Talk:Transmission line

Merging Communications cable

Although it uses the more common name, Communications cable is presenting the same subject as Transmission line#Practical types of electrical transmission line and it's children. I would suppose that if Communication Cable is turned into a referential list of types of Comms Cables, then it would be useful to have. However, at the moment it's existence seems surplus. Your thoughts? Foxhill 07:05, 20 November 2006 (UTC)

Since Communications cable right now redirects to Transmission line, I think it would be a good idea to do as you propose: to have Communications cable mainly as a referential list, and to keep the mathematical stuff on Transmission line. Then Transmission line can be kept scientific, and Communications cable can contain commercial aspects of cable. Note also that there are a lot of aspects on Communications cable that has nothing to do with physics. For example dimension standars, connectors, environmental issues, etc. --HelgeStenstrom 12:52, 31 January 2007 (UTC)

The article is very long and I would be happy to put the signal transmission stuff into a separate article. There are thound and 1 article about communication, but they make the topic look unnecessary complicated. Arnero 16:57, 25 February 2007 (UTC)

Typo or Contradiction ?

Transmission Line page has a confusing aspect. See the following two excerpts within quotes.

"Transmission Line vs Wire ...A common rule of thumb (justified in the input impedance section) is that the cable or wire should be treated as a transmission line if the length is greater than 1/100 of the wavelength.

Input impedance of a transmission line ...This occurs when either the length of the transmission line is at least 100 times smaller than the wavelength.... The physical significance of this is that the transmission line can be ignored (i.e. treated as a wire) in either case."

To me, "at least 100 times smaller" in the second excerpt sounds like the length of the transmission line is *less* than 1/100 of the wavelength. But the first excerpt says "greater than 1/100". Which is correct? 75moore 21:35, 25 May 2007 (UTC)

The first one says that the cable is a transmission line if it's longer than 1/100 of a wavelength. The second one says that the cable is not a transmission line if it's shorter than 1/100 of a wavelength. Sorry about the double negative, but the two statements agree with each other. --Heron 14:11, 26 May 2007 (UTC)

Merge with Transmission medium: absolutely not

Whoever suggested this has not given a signed proposal, but there is no reason to merge these articles. 'Transmission medium' is a scientific topic in physics which deals with any kind of waves transmitting through any medium and has has very diverse applications in many brances of science and technology, not just the narrow application of a transmission line in telecommunications. Rexparry sydney 02:46, 10 August 2007 (UTC)

Merge with Single-circuit transmission line

The single-circuit transmission line article does not present sufficient information to stand on its own. It should be merged here. Neelix (talk) 20:26, 22 August 2009 (UTC)

Nah, this article is aimed at telecomms (although its principles apply to power transmission as well). You want electric power transmission. The article also has a sister, double-circuit transmission line and they would like to go on a double date together if they are going out anywhere. SpinningSpark 20:57, 22 August 2009 (UTC)

If it is to continue to exist, this article needs to be improved. Spinningspark says it is about communications, but it looks to me as if it is about electric power transmission. Who ever heard of a three-phase communications transmission line? --Jc3s5h (talk) 13:51, 23 August 2009 (UTC)

Clarification. By "this article" I meant the article here transmission line which is indeed slanted to telecomms and is inappropriate for single-circuit transmission line or double-circuit transmission line to be merged into. I am suggesting as a target electric power transmission which is much more suitable. I should have known it was a mistake to try humour on Wikipedia, there's always someone who doesn't understand. Both articles together could be reduced to a few short sentences: something like "pylons commonly carry two circuits" captures the essence of it. SpinningSpark 14:36, 23 August 2009 (UTC)

and waveguides?

I reckon this page may be spreading the definition of transmission line a bit far. Transmission lines are sometimes regarded as waveguides, but waveguides aren't usually described as transmissions lines (lines possibly). The IEEE Standard Dictionary of Electrical and Electronics Terms 6th Ed. has:

"transmission line (1) (A) (data transmission) (signal transmission system) The conductive connections between system elements which carry signal power. A waveguide consisting of two or more conductors [my italics] (B)(data transmision)(electric power) [...etc....]. (C)(data transmission) (electromagnetic wave guidance) A system of material boundaries or structures for guiding electromagnetic waves, in the TEM (transverse electromagnetic) mode [my italics]. Commonly a two-wire or coaxial system of conductors. (2)[...etc....] (3)(waveguide) [...etc....] in the TEM mode [my italics] [...etc....]"

(Sorry that was a bit long). I'm loathe to agree with this publication (it's a bit like finding myself in agreement with Mrs Thatcher), but on this occasion it suits my purpose - so can we lose the waveguides? --catslash (talk) 21:26, 11 November 2009 (UTC)

I think we have to face the fact that many authors do not go along with that definition, examples [1][2][3][4][5]. It might be fair to say that possibly most authors include only pairs of conductors as transmission lines, but even that statement immediately runs into a problem with acoustic transmission lines, a widely used concept in acoustics which are more often than not of the hollow waveguide type, and yet the analysis in acoustics will often proceed by analogy with the electrical transmission line theory. Limiting the scope to only those forms typically included in an electrical analysis textbook (TEM) would suggest a disambiguated title of transmission line (electrical) leaving this title free for a more general discussion. SpinningSpark 10:11, 2 July 2010 (UTC)

You have anticipated my next objection: acoustic transmission lines. These have as section in the body of the article, but it consists of little more than a link to acoustic transmission line, and so could be moved to a hatnote. Incidentally, there is also a waveguide (acoustics) page. As there is no contention for the transmission line title, and as electric transmission line is probably the most common meaning, a dab page or an (electric) qualifier seems unnecessary. The only other common meaning I can think of is specifically electric power transmission lines, but there does not appear to be a page devoted to just the lines themselves overhead power lines. --catslash (talk) 16:00, 2 July 2010 (UTC)

Proposal: Correcting Language in the section "The four terminal model"

In this section there is the statement "Ensuring the source impedance matches Z0 will maximize power transfer from the source to the transmission line". It is correct to say that if the source impedance is fixed, that matching the load (or the transmission line impedance) maximizes the power transferred to the load, but the converse is not true. If the load or input impedance of the transmission line is fixed and the source is a voltage source with a series impedance, then minimizing the source impedance maximizes the power transfer. If the source is a current source with a shunt impedance, then maximizing the source impedance will maximize the power transferred to the load. One reason to match the source to a fixed impedance transmission line is to help control reflections generated at the load end of the transmission line. This is often seen in printed circuit boards where there is one source and one load and the load is high impedance. I think the entire sentence should be removed. Constant314 (talk) 01:03, 5 October 2010 (UTC)

It depends what parameter of the source is held constant as the impedance is varied. If you hold constant the available power (that is, you vary the source impedance by putting a transformer on the source), then the statement is true. However, I expect most readers would assume that the e.m.f. is held constant, and so I have deleted the sentence. --catslash (talk) 09:12, 5 October 2010 (UTC)

Solutions of the Telegrapher's Equations as Circuit Components

Is this standard stuff? I've never heard of it before. Also, why would one want to represent a transmission line by an active circuit? --catslash (talk) 18:18, 18 October 2010 (UTC)

Regarding the question “why”

• 1. You can insert it into your SPICE circuit simulator for robust simulation of the transmission line, its load, its source and all the other circuitry including equalizers, filters, pads, amplifiers and other signal conditioning. It also lets you readily simulate such things as multiple bridge taps, gauge changes, splits, and corroded splices. You can simulate complex structures such as customer wiring which might have bridge taps with bridge taps, on-hook telephones, off- hook telephones, etc. If you can draw it as a circuit, then you can simulate it with SPICE. Check out the link about SPICE simulation for more details.

http://www.eetimes.com/design/microwave-rf-design/4200760/SPICE-Simulation-of-Transmission-Lines-by-the-Telegrapher-s-Method-Part-1-of-3-

• 2. Thanks to Linear Technology Corporation, SPICE is available to almost every one with a very general and free license at

http://www.linear.com/designtools/software/ltspice.jsp

• 3. This is subjective, but I find this diagram to be highly intuitive. If the load side is open, it is obvious that the pulse voltage doubles and is reflected. If the load is shorted, the pulse current doubles and is reflected. If the load is matched, then there is no reflection. For a sanity check, assume that the line is lossless but has delay (T = pure delay) and then try various combinations of impedance at each end (infinite, matched, and zero) and verify that you get the expected behavior. Constant314 (talk) 13:25, 19 October 2010 (UTC)

Regarding the question: “Is this standard stuff?”

• The solutions to the Telegrapher's Equation are, of course, standard stuff.
• Representing the transmission line as a two-port is also standard stuff.
• Using the solutions of the Telegrapher’s equations to derive the two-port parameters is standard stuff.
• Using circuits to implement equations is standard stuff.
• The pararmeters of any of the standard two-ports can be derived from this circiut.Constant314 (talk) 23:23, 19 October 2010 (UTC)

But, is this circuit new? The mathematical equations that the circuit implies have been around for decades. So, is this standard stuff? In my opinion, yes it is because it translates precisely to standard stuff (the Telegrapher’s equations). But representing a transmission line as this circuit instead of a list of equations may be new. Which is to say, I haven’t seen this particular circuit anywhere, although I recall seeing crude equivalent circuits buried in the back of very old (from the 90’s or older) SPICE manuals. Constant314 (talk) 15:28, 19 October 2010 (UTC) Constant314 (talk) 15:32, 19 October 2010 (UTC)

Constant314 (talk) 20:03, 19 October 2010 (UTC)

For use in circuit solvers that don't support transmission lines is a pretty good why, and equivalent circuits (in general) are certainly standard stuff. The 2 in the version on the telegrapher's equations page reminds me that the Thèvenin equivalent for one just end of a transmission line is the characteristic impedance in series with a voltage source equal to twice the arriving signal. This is something I use all the time, and is standard stuff because it can be found in books like this. --catslash (talk) 20:47, 20 October 2010 (UTC)

And of course a ladder is commonly used approximate equivalent for a finite transmission line. --catslash (talk) 21:18, 20 October 2010 (UTC)

Proposal: Change upper case gamma to lower case gamma to conform to references.

I'd like to change the propagation constant from upper case to lower case.

I have two reasons:

1. To conform with the symbols used by the references. I've looked at about 7 sources and all of them use lower case gamma instead of upper case gamma.

2. To free up the upper case gamma for its traditional use as a reflection coefficient which is a section I wish to create. —Preceding unsigned comment added by Constant314 (talk • contribs) 12:32, 21 October 2010 (UTC)

Wrong Schematic

In the schematic , the shunt resistor is labeled G, but everywhere in the article G represents the shunt CONDUCTANCE. So, that resistor should be labeled 1/G. — Preceding unsigned comment added by 129.105.6.129 (talk) 15:38, 25 May 2011 (UTC)

Rocketman768 (talk) 15:39, 25 May 2011 (UTC)

Nah, this is absolutely the conventional representation. There are not separate graphic symbols for resistance and conductance. The symbol G has a conventional meaning of conductance which makes the meaning of the diagram perfectly clear. SpinningSpark 16:32, 25 May 2011 (UTC)

Electric power transmission lines

kindly tell me in detail about technical terms used in transmission line design.

and is there any specifications of transmission line towers? — Preceding unsigned comment added by 115.186.48.194 (talk) 08:14, 17 August 2011 (UTC)

This article is about transmission lines for radio frequency signals. For electric power transmission lines used by electric utilities, ask at the Wikipedia:Reference desk/Science. Or see our article, Electric power transmission. Jc3s5h (talk) 09:35, 17 August 2011 (UTC)

"The four terminal model"'s statement of 3dB loss halving the Power?

I always thought that 3dB loss are equivalent of about 30% loss of power. At least the following formula leads me to that conclusion: P[dB] = 10lg(P[W]); If I'm not right, do tell me. I want to know. — Preceding unsigned comment added by Nikolageneshki (talk • contribs) 14:57, 24 January 2012 (UTC)

Let's do the math. A signal has a power, P₀ of 0 dB which we define in this case as 1 watt. It passes through something that causes a loss of 3 dB, so the new power, P₁ is -3 dB.

P[dB] = 10lg(P[W])

-3 dB = 10 log(P₁ W)

-0.3 B = log(P₁ W)

Using each side of the equation as the exponent of 10 we get

10^-0.3 W = P₁ W

0.501 W = P₁ W

Jc3s5h (talk) 15:55, 24 January 2012 (UTC)

The simple version: you are wrong, and you are making a classic confusion about voltage and power. A 3dB loss halves the power, but it only reduces the voltage by 30%. Power (when the load resistance is fixed) is proportional to the voltage squared (P = I * V = (V/R) * V). Strangely enough, if you lose about 30% of the voltage, then you still 70% of the voltage, and 0.7 * 0.7 = 0.49 -- which is about half the power. The actual fraction is not 0.7 but rather √2/2. Glrx (talk) 21:17, 25 January 2012 (UTC)