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Hi,

I am trying to get to grips why I am not getting the same Smith Chart results with my NanoVNA as I am with my DG8SAQ VNA. I have got to the stage of trying to make everything as simple as possible. I am using the same connectors, cables and calibration standards.

To make things even simpler I am just using a quality HV mica capacitor which measures 442 pf. According to Pasan software this should be 0 -j50 ohms at 7.2MHz. When I use the DG8SAQ VNA I get milli ohms -j46 ohms, which to me is close enough.

Using the NanoVNA with an identical setup I get 11 ohms -j49 ohms. Which is pretty useless. What is going on ?

T.I.A.

Stewart G3RXQ

Using the NanoVNA with an identical setup I get 11 ohms -j49 ohms. Which is pretty
useless. What is going on ?

Indeed 442 pF at 7.2 MHz, that's 0-j50 ohms No need for software to calculate that.
Z=1/Cw where
w=2*pi*F
C in Farad
F in Hz
pi 3.14

If with your NanoVNA you read: 11 ohms -j49 ohms, it's probably because you have badly calibrated your nanoVNA. You have to do OSL at the location where you place your capacitor
--
Fran?ois

-----Message d'origine-----

De la part de Stewart Baker
Envoyé : samedi 8 mars 2025 15:51

Hi Stewart

Even at 7MHz it is unlikely that you will not be seeing some stray inductance and stray capacitance, so I am more surprised, at the 0 -j50 ohms. Unless your test jig is very good and the actual capacitor is leadless you are unlikely to get exactly 0 -j50 ohms.

73
Jeff G8HUL

________________________________________
From: [email protected] <[email protected]> on behalf of Stewart Baker <baker@...>
Sent: 08 March 2025 14:51
To: [email protected]
Subject: [nanovna-users] Smith Chart accuracy - what am I missing ?

Hi,

I am trying to get to grips why I am not getting the same Smith Chart results with my NanoVNA as I am with my DG8SAQ VNA. I have got to the stage of trying to make everything as simple as possible. I am using the same connectors, cables and calibration standards.

To make things even simpler I am just using a quality HV mica capacitor which measures 442 pf. According to Pasan software this should be 0 -j50 ohms at 7.2MHz. When I use the DG8SAQ VNA I get milli ohms -j46 ohms, which to me is close enough.

Using the NanoVNA with an identical setup I get 11 ohms -j49 ohms. Which is pretty useless. What is going on ?

T.I.A.

Stewart G3RXQ

Hi Jeff,

Yes, you are right about the strays of the test jig, and about the capacitor lead length. However, I am using the same test jig and capacitor that, although not perfect gives milli ohms resistance when using the DG8SAQ VNA and 11 ohms with the NanoVNA. So I am comparing Apples with Apples, and not with Oranges.

73 Stewart G3RXQ

I get accurate results when measuring capacitors with my NanoVNA so I assume it is something with your setup or calibration. Without photos or screenshots we don't have much to work with here.

Here is a measurement of a ceramic cap marked 485 pF that I made with a NanoVNA. I compared it to a measurement made with a DE-5000 LCR meter at 100 kHz. The estimate of Rs is poor under 500 kHz when the reactance is very high. But capacitance measurement is quite good.

Hi Stewart,

Out of grins and giggles I took a 470 pF silver mica cap which actually measured 454pf using smart tweezers. I then just used a test jig with my NanoVNA which was a white plug in experimenter board. Using an 18inch cable connected to my NanoVNA on one end and an SMA adapter on the other with 0.75 inch wires soldered to the SMA adapter that I plugged into my experimenter board. I then just did a simple open, short, load calibration in which the open was just that (complete open), then I used a short jumper wire plugged into the board, and then a 50 ohm resistor plugged into the experimenter board. I used a resistor with legs versus a nice calibration resistor that comes with the NanoVnA because I wanted the calibration to include the component legs (zero out the effects of the complement legs). I then measured my silver mica cap and it measured 845 m - 46.29 j ohms at 7.285 MHz which is pretty close to the Xc value of 48.12 ohms based on a 454pf cap. Pretty crude fixture but does a decent job at 7 MHz.

P.S. I’m using a NanoVNA-F V3.1

Just FYI,
Don wd8dsb

Hi Donald,

Thanks for that, it was pretty much what I was expecting, and ties in with what I am getting with my DG8SAQ VNA. I am also using a silver mica capacitor with about the same lead lengths. Also when using a a Sark 110 VNA I am able to get sensible readings. To get 11 ohms resistive just doesn't make any sense, and I am beginning to suspect that I have a defective NanoVNA.

Thanks for the info.

73 Stewart G3RXQ

Hi
What Smith measurements given with the reference load, the short circuit and open terminaison ??? It can help to better understand what can be the issue
73s Nizar

Hi All,

In the post title it says "what am I missing ?". Well, I now know what I missed - tapping the "Done" key when calibrating !

I am so used to calibrating my DG8SAQ and Sark 110 VNA's that I thought the NanoVNA worked the same. It would seem that, with those VNA's, exiting the calibration routine screen automatically applies the cal. Not so with the NanoVNA.

Observing that the load resistor was not measuring 50 ohms after what I thought was a correct calibration gave the clue - DUH !

Many thanks to those who gave their help.

73 Stewart G3RXQ