Why FT8 is a strictly time-synchronized mode: transmissions start at the top of each 15-second interval, and the decoder searches only a narrow window around that expected start time. Every station therefore needs its PC clock synced to UTC, typically with an internet time service (NTP) or GPS, to within roughly a second. The audio modulation and demodulation are done in software by programs like WSJT-X, so no external hardware modem is needed, just a sound card interface.
Watch out The special hardware modem choice describes older packet or RTTY setups using a TNC; FT8 does all of that in software. Antenna polarization and receiver attenuator settings have nothing to do with the protocol, though keeping received audio at a modest level does help decoding.
FT8 = 15-second slots, so keep the clock within about 1 second. Sync the PC, not the antenna.