Cellular telephones communicate with stationary base units using radio waves. The frequencies of these waves are near 850 MHz, or 850,000,000 cycles per second.
1. Assuming that your cellular telephone is transmitting a radio wave at exactly 850 MHz and that this wave is traveling through empty space, what is the wavelength of the wave?
Answer: 0.353 meters
Why: The product of the wave's frequency times its wavelength must be the speed of light. For a wave of frequency 850 MHz, the wavelength turns out to be about 0.353 meters.
2. The antenna of your cellular telephone is made of metal. Why can't the antenna be made entirely of plastic? 2.92118577228705|q
Answer: Plastic doesn't conduct electricity.
Why: In order to be able to push charge up and down on the antenna, the charge must be able to move through the antenna. Plastics general don't conduct electricity and thus won't work as antennas.
3. Electric charges are what emit radio waves, so why can't the cellular telephone emit a radio wave by just putting a positive charge on its antenna permanently?
Answer: The permanent charge doesn't accelerate and thus doesn't emit any electromagnetic waves.
Why: Only accelerating charge emits radio waves. Since this charge is permanent and the antenna is vibrating mechanically, the charge isn't accelerating and doesn't emit radio wave.
4. If you took a positively charged antenna and shook it up and down 850 million times a second, would it emit a radio wave. If so, why? If not, why not?
Answer: Yes, it would emit radio waves. The charge would accelerate is you shook it and would therefore emit radio waves.
Why: Regardless of how the charge accelerates, it will emit radio waves when it does. By shaking the charge at the right frequency, you'll succeed in emitting 850 MHz radio waves.
5. The transmitted power of a cellular telephone is small so that the wave is too weak to interfere with communications outside your local cell region. What aspects of the radio wave creation process limit the power of the radio wave?
Answer: Limited amount of charge and limited acceleration of that charge.
Why: The strength of the emitted radio wave depends on the amount of charge that is accelerating and how hard that charge accelerates. To keep the transmitted power small, the cellular telephone only accelerates a modest amount of charge and the charges' acceleration is kept relatively small.
6. As you talk into an analog cellular phone, the pressure fluctuations in your voice become represented by slight frequency variations in the radio wave leaving the phone. If you could freeze the radio wave in time and examine it at your leisure, what aspect of this frozen wave would allow you to see that it contains frequency variations?
Answer: The wavelength would vary from place to place along the emitted wave.
Why: Changes in the frequency of the outgoing wave will also affect the wavelength of that wave. When the frequency increases, the wave crests will be packed more closely together so the wavelength will decrease. When the frequency decreases, the wave crests will be packed less tightly and the wavelength will increase.
7. The base unit that communicates with your cellular phone transmits and receives radio waves of both vertical and horizontal polarization. Why is that important?
Answer: The cellular telephone's antenna transmits and receives only waves that are polarized at least partially along the antenna. If the base antenna were only able to transmit and receive waves that were polarized at right angles to the cellular telephone's antenna, the base unit and the cellular telephone could not communicate with one another.
Why: If the two antennas, base and cellular telephone, are perpendicular to one another, they won't be able to affect one another. That's because the wave from one antenna will push charge across the other antenna and not along that second antenna's length. For example, a vertical base antenna and a horizontal cellular telephone antenna won't interact at all.
8. Cellular telephone base unit antennas are often located atop towers, tall buildings, power pylons, or mountains. Why?
Answer: The waves between base unit and cellular telephone travel in straight lines, so raising the base unit antenna improves the likelihood of a good line-of-sight contact between the base unit and cellular telephone antennas.
Why: Like light, higher frequency radio waves are "brightest" when they pass directly from the source to the receiver. That means that line-of-sight is important. If you can see the base unit antenna, you can surely receive its radio waves. But if you can't see that antenna, you may have to rely on reflections to convey radio waves between the base and your cellular telephone, and these reflections are much less reliable.