Yes, open or shorted gives equivalent measurements (one is phase-reversed
from the other).
And terminated in the characteristic impedance of the cable will give an
'infinite' measurement (no reflection).
How to adapt to other cables? You can do other cables the same way, even if
they are balanced or not of 50-ohm impedance. You do need to know the
velocity factor.
And you also need to know that the measurement is not exact: you will only
get within several percent of the cable length at best. This is due to the
way the nanovna (or host software) does the TDR measurement. The nanovna
measures at discrete frequencies using its frequency scan, then the
software takes those points and calculates an FFT to change to the time
domain. This inherently uses a 'bin' size, which limits the granularity of
the measurement. You can see this granularity as you step across the TDR
graph - each calculated point is separated by this minimum granularity.
If your vna firmware has the "Measure / Cable" option in the menu, it is
better to use if you want to find the length of the cable (or the distance
to a break, if you have seen one in the TDR). It still is only as accurate
as your estimate of the cable velocity factor, but is accurately measured
(it uses a different technique without any binning effect/uncertainty).
The Measure Cable function will also show the characteristic impedance of
the cable, and the loss in dB along the length of the cable.
toggle quoted message
Show quoted text
On Tue, Nov 15, 2022 at 12:26 AM tuckvk3cca <tuckvk3cca@...> wrote: Great, thanks. So you get the same results if the end is either open or
shorted right? Reverse the cable and you can confirm where the break is.
With a dummy load the cable length should read very large with no kinks
right?
How do I adapt this to other cables? Do I need a balun?
|
Some very important observations in previous responses:
1 - to phase match, you don't need any particular value, just to make them
the same for two cables
2 - at your target frequency, 5 degrees is about 1/10 of an inch, a very
exacting measurement over 20 feet of cable: just stress in the cable or
connector issues could easily change its electrical length by that much.
And the velocity factor variability in the cables could readily introduce
that much uncertainty.
3 - you do want to leave the far end of the cable open when using the
nanovna for measurements
Another important measurement issue:
The nanovna 'tdr' measurements can't measure within 0.1 inch at 20 feet.
This is because it is not a true TDR - it uses frequency scanning at
discrete frequency points, then an FFT to compute the corresponding
length. The 'bin' size of the FFT depends on the size of the frequency
scan steps (with an inverse relationship). In TDR mode you can see this as
you move the cursor across the display. The 'length' measurement will be
in steps of so many mm or cm. That is the finest granularity that the TDR
can do for the given frequency range. You can make it more accurate by
expanding the frequency span - but only to a point
One way to deal with this in your situation is to measure one cable,
trimming it by about 1/10 inch at a time just until the nanovna measurement
pops from one 'bin' to the next - meaning you are measuring at the edge of
that bin. Then do the same with the other cable, and you should be very
close. (Assuming there are not significant velocity factor differences in
the two lengths of cable).
Another perhaps better way is to not use the TDR mode, but (if your
firmware has it) the Measure / Cable function. Leave the cable open at the
end. Set the top frequency of the vna such that the smith chart trace is
just over a half-circle. Then note the length of the cable shown. Trim
the other cable to show that same length. You can ignore the velocity
factor and whether or not the length is correct to measured length, since
you only need them to match electrically. This method is usually more
accurate, because it measures the quarter-wavelength resonance of the cable
by finding the phase crossing, using excellent interpolation between
measured frequency points.
Good luck.
Stan
toggle quoted message
Show quoted text
On Tue, Nov 15, 2022 at 11:00 AM Jim Lux <jimlux@...> wrote: On 11/15/22 10:56 AM, dalerheaume via groups.io wrote:
Sorry...S11 delay. Cable terminated at one end only to allow for trimming.
Probably want end "open" so you get a reflection to measure the length,
not terminated in 50 ohms.
|
I installed the NanoVna App on my Android hoping to use it on my phone. My phone has a micro usb C socket with usb cable and I normally use this cable to connect to my PC to run NanoVNASave. I bought an micro OTG cable the connects the phone to a usb C socket. I then connect this to the NanoVna using the same cable USB C cable that I have been using before. When I ran the App and tried to connect to the NanoVNA, it says no device found. Anyone had this problem before?
|
On 11/15/22 10:56 AM, dalerheaume via groups.io wrote: Sorry...S11 delay. Cable terminated at one end only to allow for trimming. Probably want end "open" so you get a reflection to measure the length, not terminated in 50 ohms.
|
On Tue, Nov 15, 2022 at 12:50 PM, Jim Lux wrote:
You're seeing the round trip time of ~50 ns
What does the S21 measure, phase wise?
Rather than fool with TDR.. (or are you using TDR to measure the
distance to the cut end, before installing the connector?)
You can also measure S11 phase at your frequency of interest, it's twice
the one way phase.
at 1.1 GHz, roughly 1 ns per cycle, or ~8" for 360 degrees.
If you want 5 degrees, that's about 1/10th of an inch.
Are your cables at the same temperature? Do they have any other
mechanical variations (many cables "off the reel" have periodic
variations in dimensions).
THANK YOU! That helps tremendously.
|
Sorry...S11 delay. Cable terminated at one end only to allow for trimming.
|
Re: Nano VNA BASIC WORKSHOP via Zoom
The time didn't work the first time, and nothing has changed so it won't work again.
Steve
W5RRX
Las Cruces, NM
|
On 11/15/22 9:59 AM, Siegfried Jackstien wrote: I think you got it correct (almost)... But you see a double delay cause its up and back down the cable... 26.8x2=53.6...
And... You need them the same length / delay... If both are 52..or 53 does not matter but they need to be the same.. Right?!
So... Cut the longer a tiny bit shorter... And you have it...
Dg9bfc sigi
They also make cool little length adjusters, at least in SMA. I've not seen one in N or UHF. They're not cheap, at least new, but they might show up surplus. Someone with a big box of adapters might also find barrels and other adapters that differ in length by a few mm, which can be useful for phase trimming at microwave frequencies.
|
I think you got it correct (almost)... But you see a double delay cause its up and back down the cable... 26.8x2=53.6...
And... You need them the same length / delay... If both are 52..or 53 does not matter but they need to be the same.. Right?!
So... Cut the longer a tiny bit shorter... And you have it...
Dg9bfc sigi
Am 15.11.2022 17:21 schrieb "dalerheaume via groups.io" <dalerheaume@...>:
toggle quoted message
Show quoted text
I am having difficulty constructing a set of phase matched cables using my
NanoVNA SAA-2N and the tutorial available at this link
.
The cable I am using is Times Microwave LMR-400-LLPX with a nominal length
of 20 feet.? Manufacturer's specification for time delay is 1.34 nS/ft.
My cables need to be phase matched +/- 5 degrees at 1.1GHz.
The difficulty that I'm having is with the following results returned by
the NanoVNA.
Cable 1 measured time delay = 54.35 nS
Cable 2 measured time delay = 52.93 nS
Surprisingly, both values are significantly greater than the calculated
delay of 26.8 nS = 1.34 nS/ft x 20 ft
Furthermore, based on the formula in the tutorial, the phase shift appears
to be 562 degrees
What am I doing wrong?
|
On 11/15/22 8:21 AM, dalerheaume via groups.io wrote: I am having difficulty constructing a set of phase matched cables using my NanoVNA SAA-2N and the tutorial available at this link .
The cable I am using is Times Microwave LMR-400-LLPX with a nominal length of 20 feet. Manufacturer's specification for time delay is 1.34 nS/ft.
My cables need to be phase matched +/- 5 degrees at 1.1GHz.
The difficulty that I'm having is with the following results returned by the NanoVNA.
Cable 1 measured time delay = 54.35 nS
Cable 2 measured time delay = 52.93 nS
Surprisingly, both values are significantly greater than the calculated delay of 26.8 nS = 1.34 nS/ft x 20 ft
Furthermore, based on the formula in the tutorial, the phase shift appears to be 562 degrees
What am I doing wrong?
You're seeing the round trip time of ~50 ns What does the S21 measure, phase wise? Rather than fool with TDR.. (or are you using TDR to measure the distance to the cut end, before installing the connector?) You can also measure S11 phase at your frequency of interest, it's twice the one way phase. at 1.1 GHz, roughly 1 ns per cycle, or ~8" for 360 degrees. If you want 5 degrees, that's about 1/10th of an inch. Are your cables at the same temperature? Do they have any other mechanical variations (many cables "off the reel" have periodic variations in dimensions).
|
S11 or S21 delay?
On S11 2x delay (to cable end and back)
On S21 1x delay (from one port to another(
|
I am having difficulty constructing a set of phase matched cables using my NanoVNA SAA-2N and the tutorial available at this link .
The cable I am using is Times Microwave LMR-400-LLPX with a nominal length of 20 feet. Manufacturer's specification for time delay is 1.34 nS/ft.
My cables need to be phase matched +/- 5 degrees at 1.1GHz.
The difficulty that I'm having is with the following results returned by the NanoVNA.
Cable 1 measured time delay = 54.35 nS
Cable 2 measured time delay = 52.93 nS
Surprisingly, both values are significantly greater than the calculated delay of 26.8 nS = 1.34 nS/ft x 20 ft
Furthermore, based on the formula in the tutorial, the phase shift appears to be 562 degrees
What am I doing wrong?
|
Re: Nano VNA BASIC WORKSHOP via Zoom
Might I suggest that we continue to meet via Zoom on Monday evenings at 7 pm (EST) to share what we know about the VNA?
I don¡¯t see my talk as a ¡°lecture¡± to record and watch. I¡¯m not a VNA engineer.
But I can see why this technique (record and play back) might work for some who just can¡¯t make the ¡°live¡± session.
We have met three times now via Zoom and discussed these topics:
1) What is a VNA and in broad strokes how does it work?
I compared the VNA to a dolphin or a bat and SONAR - what do you learn from the reflected SOUND waves
2) Why is CALIBRATING the device over a frequency range important - and how do you do that?
The SAVER software makes this process easy - and I can save a file with thousands of data points.
We then talked about
Why an antenna has a resistance, a reactance, and an impedance?
What kinds of graphs can you plot to understand an antenna?
I did a live demo with my own 80m dipole in my yard.
3) Yesterday we talked about some of the VNA guts
Reflection Coefficient (rho) as a RATIO of reflected voltage to forward voltage and why 0<rho<1
RETURN LOSS - what is it, why is it measured in decibels, and why is it a positive value
SWR - how is this related to reflection coefficient and Return Loss
We ran out of time (8 pm EST) and stopped but there were some good questions and discussions.
Might I suggest we continue to meet via Zoom on Monday evenings at 7 pm EST.
I realize that makes it awful for other time zones distant from N.America
I hinted that I could use my own Nano VNA (model 4 H) to help me adjust my MANUAL antenna tuner for HF antennas.
I then showed how the various graphs (SWR, Return Loss) change as I mess with the capacitor/inductor setting in my ¡°TUNER¡±.
So come join in on this discussion of Nano VNA next Monday evening.
De k3eui Barry
Philadelphia PA USA
|
Great, thanks. So you get the same results if the end is either open or shorted right? Reverse the cable and you can confirm where the break is.
With a dummy load the cable length should read very large with no kinks right?
How do I adapt this to other cables? Do I need a balun?
|
Re: Nano VNA BASIC WORKSHOP via Zoom
Hello Barry,
Some of us in the NanoVNA community actually live outside continental USA.
Not sure where your QTH is, but there is also a 5 hour time difference across the continental USA as well.
It would be great to have access to your obvious vast experience in RF topics, especially when it shows the application of the fabulous NanoVNA.
Would you reconsider recording? (Even delay access to encourage as much online participation as possible)
73...Bob VK2ZRE (Downunder in Australia)
toggle quoted message
Show quoted text
On 15/11/2022 12:38 am, Barry K3EUI wrote: I have been asked many times if these Zoom sessions on Nano VNA were recorded.
NO. They were not recorded.
The reason is simple: I wanted these sessions to be discussions, not lectures.
Many folks will not ask or answer questions if the session is recorded (from my experience as a teacher/professor).
I will pause during the zoom session and ask folks to contribute (like a live class)
¡°What is reactance in an antenna?¡±
And then ask for contributions.
How do you measure reactance with a VNA from inside your shack (at the end of X feet of coaxial feed line)?
¡°Does SWR change along the feed line if the attenuation is very low?¡±
That one is interesting.
I suggest you read the Walt Maxwell books on SWR and feed lines from QST articles in the 1970¡¯s.
Sometimes (actually often) an open ended question leads to a lively debate!
I have about 200 slides (PPT) that I made over the past three years covering all aspect of VNA and antennas and Smith charts.
I¡¯d be happy to share those, as a PPT or as a PDF file.
The slides are just key points to get a discussion going.
Learning is an iterative process with a lot of mistakes along the way.
Come join in on the presentation/discussion,
de k3eui. Barry
Philly
Nov 14th
|
On Mon, Nov 14, 2022 at 12:53 AM, tuckvk3cca wrote:
When you click on the TDR in Nanovsaver. The length of the cable is already
indicated, which I presume is the physical length once you specify the coax
type or VF. There are two plots on the graph, but what do they show?
One graph shows the Time Domain response of the cable attached to CH0 (port1). The other shows the approximate impedance magnitude |Z|. In the annotated Saver graph below I measured 3M of RG-316 connected to 1.8M of 75 ohm RG-6 with an adapter. Roger
|
Re: Nano VNA BASIC WORKSHOP via Zoom
I have been asked many times if these Zoom sessions on Nano VNA were recorded.
NO. They were not recorded.
The reason is simple: I wanted these sessions to be discussions, not lectures.
Many folks will not ask or answer questions if the session is recorded (from my experience as a teacher/professor).
I will pause during the zoom session and ask folks to contribute (like a live class)
¡°What is reactance in an antenna?¡±
And then ask for contributions.
How do you measure reactance with a VNA from inside your shack (at the end of X feet of coaxial feed line)?
¡°Does SWR change along the feed line if the attenuation is very low?¡±
That one is interesting.
I suggest you read the Walt Maxwell books on SWR and feed lines from QST articles in the 1970¡¯s.
Sometimes (actually often) an open ended question leads to a lively debate!
I have about 200 slides (PPT) that I made over the past three years covering all aspect of VNA and antennas and Smith charts.
I¡¯d be happy to share those, as a PPT or as a PDF file.
The slides are just key points to get a discussion going.
Learning is an iterative process with a lot of mistakes along the way.
Come join in on the presentation/discussion,
de k3eui. Barry
Philly
Nov 14th
|
Re: Nano VNA BASIC WORKSHOP via Zoom
On Sun, Nov 13, 2022 at 10:57 AM, Barry K3EUI wrote:
No, I did not record them because I wanted to encourage questions/comments and
when you record these sessions,
(some) folks are hesitant to ask questions or respond.
That is truly unfortunate. I am sure there are hundreds if not thousands of people including myself who would love to watch the presentation, but who got late notice or were not available at the time. How many viewers fell into the category of too shy to participate? Could you not record the session, maybe have a recorded Q&A, and then have a closed Q&A at the end? I only ask because this Zoom sounds like a great thing you did, but for very limited benefit. I am reminded of school classes where a few students were too shy to speak up in class, and therefore they fell behind, eventually failing the class. If they were lucky there was a teacher who noticed and took them aside, perhaps arranging private tutoring so they would be more comfortable.
|
On Sun, Nov 13, 2022 at 09:59 PM, Ho-Ro wrote:
my LCD screens of nano and tiny do not show any artefacts
DiSlord, Correction: My tinySA shows it on the "VERSION" screen, while the NanoVNA does not. But this is also more a topic deep in the HW/FW - we should discuss this on GitHub. Are you still using your repo because there has been no traffic for a long time? Where can we follow your good work? Martin
|
When you click on the TDR in Nanovsaver. The length of the cable is already indicated, which I presume is the physical length once you specify the coax type or VF. There are two plots on the graph, but what do they show?
|
F4WCV
Barry K3EUI
Roger Need
Ho-Ro