Late night radio: why is it the AM stations one can hear thousands of miles away?

Question

I do know something about the ionosphere and how it reflects radio waves, but never really got why it should be the AM frequencies that travel so far.

Thank you, juliepel, but why should higher frequencies "break through", especially considering that the radio telescope at Arecibo uses a frequency of 410 MHz to probe the ionosphere (by reflection)?

And to fyrfyter, what makes AM more "powerful" than FM?

Answer 1

Can you speak in spanish please?

欢迎到我的党

Answer 2

you're right that that the upper atmosphere reflects all kinds of engery, the reason that am can travel so far is because it is such a strong signal, am is short for amplitude ( which is the measure of how powerful something is) modulation, where fm, short for frequency (which is the interval at which something happens) modulation, both are nothing more than terms to describe a wave, am describes a long wave heightwise, and fm describes the interval between waves, the benefit of fm is that there is more waves in the same period of time than with am, the waves are generally compact and relatively frequent, the drawback to fm is that am waves are so much more powerful even though there are fewer in the same time span as fm, however they can travel far distances around the earth before the signal breakdown becomes aparent because of the size of the wave.

Answer 3

It's because of their frequency. They reflect off the ionosphere like short wave radio. FM and TV don't reflect; they go out into space.

Answer 4

It's because they're much lower frequencies. Higher frequencies would break the ionosphere barrier very easily.

Also keep in mind that more energy is required to transmit high frequencies, causing another physical limitation there.