TX Power

TX Power

TX power (transmit power) is the strength of the radio signal emitted by a BLE device, measured in dBm (decibels relative to one milliwatt). It is one of the two primary factors -- along with RX sensitivity -- that determine the link budget and, consequently, the maximum communication range.

Typical BLE TX Power Levels

BLE devices typically support transmit power levels ranging from -20 dBm (10 microwatts) to +20 dBm (100 milliwatts). Most BLE SoCs offer a configurable set of discrete power levels. The nRF52832 supports -40 to +4 dBm in 4 dB steps. The nRF52840 supports -40 to +8 dBm. The ESP32-C3 can reach +21 dBm with an external power amplifier.

Higher TX power levels require more current from the power supply. At +0 dBm, a typical BLE SoC draws 4--8 mA during transmission. At +8 dBm, this increases to 10--20 mA. The relationship between power output and current draw is non-linear, so doubling the TX power (adding 3 dBm) does not double the current but does increase it substantially.

Impact on Range

Every 6 dB increase in TX power approximately doubles the communication range in free space. However, real-world environments with walls, furniture, and human bodies introduce multipath fading and absorption that reduce the theoretical gain. In practice, increasing TX power from 0 dBm to +8 dBm might extend range from 100 m to 150 m rather than the theoretical 200 m.

TX Power in Advertising

BLE advertising packets can include a TX Power Level AD type (0x0A) that reports the transmit power used for the advertising packet. A scanning device can subtract the RSSI from this value to estimate path loss and, by extension, the distance to the advertiser. This is the basis for proximity estimation in beacon applications and indoor positioning systems.

Regulatory Limits

Regulatory bodies cap the maximum EIRP (Effective Isotropic Radiated Power) in the 2.4 GHz ISM band. In the US, the FCC allows up to +30 dBm EIRP. In Europe, ETSI limits it to +10 dBm. These limits include antenna gain, so a device with a +3 dBi antenna and +10 dBm TX power has +13 dBm EIRP and may exceed European limits.