Talk:Reflections of signals on conducting lines

Voltage sign?
Shouldn't the voltage in figure 1 be of opposite sign since it's a reflection? — Preceding unsigned comment added by 71.61.20.140 (talk) 12:03, 2 March 2013 (UTC)


 * No, the reflection is from an open-circuit and is the same sign as the incident wave, if the termination were a short-circuit then the reflection would be negative. Consider a voltage source that when terminated with an impedance matching its internal impedance has an emf of E at its terminals.  Such a source will have an open-circuit voltage of 2E and a short circuit voltage of 0.  Thus, when connected to a transmission line, we know the open-circuit voltage at the end of the line must end up as 2E when all the transients have died away.  Assuming the line is matched to the source, the incident wave will be E.  The reflected wave must therefore be +E in order for the final voltage to be 2E.  Likewise, for a short-circuited line the reflected wave must be &minus;E in order for the final voltage to be zero.  Spinning  Spark  14:50, 2 March 2013 (UTC)


 * What is the change in phase of the reflected signal for a purely reactive load relative to? I can't see how the change in phase relative to the incident wave is -2arctan(Xl/Ro). By inspecting the Smith chart, would it not appear that the phase of the complex reflection coefficient at the load is


 * θ = π - 2arctan(Xl/Ro) for purely inductive loads


 * θ= -π - 2arctan (Xl/Ro) for purely capacitive loads (where Xl is negative)


 * So if the reactance of the load is zero (and of course the resistance of the load is zero as this is the case for purely reactive loads) the phase is ±180° and if the reactance is infinitely high the phase is 0°. Also if the normalised impedance (Xl/Ro)= 1 (inductive load) then the phase is 90° and -90° if (Xl/Ro) = -1 (capacitive). Shropshire70 (talk) 22:17, 18 June 2013 (UTC)


 * There is a voltage wave and a current wave and their product determines the direction that the power flows. When the wave reflects, power in the reflected wave is going in the opposite direction to the power in the incident wave.  To reverse power the product of voltage and current must reverse.  In the case of an open termination, the current changes sign and the voltage does not.  If the incident wave is traveling left to right and the direction of the current is left to right then in the reflected wave the direction of the current is right to left.Constant314 (talk) 15:22, 24 August 2013 (UTC)

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This is "duplicated" here for when the review is no longer transcluded.

Congratulations! This article has passed as a Wikipedia Good Article. Sincerely, North8000 (talk) 01:42, 27 September 2013 (UTC)  Reviewer

Is "wave" correct terminology
Is "wave" correct terminology for pulses being reflected in a conducting line? I see that term used extensively in the literature, but I feel it is inaccurate and in use simply because the pulse's movement would resemble a wave. Please advice. — Preceding unsigned comment added by 2001:700:300:1724:0:0:0:63 (talk) 17:59, 7 October 2015 (UTC)


 * Generally wave means a solution of the wave equation. Although we focus on current and voltage, there is literally an electromagnetic wave propagation in the dielectric between the conductors.Constant314 (talk) 20:22, 7 October 2015 (UTC)


 * As far as my understanding of this "wave" goes, it propagates due to fact that it has to charge all the capacitances in the "infinite" amount of exact-PI equivalents in series. It cannot fill C2 before C1 is full, and so on. Further, the relationship between the cable capacitance and inductance yields the propagation speed. An electromagnetic wave should be traveling in air, like visible light or radio signals etc. In this topic, I think that the correct terminology is "propagating (voltage) pulse", "reflection of (voltage) pulse" etc. The term "wave" should be left out altogether.2001:700:300:1726:0:0:0:1D (talk) 00:25, 8 October 2015 (UTC)


 * Waves propagate readily in the dielectrics such as glass and Teflon and even weakly into metal. There is a full featured electromagnetic wave in the dielectric that fully satisfies Maxwell’s equations and the wave equation.  The insulation is in effect transparent at the frequencies of the pulse. The transmission line analysis happens to exist right on the border between circuit theory and full electromagnetic field theory.  The transmission line can be solved entirely by electromagnetic field methods, but thanks to Heaviside’s learned insight the fundamental mode can also be solved as an infinite series of identical infinitesimal pi-sections.  But the ultimate arbitrators on Wikipedia are reliable sources.  There are literally hundreds of text books that call it a wave.Constant314 (talk) 00:47, 8 October 2015 (UTC)


 * Thank you for clarifying. 2001:700:300:1726:0:0:0:28 (talk) 19:38, 8 October 2015 (UTC)

Signal reflection
Would someone be able to merge some of this article to signal reflection? It would help to have some of the explanatory material from this manifestation of the phenomenon (especially since the other article doesn't have any sources), but I don't feel familiar enough with it to do it myself. Thank you. — Sasuke Sarutobi (talk) 12:49, 14 December 2015 (UTC)
 * I believe that article should be merged here. fgnievinski (talk) 01:53, 28 March 2016 (UTC)


 * Oppose merge. This article is specifically about reflections on conductors.  Signal reflection is a broader topic and includes material on optical fibres which would be inappropriate to merge here.  In principle, that article could be expanded to include non-electromagnetic waves in other media.  In addition, this page is listed as a good article and we shouldn't be merging unsourced information into it. SpinningSpark 07:25, 28 March 2016 (UTC)
 * Oppose merge. ditto. Signal reflection has no citations even though it was noted since 2007.  Perhaps it is a candidate for deletion.Constant314 (talk) 04:48, 29 March 2016 (UTC)

"Transmission coefficient" equations seem to be wrong (Section "Discontinuity along line")
Just tried doing some hand proofs and I thought the initial equation for T is wrong. I changed it from:


 * $$T=\frac {V_\mathrm t}{V_\mathrm i} = \frac {2Z_{02}}{Z_{02}+Z_{01}}$$

To:


 * $$T=\frac {V_\mathrm t}{V_\mathrm i} = \frac {2Z_{01}}{Z_{02}+Z_{01}}$$

As I understand:  Reflection Coeff + Transmission Coeff = 1

Can someone confirm this? I'm not sure how to modify the next T equation. I'm trying to find sources.

(static ip) 14.203.193.219 (talk) 06:34, 3 June 2016 (UTC)


 * I think that is wrong. Transmission Coeff = 1 + Reflection Coeff
 * Test case, shorted line → reflection coef = -1, Transmission Coeff = 0 Constant314 (talk) 08:56, 3 June 2016 (UTC)


 * Yes it is. " Transmission Coeff = 1 + Reflection Coeff " is indeed the correct equation, which leads to Z02 on top, not Z01.
 * Thanks for reverting.
 * (static ip) 14.203.193.219 (talk) 10:28, 3 June 2016 (UTC)

Real Operator
Have you looked at what it looks like? Did it always look that way?
 * $$\Re( Z_\mathrm L)$$

That looks like the symbol for the set of real numbers instead of the real part operator. My small sample of seven (Jackson, Kuo, Balanis, Kraus, Harington and two by Hayt) books, all use "Re" for the real operator instead of a script R. I had not noticed it before. Has something changed with LayTex? Constant314 (talk) 17:18, 22 July 2017 (UTC)


 * The symbol for the set of real numbers is usually the blackboard bold $$\mathbb R$$. The code   gets a Fraktur font $$\mathfrak R$$ in all three browsers I have tried it in.  You may have some odd setting in your preferences or CSS if you are getting something different.  Try logging out and see if you still have the same problem. Using   is the right thing to do here because it has the right semantic meaning in the code.  Any rendering problem needs fixing elsewhere rather than using code kludges.  I might agree that   is a more intuitive rendering, but my biggest objection to your edit was that it was not self-consistent throughout the article. SpinningSpark 18:55, 22 July 2017 (UTC)


 * I thought that I got them all, but now I see that I missed a couple. So,as long as I get all instances (I see four now), you would not object?Constant314 (talk) 19:12, 22 July 2017 (UTC)


 * I don't really have any strong opinion, but it might be wise to discuss this more generally first. I seem to remember that I started off writing   and   (but not in this article apparently) and later on other editors changed it.  I suspect that the issue will be maintaining the semantic content of the code as I said above.  Give me a day or so and I might be able to come up with examples of articles where that change was made. SpinningSpark 19:29, 22 July 2017 (UTC)


 * It was a recent edit to plane wave that got my attention. I noticed that the article on complex numbers and phasors uses \operatorname{Re}.Constant314 (talk) 22:16, 22 July 2017 (UTC)


 * I'm not really coming up with anything, I can't remember precisely what started me formatting it this way. Go ahead and change it if you want. SpinningSpark 10:03, 23 July 2017 (UTC)


 * I think I got them all changed. I found six instances.  Constant314 (talk) 18:18, 29 July 2017 (UTC)

wrong formula in the specific cases section
In the specific cases section, step voltage V\,u(t) is applied to ONE END of a lossless line, and u(t) is a function of time t. However, the given formula $$v_\mathrm i = V\,u(\kappa\,t - x)\,\! $$ and $$i_\mathrm i = \frac{v_\mathrm i}{Z_0} = I\,u(\kappa\,t-x)$$ are using u(t) as if u(t) were a function of a distance, which is quite a nonsense.

Remember that V\,u(t) indicates voltage AT one end of a line in a time-varying domain.

The correct formula for the incident voltage and current at the other end of the line which is distance x away are
 * $$v_\mathrm i = V\,u(t - \frac{x}{\kappa})\,\! $$
 * $$i_\mathrm i = \frac{v_\mathrm i}{Z_0} = I\,u(t - \frac{x}{\kappa})$$. — Preceding unsigned comment added by Widefoot (talk • contribs) 23:03, 20 February 2019 (UTC)
 * I can see both points of view. We really don't care about the units of the unit step, but it clearly specified in the preceding lines as a function of time. I agree that
 * $$v_\mathrm i = V\,u(\kappa\,t - x)\,\! $$
 * is probably an abuse of notation. Constant314 (talk) 23:43, 20 February 2019 (UTC)
 * Constant314, it is not a matter of abuse but correct or incorrect. Widefoot (talk) 00:57, 21 February 2019 (UTC)
 * Looks like this article has been around for a while. I am quite surprised that this simple math problem has not been reported by anyone so far. And, I wonder whether Wikipedia has a means to check when someone raises a serious question about correctness of information.  For Wikipedia to gain more authoritative status, it will need groups of knowledgeable people in different areas who are willing to participate in checking validity issues when questions are raised. Widefoot (talk) 07:42, 21 February 2019 (UTC)
 * Widefoot, there is no body experts because WP has been bitten by so called experts who were not who they said they were or did not have the credentials that they claimed. WP is not an authority; it is an aggregate of what is said in reliable secondary sources.Constant314 (talk) 19:26, 21 February 2019 (UTC)

Ok, I didn't want to go this far, but I will try to explain. V\,u(t) describes the incident voltage at the one end of the line in consideration. Let's say the other end of the line in consideration is distance x away from the one end. Also, if the speed of propagation is \kappa, then it takes time for a change at the one end to reach the other end. That is, the voltage seen at the other end is the voltage seen time ago at the one end. Therefore, in math form, the voltage at the other end is. — Preceding unsigned comment added by Widefoot (talk • contribs) 20:26, 21 February 2019 (UTC)
 * Please pay attention to the terminology used here. The article does not define the unit step function as a function of time.  It is no more true to say that $$u$$ is a function of time, than to say the quadratic equation is a function of time.  $$u$$ can be a function of any variable we choose.  Certainly $$u(t)$$ is a function of time, and the applied voltage at the input of the line is defined as a function of time.  But we are next interested in the value of incident voltage at some distance from the start of the line.  For this a function of distance is more appropriate.   For that, $$v_\mathrm i = V\,u(\kappa\,t - x)\,\! $$ is not an incorrect expression. <b style="background:#FAFAD2;color:#C08000">Spinning</b><b style="color:#4840A0">Spark</b> 12:10, 21 February 2019 (UTC)
 * I support $$v_\mathrm i = V\,u(t - \frac{x}{\kappa})\,\! $$ because, to me, it more obviously says that $$v_\mathrm i$$ is zero until a time such that $$t > \frac{x}{\kappa} \,\! $$ Constant314 (talk) 19:26, 21 February 2019 (UTC)
 * SpinningSpark, The article DOES define the unit step function as a function of time : " u(t) is the unit step function with time t ." If you are in doubt, I suggest that you go ask some math professor what this statement means.
 * I was never doubting the correctness of that expression, only whether it is the best way to present it. I've decided it's not really worth getting into a stew over this and withdraw my objection. <b style="background:#FAFAD2;color:#C08000">Spinning</b><b style="color:#4840A0">Spark</b> 20:29, 21 February 2019 (UTC)


 * Widefoot, you have a talk page. To access it, look above where you will find your name in red followed by "(talk)".  Click on talk to see your talk page. Constant314 (talk) 20:42, 21 February 2019 (UTC)

Merge with three (or more) articles
There is one stub article
 * Signal reflection

and two sub-articles
 * Transmission coefficient
 * Reflection coefficient

which appear to have a great deal of overlap. I suggest that Reflections of signals on conducting lines and Signal reflection should be merged, perhaps under the shorter title used by the stub. It also seems to me that at least part of the explanatory text in Transmission coefficient and Reflection coefficient should be merged into this article (or its successor) if not already present, and replaced with a "see also" notice referring to this article. Astro-Tom-ical (talk) 02:57, 5 December 2019 (UTC)
 * This was proposed before (and failed). See further up the page for the rationale. <b style="background:#FAFAD2;color:#C08000">Spinning</b><b style="color:#4840A0">Spark</b> 11:24, 11 December 2019 (UTC)