Coaxial cable loss measurement, right way?

My coaxial cable often work with high power (ie 1KW), I've just measure cable loss by using VNA.
But I wonder the value mesured by VNA is not correct because of low power input ( 20dBm).

Do you know cable loss vs input power?

the insertion loss should be the same, since you are not using any energy storage or magnetic devices (like a ferrite circulator that my saturate).

What u are not measuring is any high power effects though. Lets say your cable is poorly designed, and the high power is arcing over, or causing little carbon tracks to develop on the insulator...then, of course, a low power measurement taken previously is not longer valid.