On Fri, Jul 30, 2021 at 02:38 PM, ward harriman wrote:
It occurs to me that it might be easier to characterize a longer piece of line
(which allows one to work at lower frequencies and have fewer capacitance
problems) and then cut the desired length¡¡.
Just thinking¡¯ outloud¡¡
I agree with you. I have been doing some tests on a 38.75 foot piece of 300 ohm twin lead with the intent of creating a 1/4 wavelength stub. I used a signal generator with a fast pulse and a digital scope to perform TDR. The round trip delay was 103.2 nanoseconds. This works out to a velocity factor of .763. Next I used the Transform mode on a NanoVNA-H4 and got 102.0 nanoseconds which is a VF of .772 which is pretty close considering the NanVNA is using a IFFT method and not an actual pulse.
Once we know the VF to some degree of precision a 1/4 wavelength piece can be cut . A 20 inch piece with a VF of .763 is 1/4 wavelength at 112.49 MHz. With the NanoVNA calculated VF of .772 that same 20 inch piece is 1/4 wavelength at 113.81 MHz. So a difference of 1.32 MHz.
Next step was to cut a 20 inch piece and see what the NanoVNA measures on the Smith Chart using a short or an open on the end. A photo of the test setup showing the jig and the ferrites on the short coax cable is attached. Without the ferrites the common mode was really affecting the measurements as shown in the attached screenshots. This was particularly the case when the termination was shorted and high impedance was being measured by the NanoVNA. 1/4 wavelength frequency was measured with reactance as close to zero as possible.
A comparison of the results (20" 300 ohm twinlead) 1/4 wavelength frequency..
Using TDR method (calculated) 112.49 MHz.
Using NanoVNA Transform method (calculated) 113.81 MHz.
Open termination - NanoVNA - without ferrites 112.02 MHz.
Open termination - NanoVNA - with ferrites 112.02 MHz.
Short termination - NanoVNA - without ferrites 80.529 MHz. ** see screenshot
Short termination - NanoVNA - with ferrites 108.02 MHz.
Conclusions
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1. Common mode is an issue and adding ferrites gave better results. I used 3 mix 31 after trying 1 and 2 with poorer results. Mix 43 have higher permeability and would be better but I didn't have any snap-on ones.
2. Results are closer to the TDR method when an open termination is used. The impedance being measured is low and in a better range for the NanoVNA. With a low impedance there is less common mode.
3. A proper balun was not investigated and takes more effort to construct and test.
Roger
Roger Need
