Is it possible to set Tx between -20 and -30 dBm?

[Daliwan]

We have a project which need to control the distance between the slave and the master to be less than 10m, say 5m. We tried -20 and found it a little longer than we expected and also tried whisper mode, -30, than it can work when the module stands alone, but almost can not connected when it is on board. I don't know what influence this when it is on board and I want to know if it is possible to use the number between -20 and -30 so that I can get an accurate distance. Thanks.

[endnode]

No, all nRF5x chips support only values listed in RADIO->TXPOWER register of respective Product Specification. In general just values between -20 and +4dBm with 4dBm steps are usable. However what makes precise location impossible are neither TXPOWER values neither receiver's sensitivity (they typically work with 1dBm resolution in -90~+10dBm range so it doesn't matter if you transmit at 0dBm or -12dBm it should be the same usable effect in close range of 10-20m) but terrible dependency of 2.4GHz radio on surrounding topology. In other words even small variations like micro movements of transmitter and objects around can change it by +-10dBm (or even more) in next TX burst so you believe that device move quickly but it's staying on the spot, just shaking a bit.