You needed to sweep up to a higher frequency to see your impedance null.
A method to calculate the VF of any feedline, that will work with a VNA, antenna analyze or SWR bridge.
(this sounds complicated, but only takes a minute to do if you have already measured the actual feedline length):
Leave one end of the feed line open (or shorted -not terminated with the characteristic impedance).
Sweep over a wide frequency range, starting with a bit lower than MHz=150/(feedline length in meters),
and stopping after 2 (preferably 4) multiples of the starting frequency.
Carefully note the frequencies where you get a peak and a null.
You need to sweep a wide enough frequency range to to get at least two peaks or two nulls.
It will be apparent from the frequencies recorded, which is the fundamental, 2nd harmonic, etc.
Calculate the free-space length (meters) using 299.8/(2 x frequency x harmonic).
Your actual length / (calculated free-space length) is the exact velocity factor.
VF can vary 1 percent between batches of the same type coax, less so for parallel line.
I used this technique to get the exact VF of a 500 foot length of LMR400,
and knowing this was later able to locate a center conductor break with 1/8 inch accuracy.
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N0YWB
N0YWB