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A NanoVNA calculates the SWR for you. If this SWR is so important for an OM at the output of its Tx, it is not a very relevant measurement produced by a NanoVNA.

SWR is a number and only *one*

A NanoVNA is a measuring device that allows you to measure a couple of *two* numbers as a function of frequency using only port 0. The NanaoVNA will present these two numbers to you in multiple forms adapted to a particular use. The two most important:
- The impedance in the form A+jB (A et B in ?)
- Parameter S11 in the form again a+jb

For example, if you notice that at the output of your Tx you have a high SWR, not very compatible with a Tx with transistors, what is relevant is to correct your installation to reduce this SWR. This SWR is the consequence of the fact that the impedance brought back by your antenna + the transmission line is not 50 + j0 or 50. You must correct your installation and the NanoVNA will help you calculate the device to be implemented.

A simple doublet resonating at the frequency of use never presents an impedance of 50. Do not stop at the evaluation of the SWR by your NanoVNA. The SWR is only a sterile observation.
--
F1AMM
Fran?ois

Thanks for trying to be a help brother. I watched both those videos before but something in them just didn't click.
I just sat down and read the beginner's guide start to finish and setting this one up is no different the the old one i had! Lol
I think trying to read the (official) factory guide get me all twisted up trying to follow it. Smh I bet tomorrow I have my antenna up and running finally ?

Mike,

I found these short videos very helpful for doing SWR readings on antennas.

How to read SWR on nanoVNA.


How to calibrate nanoVNA.

On 6/9/23 6:19 PM, Anne Ranch wrote:

On Fri, Jun 9, 2023 at 04:46 PM, Roger Need wrote:

The TDR feature in NanoVNA Saver has some issues

Allow me to discourage you from such posting... you will get crucified by "guardians of purity of nanoVNA" , but I assume you are an adult and can take it.
Yes, the nanoVNASaver is a challenge , to put it mildly. ( and I will leave it at that...)
I am still searching for a clue where to find at least some of the TDR code.
I got sidetracked actually calculating the REAL RG6 cable length to act as "in line transformer".
And it is too hot to work on the antenna....
-=-=

The calculations are in ./Windows/TDR.py (not ./Charts/TDR.py, that's what actually does the plotting)

It's pretty straightforward in function updateTDR() - blackman window, ifft, convolution with 1s to turn impulse response into step response, then calculate Z from the step response
self.step_response_Z = 50 * (1 + self.step_response) / (1 - self.step_response)


I posted the source snippet an hour or so ago.

The length you posted seems to be about right, if you had entered 0.66 for the VF and the RG-6 was actually foam dielectric with a VF of 0.84


__init__() sets up the defaults and populates the cable type dropdown with corresponding velocity factors.

the only tricky part is where it concatenates a series of smaller sweep segments.
s11 = []
for d in self.app.data11:
s11.append(np.complex(d.re, d.im))

app.data11 is a list of data sets.

One area where it could foul up is if the points aren't equally spaced, or if the segments have any overlap. The other place where a hiccup could occur is if there's a truncation issue in setting the frequencies of the segments. I've been bitten by that when trying to do log sweeps and the step size at the low end is < minimum step size.

On Fri, Jun 9, 2023 at 04:46 PM, Roger Need wrote:

The TDR feature in NanoVNA Saver has some issues

Allow me to discourage you from such posting... you will get crucified by "guardians of purity of nanoVNA" , but I assume you are an adult and can take it.
Yes, the nanoVNASaver is a challenge , to put it mildly. ( and I will leave it at that...)
I am still searching for a clue where to find at least some of the TDR code.
I got sidetracked actually calculating the REAL RG6 cable length to act as "in line transformer".
And it is too hot to work on the antenna....

On Fri, Jun 9, 2023 at 01:26 PM, Jim Lux wrote:

It's useful to know how to make the TDR work - I've found more than one
feedline problem by using TDR - yeah, sometimes if there's combinations
of solid, foam, and air dielectric, the distances don't work out, but
you can still see the discontinuities from connectors.

The TDR feature in NanoVNA Saver has some issues. In 5.5 it does not plot the time graph correctly. In all versions the impedance graph was not correct in the initial release and still has problems. I find doing TDR on the NanoVNA itself is easier and the NanoVNA app TDR display plots impedance much better. Take a look at the attached plots from a NanoVNA-H4 , NanoVNA App and Saver. The first scan is 25 feet (7.62 meters) of RG58 and the second is with 8 feet (2.4 meters) of RG6 added on the end. Nominal impedance of the RG58 is 50 ohms and the RG6 is 75. Note how NanoVNA App resolves the impedance of the two transmission lines

Roger

There is an Absolute Beginners Guide to the NanoVNA in the Files section of this group

/g/nanovna-users/files/Absolute%20Beginner%20Guide%20to%20The%20NanoVNA/Absolute_Beginner_Guide_NanoVNA_v1_6.pdf

Roger

I bought a NANO VNA when they first came out to use as a antenna analyzer. I sold it after I bought a regular analyzer and since bought another VNA
This is new updated software is confusing compared to the fist one I had ?
Trying to read the manual im getting more confused trying to figure out the basics of swr measurement. Can anyone give me a dumbed down step by step version how to set the new software for SWR?
It would be much appreciated ?

On 6/9/23 12:01 PM, Roger Need via groups.io wrote:

On Fri, Jun 9, 2023 at 09:21 AM, Anne Ranch wrote:

I shall repeat
the task is -
is the unknown length of cable in multiples of 1/4 wavelength @ 14 MHz band,?

hence

what should the sweep frequencies be ?

Why are you using TDR to do this task? Just sweep from 50 kHz to 20 MHz and set yellow trace to s11 Phase and Green to Smith chart. Then just spin the marker and you can see what you want. Attached is a plot of a 9.4 meter long piece of RG58. The markers show the 1/4, 1/2, and 3/4 wavelength frequencies. At 14.16 MHz. this cable is about 0.5 +.25(144/180) = 0.7 wavelengths long

It's useful to know how to make the TDR work - I've found more than one feedline problem by using TDR - yeah, sometimes if there's combinations of solid, foam, and air dielectric, the distances don't work out, but you can still see the discontinuities from connectors.

Attached is a TDR plot with a feedline to an S-band antenna with multiple segments, and the actual antenna at the end.

On 6/9/23 11:12 AM, Anne Ranch wrote:

Here is a new twist ....
I replaced current , unknown type , unmarked type of coax with
known, good RG6 length plainly marked in cubits and converted them to meters - approximately 35 meters.
Now when I set sweep to 10 to 60 MHz TDR "estimated length" is around 28 meters - with open ended coax.
( Not even close to 35 meters ...)

Is your coax foam or solid dielectric - RG-6 comes in both forms.

What does the trace look like? (can you save it and post it?) I'd expect around 41-42 meters if set to VF=1 (or 100%).

if it's set to, say, 0.66 VF and it's actually 0.84 (first google hit for RG-6 VF), then it will give you a shorter than real length
(35 *.66/.84 = 27.5 meters which is pretty close to what you got.

some RG-6 is foam dielectric so the epsilon of the dielectric is smaller.

The other way it can get fooled is if there's a place where the cable is damaged, causing a discontinuity that's big enough to fool the algorithm.



The algorithm does a simple argmax to find the peak
s11 = []
for d in self.app.data11:
s11.append(np.complex(d.re, d.im))

window = np.blackman(len(self.app.data11))

windowed_s11 = window * s11
self.td = np.abs(np.fft.ifft(windowed_s11, FFT_POINTS))
step = np.ones(FFT_POINTS)
self.step_response = signal.convolve(self.td, step)

self.step_response_Z = 50 * (1 + self.step_response) / (1 - self.step_response)

time_axis = np.linspace(0, 1/step_size, FFT_POINTS)
self.distance_axis = time_axis * v * c
# peak = np.max(td)
# We should check that this is an actual *peak*, and not just a vague maximum
index_peak = np.argmax(self.td)

cable_len = round(self.distance_axis[index_peak]/2, 3)
feet = math.floor(cable_len / 0.3048)
inches = round(((cable_len / 0.3048) - feet)*12, 1)

self.tdr_result_label.setText(f"{cable_len}m ({feet}ft {inches}in)")
self.app.tdr_result_label.setText(str(cable_len) + " m")
self.updated.emit()

I started looking at the software and so far it looks as there is "the original " nanoVNASaver and several "forks", some for windoze only.
It would be nice if somebody who KNOWS where " TDR" software resides help me and point me in the right direction.
YES, I can download all and then I can search but if somebody already knows where to look , let's share that info, please...
and thanks
...

If it is indeed a USB communication error as others have suggested, have you tried using a different USB cable between the NanoVNA and PC? Maybe even try a different USB port on the PC. Are you connecting to an older USB-2 port, or ‘newer’ USB-3 port? Some software and/or hardware can act unpredictably on USB-3 ports if the hardware and/or software was designed on a USB-2 platform. Granted, USB-3 is supposed to be backward-compatible with USB-2, but USB-2 hardware+software may not be “forward”-compliant with a USB-3 port.

Is this a direct connection to the PC, or are you connecting through a USB hub? If connecting through a hub, perhaps try a direct connection.

Just reaching for other possibilities. Good luck.

Ken -- WB?OCV

On Fri, Jun 9, 2023 at 09:21 AM, Anne Ranch wrote:

I shall repeat
the task is -
is the unknown length of cable in multiples of 1/4 wavelength @ 14 MHz band,?

hence

what should the sweep frequencies be ?

Why are you using TDR to do this task? Just sweep from 50 kHz to 20 MHz and set yellow trace to s11 Phase and Green to Smith chart. Then just spin the marker and you can see what you want. Attached is a plot of a 9.4 meter long piece of RG58. The markers show the 1/4, 1/2, and 3/4 wavelength frequencies. At 14.16 MHz. this cable is about 0.5 +.25(144/180) = 0.7 wavelengths long

On Fri, Jun 9, 2023 at 09:21 AM, Anne Ranch wrote:

the task is -
is the unknown length of cable in multiples of 1/4 wavelength @ 14 MHz band,?

To directly answer this, what you need to know is the electrical length of the cable, not the physical length. And nanovna-saver can directly display this without the complication of TDR and VF estimates.
Select a graph type of S11 phase, and connect your cable to the nanovna with the far end of the cable open. Then first select a frequency range like 50kHz to 15MHz. You will see the phase start at near 0, then the phase will reverse every quarter wavelength (based on the true electrical cable length). The waveform looks like a sawtooth, because of the open cable end. Count the number of quarter wavelengths up to your desired operating frequency. Then change the frequency range to zoom in on the 14MHz band to get a detail view of the additional phase offset. Then you can trim or add to your cable to move the quarter wavelength point to your desired frequency.
I attached photos of this approach using an RG8 foam cable approximately 30M long.

Here is a new twist ....
I replaced current , unknown type , unmarked type of coax with
known, good RG6 length plainly marked in cubits and converted them to meters - approximately 35 meters.
Now when I set sweep to 10 to 60 MHz TDR "estimated length" is around 28 meters - with open ended coax.
( Not even close to 35 meters ...)
I started looking at the software and so far it looks as there is "the original " nanoVNASaver and several "forks", some for windoze only.
It would be nice if somebody who KNOWS where " TDR" software resides help me and point me in the right direction.
YES, I can download all and then I can search but if somebody already knows where to look , let's share that info, please...
and thanks
...

I tried a couple of ferrite beads also tried updated software but no luck. I need to get a few more beads to see if that works. Like Stan, I am suspicious that it might be RF. May be a few weeks but will advise results of more beads.

On 6/9/23 9:21 AM, Anne Ranch wrote:

On Fri, Jun 9, 2023 at 08:57 AM, Maynard Wright, P. E., W6PAP wrote:

you need "all frequencies" (or at least enough frequencies).

OK, the discussion did covered the theory , now is the time to apply it and be practical.
.
I shall repeat
the task is -
is the unknown length of cable in multiples of 1/4 wavelength @ 14 MHz band,?
hence
what should the sweep frequencies be ?

Step size (span/number of points) sets the "unambiguous range" - if the steps are too far apart, then the true length might be a multiple of 1/step size longer.

So, if you've got, say, 100 ft of coax - that's about 150 nanoseconds one way, and it's a two way measurement, so you want your step size to be smaller than 1/300 ns or around 3 MHz

The span sets the resolution (although one can interpolate) - 100 MHz span is 10 ns resolution.

Its probably best to avoid crossing the 300 MHz "change in harmonics" boundary.

With a 101 point NanoVNA, then doing 1 MHz to 201 MHz gives you 5 ns (~1.5 ft) resolution (two way time) and allows for more than 100 ft.

If you've got a 401 point NanoVNA, go for 1-300 MHz and whatever step size that gives you.

On Fri, Jun 9, 2023 at 08:57 AM, Maynard Wright, P. E., W6PAP wrote:

you need "all frequencies" (or at least enough frequencies).

OK, the discussion did covered the theory , now is the time to apply it and be practical.
.

I shall repeat
the task is -
is the unknown length of cable in multiples of 1/4 wavelength @ 14 MHz band,?

hence

what should the sweep frequencies be ?

On 6/9/23 07:52, Jim Lux wrote:

On 6/9/23 6:49 AM, Anne Ranch wrote:

Coax is, of course, not identical to paired cable, but using a low end of 50 kHz makes me wonder whether the velocity might be lower than the published high frequency value sufficiently to skew the results a bit. Maybe using a higher frequency low end would be better.? I don't know enough about the characteristics of coax at low frequencies to be sure, just raising a caution here.

This would be known as "dispersion" and would cause problems with either the pulsed (time domain) or swept frequency (synthetic time domain) approach.? A fast pulse (as used in TDR) has all frequencies in it: feed it into a dispersive medium (different velocity for different frequencies) and the envelope of the pulse will be stretched out - it won't be a sharp pulse any more.
The swept frequency technique (e.g. as used in a VNA) measures the amplitude and phase at a series of frequencies, does an inverse Fourier Transform to get that back into time domain, and you'll see the same smeared out pulse.

This is true. Setting the low frequency such that the high frequency VF is closely approximated throughout the range of frequencies used would minimize the spreading and make determination of the length of the cable a bit more precise.

Radio folks are generally familiar with the high frequency approximations to the complex characteristic impedance and complex propagation constant for transmission lines, but there are also low frequency approximations used by telephone engineers that are pretty accurate at voice frequencies. 50 kHz is in the transition region of frequencies where neither the high or low frequency approximations will be very accurate and calculations may need to be made using the full set of primary constants.

During measurements, the characteristic impedance can be seen to become increasingly complex, rather than nearly real, as the frequency is lowered and the values of the attenuation and phase shift (in neper and radians per unit length) will tend to converge toward each other until their angle is almost 45 degrees at voice frequencies.

Although there is a wealth of information out there on telephone cable pairs for lower frequencies, I haven't seen anything similar for coax, maybe because coax is seldom used for voice frequency transmission. So 50 kHz brings up concerns in my mind, but it would be interesting to see something about how coax actually behaves at such a frequency.

I expressed my concerns about the low frequency of 50 kHz due to previous experience with another type of transmission line, but I can't help with nanoVNASaver because I use my nanonVNA only by itself so far.

73,

Maynard
W6PAP

BEFORE I get "analyzing" the TDR (nanoVNASaver)? source code? , allow me to take another stab at this .

In basic steps - how do I setup to measure length of? KNOWN type of coaxial cable using TDR? - in nanoVNASaver ?

Assuming that TDR uses specific frequency to perform the task (?)

It uses a range of frequencies

Setup? nanoVNASaver sweep? starting frequency
setup nanoVNASaver sweep? ending frequency
run sweep
open / push "Time Domain Reflectometry..."? in TDR? "window"
observe TDR graph and "ESTIMATED cable length:"
change / verify the cable type (!) - changes VF according to cable type
optionally fiddle with TDR graph X-axis? whose parameters are in "meters" , not in nS.

Length is measured in meters (duh) but TDR calculates using "time" and reflection time is related to frequency used.

Not exactly.. it makes use of the frequency domain<>time domain duality from a Fourier transform. It measures in frequency domain, and then *displays* in time domain.? So you need "all frequencies" (or at least enough frequencies).

Does all this makes some sense or am I missing IMPORTANT part / option / parameter?

All this using nanoVNASaver? - no other tool , for time being...

Same idea whether done in the box or in software afterwards.

I have a NanoVNA H4 and it's great for measuring antennas from inside the shack, but not quite rugged enough to used outside up a ladder. Also the display is hard to read in the sunshine.

For outside use I have one of these:



It's sturdy and easy to read, although the menu system is a little cumbersome. My favourite feature for multiband antennas is a simulatnaeous view of 5-band VSWR.

--
Mike G8GYW

On 6/9/23 6:49 AM, Anne Ranch wrote:

Coax is, of course, not identical to paired cable, but using a low end of 50 kHz makes me wonder whether the velocity might be lower than the published high frequency value sufficiently to skew the results a bit. Maybe using a higher frequency low end would be better. I don't know enough about the characteristics of coax at low frequencies to be sure, just raising a caution here.

This would be known as "dispersion" and would cause problems with either the pulsed (time domain) or swept frequency (synthetic time domain) approach. A fast pulse (as used in TDR) has all frequencies in it: feed it into a dispersive medium (different velocity for different frequencies) and the envelope of the pulse will be stretched out - it won't be a sharp pulse any more.

The swept frequency technique (e.g. as used in a VNA) measures the amplitude and phase at a series of frequencies, does an inverse Fourier Transform to get that back into time domain, and you'll see the same smeared out pulse.

BEFORE I get "analyzing" the TDR (nanoVNASaver) source code , allow me to take another stab at this .
In basic steps - how do I setup to measure length of KNOWN type of coaxial cable using TDR - in nanoVNASaver ?
Assuming that TDR uses specific frequency to perform the task (?)

It uses a range of frequencies

Setup nanoVNASaver sweep starting frequency
setup nanoVNASaver sweep ending frequency
run sweep
open / push "Time Domain Reflectometry..." in TDR "window"
observe TDR graph and "ESTIMATED cable length:"
change / verify the cable type (!) - changes VF according to cable type
optionally fiddle with TDR graph X-axis whose parameters are in "meters" , not in nS.
Length is measured in meters (duh) but TDR calculates using "time" and reflection time is related to frequency used.

Not exactly.. it makes use of the frequency domain<>time domain duality from a Fourier transform. It measures in frequency domain, and then *displays* in time domain. So you need "all frequencies" (or at least enough frequencies).

Does all this makes some sense or am I missing IMPORTANT part / option / parameter?
All this using nanoVNASaver - no other tool , for time being...

Same idea whether done in the box or in software afterwards.