When I understand the objective correctly, the task is to capture data. For that purpose, you probably set youyour Wi-Fi interface into monitor mode and capture the frames being received.
If that is correct, fiddling with the Tx power (=transmit power) won’t help at all. You would either have to change the receiver sensitivity or the attenuation of the antenna.
Receiver sensitivity is a problem. Without development access to the Wi-Fi chip configuration, you are not likely to be successful.
Attenuation of the antenna is not a big problem. You could use an antenna with less antenna gain than your current model or simplesimply put some meters of antenna cable between the antenna jack on the device and the actual antenna.
However: Why would you want to do that at all? If your Wi-Fi capture contains totoo many networks that are not of interest to you, simply filter them out while or after capturing.
Keep in mind: Making your sniffer worse will not only reduce its operating range, but it will also cause it to miss frames in the near field.
