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| Radio
Waves In order to see a program on your television, you must receive a source signal that the TV will eventually translate into audio and video. A common way to receive this signal is by radio waves. Radio waves belong to a class of waves called electromagnetic waves. Examples of electromagnetic waves include radio waves, visible light, X-rays, gamma rays, and microwaves. All electromagnetic waves travel in straight lines (assuming they are moving through a constant medium) at the speed of light, c. Electromagnetic waves consist of a magnetic field wave and an electric field wave at right angles to each other. Unlike other waves, electromagnetic waves apparently do not need a medium to travel through (i.e. they can travel through a vacuum). Radio waves that carry TV channels 2-6 range through 54-88 MHz (channels 7-13 use 174-214 MHz and channels 14-83 use 470-890 MHz). |
| Generating a Signal Radio waves are produced in a way similar to how electric lights work. As energy is applied to a material (by electricity, for example), the electrons on individual atoms are excited into a higher energy state. When the electrons move back down to a lower energy level, they release photons with energy corresponding to the amount of energy the electron gives up (the energy required to boost electrons to higher energy levels varies between atoms). Since the energy of a photon can be found by using E = hf (where E is the energy, h is Planck's constant, and f is the frequency), the amount of energy required to generate any of the photons for electromagnetic waves can be found. This process of exciting electrons and causing them to move down to their original energy levels and release photons is how broadcasting stations broadcast the radio waves that your television picks up. |
| How
the Signal Travels Radio waves are at such low energy (compared to, say, light, which has frequencies around 6x1014 Hz or 6x108 MHz) that they are not absorbed by the frame of your house (since the energy of the photons are not high enough to boost the electrons of atoms in wood to higher energy levels). Because of this, they are able to pass on through your house and into your living room where your television sits. So the frame of your house poses no problem in getting a TV signal from a broadcasting tower to your house. Unfortunately, radio waves only travel in straight lines away from their source. Line of sight restrictions clearly would prevent a signal from being able to be case from a long distance to your house... ...Unless of course the radio waves were reflected. About 90,000 meters above the earth's surface is the ionosphere, a layer of the atmosphere. Low frequency radio waves (such as the ones for channels 2-6) that hit this layer of the atmosphere with a small angle of incidence are reflected back down onto the earth. Because of the ionosphere, a TV broadcasting station could theoretically be on the other side of the earth and still be able to send a signal to you. However, a large portion of the radio waves that hit the ionosphere have larger angles of incidence than what is required instead fly off into space. It is important to note, however, that radio waves are not simply reflected from the ionosphere; it is more of a gradual bending. But since reflection is a good enough approximation (and easier to understand), it is often explained in this manner. This reflection of radio waves only applies to the lower frequency waves. UHF, VHF, and SHF signals are too high of energy waves and are therefore never reflected by the ionosphere You must be within line-of-sight of a TV broadcast antenna in order to pick up these signals. |
| Receiving the Signal Since humans have no way of perceiving radio waves, simply broadcasting TV signals accomplishes nothing. TVs have antennas for the purpose of intercepting radio waves. A simple receiving antenna is basically a loop of wire. As an electromagnetic wave that is oriented parallel to the normal of the wire loop passes through, a change in flux (magnetic field times area) of the loop of wire is produced. In accordance with Lens's Law, this change in flux is opposed by an induced emf inside the wire. The amount of current that is induced in the wire loop (or antenna) can be measured and decoded into a TV signal that tells the Cathode Ray Tube how to produce the image. |