Is there firmware for the NanoVNA-H that supports selecting the port impedance (as is available on the VNA-H$)?

So why not just use it with 50-ohm center on the Smith Chart? It will
easily show 75-ohms to the right of center on the central horizontal line
with a non-reactive resistor. By design, the NANOVNAs are 50-ohm devices
at both ports as are high-$$$ units. Pretty much only the TV and CATV
people use 75-ohms. The rest of the RF and µW industries use 50-ohms.

If you absolutely require 75-ohm ports, you could construct a loss matching
pad from 50-ohms to 75-ohms. You will lose a bit of dynamic range in doing
so.

Dave - WØLEV

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On Thu, Nov 28, 2024 at 6:36 PM John via groups.io <radio=
netins.net@groups.io> wrote:

Is there firmware for the NanoVNA-H that supports selecting the port
impedance (as is available on the VNA-H$)?

--

*Dave - WØLEV*


--
Dave - WØLEV

With a "normal" nanoVNA (50 ohms), if, during the SOL calibration, instead of using 50 ohms during the LOAD you place a load of 75 ohms, it works. The central point of the Smith will correspond to 75 ohms. On the other hand, it will be necessary to make a rule of 3 for the values ​​calculated by the NanoVNA. It is only the measured ROS that is correct and the Smith only displays this ROS.
--
F1AMM
François

In this firmware https://github.com/DiSlord/NanoVNA-D/releases/tag/v1.2.40
is a option DISPLAY/PORT-Z where you can specify impedance, ex 75, or 100 etc.
Calibrate normally before entering PORT-Z with a 50-ohm set.

Hi All

Rgarding its price , May be we can owne two NanoVNA's H4. , one for 50 Ohm nerworks , and the second for 75 ohm networks just with bridge modds to 75 ohm of port1. And resistors devider of port2 to have 75 ohm input impedance , May be with the same Dislord firmware, Calibration will be done with 75 ohm load obviously, then no more compromise of specific impedance , we may use a very good resistor quality as 0.5% accurate resistors and 1206 format to ensure accurate measurement and calibration stability .
73s Nizar

Without changes in the firmware it will not work properly. Of course, if you're only interested in SWR it's probably enough, but you'll see 50 ohms instead of 75 when measuring impedance.

Hi

For using DiSlord Coax function, I guess that it's rather to start with the minimum frequency of the NanoVNA-H4 does ,
- around 10Khz start frequency .
-to stop with just the frequency given a little more then Half turn of smith Shart .
- calibration with 401 Sweep points , done just before Coax measurement at this optimum Start/Stop band.
- accuracy should also depend on the long of the Coax, will be better with longuer coax as good as we have the external physical length value given by a classic meter accurately. Vp setting should be adjusted to display the exact phisical length with three significant digits at least.

Still no enought infos about losses values measurement accuracy of DiSlord Method.... it should be at least related to the cautions above.

73's Nizar

Your observations are mostly correct, Nizar.

However, please note the following:
1) The velocity factor setting affects only the reported length measurement, and nothing else. And setting the velocity factor to 3 significant digits is, in my opinion, never needed. This is because manufacturing tolerances of coaxial cables result in velocity factors across a much larger range than that. Even from the same manufacturer, two batches of cable of the same type can vary in velocity factor by a few percent. So if you need accuracy better than a few percent, you must physically measure each cable length to determine its precise velocity factor. Which means that the nanovna measure cable function is excellent for getting an estimate of the cable length within a few percent, but can only get better than that if you physically measure the cable anyway, which defeats the purpose of having the nanovna measure it for you.

2) As a side-note, when you require a precise length measurement for something like a tuning stub, the nanovna can do that with high precision, since you measure and trim for the electrical length, which is not subject to velocity factor variations.

3) While you may achieve the highest precision by adjusting the frequency range to make just over a half-circle on the Smith chart, please note that the nanovna 'measure' firmware does an excellent internal (cubic spline) interpolation between those 401 measurement points. So even if you set your upper frequency such that 3 or 4 full circles are drawn on the Smith chart, you will still get a highly accurate data. Just try it. Look at the data measured with one half-circle, then change it to several half-circles or full-circles and compare. It does so well at the interpolation that I rarely change my upper frequency (which is typically 30MHz for HF use) unless the Smith chart draws many circles and I need to verify the accuracy. (This is true at least for cable lengths of a few feet or more, which can be measured with a 30MHz upper frequency - I haven't evaluated this for very short cables - but the interpolation is the same, so I'm sure it will still achieve good results).

4) As for the loss measurements, early versions of the 'measure' firmware used a simple calculation: 1/2 of the measured two-way reflection loss of the open-ended cable, at the cursor frequency. This is the same measurement you would get with the normal S11 logmag. This works quite well, but for some scenarios, a more accurate loss measurement can be made by doing both an 'open' and a 'short' measurement, and averaging the two, or by connecting both ends of the cable to the nanovna and doing an S21 'through' measurement. Recent versions of the 'measure' firmware were updated to use a more complex and accurate loss calculation (discussed a few months ago in detail on the nanovna-beta group). The nanovna-H4 implements the first (and dominant) quadratic part of this calculation, while the V2 devices (with more firmware/memory space) include the full calculation for even greater accuracy. This method again is based on the S11 reflection measurement, but uses an advanced mathematical model on what loss that represents.

I hope this information is useful to you...
Stan KC7XE

For velocity constant I still prefer measuring the frequency of the first
short circuit for both an open (reflects as a short) and a shorted
(reflects as an open) cable. Average those two. Then take an accurate
measurement of the physical length of the cable. Ratio the short/open
1/4-wavelength against the physical length. That's the velocity constant
of your length of coax cable.

Dave - WØLEV

On Tue, Dec 3, 2024 at 8:20 PM Stan Dye via groups.io <standye=
gmail.com@groups.io> wrote:

Your observations are mostly correct, Nizar.

However, please note the following:
1) The velocity factor setting affects only the reported length
measurement, and nothing else. And setting the velocity factor to 3
significant digits is, in my opinion, never needed. This is because
manufacturing tolerances of coaxial cables result in velocity factors
across a much larger range than that. Even from the same manufacturer, two
batches of cable of the same type can vary in velocity factor by a few
percent. So if you need accuracy better than a few percent, you must
physically measure each cable length to determine its precise velocity
factor. Which means that the nanovna measure cable function is excellent
for getting an estimate of the cable length within a few percent, but can
only get better than that if you physically measure the cable anyway, which
defeats the purpose of having the nanovna measure it for you.

2) As a side-note, when you require a precise length measurement for
something like a tuning stub, the nanovna can do that with high precision,
since you measure and trim for the electrical length, which is not subject
to velocity factor variations.

3) While you may achieve the highest precision by adjusting the frequency
range to make just over a half-circle on the Smith chart, please note that
the nanovna 'measure' firmware does an excellent internal (cubic spline)
interpolation between those 401 measurement points. So even if you set
your upper frequency such that 3 or 4 full circles are drawn on the Smith
chart, you will still get a highly accurate data. Just try it. Look at
the data measured with one half-circle, then change it to several
half-circles or full-circles and compare. It does so well at the
interpolation that I rarely change my upper frequency (which is typically
30MHz for HF use) unless the Smith chart draws many circles and I need to
verify the accuracy. (This is true at least for cable lengths of a few
feet or more, which can be measured with a 30MHz upper frequency - I
haven't evaluated this for very short cables - but the interpolation is the
same, so I'm sure it will still achieve good results).

4) As for the loss measurements, early versions of the 'measure' firmware
used a simple calculation: 1/2 of the measured two-way reflection loss of
the open-ended cable, at the cursor frequency. This is the same
measurement you would get with the normal S11 logmag. This works quite
well, but for some scenarios, a more accurate loss measurement can be made
by doing both an 'open' and a 'short' measurement, and averaging the two,
or by connecting both ends of the cable to the nanovna and doing an S21
'through' measurement. Recent versions of the 'measure' firmware were
updated to use a more complex and accurate loss calculation (discussed a
few months ago in detail on the nanovna-beta group). The nanovna-H4
implements the first (and dominant) quadratic part of this calculation,
while the V2 devices (with more firmware/memory space) include the full
calculation for even greater accuracy. This method again is based on the
S11 reflection measurement, but uses an advanced mathematical model on what
loss that represents.

I hope this information is useful to you...
Stan KC7XE

--

*Dave - WØLEV*


--
Dave - WØLEV

Hi Stan and Dave

Thank you very much for this very usefull infos and comments about the advanced DiSlord function for coax measurements .

I will try it and will enjoy it soon.

73's Nizar.