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Re: Proper way to measure cable length
William Smith
Yes.
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73, Willie N1JBJ On Feb 7, 2022, at 12:10 PM, Clyde Spencer <cftr01b@...> wrote: |
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Re: Proper way to measure cable length
On 2/7/22 9:45 AM, Toad Laurence via groups.io wrote:
Dear all,I can give a practical use - you have a box of cable (whether Cat 5 or coax or HV neon wire) where it's not obvious how much is in the box (since you didn't buy the stuff with "foot markings" on the jacket) - and you want to know whether you need to buy another box before starting on the project. The "how much is actually on the reel" is often a real question (although usually, you'll know what's on the reel, and so, will know the VF). There used to be quite the traffic in "spool ends" of coax from Cable TV companies (may still be, but people don't talk about it as much). |
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Re: Proper way to measure cable length
Dear all,
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Could someone answer this:- Why is there this obsession to electrically measure the physical length of an arbitrary length of coax? Unless you are trying to make a long distance physical link (when an approximate velocity factor will suffice if you have measured your unmarked, dodgy drum of coax electrically), surely we are more interested in electrical length of coax on most occasions anyway? Yours, Mystified, G7PSZ Sent from my iPad, complete with speeling mistakes On 7 Feb 2022, at 17:16, Jim Lux <jim@...> wrote:On 2/6/22 8:33 PM, lobos305 via groups.io wrote:Kind of circling back around to *this* question. |
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Re: Proper way to measure cable length
On 2/6/22 8:33 PM, lobos305 via groups.io wrote:
What velocity factor would you use to measure the physical length of a cable when you don't know the cable type?Kind of circling back around to *this* question. You've got a spool or coil of coax sitting in front of you.? How do you measure the physical length without actually unspooling or measuring the spool and hoping algebra works. TDR (or using a VNA's TDR function) will give you the electrical length in seconds.? But without the velocity factor, that doesn't tell you physical length. a) you can cut a known shorter length and *measure* the velocity factor. Then, Phys length = Elec Length * Vf b) you can look at the dielectric and estimate - Solid = 0.66-0.70, foam plastic 0.75-0.85, spacer but mostly air (heliax and similar), 0.95 You might do better if you can know the kind of plastic - PE vs PTFE, for instance.? One melts and burns, the other, not so well. If you've got silica dielectric, well, it's so expensive, you should probably measure it with a tape measure. If you've got special delay line coax, with the ferrite loaded or coiled center conductor, measure a sample, as in #1 It's also not unheard of for the Velocity factor to change in a spool, particularly if it's been sitting a good long while (that surplus coax from 1950) |
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Re: Proper way to measure cable length
"Scavenger Time Domain Reflectometer Coaxial Cable Tester," Stan Johnson, W0SJ. QST, May 2021, pps. 30-33.
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73, Vince N2JRS -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of William Smith via groups.io Sent: Monday, February 7, 2022 11:48 AM To: [email protected] Subject: Re: [nanovna-users] Proper way to measure cable length Sounds intriguing, do you have more details, or maybe a pointer to the QST project? If you made your boards with OSHpark, you can designate them as public, so anyone can order them. Not sure if other vendors offer the same service. 73, Willie N1JBJ On Feb 7, 2022, at 11:35 AM, Syd via groups.io <nhuq1@...> wrote: |
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Re: Proper way to measure cable length
Does this kit TDR require an oscilloscope to make the measurement?
On Mon, Feb 7, 2022, 12:06 PM Clyde Spencer via groups.io <cftr01b= [email protected]> wrote: The latest versions of the NanoVNAs provide the TDR function. |
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Re: Proper way to measure cable length
The latest versions of the NanoVNAs provide the TDR function.
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On Mon, Feb 7, 2022, 11:48 AM William Smith <w_smith@...> wrote:
Sounds intriguing, do you have more details, or maybe a pointer to the QST |
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Re: Proper way to measure cable length
Syd,
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I would like a board of kit if it is still available. Please provide details. Thanks, Mike N2MS On 02/07/2022 11:35 AM Syd via groups.io <nhuq1@...> wrote: |
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Re: Proper way to measure cable length
William Smith
Sounds intriguing, do you have more details, or maybe a pointer to the QST project? If you made your boards with OSHpark, you can designate them as public, so anyone can order them. Not sure if other vendors offer the same service.
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73, Willie N1JBJ On Feb 7, 2022, at 11:35 AM, Syd via groups.io <nhuq1@...> wrote: |
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Re: Proper way to measure cable length
Syd
In an older publication of QST there was a TDR project. I made it and it works quite well for measuring electrical length of a coax and from that you can calculate the velocity actor. If you can find it in QST, probably about 1 year ago, it is a very simple project. Since my original construction of this project, I decided to manufacture a PCB to make this using an online tool for creating a 2 sided PC board. A nice learning experience. Now I have the original, working and wired project board with parts, and 2 empty PCBs for this project. If anyone wants either the wired project or one of the 2 empty PCBs, let me know.
73 wt1v |
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Re: Proper way to measure cable length
Try 0.66 and measure a piece of the same cable with a ruler and the VNA.
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Adjust VF until the two agree. If you can see the dielectric, use the VF for another cable with the same dielectric. Glenn On 2/7/2022 10:15 AM, Jim Lux wrote:
On 2/6/22 8:33 PM, lobos305 via groups.io wrote:What velocity factor would you use to measure the physical length of a cable when you don't know the cable type? --
----------------------------------------------------------------------- Glenn Little ARRL Technical Specialist QCWA LM 28417 Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178 QTH: Goose Creek, SC USA (EM92xx) USSVI, FRA, NRA-LM ARRL TAPR "It is not the class of license that the Amateur holds but the class of the Amateur that holds the license" |
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Re: Proper way to measure cable length
How many answers need to be posted about this? I posted a link that gives
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the info for the most common materials used, including foam polyethylene and foam polystyrene. Zack W9SZ <> Virus-free. www.avg.com <> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> On Mon, Feb 7, 2022 at 8:11 AM Clyde Spencer <cftr01b@...> wrote:
Look at the end of the cable. If the insulation between the outer shield |
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Re: Proper way to measure cable length
On 2/7/22 5:37 AM, Zack Widup wrote:
Most of the commonly found cables have a velocity factor of 0.66.Most *solid dielectric* cables are close to 0.66 Foam dielectric tends to be faster (because the Vprop is related to the permittivity of the dielectric, and foam is lower than solid)
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Re: Proper way to measure cable length
On 2/6/22 8:33 PM, lobos305 via groups.io wrote:
What velocity factor would you use to measure the physical length of a cable when you don't know the cable type? You have no way to know, unless you make another measurement to compare physical and electrical length. |
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Re: Proper way to measure cable length
William Smith
Not that there can¡¯t be reasons for doing this (buying x feet of new coax to replace it with), but if you state the reason for wanting this measurement, maybe there¡¯s a different way to solve the problem.
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This is just one step removed from ¡°How long is a piece of string?¡±, velocity factor can vary significantly, and so can the end result. 73, Willie N1JBJ On Feb 6, 2022, at 11:33 PM, lobos305 via groups.io <lobos305@...> wrote: |
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Re: Proper way to measure cable length
Look at the end of the cable. If the insulation between the outer shield
and center conductor is a hard and solid material the velocity factor will probably be 0.66. If this insulation is a soft foam material it will be between .72 and .80. Is the cable the size of RG8/213 or LMR400? If the cable has a thin foil layer on the inside insulator then it will most likely be LMR400. RG8/213 has no foil lining. *Clyde K. Spencer* On Sun, Feb 6, 2022 at 11:33 PM lobos305 via groups.io <lobos305= [email protected]> wrote: What velocity factor would you use to measure the physical length of a |
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Re: Proper way to measure cable length
Here's more data:
Zack On Mon, Feb 7, 2022 at 7:38 AM Zack Widup via groups.io <w9sz.zack= [email protected]> wrote: Most of the commonly found cables have a velocity factor of 0.66.<> Virus-free. www.avast.com <> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> |
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Re: Proper way to measure cable length
Most of the commonly found cables have a velocity factor of 0.66.
Zack W9SZ <> Virus-free. www.avast.com <> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> On Sun, Feb 6, 2022 at 10:33 PM lobos305 via groups.io <lobos305= [email protected]> wrote: What velocity factor would you use to measure the physical length of a |
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