Most VHF and UHF activity is FM through repeaters, but the bands also hold a quiet world of weak-signal operating: SSB, CW and digital modes at the very edge of what a receiver can detect, on paths that go far beyond ordinary line of sight. It rewards good antennas, patience and careful listening.
Why narrow is quiet
Noise floor in each mode's own bandwidth (power in dBm). The narrower the signal, the less noise gets in alongside it.
Narrow CW vs SSB13.8 dB quieter
Noise power is proportional to bandwidth. A signal 2.4 kHz wide (SSB) lets in 2.4 kHz of noise; a 100 Hz CW signal lets in 24 times less, about 14 dB. Digital modes go further: they pack the message into a few tens of hertz and use error-correcting codes, so a decoder can recover a message even when the signal is below the noise in a normal 2.5 kHz voice bandwidth. Narrower signals also need the transmitter and receiver to stay on frequency, so a stable oscillator matters.
Making the rest of the difference
Every decibel counts, and the choices are the same ones that decide any link budget:
Antenna gain: a long Yagi, or several stacked, sharpens the beam and adds gain at both ends. Weak-signal operators use horizontal polarization, which matches the other stations and reduces pickup of local vertically polarized FM signals and noise.
Low feed-line loss: at VHF and UHF, a long coax run eats the signal before the receiver sees it. See Feed line loss and velocity factor.
A low-noise preamp at the antenna, ahead of the feed line, so the first stage sets the noise figure and not the cable.
A quiet site away from household electronics, because man-made noise at VHF raises the floor.
Where the signals come from
Pick the mode for the reflector: MSK144 for meteors, Q65 for the Moon.
Weak-signal operators work far beyond the horizon using tropospheric enhancement and scatter, sporadic E, meteor scatter, aurora and moonbounce. Many of these paths are brief or random, so people gather near calling frequencies, and arrange contacts with each other online. They report their location as a grid square (the Maidenhead locator), which also drives contests and awards. See Tropospheric ducting and scatter.