On Sat, Jan 25, 2020 at 10:01 PM, Bob Albert wrote:
"... When I measure 50 Ohm coaxial impedance with the 1/8 wave method, I always get 50 Ohms. I tried some 75 Ohm cable and still get 50 Ohms!.."
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Bob,
I haven't tried the 1/8 wave method with my NanoVNA-H4 yet so your question gave me a reason to do so. See the attachment for details.
1. I connected a 2 mtr length of RG-59 (75 ohm) cable to CH0. My calibrated range was 50k-1G and I reduced the stop frequency until I had one full revolution of the smith chart.
2. I set <marker 1> to half-way around the smith chart (1/8 wave length) and read the frequency (24 MHz)
3. I set <marker 2> to half that frequency (~12 MHz) and read the impedance (~75 ohms).
I could not set my second marker to exactly 12 MHz because my original calibration points landed me on either side of that frequency. One side was 76.4 ohms and the other side was 73.2 ohms. If I had done a 101 point cal from 50k-50M I could have landed on 12 MHz (~75 ohms).
I've used this method since it was introduced to the group and its always worked for me.
- Herb
