Transmitter Output Power (Tx Power)
Transmitter output power is the RF power the power stage delivers to the antenna connector, written in dBm or mW. In short-range devices typical values are between 0 dBm and 20 dBm, that is 1 mW to 100 mW. The effective power that goes on the air is not this figure but the ERP/EIRP found by subtracting cable loss and adding antenna gain.
Increasing output power is the first solution that comes to mind for range problems, but the returns fall off quickly. In free space, doubling the range requires 6 dB, that is four times the power; quadrupling it requires 12 dB, sixteen times the power.
- Regulatory limit: band limits do not permit such an increase anyway.
- Battery cost: the current during transmission rises with power. Although what really determines total battery life in remote controls is the sleep current, degradation of the output power at low battery is seen as a loss of range.
- There are cheaper gains: bringing the antenna out of the enclosure, raising it and matching polarisation gain more dB on most sites than a power increase.
For AXI's ST, SF, HCT and SAROZ kits the output power value is declared in the product technical file; with range problems the correct first step is not to fiddle with power but to review antenna placement and noise sources.
Context where this term is used: Wireless remote control installation: pairing, relay, safety