Topband: Measure dipole on ground to get VF of BOG.

[Mikek]

I laid down 260ft of wire as a dipole on the ground, it is resonant at 
1190kHz.
I calculate this as a VF of 0.685, my calculation is actually 0.315 but 
I don't think that's right,
so I subtracted from 1. Can someone post the proper formula to calculate 
the VF using Resonant frequency and Length.
260ft = 79.248 Meters  Frequency is 1190kHz.

                        Thanks, Mikek KF4ITA