This is a frequently met problem when people setup a RF simulation. The root cause is the default simulation time in FDTD. By default it assumes the source is in optical frequency, so the default simulation time is 1000fs, which corresponds to frequency in a few hundreds of THz. At GHz range, The frequency range is 10^5 lower. Thus you will need to change the simulation time much longger, till the signal is fully showing the pulse.
To avoid very large numerical numbers, you can change the units to be used from "Setting" :
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