| MadSci Network: Engineering |
Hi Irene, That's a really good question, but the answer is a bit involved. I'll try and explain it as well as I can without getting into a lot of complex theory. To answer the first part of your question, radio waves carry information through some kind of modulation. Modulation means to vary some characteristic of a carrier wave with information. In the case of AM radio, the information is the sound signal. The carrier wave (550 kHz - 1650 kHz in the US) is amplitude modulated (AM). This means the strength or the amplitude of the carrier signal is increased or decreased in proportion to the sound signal. This link discusses modulation in general: http://en.wikipedia.org/wiki/Modulation Here is a link that shows some diagrams of amplitude modulation (AM) at work: http://en.wikipedia.org/wiki/ Amplitude_modulation For AM broadcast radio, the sound waves are much lower in frequency than the carrier wave, typicially limited to about 5 kHz. This is about 1000 times lower than the carrier wave frequency. Our ears are capable of hearing signals in the range of 20 Hz - 20 kHz, so AM radio reproduces only one third to one quarter of human hearing. This is one of the reasons why AM radio does not sound as good as other types of radio, such as FM. FM radio uses frequency modulation. This link show a diagram of FM at work: http://en.wikipedia.org/wiki/ Frequency_modulation FM is a little more complicated for many reasons. The benefit is that the quality of information carried (sound in our case), can be better than AM. In addition, FM broadcast radio is capable of reproducing sound signals up to about 15 kHz, which is about the limit of most peoples hearing. For these reasons, FM sound quality is much better than AM. To answer the second part of your question, we have to talk about how transmitters and radios work. In it's simplest form, a transmitter takes an audio signal and modulates a carrier wave. This carrier is amplified (increased) many times. At this point, the modulated carrier wave can be coupled to a transmitting antenna. The shape and dimensions of the antenna are such that the carrier signal is converted into electromagnetic radiation. This sounds complicated, and mathematically it is! An easy way to picture electromagnetic radiation is to toss a small rock into a pond of still water. You should see a series of waves that spread from where the rock hits the water. The further the waves go, the smaller they get. At some point the waves are so small, they seem to disappear. Electromagnetic waves work in a similar way. The antenna is like the rock dropped into a pond. We can't see that waves produced by the antenna, but a receiving antenna can. Incidentally, visible light is also a form of electromagnetic (EM) radiation, but the frequency is much, much higher than radio waves. Here's a link that describes the electromagnetic spectrum in a little more detail: http:// imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html A radio receiver picks up the carrier signal produced by the transmitter at the receiving antenna. The signal is typically thousands of times smaller than it was at the transmitter. It's like a rock was dropped in the middle of a very large pond we measured the wave a very long distance away. The receiving antenna does the exact opposite of the transmitting antenna: it converts the EM radiation back to an electrical signal. In most areas, there are literally thousands of different carrier waves hitting a receiving antenna at difference frequencies and modulation type. The radio selects a specific frequency using a tuner. The tuner is an electrical circuit designed to select only certain frequencies and reject all others. The tuning control on a radio adjusts the center frequency of the tuner. Once the carrier we want is selected (which corresponds to a particular radio station), the carrier signal is demodulated. Demodulation is the process of recovering the information from the carrier. Since the transmitter modulated the carrier with audio signals, the demodulator recovers the same audio signal. The audio signal at this point is also very small - we couldn't hear it. The audio is passed into an audio power amplifier which makes it strong enough to drive a speaker. The speaker is like a linear motor. Applying an electrical signal on the speaker wires causes the speaker cone to move. This motion changes the air pressure which our ears are capable of hearing. This link explains speakers in a little more detail: http://money.howstuffworks.com/ speaker6.htm It turns out that a speaker will also work as a microphone. A microphone is like a speaker in reverse: it takes sound waves and converts them into electrical signals. There are lots of details I have left out in this explanation. The reason is that it takes several years of undergraduate university study to fully understand how everything in a radio works, but don't be discouraged! You can understand the ideas without understanding all of the details. I encourage you to ask more detailed questions, this will probably help you understand the parts that interest you the most.
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