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When you do a calibration

open/short/50,

Stimulus from 3 to 30 Mhz.

it should then be accurate between 3 and 30 Mhz correct?

OK you calibrated with the 3 to 30 Mhz scan width.

Now if you change the stimulus width to like 14 to 14.5 Mhz.
and not do a new calibration,

is the calibration still good since this narrower window is still inside the original scan width/

Joe

On 10/17/21 6:28 AM, Joe WB9SBD wrote:

When you do a calibration

open/short/50,

Stimulus from 3 to 30 Mhz.

it should then be accurate between 3 and 30 Mhz correct?

OK you calibrated with the 3 to 30 Mhz scan width.

Now if you change the stimulus width to like 14 to 14.5 Mhz.
and not do a new calibration,

is the calibration still good since this narrower window is still inside the original scan width/

Joe

Yes, for the most part.? Let's say you have the original NanoVNA which uses 101 points. Your original cal is 101 points over the 27 MHz between 3 and 30, so about 270 kHz/point.? If you "zoom in" to 14 -14.5, the VNA interpolates between the 3 or 4 points covering that range.? Since it's unlikely that the VNA radically varies over that bandwidth, you'll get good measurements.


A notable exception might be if you're calibrating at the end of a longish feedline.? Then, there could be significant variations over a narrower frequency range (for instance, if you happened to be exactly 1/4 or 1/2 wavelength right in the middle of the band).

Hi Joe,

YES. However if you desire to improve accuracy, then do the calibration again over the smaller freq span.

For your work, you might consider calibration over each of the ham bands and store each cal sweep in each of the 5 memory locations. Probably overkill, but not a big deal.

So consider doing a 5 MHz sweep over each ham band as a start.

I had wondered about this. I will calibrate over several different ranges
(e.g. 143 to 147 MHz, 431 to 433 MHz, etc.) and save each one. I am
assuming the calibration is saved each time one of the frequency ranges is
saved.

73, Zack W9SZ

Now when you save a calibration to a memory.

Is that good still after a power down and then fired up again some other day?

Joe

YES.

Please read ANY of the user guides and learn what the letters on the left side of the display mean.



On Sun., 17 Oct. 2021 at 10:18 a.m., Joe WB9SBD<nss@...> wrote: Now when you save a calibration to a memory.

Is that good still after a power down and then fired up again some other
day?

Joe

--- On Sunday, October 17, 2021, 09:28:17 AM EDT, Joe WB9SBD <nss@...> wrote:

When you do a calibration open/short/50, Stimulus from 3 to 30 Mhz.
it should then be accurate between 3 and 30 Mhz correct?

OK you calibrated with the 3 to 30 Mhz scan width.
Now if you change the stimulus width to like 14 to 14.5 Mhz and not do a new calibration,
is the calibration still good since this narrower window is still inside the original scan width?

When you choose a set of frequency limits, and a set of frequencies in that span (number of frequency steps), then only and exactly that set of frequencies is calibrated. If you then make a measurement on that range, the perfect calibration is used for each frequency.

If you change the frequency range, or the number of frequency steps, then interpolation must be performed, and the calibration is not as good as it could be.

If you want the best results, always calibrate every time you change frequency range or the number of frequency steps.

73,
Ken N8KH

On 10/17/21 1:48 PM, Kenneth Hendrickson via groups.io wrote:

--- On Sunday, October 17, 2021, 09:28:17 AM EDT, Joe WB9SBD <nss@...> wrote:

When you do a calibration open/short/50, Stimulus from 3 to 30 Mhz.
it should then be accurate between 3 and 30 Mhz correct?
OK you calibrated with the 3 to 30 Mhz scan width.
Now if you change the stimulus width to like 14 to 14.5 Mhz and not do a new calibration,
is the calibration still good since this narrower window is still inside the original scan width?

When you choose a set of frequency limits, and a set of frequencies in that span (number of frequency steps), then only and exactly that set of frequencies is calibrated. If you then make a measurement on that range, the perfect calibration is used for each frequency.

If you change the frequency range, or the number of frequency steps, then interpolation must be performed, and the calibration is not as good as it could be.

Barring pathological cases (you've got things with narrowband phenomena, inside the calibration plane, like a stub that's a significant fraction of the wavelength),? I don't think the cal performance is degraded very much.? The cal parameters don't vary in a radical or fast way, most of the time, and the calculations are done with sufficient precision, that the limit on accuracy is something else (SNR of the measurement).? The interpolation is done in floating point, as are the calibration calculations.

There are places where I'd watch out - if you cross a harmonic switchover boundary, there's likely to be a glitch, because gains, etc. all change, and interpolation might not be valid.

If you want the best results, always calibrate every time you change frequency range or the number of frequency steps.

On Sun, Oct 17, 2021 at 02:11 PM, Jim Lux wrote:

Barring pathological cases (you've got things with narrowband phenomena,
inside the calibration plane, like a stub that's a significant fraction
of the wavelength),? I don't think the cal performance is degraded very
much.? The cal parameters don't vary in a radical or fast way, most of
the time, and the calculations are done with sufficient precision, that
the limit on accuracy is something else (SNR of the measurement).? The
interpolation is done in floating point, as are the calibration
calculations.

There are places where I'd watch out - if you cross a harmonic
switchover boundary, there's likely to be a glitch, because gains, etc.
all change, and interpolation might not be valid.

Jim,

I agree with you. Here are some measurements of a 20M dipole. First measurement was made using calibrated range of 1 to 30 MHz. Next was using same calibration but "zooming in" to 13 to 15 MHz. Last was calibrated from 13 to 15 MHz. Interpolation worked very well for this experiment.

Roger