I am using a 5D-2V calbe for my dipole. V1 and V2 are measured at both ends of the cable.
V2(blue)/V1(green) = 0.926371240154 V2(blue) phase advance against V1(green)= -253.07921443 deg. (V2 is in advance if positive)
The electrical length of the cable is (300.0m/7.001MHz)*(253.08/360.0)=30.12m, and the physical length 20.18m, assuming the velocity factor of 0.67.
Measuring the length with different frequencies gives this result, where the numbers 1 through 8 corresponds to the 3.5MHz/7MHz/10MHz/14MHz/18MHz/21MHz/24MHz/28MHz band, respectively.