Much coaxial cable is of unknown origin. Even what appears to be legitimate cable may be counterfeit. For an application reliant on velocity factor, such as element lengths in a coaxial collinear antenna, knowing the actual velocity factor (VF) is essential.
Measuring coaxial cable VF is relatively straightforward with a vector network analyzer (VNA). The following example was performed at HF, but the exact same technique can be used at higher frequencies, using a shorter length of cable. Coaxial cable VF does not usually change much with frequency, so it isn’t necessary to perform the measurement at 1090 MHz.
Caveat: in order to obtain an accurate VF measurement it is essential to establish the VNA measurement plane, aka calibration or reference plane. For new VNA users, this excellent tutorial explains VNA calibration.
Although coaxial cable VF measurement can be performed with a lower frequency VNA, such as the original NanoVNA used in the above tutorial, I would encourage ADS-B hobbyists to obtain a VNA capable of at least 1090 MHz so that completed antennas can be measured. With a 2-port VNA, filters can be characterized as well. I believe the NanoVNA V2 is currently the least expensive 2-port VNA suitable for 1090 MHz (it works from 50 kHz to 3 GHz).