About controlling txpower in Raspberrry Pi 4

I did a test to control the radio transmission power of the raspberry pie that installed OpenWRT.

[How to test]

  1. Raspberry Pi (client) - Raspberry Pi (server)
  2. Upload files from client to server via scp command
  3. Change txpower through the command "iw dev wlan0 set txpower fixed xxx" (uci did not work, so we replaced it with a command.)
  4. Check the txpower set through the iwinfo command.
  5. The input cable was connected to the Spectrum Analyzer (8595E) and the other end of the cable was secured around the Raspberry Pi antenna chipset to measure the frequency.

Result : Frequency does not change significantly. For 0dbm -> 31dbm, the signal increased by 5dbm.

Usually, I think the signal should increase by 1 dBm every time txpower increases by 1 dBm, but it seems too small.

Is there another command or actual maximum txpower on Raspberry Pi4 that controls txpower?

please advice