Re: Proper way to measure cable length
Well, yes, but the OP didn¡¯t specify exactly what he wanted to do, so we¡¯re left guessing. Does he want to measure the physical length of a cable that¡¯s already installed, so he can order a replacement? Does he want to know how much is left on a partial spool? Does he want to make phasing lines? How accurate does he want to be? Various people are answering all the above questions and more, but we don¡¯t know, for instance, if he can cut a few feet off and measure the known length. [And as a consequence, most of the answers will be wrong, because they are answering the wrong question, but we don¡¯t know what the right question is.]
That said, there¡¯s a lot of knowledge in the answers that _have_ been given, so the entire discussion is worthwhile, and very educational.
73, Willie N1JBJ
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On Feb 8, 2022, at 5:16 AM, Arie Kleingeld PA3A <pa3a@...> wrote:
Measuring the cable length comes down to measuring the Velocity Factor (VF) first.
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Re: NanoVNA for the Blind?
Hi Austin. I will contact you directly to help you get going. In that process we can update the documentation so that it is completely descriptive and then report back to this group. My goal is to make it so that anyone can get up and running easily, but I recognize that it's not there yet. :-)
73s, Bruce KC1FSZ
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Re: Proper way to measure cable length
I have reentered this group after a long absence to see what's happening now.
Measuring the cable length comes down to measuring the Velocity Factor (VF) first.
The simple and basic way to go is take 5m of transmissionline. Knowing the VF will be between (wide boundaries) 0.5 and 1, we can conclude that this line will be quarter wavelength long somewhere between 15 MHz and 7,5 MHz.
Connect the VNA to the line and leave the other end open. Sweep S11 (R+jX) between 7.5 and 15MHz and find the frequency where the impedance is at its minimum and X = 0. At this frequency the line is a quarter wavelength long.
As an example:
- the measured frequency = 11.7 MHz
- a full wavelength for this frequency = 300 / 11.7 = 25,64m
- a quarter of this is: 25,64 / 4 = 6,41m
So the electrical length of the 5m line = 6,41m and physical length = 5m
Therefore the VF = 5 / 6,41 = 0.78
Now measure the unknown length of transmissionline the same way to find the frequency where the line is a quarter wavelength long.
Sweep S11 starting at a low frequency for example 500 kHz for a long transmissionline ** to find the quarterwave frequency. Calculate the quarter wavelenght in meters; divide it by the VF that you found above and then you will know the length of the line.
(** A quarter wavelength at 500 kHz is 600m / 4 = 150m and taking into account the VF, the electrical length will be 150/0,78 = nearly 200m, should be enough :-)? )
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Re: PC Won't Recognize Unit in DFU Mode so Can't Update Firmware
Generally I would say, if the nano is showing up on a com port when it is connected either it isn't actually in DFU mode.If it is an old model setting dfu mode is accomplished by shorting an internal connection while powering on. With newer?models, you hold the thumbwheel depressed while powering on. The screen should be blank. So, question 1: Are you?actually getting the nano into dfu mode? If it is in dfu mode, (it should not show up on a com port) Instead, it should be?under a different category in windows device manager. Universal Serial Bus Controllers, AS STM device In DFU mode(see attach below) So, if it is on a com port, that probably means windows loaded an incorrect driver. But, in my experience?asking windows to update the driver won't work. It will insist it already has the best driver; despite the fact it isn't working.the only cure is to deinstall the device using something like zadig (a program you can install) to get it listed in the correct category, with the right driver.
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On Monday, February 7, 2022, 01:13:23 PM EST, David <dokrent@...> wrote:
I tried STCUBE, but it too didn't pick up the nanovna either. The computer
sees it on com4.?
Home Email: dokrent@...
W7DAO
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Re: Proper way to measure cable length
Very important if you are making phasing lines for stacks of Yagi antennas.
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On Monday, February 7, 2022, 11:46:08 AM CST, Toad Laurence via groups.io <g7psz@...> wrote: Dear all, 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:
What velocity factor would you use to measure the physical length of a cable when you don't know the cable type?
Pat
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: NanoVNA for the Blind?
I thought of something. If I buy a NanoVNA, will I require sighted help to get it up and running to the point where I can use the web interface? Or do I just need to connect it up and start the program? Thanks.
PS: I also have programming experience, though I mainly use Ruby. Have they made a guide for programmers? I'll look around your code a little.
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Re: NanoVNA for the Blind?
Wonderful! I'll check it out.
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Re: Proper way to measure cable length
In my firmware exist option
Measure->Cable
User need select correct frequency range (minimum/maximim frequency depend from cable length on smitch chart must rotate by 180 degree)
As can see in this mode Nano measure impedance, length (need set correct velocity factor) and loss (at marker point, use s11 logmag / 2)
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Re: PC Won't Recognize Unit in DFU Mode so Can't Update Firmware
I tried STCUBE, but it too didn't pick up the nanovna either. The computer
sees it on com4.
Home Email: dokrent@...
W7DAO
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Re: Proper way to measure cable length
For an example, it is extremely helpful when troubleshooting a transmission line fault. I have an Anritsu Sitemaster that I use to do DTF (Distance to Fault) tests. I can locate a problem, usually within a few feet, and save lots of time taking things apart that are not the problem.
I sometimes test a spool of hardline that we suspect has been shipped improperly on it's side and dropped. This can cause the cable to deform and cause an impedance bump in the line. It's hard for a supplier to deny that the cable is bad when it's still on the spool and never unreeled.
Joe
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On 2/7/2022 12:45 PM, Toad Laurence via groups.io wrote: Dear all,
Could someone answer this:-
Why is there this obsession to electrically measure the physical length of an arbitrary length of coax?
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Re: Proper way to measure cable length
Yes.
73, Willie N1JBJ
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On Feb 7, 2022, at 12:10 PM, Clyde Spencer <cftr01b@...> wrote:
Does this kit TDR require an oscilloscope to make the measurement?
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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,
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? 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,
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
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On 7 Feb 2022, at 17:16, Jim Lux <jim@...> 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?
Pat
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
See also a couple of videos from Alan W2AEW on this subject, including a simple TDR circuit which I have built and works adequately for my needs.
Chris G4CWS
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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?
Pat
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.
73,
Vince N2JRS
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-----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:
In an older publication of QST there was a TDR project.
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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.
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
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= [email protected]> wrote:
In an older publication of QST there was a TDR project.
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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
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= [email protected]> wrote:
In an older publication of QST there was a TDR project.
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Re: Proper way to measure cable length
Syd,
I would like a board of kit if it is still available. Please provide details.
Thanks,
Mike N2MS
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On 02/07/2022 11:35 AM Syd via groups.io <nhuq1@...> wrote:
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
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
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On Feb 7, 2022, at 11:35 AM, Syd via groups.io <nhuq1@...> wrote:
In an older publication of QST there was a TDR project.
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Arie Kleingeld PA3A
vh2