Why is my iPhone transmitting in airplane mode?

I switched my iPhone 8 into airplane mode, but it continued periodically transmitting on 1.8 GHz, as measured by an electrosmog meter . Why? Isn't airplane mode supposed to turn off the radio transmitters? Or is airplane mode on an iPhone only a low-power mode? After enabling airplane mode, I powered the phone off and booted it in a Faraday cage. It still transmitted on 1.8 GHz, despite not being able to receive any cell tower signals.