Why can I hear an FM station deep inside a concrete garage, but not AM?

Question

I am able to clearly hear FM stations deep within my parking garage 2 stories underground which is sheilded with concrete..but not AM. Why is this?

Is it possible the vents/pipes are somehow acting like waveguides for FM but not AM?

Its quite amazing really. Any creative thoughts?

Thanx

Answer 1

Frequency modulation (FM) is more robust than Amplitude modulation (AM). Amplitude modulation must maintain a large enough signal amplitude in presence of noise in order to be demodulated and received successfully. Materials intervening will attenuate the amplitude and make for poor reception for AM. FM on the other hand allows the receiver to demodulate the signal even if its amplitude is degraded by attenuation and noise. The FM receiver may use an electronic function to limit the received signal so that the frequency variations of the modulation are easier to separate and lock onto. This makes FM more immune to noise and the pseudo AM modulation that materials may cause to the radio signal, since it is less likely for the FM radio signal to be frequency distorted by intervening materials.

Answer 2

One difference between AM and FM is that FM broadcast frequencies are higher, around 100MHz. So FM waves are around 3 metres long. So your waveguide explanation makes sense. High frequencies don't penetrate earth, concrete and water as well as lower frequencies. So either your explanation is correct or the FM stations are closer or more powerful than the AM ones where you live.

Answer 3

Previous answer is correct but more complex than needed. FM signals are sent out with more power. AM signals are lower power. Higher power signals can penetrate through more matter.

Answer 4

AM waves are 20 feet high so they get cut off if your roof is smaller.

Answer 5

because of the frequency