Talk:Twin-lead

I don't think the color vs. black and white distinction is relevant. A balanced line directly from the antenna to the TV is actually superior to a connection requiring a balun, as the balun has some loss, and the coax is (in general) more lossy than the twin-lead line. However, if the line has to pass through a wall that has a lot of rebar or other metal, it will interfere with the twin-lead, and coax is better. Also, the old screw type antenna connections are notoriously loose and have poor connections. Coax connectors are a bit more robust and smaller (i.e., easier to install on the assembly line) and so would probably shave a few cents off manufacturing cost, as well as be more maintainable for the consumer, and so would be more likely on a modern mass market TV. -ssd 04:42, 3 January 2006 (UTC)
 * Twin lead did actually had less loss than coax under ideal conditions, but had serious problems.... It was unsheilded, it interacts with metal objects, and it was sensitive to impedence changes caused by weather.  Cable companies couldn't use it because it would leak signals, both in and out.  Even with a coax hooked to balun the length of twin lead inside the tv between the tuner circuit and screw terminals would let sources of interference leak in.  Strong broadcast TV signals would cause ghosting on cable TV channels.  Problems with cable TV seems to spell the end of use of twin lead terminals on TV sets.  Big E 1977  01:19, 24 July 2006 (UTC)

In order to match 75 to 300 ohms, one needs a turns ratio of 1:2, not 1:4 —Preceding unsigned comment added by Jacks5kids (talk • contribs) 21:37, 10 February 2008 (UTC)

I've corrected the balun turns ratio from 4:1 to 2:1. Wouldn't surprise me if some beginner changes it back. 86.4.152.167 (talk) 01:34, 27 January 2009 (UTC)

I'm planning on expanding on this - I'll do it later this week unless someone disagrees strongly - yes, from an engineering perspective its 2 turns to 1 to go from 75 to 300 - but that's not how baluns are marketed - they're marketed by they OHM conversion, so you buy a 4:1 balun to go from 75 to 300. Ygbsm (talk) 20:16, 4 January 2011 (UTC)

Date
This stuff became more or less obsolete in the 1960s in the UK, not recently. Since then it has occasionally been seen on FM rabbits ear antennas, but not often. Tabby (talk) 12:21, 1 March 2008 (UTC)

Shielded twin-lead?
I am very suprised to see this stuff described as better than coaxial. Twin-lead seems to have been about as "useful" and "reliable" as using an old wire coathanger as an antenna. Long lengths of twin-lead seems to work great as an antenna, meaning it sucks up interference from all directions to such a degree that the yagi on the roof becomes drowned out in the noise.

I tend to assume people used it back in the 1950s, only because they could not afford the better coaxial system. I know that the old coaxial was usually huge, about a half-inch wide, and very stiff.

Is there an equivelant form of twin-lead that is very low-loss, and as well-shielded against interference as coaxial?

DMahalko (talk) 06:23, 22 February 2009 (UTC)

Merge
I think that the page on Ladder Line (http://en.wikipedia.org/wiki/Twin-lead) should be merged into / redirected to this page, which is more general and more complete. However, I don't know how to do the banner at the top of the ladder line page proposing same. 70.232.79.15 (talk) 01:46, 31 March 2010 (UTC)
 * Done. - LuckyLouie (talk) 01:54, 31 March 2010 (UTC)

Table of loss at the end of the article is not very helpful
The table at the end of the article can't be correct. 75 ohm and 300 ohm lines would not have the same loss and these values differ from the few sources I checked. I will research various authorities and see if I can come up with some more reliable values. I appreciate any comments from any and all. JNRSTANLEY (talk) 18:47, 18 July 2013 (UTC) Looking further, I realize that the most serious problem is that the actual 300 and 75 ohm lines are not identified. Of course there will likely be some 300 ohm line and some 75 ohm line with the losses shown. But what is the wire size, spacing, etc that gives those values? A 300 ohm line made with larger wire will have less loss than one made with smaller wire, assuming the loss mechanism is copper loss and some dielectric loss, and not radiation. If radiation is included the larger one might have greater loss. I think it best to pick perhaps two or three actual lines that are presently available and list their losses. Finding a presently available 72 ohm line might be difficult. I am not going to rush into this, but I think the table as shown, while useful for the capacity and VF values, is not very helpful for the loss values. It seems to imply that 72 and 300 ohm lines have similar loss. For the same wire size, this is very misleading to someone who might be considering building a line. We might need to also change the statement earlier in the text stating that the losses in parallel lines is an order of magnitude less than in coax, although this is kind of an apples and oranges comparison. One big use of parallel lines is in HF high power, so data for 10 MHz should also be included. It also occurs to me that there is no consideration given to open wire lines used in high power SW broadcast, which happens to be my area of expertise. Any interested party please comment. JNRSTANLEY (talk) 20:23, 18 July 2013 (UTC)
 * Not sure what you mean by VF values. If you mean the phase velocities (0.8c and 0.71c), then these depend on the thickness and dielectric constant of the insulation on the wires. The capacitances likewise depend on the details of the dielectric. If the capacitances and phase velocities are consistent, then we should have C = 1 / (Z vphase) in each case (so Z = 300 Ω, vphase = 0.8c => C = 11.1188 pF/m). In any case, since the details of the dielectric sheaths are not given, then these values are -like the losses- somewhat meaningless. To be fair, the section is headed typical values, so it doesn't make any claims that the data has any particular significance. However, I reckon your objection is valid and so the table should go. I shall delete it. --catslash (talk) 23:41, 18 July 2013 (UTC)


 * Yes, VF is the same as phase velocity. And you are quite right, it also depends on the thickness and type of insulation.  300 ohm open wire lines can have a VF near .95.  Better to delete for now. Thanks.JNRSTANLEY (talk) 12:04, 19 July 2013 (UTC)

Bad introduction quiz
This article has a bad introduction. It's been bad for a long time, there has been plenty of opportunity for editors to improve it, but apparently nobody noticed. I am going to improve it after writing this, so here is a link to the previous bad version. Take a look. Can you tell what's wrong with it?

(spoilers)

The introduction doesn't say what twin-lead is used for. The only information is the lead sentence, which says it is "...used...to carry radio frequency (RF) signals." The general readers who come to this article will include elementary school students, high-school dropouts, English majors, and overworked single mothers. Unless they are radio hobbyists, they will probably not know what "radio frequency" is and what "signals" are. Besides, this sentence alone is misleading. Twin-lead is not used in general to carry radio frequency signals, it is mostly used for one purpose, to connect radio transmitters and receivers with their antennas. This fact should be in there. The non-technical readers above are arguably the ones who most need a Wikipedia article like this. They may be looking for the simplest possible explanation of twin-lead. The article body may not be comprehensible to them, but we should try to make the introduction understandable to as wide a range of readers as possible (WP:EXPLAINLEAD, WP:UPFRONT).

Second, the lower half of the introduction focuses with geeky WP:UNDUE WEIGHT on one tiny topic: the comparison with coaxial cable. There are 8 sentences on this subject. It doesn't say why twin-lead should be compared to coax, it doesn't say that coax is the main alternative for antenna feedlines. The last sentence in the introduction is "The change to coax was driven by the fact that TV tuners were switching to 75 ohms." Wait - what change??? Twin-lead was used with TVs? There is absolutely no context to this statement, which will be incomprehensible to anyone who is not familiar with electronics. The facts needed for a general reader to understand this sentence are: These facts should either be added to the introduction, or preferably this little tech infodump should be moved into the article body. Cheers --ChetvornoTALK
 * Twin-lead is used to connect radio receivers to their antennas.
 * In particular, it was previously used to connect TV antennas to TVs.
 * TV antennas are now connected to TVs with coaxial cable.
 * Transmission line has a characteristic impedance, which must be matched to the TV input for the strongest reception.