These are periodic Gaussian pulses of period 100ns.Pulse duration is 1ns.Two possible thoughts are:

1 Since the Fourier Transform of a single pulse have a central frequency of 4GHz and bandwidth of 2GHz,wavelength is light speed in medal(copper)/4Ghz.

2 Since wavelength is the distance of pulse travels in one period,it will be pulse duration*light speed in medal.

Obviously these two are different.

Please help me.

Thanks a lot!