On Fri, Sep 27, 2019 at 10:52 PM, hwalker wrote:
With your access to better equipment, the nanoVNA's TDR function probably does
seem like a toy, but if the user realizes its limitations and doesn't expect
laboratory quality from a $50 device, the function is still a worthwhile tool
and learning feature.
Unfortunately I don't have access to better equipment. The only VNA that I have is NanoVNA and EU1KY single port VNA.
If the TDR function tells me that a reel of cable marked
25 meters is 24.5 meters that's close enough for me to feel confident
purchasing it.
Such cipher 25 "meters" or 24.5 "meters" is completely useless, because you're just don't know the real cable velocity factor. You will not be able to measure velocity factor of the cable at hamfests selling. When you don't know velocity factor of the cables, you cannot compare two cables and cannot measure it.
For example, you're want to buy the cable with specific length and needs to select proper length from two pieces of cables A and B:
- cable A has 11 meters length (physical length), velocity factor 0.60, but you're don't know that
- cable B has 7.4 meters length (physical length), velocity factor 0.90, but you're don't know that
You're trying to measure cable length with NanoVNA TDR. Since you're don't know the real velocity factor (which actually depends on manufacturer, environment temperature and frequency) of these cables, you're using default setup for velocity factor 0.66.
NanoVNA shows you:
- 9.1 "meters" for cable A
- 10.1 "meters" for cable B
You're needs to select 10 meters (physical length) piece.
Which one you will select, based on NanoVNA TDR readings? :)
