开云体育

On Wed, Nov 13, 2019 at 07:16 AM, Larry Rothman wrote:

"There is an interesting discussion on hugen79's NanoVNA Issues page on Github

regarding circuit changes/mods to reduce overall noise levels."

-------------------------------------------------------------------------------

Larry,
Hugen posted a chart a few hours ago showing the results of a new PCB with some of the recent improvements installed (see attachment). S11 looks to be <= 40 dB to 1500 MHz and S21 dynamic range is >= 50 dB to 1500 MHz.

- Herb

On Fri, Nov 15, 2019 at 06:33 AM, Kurt Poulsen wrote:

" I have done a lot of such measurements in the past and eliminated the inductance of the wire thru the core.

In your setup I think you get even better result by doing a SOL calibration at the end of the BNC adaptor using the Shorting wire thru the core without the core as short and place a leaded 50 ohm resistor as load and nothing for the open. The you get eliminated the "funny" impedances of the adaptors in the calibration process "

-------------------------------------------------------------------------------------------------------------------------------
Kurt,
The one turn loop gave me a method of sorting my ferrites by visual signature, but I could never get a good SOL calibration, per your suggestion, that extended beyond a few MHz.

Since most of my unidentified ferrites are clamp-on style, I decided to "go down the rabbit hole" and try to build a fixture using some of the references you provided and material that I had on hand.

Attached is a photo of the fixture I built. I tried to construct a coaxial type fixture where I could easily get access to the center conductor. I performed an OSL calibration from 1 - 500 MHz with the fixture cover in place and no ferrite installed.

I made a reference plot with the fixture terminated in 50 ohms. The S11 impedance plot showed a straight 50 ohms with a slight bump at the 300 MHz band switch over point (see red trace in attached plot).

I then replaced the 50 ohm load with the short standard used during calibration, installed a Fair-rite 0443164151 ferrite on the inner conductor (see attached photo), replaced the fixture cover and made a test run. The S11 impedance plot showed a rise in impedance with at least 100 ohms of impedance between ~17 MHz and 400 MHz (See black trace in attached plot) .

The slight bump at 300 MHz during the reference run turned into huge outlier during the test run. I would typically remove it from my data set. The typical impedance data in the chart provided by Fair-rite does not closely match the data I obtained (see embedded chart data).

I discovered that the coaxial fixture could also be used to measure the transfer impedance of an rf current probe that I had constructed from ferrites, so there was a secondary benefit to its construction.

- Herb

Thanks. I'm using a cross-needle meter without any tuner, so I'm matching
the same antenna as measured with the Nano.

On Fri, Nov 15, 2019 at 01:01 PM, <erik@...> wrote:
" Rune,

extend the s11 with the the complex conjugate of the s11 as input to the fft

Then you can use only the real values as impulse response with the sign being correct.

All imag values will be zero
No need to apply any windowing so you do not lose any information."

--------------------------------------------------------------------------------------------------------------

Eric,
I haven't actually looked at Rune's Python code but I believe he credits nuclear_rambo for the tdr and the step impedance functions. Nuclear_rambo calculates the initial impedance step correctly but then sign information is lost for additional impedance steps along the length of the cable. I've examined his Python script before but don't have the background to suggest corrective changes to the code. The script is as follows:

# Create step waveform and compute step response
step = np.ones(NFFT)
step_response = np.convolve(td, step)
step_response_Z = Zo * (1 + step_response) / (1 - step_response)
step_response_Z = step_response_Z[:16384]

# Calculate maximum time axis
t_axis = np.linspace(0, 1 / cable.frequency.step, NFFT)
d_axis = constants.speed_of_light * _prop_speed * t_axis

# find the peak and distance
pk = np.max(td)
idx_pk = np.where(td == pk)[0]
print(d_axis[idx_pk[0]] / 2)

# Plot time response
fig, ax1 = plt.subplots()
ax2 = ax1.twinx()
ax2.set_ylim([0, 500])
ax2.yaxis.set_ticks(np.arange(0, 500, 50))
ax1.plot(d_axis, td, 'g-')
ax2.plot(d_axis, step_response_Z, 'r-')
ax1.set_xlabel("Distance (m)")
ax1.set_ylabel("Reflection Magnitude")
ax2.set_ylabel("Impedance (Ohms)")
ax1.set_title("Return loss Time domain")
plt.show()

- Herb

Never mind; I found them cheap on ebay.? Will order soon.
Bob

Herman,
Thank you very much for the information.

Marco

Good afternoon!

Depending on how everything in your rig is connected and/or grounded, switching from a rig that is connected to the powerline ground to the NanoVNA that is floating could change things so that both of your readings are correct. If that sounds possible, you could tie the shield of the transmission line to the ground/chassis/etc of your xmtr while the NanoVNA is connected. That would eliminate that as a possible issue.

Just a thought...

Tom
AE5I

Rune,
extend the s11 with the the complex conjugate of the s11 as input to the fft
Then you can use only the real values as impulse response with the sign being correct.
All imag values will be zero
No need to apply any windowing so you do not lose any information.




--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

Rune,

I think that the TDR function is missing a trick in computing the observed impedance along a line. It looks like you are integrating the return impulses to get the impedance values, but ignoring the sign of the impulses. As a result, the impedance value always goes up, even if the cable is shorted at the end. It would probably be a good idea to include polarity in the impulse plot anyway, since that can help show the nature of the discontinuities along the line.

--John Gord

I did measure both with another vna. Ch1 is indeed worse but I found both to be closer to 50 ohm. Currently traveling so don't access to the exact numbers
Next to the resistance difference ch1 has a relevant reactive component. Best to use a 10dB attenuator at ch1
Calibrate the thru with the 10dB to remove the calibration impact
--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

New firmware from hugen:

NanoVNA-H version compiled on November 15, 2019

Using the code of nanoVNA-Q of qrp73, the driving of si5351 and aic3204 is more reasonable. By judging that the si5351 locking state is more reasonable than simply setting a delay, it can effectively avoid the noise caused by the unstabilization of si5351.

Unlike the compilation optimization of QRP73 and edy555, inline optimization is not disabled, and the refresh efficiency is better. Due to limited flash space DUMP, SCANRAW, COLOR commands are not available.

Optimized for AA version display.

Hi Ron
If it is the SWR indication build into your radio you are referring to, and your radio has an internal antenna tuner (or I would rather call it a impedance matching network), then the radio is matching to the actual antenna impedance when tuning so the PA stage see a close to 50ohm load impedance. Thus you can newer compare a NanoVNA SWR measurement of the antenna with a SWR indication in the radio display.
Kind regards
Kurt

-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Ron - An Old Ham in Utah
Sendt: 15. november 2019 16:45
Til: [email protected]
Emne: [nanovna-users] SWR...Nano versus Transmitting

This is my first post after lurking since October, so I'm quite new to the Nanovna. The Nanovna measurements of SWR are considerably lower than what I'm seeing when I transmit on-the-air. What does this indicate? Calibration problems, or errors in my other equipment? Any insight would be greatly appreciated. Thanks.
--
Ron, K7UV

These figures for the impedance of CH0, CH1 will vary considerably with frequency.

Jerry

What are you using to measure VSWR while transmitting. If you use a meter
that simply changes scales with no re-normalization for any specific power,
they are quite prone to read improperly when less than full scale power is
sourced from the transmitter. Cross-needle meters do not have this
problem, in general.

Dave - W?LEV

On Fri, Nov 15, 2019 at 3:44 PM Ron - An Old Ham in Utah <k7uv@...>
wrote:

This is my first post after lurking since October, so I'm quite new to the
Nanovna. The Nanovna measurements of SWR are considerably lower than what
I'm seeing when I transmit on-the-air. What does this indicate? Calibration
problems, or errors in my other equipment? Any insight would be greatly
appreciated. Thanks.
--
Ron, K7UV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

On Fri, Nov 15, 2019 at 11:44 AM, QRP RX wrote:

I think you can use 220 Ah battery (220'000 mAh) and it should works ok

It's a shame they all appear to be 12V ;)

On Thu, Nov 14, 2019 at 06:18 PM, Larry Rothman wrote:

Any 18650 cell will be way more than enough for the nano's needs.

Thanks Larry. I'll try and find a "quality" one.

Thanks for the additional explanation, Herb. Sorry if I was too unclear.

It would perhaps be possible for the software to have an 'auto' mode that
started at a low frequency and increased it until the reflection came
conveniently close to the limit of the range for best accuracy. But I don't
really like auto procedures that hide what's really going on : what if you
just wanted to look for partial reflections due to stubs or joints ?

The critical thing is that you obviously felt you should test it at the
working frequency of the system, and I can see why you'd start there. But
as Herb says, what you actually need is to test at a frequency where the
wavelength is comparable with the cable length. Presumably if you tried to
display the results of something more than a wavelength away, you'd get
multiple responses (repeated one wavelength apart).

Rune,
I find myself quite often having to use an external graphics program to add descriptive text to the NanoVNA-Saver chart images I save (See attachment). Is it feasible to add a comment or title box under "Display set-up …" where text can be entered that is placed at the top of the chart as shown in the attachment?

I also inquired on the NanoVNA-F group if there was a corresponding command for NanoVNA's screen capture. BH5HNU responded, "Currently not supported, this is a very useful feature, we consider adding it in the next firmware release." So if you were to implement screen capture in NanoVNA-saver, at least for now it would not work with NanoVNA-F.

- Herb

On Fri, Nov 15, 2019 at 05:36 AM, Barry Jackson wrote:

"If a particular frequency span setting, sweep mode etc. is needed for TDR measurements, should not the software set those modes temporarily when the TDR button is clicked and then revert back to the previous settings on leaving the TDR screen?"
-----------------------------------------------------------------------------------------------
Barry,
Expanding a bit on what Adrian Godwin responded. This is one of those situations where its better to think of frequency like a ham in meters instead of Hz. That helped me to wrap my head around the fact that when measuring meters of cable I needed to change my stop frequency to a value that was equally long in meters for the TDR function to yield good data. If you have firmware installed that has the TDR menu option, you will run into the same thing using the NanoVNA in the field.

Also, as Adrian said, neither the NanoVNA or any software program knows what length of cable you are attempting to measure, so just as you need to manually enter the velocity factor, you may also need to manually change your stop frequency for accurate results.

- Herb

This is my first post after lurking since October, so I'm quite new to the Nanovna. The Nanovna measurements of SWR are considerably lower than what I'm seeing when I transmit on-the-air. What does this indicate? Calibration problems, or errors in my other equipment? Any insight would be greatly appreciated. Thanks.
--
Ron, K7UV