Arduino accidentally transmitting FM

Question

I was trying to make an FM transmitter and, because I have no mic, I thought I could just use an Arduino (Uno) to generate some tones for testing. It turned out I never got a signal from the circuit but from the Arduino alone.

I'm connecting it to pin 9, battery- to GND, battery+ to Vin and no other connections. The transmitter circuit is gone.

I tested it with multiple antenna lengths. Every time, it produces a signal that can clearly be picked up by my radio on multiple different FM frequencies. I know this radio works, since it can receive multiple radio stations in my area.

How could this be happening?

Here is the code I'm producing the sound with. Both times, the same thing happens.

void setup() {
  pinMode(9,OUTPUT);
}

void loop() {
  while(millis()%1000<500){
  delayMicroseconds(1136);
  digitalWrite(9,HIGH);
  delayMicroseconds(1136);
  digitalWrite(9,LOW);
  }
  while(millis()%1000>=500){
  delayMicroseconds(757);
  digitalWrite(9,HIGH);
  delayMicroseconds(757);
  digitalWrite(9,LOW);
  }
}

void setup() {
  pinMode(9,OUTPUT);
}

void loop() {
  delay(500);
  tone(9,440);
  delay(500);
  tone(9,660);
}

Answer 1

You're probably generating or monitoring the audio with a class D amplifier or other PWM system. This is the amplifier equivalent of a buck power supply, using the duty cycle of its switched output to modify the effective instantaneous amplitude. Many are unfiltered, and the leads to earphones or other circuits make excellent antennas.

Although they largely operate in the sub-MHz range, they can generate an impressive comb of harmonics easily reaching into FM broadcast or shortwave bands and be picked up by radios.

Now, modifying the duty cycle in accordance with the signal is actually an implementation of phase modulation, the lesser known cousin of frequency modulation. It's close enough that it can be detected by most FM receivers, if the bandwidth of the switched signal multiplied by the harmonic number is still within the channel spacing of the receiver.

And voila! you're an FM broadcaster on multiple channels at once! Mr. Microphone could only wish for such power.