WiFi.setOutputPower setting ignored after some time

[Teddyz]

Setup

• Module: Generic ESP8266 Module
• Flash Mode: dio
• Flash Size: 4MB
• lwip Variant: v2 Lower Memory
• Reset Method: [ck|nodemcu]
• Flash Frequency: 40Mhz
• CPU Frequency: 80Mhz
• Upload Using: SERIAL
• Upload Speed: 512k
• esp8266/Arduino v2.5.2

Problem Description

Decreasing output power by WiFi.setOutputPower( 0.0 ); is not permanent.
High output level is restored after some time, less than 30 minutes.

How to reproduce

1 Upload code below.
2 Place esp8266 some 10 meters from AP
3 view esp's signal strength using AP internal web server.
4 after one minute the signal strength go lower (OK)
5 within some 30 minutes, the signal strength will be high again (NOK)

#include <ESP8266WiFi.h>

const char* ssid     = "myssid";
const char* password = "mypass";

uint32_t millisStartLoop;
bool doneSetOutputPowerToLow = false;

void setup() {
  Serial.begin( 74880 );
  // We start by connecting to a WiFi network
  WiFi.mode(WIFI_STA);
  WiFi.hostname( "setOutputPower" );
  WiFi.setOutputPower( 20.5 );

  Serial.print("Wait for WiFi... "); 
  WiFi.begin( ssid, password );
  while ( !WiFi.isConnected() )   {
    Serial.print(".");
    delay(1000);
  }
  Serial.println("\nWiFi connected to");
  Serial.println(WiFi.localIP());
  millisStartLoop = millis();
}

void loop() {
  if ( !doneSetOutputPowerToLow && (millis() - millisStartLoop) > 60000 ) {
    WiFi.setOutputPower( 0.0 );
    Serial.println( "WiFi.setOutputPower( 0.0 )" );
    doneSetOutputPowerToLow = true;
    }
}