What is SSB? Why is SSB used in HF radio transmission? And why is SSB is not used in VHF?

Single sideband is the filtering of one sideband from a typical AM modulated signal.(DSB)

The carrier is suppressed, reinserted at the receiver BFO frequency to demodulate the signal.

For HF, this has a much improved efficiency and half the noise by comparison.

SSB puts all the power with the intelligence to be transmitted in one sideband, effectively doubling the ERP over AM DSB

SSB is used into the VHF spectrum, but of no benefit above 150 MHz.

Modern communications makes use of digital spread spectrum above 70 Mhz. Digital is possible below this but the data rate becomes limited.

The old terrestrial TV broadcasting NTSC used vestigial sideband, a reduced carrier at center, the video in one sideband to 4 Mhz bandwith, the sound in the other sideband with total 6 Mhz channel.

The new ATSC digital can squeeze up to 10 sub channels into this 6 Mhz channel bandwidth.

Two primary forms of digital spread spectrum are direct sequence and frequency hopping.

SSB aka single side-band is a way to transmit without using a carrier-wave. It greatly reduces the energy-requirements compared to AM (amplitude modulation). and uses less bandwidth.

why is SSB is not used in VHF?

two reasons: because it's harder to tune your receiver, and because VHF (30-300 MHz) is mainly used for high-bandwidth contacts, like data, encrypted speech, high-fidelity music and television

SSB TRANSMISSION WAS THE MOST SIMPLE WAY IN THE OLD DAY TO REACH VERY FAR AWAY DISTANCE WITH LITTLE RF POWER, LIKE 10W TRANSMITTER POWER ABLE TO REACH OVER HUNDRED MILES AWAY. WHEREAS SAME RF POWER IN VHF BAND ONLY ABLE TO REACH NOT MORE THAN 20 MILES.