Why does AM radio transmit over a longer distance than FM and why is AM disrupted easier than FM by things like underground carparks or tunnels?
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AM over mediumwave frequencies can curve around the Earth to a limited extent to reach over the horizon, it can also go around large structures. Longer wavelengths work better. At night the signal is trapped between the surface of the earth and the ionosphere and can be received across a continent if not encountering interference from other transmitters.
I can’t explain about carparks. My experience with FM is the opposite inside concrete buildings on the ground floor.
AM stands for Amplitude Modulated which means the frequency of the waves never changes but the relative strength does to denote changes in the broadcast. Waves with a lower frequency, like very low sounds, just travel distances better because the waveform is so long that it takes a lot to disrupt it. Where as a shorter waveform, like a high screeching sound, is piercing but doesn’t travel that far because the short waveform is easier to disrupt
Conversely though, low frequency/long waveforms, don’t have the penetrating power that shorter/higher frequency waveforms do.
FM stands for Frequency Modulated. So the amplitude, or “height,” of the waveform doesn’t change but the frequency, or “length,” does. Because the overall energy of the wave remains the same (amplitude) this allows for a much cleaner and broader range of data you can transmit. This is why music sounds better on FM than it does on AM.
FM bandwidth is a much much higher range of frequencies than its AM counterpart. So as I mentioned above, higher frequency allows for greater penetration but a lower overall distance the waves can travel. That’s why FM radio stations aren’t as affected by physical barriers but do have a pretty limited range
AM radio uses lower frequencies. About 1/100 the frequency of FM. This means that it is more capable of bouncing off of the upper atmosphere and curving around obstacles.
This isn’t *necessarily* true for all AM broadcasts, but it happens that public AM broadcasting frequencies are in this lower range.
AM sends information based on the strength of the radio waves, which makes it fairly easy to confuse if some other source exists for waves of a similar frequency. FM sends information based on the frequency, and so unless you have another nearby source that sends a slightly-changing frequency, there is much less interference.
It’s not AM vs FM that makes the difference, but the frequencies we use for public broadcasting. AM radio stations use around 1 MHz whereas FM ones use around 100 MHz. The lower frequencies bounce off the ionosphere, high above the atmosphere, and so can reach over the horizon to be received far from the transmitter. 100 MHz is too high to be reliably reflected by the ionosphere so you can’t be too far over the horizon to receive it.
The lower frequencies also have longer wavelengths, about 300 metres for AM stations. That’s too big to fit into small spaces so it doesn’t work well inside a tunnel whereas the 3-metre wavelengths of FM stations can.
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