> In technical terms, it’s basically a diode detector followed by a high-gain audio amplifier, which makes it safe to use without a risk of spurious harmonics o

> In technical terms, it’s basically a diode detector followed by a high-gain audio amplifier, which makes it safe to use without a risk of spurious harmonics or any other emissions from the (non existent) local oscillator. the down side is that the signal needs to be very strong to be detected. that means I should be physically very close the the transmitter to hear anything. in our case, I could hear the pilot, but not the ground station (pilot usually acknowledge what was communicated to them, so you can get the gist). you can also hear any other electro-magnetic energy such as atmospheric cracks, cell phones (not switched to flight mode) searching for a tower and engine roar, among others. So this is an RF detector circuit. If you happen to be close to an AM transmitter (like an air band radio) you'll get to hear the signal. If you are close to two AM transmitters you'll hear them both at once given that this has no tuned elements.