measuring a wires length - Page 1
Forum section: electronics / beginners

author: p.larner
author_rank: Frequent Contributor
timestamp: November 23, 2023, 06:17:58 am
content: is there any way to measure a single wire from one end ,ie tdr etc?.

author: pqass
author_rank: Super Contributor
timestamp: November 23, 2023, 06:36:51 am
content: TDR works only with coax or a  twisted pair open at the far end; needs a parallel return.Searchwww.youtube.comfor W2AEW, video #88.At least for the unsophisticated TDR presented in the video.

author: radiolistener
author_rank: Super Contributor
timestamp: November 23, 2023, 06:40:21 am
content: Yes, but you're needs to know wave propagation speed in wire isolation and envirionment around wire and put the wire in a straight wire and keep it far away from any material (like ground, buildings and other objects). It's more easy to measure it with usual roulette meter or laser meter

author: Andy Chee
author_rank: Super Contributor
timestamp: November 23, 2023, 06:59:08 am
content: I once measured a reel of RG-58 coax, by counting the turns per layer, and estimating the diameter of each layer (obviously because it spirals, the diameter of each layer progressively gets smaller as you go from outside to inside).

author: donlisms
author_rank: Frequent Contributor
timestamp: November 23, 2023, 09:17:49 am
content: CuriousMarc, Apollo Guidance Computer Part 7: Memory Module B12, at about 9:00 and following.

author: armandine2
author_rank: Frequent Contributor
timestamp: November 23, 2023, 11:06:57 am
content: https://www.nutsvolts.com/magazine/article/the_capacitance_connectiongood comments about a confusing article

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2023, 02:14:54 am
content: https://en.m.wikipedia.org/wiki/Velocity_factorhttps://owenduffy.net/transmissionline/concept/mvf/index.htm

author: ArdWar
author_rank: Frequent Contributor
timestamp: November 24, 2023, 02:52:57 am
content: I don't know if it's even theoretically, physically possible for a truly single wire (i.e. no return wire, no earth return, no environment to interact with(?)). You can send a packet of photon down a single fiber optic, but afaik you can't just send a packet of electron down a single wire.Interestingly, acoustic method might theoretically work, if impractical.

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2023, 05:56:55 am
content: Quote from: ArdWar on November 24, 2023, 02:52:57 amyou can do it for wire, this is how asymmetric antennas works.The packet of electrons (actually electrons density wave) is named electric pulse

author: DavidKo
author_rank: Frequent Contributor
timestamp: November 24, 2023, 07:23:01 am
content: What about using the sound wave. It propagates quitequick. You can try to use piezo transducer as speaker/microphone.

author: radiolistener
author_rank: Super Contributor
timestamp: November 24, 2023, 07:29:45 am
content: Quote from: DavidKo on November 24, 2023, 07:23:01 amyes, it also can be used for measurement, but you're still needs to know sound wave propagation speed in the wire...And I think the sound wave will have high attenuation, so it cannot be used for very long wire

author: MarkT
author_rank: Frequent Contributor
timestamp: November 24, 2023, 10:03:46 pm
content: Presumably you don't have access to the rest of the wire (or you could weigh it and divide by the mass/length figure - says messing with length measurements the hard way).TDR will do something reasonable if its not super long as everything has an impedance to free-space if nothing else. However you may be violating radio spectrum regulations sending fast pulses down long single wires...If you have access to a VNA you can do the frequency-domain equivalent (same caveat applies).

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2023, 09:26:09 am
content: Quote from: radiolistener on November 24, 2023, 07:29:45 amWires are usually copper and the speed of sound in copper is well known.The TDR method also works with other cables, like mains installation cable.The easy method is usually the capacitance. There is some variations in the capacitance, but not that much as it goes logarithmic with the isolation diameters. So the 100 pF/m rule of thumb works OK for coax and mains cables.Another option may be to just go by weight if one has suitable scales.

author: iMo
author_rank: Super Contributor
timestamp: November 25, 2023, 11:46:56 am
content: Quote from: p.larner on November 23, 2023, 06:17:58 amSo you want to measure your dipole's leg length, do you?To know the length of your dipole legs will not help you much, as there is a lot of parasitics in your setup.Just cut the wires down slowly on both legs till you come into the 80m band..PS: a recommendation - do not create new threads when still dealing with your antenna and stuff around it (ie. your 24 new threads in last month or so).You may get a better help when people know what you are actually doing.Otherwise you just generating questions with little or no sense..

author: CaptDon
author_rank: Super Contributor
timestamp: November 25, 2023, 03:50:19 pm
content: Can you ground the far end to a solid ground like a pipe in the ground or a ground rod etc? You will see a bobble at the far end even if your simple TDR is grounded locally. Is the wire in a coil close to your TDR?

author: Smokey
author_rank: Super Contributor
timestamp: November 25, 2023, 05:28:31 pm
content: You need to know the spec for that specific wire. but ya. VNA.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 25, 2023, 06:16:09 pm
content: You only need the wire spec if you need an accurate measure of the wire length. If you are trying to find a break in an underground wire accuracy determines how much digging you have to do. But even a fifty meter error in a 300 meter wire is a big reduction in digging, even if you further optimize with half interval techniques.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 25, 2023, 07:33:09 pm
content: the idea was to find the length of my doublet antenna legs,bit of a disaster realy,my swr was mental so started pruning the legs only to findout that one leg of the windowline had a bad join in it,so dont know where i am now as lost track of what i pruned from where,i could take it down and measure but was after a quicker way to measure it electricaly if possible,thought of usng my vna to measure each legs lowest swr freq and go from there,i just want them near balance if poss,