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Is the nanoVNA able to measure the characteristic impedance of a short stripline?

For dead-bug builds it is sometimes required to have a 50ohm stripline to connect two adjacent parts of the circuit.
One approach is to use a small coax but according to the stripline calculator that can be found here
using a small isolated wire and some copper foil should also give about 50 ohm.
So I made a test PCB and added a rather high-tech stripline consisting of a thin insulated wire and some copper foil covering the wire and keeping it close to metal layer of the PCB.
One end of the wire is connected to the center pin of a SMA connector and the other end is connected to ground
Then I connected the nanoVNA to the SMA connector using a male-male and tried to use the TDR function of the nanoVNA to measure the characteristic impedance of the stripline.
It is possible to see the effect of adding the copper foil as the maximum impedance nicely reduces from above 150ohm to 67 ohm but the resolution of the TDR is on the edge of being able to see something.
The measurement point selected was the point with the highest impedance when no foil was added.
Replacing the test PCB with a 50ohm load gives 50ohm at the same measurement point and using a 1k5 resistor to ground on another test PCB showed the measurement point to be the highest resistance.
Replacing the test PCB with a short give -4ohm

Is there a better way to measure sripline impedance with the nanoVNA, or is this the maximum possible?


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

Thinking "out loud":

Any method employed with coax is applicable to stripline. They are both
transmission lines and should render measurement of their respective
parameters rather identical.

The caution is that the stripline or coaxial cable be long enough to be
well inside the frequency range of the VNA. For example, a 1/4-wavelength
at 450 MHz on FR-4 0.0625" board is

Lambda = {Lambda (freespace) / [εr]^1/2} / 4 = 3.2 inches

So, your test microstrip must be at least that long. This implies using
the lambda/8 method. The TDR method should also work. 450 MHz is well
within the range of the NANOVNA. Possibly a longer line and
correspondingly lower frequency might yield better results?

Dave - W?LEV

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

Thanks , it worked out very well.
Given a 900MHz max frequency I was able to use a 2 inch stripline, find the frequency for its 1/4 wavelength and then measure the reactance at 1/2 the 1/4 wavelength frequency.
It is all exactly the same as when measuring cable impedance thanks to the high upper frequency of the nanoVNA
I could not get unambiguous TDR output at this short length.

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

It is about as good as you can get without a good TDR instrument.

The objective of looking for high Z resonance peak at quarter wavelength is to verify Vp. Stray reactance on the 50 ohm port will create some error in this measurement. Inductance of stub grounding also needs to be minimized.

To minimize effects of these strays it is generally better to do the Zo measurement at lower frequency (1/8 to 1/2 of intended use freq) with longer length stub line.

5% off on Vp yields about 8% off on Zo calculation.
10% off on Vp yields about 18% off on Zo calculation.