Nanovnasaver plots
|
On Sat, May 28, 2022 at 03:20 PM, tuckvk3cca wrote:
Does anyone know to improve the plots? They look faint and grainy. I triedJust click "Display Settings" and then select "Show Lines" Then select your line width and dot size. See attached screenshot. To import into Excel just create a .s1p or .s2p Touchstone file. Import it into Excel as a CSV type text file but use space as the field delimiter instead of a comma. Roger |
|
On 2022-05-29 00:50:-0700, you wrote:
Great thanks. Any body has a spreadsheet then to convert S21 gain and phase data into impedances?You might find this useful...I did it with real data from my -H4 and my EF 40 m wire. You can make a copy of this spreadsheet so that you have a backup. Then open it, and start a new spreadsheet. Select Data > From Text In the File Selector mask drop box, select all files *.* Browse to your s1p / s2p files import one start at line ? depending on what line data starts in your snp file. should read right in. ----- either copy your data in cols to this spreadsheet, noting that it has a col inserted where freq is converted to MHz if your data is shorter (fewer rows) delete the extra rows -----or copy the formulas from this spreadsheet to your spreadsheet, paying attention to changing cols It's just some thing I tossed together to look at some data but S11, SWR, Linear, SWR:Linear, Z, |Z|, Q are all good calculations ~R~ 72/73 de Rich NE1EE The Dusty Key On the banks of the Piscataqua |
|
On Sun, May 29, 2022 at 12:50 AM, tuckvk3cca wrote:
There are two methods to Convert S21 to Z. The series method is used for "high impedances" and the "shunt method" for low impedances. These are built into the latest firmware for H/H4 and in NanoVNA Saver and NanoVNA app. You need a very good test jig for S21 series or the phase angle will have errors and the impedance measured will be way off. Also the H/H4 do not have precise 50 ohm source and load impedance so you get errors from that as well. But to directly answer your question here is is a link to a Web site with a spreadsheet. The link is in the last paragraph. Here is a link to another one.... /g/nanovna-users/message/17807 Roger |
|
On Mon, May 30, 2022 at 12:50 AM, tuckvk3cca wrote:
One port S11 measurement on a NanoVNA-H4 gives good results up to several thousand ohms (see attached 3K ohm example). It is better than using the S21 serial method. The S21 serial method does not work that well on a NanoVNA. The reason is that the Ch0 and CH1 impedances are not 50 ohms across a wide range of frequencies and the NanoVNA does not do a 12 point calibration. There have been several discussions in this group about measuring chokes, baluns and impedance using various methods. Lots of good info if you do a group search. Roger |
|
On Tue, May 31, 2022 at 07:54 AM, AG6CX wrote:
I¡¯ve always thought of the conventional antenna parameter measurement as theEd, Yes most antenna analysis is done using the serial R+/-jX form for impedance. However as Jim Lux points out in another post today ( /g/nanovna-users/message/28231 ) there are cases where the parallel form is useful in antenna measurements. The intent of my post was to reply to comments by Victor Reijs and G3TXQ (sk) that the S11 mode was not suitable for measuring high impedances. When it comes to the NanoVNA this is not true and actual measurements like the one I posted can be used to prove my point. Attached is a simplified resistor model showing that R, L and C are present in physical resistors. For low values of R the inductance is the predominate reactive factor but for high values of resistance the parallel capacitive reactance across the resistor needs to be considered. Attached are some plots taken with a test jig designed to measure 0805 components with a 1K resistor mounted in the fixture. Someone new to VNA measurements would look a the R+jX plot and see that the resistance measured looks close to 1000 ohms at very low frequencies (997 ohms) but measures 950 ohms at 250 MHz. The erroneous conclusion is that the 5% error is due to poor measurement capability. However this is not the case because the decrease in R is due to the effect of parallel capacitance. The parallel capacitance is small (less than a pF) but we can measure it and see the actual resistance of the 0805 SMD component by looking at the S11 data in parallel format. The attached plots show that measured R varies from 996 to 1008 ohms over the range of 10 kHz. to 280 MHz. which is quite impressive for a $100 instrument. The capacitance calculated from the parallel reactance X is 0.14 pF across the frequency range. From the above (and my previous 3K resistor plot) the S11 reflection method has been shown to make reasonably accurate measurement of impedances up to several thousand ohms. That makes it useful for measuring devices like inductors, capacitors and common mode chokes. The other method which is usually suggested for high impedance measurement is the S11 series method. This method assumes 50 ohm CH0 (port 1) and CH1 (port 2) impedance. The NanoVNA varies from 50 ohms over its frequency range and this leads to errors in the measured data. Since the NanoVNA is a two port, one-way device it does not have 12 point error correction to compensate for the deviation from 50 ohms. Owen Duffy has posted several interesting articles on the S21 series method using the NanoVNA on his blog. Measurements are shown to illustrate his points. My own measurements using the S21 serial method have shown that the quality of the test jig, measurement frequency span and complex impedance range have a significant effect on the accuracy of the results. I tend to use the S11 reflection method most of the time and for cross-checking S21 results. Roger |
|
On Tue, May 31, 2022 at 01:26 PM, AG6CX wrote:
I wonder if you¡¯ve had the same experience with the VNWA as with the nanoVNAI do not own a VNWA so I have no first hand knowledge to share. From what I have read it has better specifications than a NanoVNA. You could join the VNWA group and pose your question there... /g/VNWA/topics Roger |
|
Thanks Roger for pointing this out. Knowing one's measurement tools is the
toggle quoted message
Show quoted text
basic rule here. Thanks for pointing out. I have also checked my NanoVNA. If I measure the Zload of port 2 (using Port 1, so using a calibration Through connection and measuring port2's S11). I get the following |Zload| of port 2: [image: afbeelding.png] This is ok-ish IMHO (variation some 0.1ohm (less than 0.2%). There is a deviation from 50ohm (around 05%), but this could be partly due to inaccuracy calibration Load (measured with DVM: 49.7ohm). I don't think the Zload (|Z| of port2/load) is problematic. If I measure metal resistors (averaged between 300 and 1800kHz) using Reflection and Series S21 I method, I get the below |Z| values. I also give the R values measured with DVM [image: afbeelding.png] [image: afbeelding.png] So indeed |Zrefl| does not deviate much from the DVM value (<2%). The |ZseriesS21| though differs quite a lot (~9%) from DVM value and looks to be proportional to the resistor value. Where is this 9% error coming from? A 9% error is large! I don't think the deviation of the port2 load (Zload) from 50ohm is problematic (only some 0.2%). Looking at the Zsourse (the impedance of port 1): would that not be close to Zload (impedance of port2, I hope/assume...). But that one is compensated using the SOLTI method, or not? I understand the missing of 12-term correction is the other problem (as stated by Owen Duffy: ). Who has a procedure for this? I could not find this on the web (Owen Duffy refers to it, but I don't see a procedure). Something must deviate some 9% from the 'my norm', but what? I tried to simulate this by varying Zo, Zload, Zsource; but I can't find really realistic values to cater for the variation see in ZseriesS21 (I need to bring Zload back to some 40ohm to cater for this 9%, but then some other Z values (|ZseriesS11|) don't match properly [and it is not measured, see above!];-). Any help to understand this is appreciated. Thanks. So at this moment I agree that 2port Series method does not produce good values. All the best, Victor Op di 31 mei 2022 om 23:07 schreef Victor Reijs via groups.io <pe1atn.victor.reijs@...>: Hello Roger, |
|
On Tue, May 31, 2022 at 02:07 PM, Victor Reijs wrote:
It is not me butThe accuracy of any measurement is important and an excellent paper on this subject was written by Brian Walker of Copper Mountain Technologies (a VNA manufacturer). "Make Accurate Impedance Measurements Using a VNA" available from This paper has been discussed on groups.io before and has been subject to misinterpretation by myself and others. So I contacted the author and he was kind enough to answer my questions and send me his spreadsheet for error analysis. The paper goes into considerable depth on how to calculate the impedance measurement and an error equation is derived to calculate ¦¤S11(max). The author then goes on to derive ¦¤S11 for a Copper Mountain Technologies S5065 VNA which can measure from 9 kHz to 6.5 GHz with a reflection accuracy (S11) specified to be ¡À0.4 dB for measurements from ?15 to 0 dB. The curves and charts in his article are all based on THIS specification. They are NOT applicable to the NanoVNA which has better performance when used over a much narrower frequency range. I wrote a technical note that provides more detail on his paper and how it can be applied to the NanoVNA. I hope you find it informative. It can be downloaded from my Box account at the link below" Roger |
|
On Mon, Jun 6, 2022 at 02:56 AM, Victor Reijs wrote:
I have also checked my NanoVNA.The calculations for S21 series method are based on the Port1 (Ch0) and Port2 (CH1) impedances Zref being 50+j0 (not |Z| = 50). See attached equations. I used the 15 cm Cable that came with my -H4 and calibrated it using SMA loads. The DC resistance of the 50 ohm load was 49.87 ohms. I then calibrated at the end of this cable to establish the reference plane (photo attached). Next I attached the cable to Port2 (CH1) and measured the complex impedance R +/-jX fro 10 kHz to 250 MHz. The attached plots show that there is a considerable change in resistance over this range and an unwanted capacitive reactance. One "trick" that works is to use a quality SMA attenuator and attach it to Port2 (CH1) and this will give a better impedance match. I have a good 6 dB one and you can see the improved performance with it attached. When calibrating for S21 with it attached you get a better impedance match but reduce the S21 dynamic range by 6 dB. What I have not tested is how the impedance of Port1 (Ch0) varies with frequency. This is not as easy a measurement and I have not done it. The attenuator "trick" could be used here as well but needs further investigation. If I measure metal resistors (averaged between 300 and 1800kHz) usingYes the S11 reflection method gives good results at resistances in the thousands of ohms. What happens with the S21 series method is that as you increase the DUT resistance you get more error in the S21 gain AND S21 phase. This translates to larger differences in the expected complex impedance of the DUT. I believe there are many sources of error such as the following: - Port1 and Port 2 impedance is not 50 +j0 as pointed out above - Stray inductance and capacitance of the test jig have a considerable effect on the magnitude and phase of the S21 measurement. I have tried several different test jigs and keep trying to improve. The attached photo shows one I use but still needs work because the S11 reference plane is too far from the DUT and I get an unwanted phase shift - To calculate S21 the NanoVNA needs to know the power level generated on Port 1. I have a hunch that this power level fluctuates as the Return Loss decreases due to a higher DUT impedance. I understand the missing of 12-term correction is the other problem (asFor more discussion on this topic here are some old post links... /g/nanovna-users/topic/67738316#7993 /g/nanovna-users/topic/hardware_deficiencies_when/80639862?p= With care it is possible to get S21 results that compare well with S11 measurements. Here is a post I made earlier on the subject. /g/nanovna-users/message/24390. But I just use S11 method most of the time for ease of use. Roger |
F4WCV
Roger Need
Rich NE1EE
