I do not have time to test before next week, and regardless I do not either have any real RF measuring equipment to test whether or not this firmware produces a stronger transmission signal strength (measuring power current in mA/Milliampere) when cranking up the TX power output configuration to the higher and higher-level setting.
Just looking at reported LQI (Link Quality Indicator) and RSSI (Received Signal Strength Indication) values for Zigbee devices is just an indication of antenna gain (i.e. reception and not transmission) and not imperial proof that CC2652P is actually increasing the TX power output to extend transmission range.
Anyway, sure we can all test different transmitter power output configurations, (“transmit_power” in Zigbee2MQTT and “tx_power” for zigpy-znp in ZHA), with our devices and unique real-world setups but without proper RF measuring equipment tools that you can hook up to the SMA-port instead of the antenna and measure actual transmission power output we can only make assumptions that it “works better” if higher TX power configuration produces improved LQI and RSSI values.
Zigbee2MQTT adapter settings:
ZHA integration advanced configuration:
Also, looking at zigpy-znp configuration example sounds as if 19 dBm is highest can set for CC2652P?
5 versus 9 in
9 versus 14 in
14 versus 19 in
My guess is that setting the TX power to somehere between 9 and 14 (and not all the way to 19 or 20) will produce the best LQI and RSSI CC2652P/CC1352P based adapters. The reason for that prediction is that the higher TX power is then the more its own transmission is also interfering with its own reception capability, as transmitting higher power will produce more signal noise.
Remmeber that raising the TX power will only make it “shout louder” and not “listen better” so it could have the negative effect in the way that it will reach devices far away but not able to hear a weak reply.
By the way, recommend hobbyist enthusiasts with time and a little more seriously interest in inexpensive RF testing hardware equipment tools check out these two blog articles: