance for a wave on a string, or through gas in a tube. Gah4 (talk) 12:26, 1 June 2022 (UTC)