MadSci Network: Physics |
Your question applies as well to television programs... the answer is that the program material is distributed through some alternate channel. This may be leased telephone lines or, increasingly, satellite links. The telephone lines that are used provide better signal quality and bandwidth than ordinary dial-up lines. Back in "olden days", circa WW-II, a combination of telephone lines and shortwave links were most likely used as the distribution medium. By the way, it's not the transmit power of shortwave that allows continent-hopping or even hemisphere-hopping links; it's the fact that at shortwave frequencies, the so-called "high frequency" or HF band (3-30 MHz), the ionosphere is able to refract signals back towards the earth. That is, shortwave signals bounce off the ionosphere and can be picked up far beyond the transmitter's horizon. Using Morse code, for example, it's possible to communicate thousands of miles with tens or a few hundred watts of power at shortwave frequency. By comparison, some FM broadcast stations transmit 50,000 watts but have an effective range of about 100 miles. Some TV stations transmit up to 5,000,000 watts with no more range! Finally, did you know that AM broadcast stations have tremendous range at night (thousands of miles) but relatively short range during the day? When the sun goes down, the lowest layer of the ionosphere disappears, which tends to absorb AM radio waves I think there are still a few "clear channel" AM stations that can be heard nationwide at night. They're called "clear channel" because they are the single authorized transmitter on the continent at that frequency. This eliminates interference from other transmitters on the same frequency hundreds or thousands of miles away. For a surprise, try tuning through the AM broadcast band at night. Even a car radio with the engine off (to eliminate radio noise from the ignition system and alternator) will often give surprising results. Better yet, invest in a cheap shortwave radio, or ask around and find a licensed amateur radio operator. There's an amazing world out there beyond the internet! of
Try the links in the MadSci Library for more information on Physics.