The n-th natural frequency for a medium with length L fixed at one end and free at the other end is given by the condition:

Where λ_n is the n-th natural wavelength. On the other hand, every transverse wave satisfies the following:

Where v is the speed of the wave, and f is the frequency of the wave.
Then:

Isolate 1/λ_n from the first equation:

Then:

Replace L=0.681m, v=25.5m and n=1,2,3 to fin the first three natural frequencies of the antenna:

Therefore, the first three natural frequencies of the antenna are 9.36Hz, 28.1Hz and 46.8Hz.