How Does FT8 Work?

FT8 is a digital communication mode that uses a modulation technique called frequency shift keying (FSK) to transmit data over the radio waves. FSK works by shifting the frequency of the radio signal between two or more frequencies to represent binary data. In the case of FT8, the data is represented using a series of frequency shifts that correspond to specific data bits.

The FT8 signal is transmitted in a series of 15-second time slots, each consisting of 79 symbols (72 of which are used for data and 7 of which are used for synchronization and other purposes). Each symbol is 18.5 milliseconds long and contains 8 FSK signals. These FSK signals represent a combination of two different frequencies, each separated by 6.25 Hz.

To transmit data, the FT8 software breaks the message up into packets, each containing 13 data bits and 2 error correction bits. The software then encodes each data bit as a combination of two frequency shifts, using a predefined table to map each combination of frequency shifts to a specific data bit.

When receiving an FT8 signal, the receiving station uses specialized software to decode the signal. The software first synchronizes with the incoming signal, then looks for the frequency shifts that correspond to each data bit in the packet. Once the data bits have been decoded, the software uses error correction techniques to correct any errors that may have occurred during transmission.

One of the key advantages of FT8 is its ability to decode weak signals. This is achieved through a combination of advanced error correction algorithms and a specially designed modulation scheme that maximizes the use of available bandwidth. FT8 can reliably decode signals with a signal-to-noise ratio (SNR) as low as -28 dB, making it an ideal mode for use in weak-signal conditions.

Another advantage of FT8 is its ability to coexist with other modes on the same band. This is because the FT8 signal is highly narrowband, occupying only about 50 Hz of bandwidth. This allows multiple FT8 signals to be transmitted simultaneously on the same band without causing interference to each other or to other modes.