Since this chapter is on amplitude modulation, I think it's good to talk about the applications of it.
Specifically, AM and FM radio. While we haven't talked about Frequency Modulation (FM) we have talked about AM (Amplitude modulation).
AM radio, that you're used to listening to, is carried at frequencies of 535 to 1705kHz. There's a lot out there about AM radio already written on the web. While we haven't talked about frequency modulation, there are some links here about it. I would suggest you check out:
The reason I include ionospheric refraction is that it causes the frequencies that carriers often use for AM radio to be able to travel long distances. This is why you can pick up AM radio stations much farther away than FM radio signals. Check out the website, it's interesting.
I also encourage anyone to contribute to this page to expand it.
I found the Ionospheric Refraction to be very interesting, so I'm going to expand on it a bit.
I found another website that gives a few more details: http://www.ycars.org/EFRA/Module%20C/EMSkyWave.htm .
Basically, the Ionosphere is a region 50-300 miles above the surface of the earth, made up of millions of charged particles (hence the name). The charged particles are created when the sun ionizes the upper levels of the atmosphere. Thus the Ionosphere is dependent on the time of day and the season. So the Ionosphere is more prevalant during the day vs. the night, and the summer vs. the winter.
Tom pointed out that AM Radio can transmit farther because of the Ionspheric Refraction because of the frequency ranges. The Ionsophere will bend and attenuate signals less than 30 MHz. Signals at higher frequencies (the website specifies f >200 MHz) are not affected by the ionosphere because the ionosphere appears completely transparent at these frequencies.
A particularly interesting event which can cause drastic effects on the Ionosphere is called a solar flare [1]. A solar flare, basically a sudden, rapid, and intense variation in brightness, can be so powerful, that the Ionosphere can become so dense, that communications depending on the Ionospheric Refraction will be completely cut-off.
