On 9/22/20 8:00 AM, Jerry Gaffke via groups.io wrote:
John wrote:
# I noticed an interesting effect: If you set the Start frequency at half of the step size
# (say 15MHz to 3000MHz, 101 points), the impedance steps at the first (and every odd)
# "delay overflow" are inverted!! Probably makes perfect sense to someone (but not to me).
That's very curious indeed!
Perhaps with some head scratching, one might use this effect to distinguish between
ambiguous regions on long runs of cables.
Though of course, far better to figure out how to process more data points.
Perhaps with nanovna-saver, or a nanovna-SAA2
At the top of Bryan's document, Step 2 states:
"I changed the nanoVNA stop frequency to 130 MHz which gives a maximum length that can be observed in the nanoVNA of about 31.5 m. "
Speed down the cable is 3e8*0.66 meters/second, assuming a velocity factor in his cable of 0.66.
So Bryan's 31.5m might represent a delay of 31.5/(3e8*0.66) = 160 ns.
If we assume Bryan did not take the velocity factor into account, that's 31.5/3e8 = 105 ns.
Neil's formula gives a delay of tmax = 39/fmax = 39/130e6 = 300 ns
John's formula gives us a delay of tmax = 100/fmax = 100/130e6 = 769 ns
I prefer the looks of John's formula, the others appear to have been arrived at empirically.
But that's a rather large spread.
Jerry, KE7ER
The longest cable is more a function of the step size than the highest frequency. It has to do with ambiguity.
