Sub-GHz Band
Sub-GHz is the common name for the short range device bands below 1 GHz; the main ones in Europe are 433.05-434.79 MHz and 863-870 MHz. Because the wavelength is on the order of 35-69 cm, it bends around obstacles better by diffraction and produces less free space loss than 2.4 GHz at the same distance; this difference is roughly 15 dB at 433 MHz and roughly 9 dB at 868 MHz.
The advantage of sub-GHz comes from two physical effects. Because free space path loss increases with frequency, a lower frequency goes further with the same link budget; in addition, the long wavelength leaves a shallower shadow behind obstacles such as building corners and columns.
- The cost is bandwidth: the 433 MHz band is only 1.74 MHz wide, and there is no room for a high data rate.
- The second cost is congestion: garage remotes, vehicle keys and telemetry devices use the same band.
- The third cost is duty cycle: within the European framework you are not allowed to transmit continuously.
In a remote control application the data carried is small, so the bandwidth limit causes no problem; the real gain is range. In return, 2.4 GHz offers more comfortable room for features such as bidirectional telemetry, frequency hopping and encrypted sessions. AXI's HCT400/HCT402 sets work with 2.4 GHz XRF; for the band of the ST and SF series, the product label is authoritative.
Context in which this term is used: 433 MHz or 2.4 GHz?