As a DJ at a very small local
radio station, you often find yourself involved in technical issues for the
station’s two channels, one of which is AM at 1020 kHz and the other of which
is FM at 89.5 MHz. The station is in the process of building a new transmitting
system.
1. To save money, the director wants to use a single antenna for both channels. You warn him that the AM channel needs a taller antenna than the FM channel. Why is that true?
2. The director had planned to put the antennas next to the station, which is in a valley at the base of a small mountain. You suggest putting them at the top of the mountain, despite the extra cost of wires. Why is altitude important, particularly for the FM antenna?
3. The AM channel must be careful not to “overmodulate” the radio wave during very loud passages because it distorts the sound people hear in their radios. You explain this effect as due to moments when the transmitter actually turns itself completely off. Why would the transmitter stop transmitting any wave at all?
4. The FM channel must also avoid overmodulation during loud passages because it will get in trouble with the FCC. Other FM stations in your area will also be angry at your station for spoiling the reception of their transmissions. How can your FM station affect those other FM stations when they operate at different carrier wave frequencies?
You’re tired of sitting in
restaurants, listening to people talk on their cell phones. To solve the
problem, you build a small device that emits a strong radio wave at a frequency
you can select. You wait for a talker to pull out a cell phone and then turn on
your little transmitter.
5. Your device has a short antenna and you carefully align that antenna parallel to the talker’s antenna. Why is this important in trying to affect the talker’s cell phone?
6. You adjust the frequency of your transmitter until it matches that of the talker’s cell phone. Now it’s time to get the talker’s attention! The talker’s cell phone is using the amplitude modulation technique to communicate with the telephone company. What should you do to your transmitter to make noise in the talker’s ear?
7. Another person enters the restaurant with a cell phone that uses the frequency modulation technique to communicate with the telephone company. What should you do to your transmitter to make noise in this person's ear?
8. The police frown on your little trick, so you decide to use passive gadgets instead. You purchase several large metal sheets and find that if you position them just right around a talker, the radio wave that reaches the talker’s cell phone is surprisingly weak. You aren’t blocking the radio wave entirely, merely reflecting pieces of it toward the cell phone from different directions. The pieces of the radio wave are all reaching the cell phone, so why do they result in such poor reception?