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?
Answer: The AM station involves a much lower wavelength radio wave and an efficient antenna for this longer wavelength wave must itself be longer.
Why: The ideal length for an antenna is 1/4 of the wavelength of
the wave it transmits. Since the AM wave has a longer wavelength, it needs a
longer antenna.
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?
Answer: Radio waves (particularly shorter wavelength ones) travel in straight lines. Increasing the altitude of the antenna allows the waves to reach more distant radios without hitting anything.
Why: Shorter wavelength radio waves follow line-of-sight paths to
your radio. If you can't see the transmitting antenna, at least a little,
you'll have trouble receiving its radio transmission.
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?
Answer: The AM station signals your radio to move its speaker toward and away from you by varying the amount of charge it moves up and down its antenna. If it tries to move too little charge up and down the antenna as part of this signaling, it may reach the point where it moves no charge up and down at all.
Why: In amplitude modulation, it is the amount of charge moving up
and down the antenna that indicates how to move the radio's speaker. While
there is no upper limit to the amount of charge that can be moved up and down
the antenna, there is a lower limit: zero.
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?
Answer: The FM station signals your radio to move its speaker toward and away from you by varying the frequency with which it moves charge up and down its antenna. If it shifts that frequency too far above or below its normal operating frequency, it will enter the frequency range allocated to another station and the two stations will compete for the attention of your radio.
Why: In frequency modulation, it is the frequency with which
charge moves up and down the antenna that indicates how to move the radio's
speaker. If that frequency changes too much, it will reach frequency ranges
that are supposed to be used only by other stations.
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?
Answer: The radio wave your device's antenna emits is polarized parallel to its antenna. This radio wave will be most effective at pushing charge up and down the talker's antenna if the talker's antenna is parallel to the radio waves polarization. Overall, the effects will be strongest when the two antennas are parallel to one another.
The electric field in a radio wave emitted from a single straight
antenna points parallel to that antenna. This electric field is most effective
at pushing on charge on an antenna that is parallel to it. In total, the two
antennas should be parallel to one another for maximum effectiveness.
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?
Answer: Turn up and down the strength (or amplitude) of the transmission.
Why: The talker's phone responds to changes in the amplitude of
the radio wave. By varying the amplitude of your transmission, you get the
talker's phone to respond by emitting sound.
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?
Answer: Shift the frequency of the transmission up and down slightly.
Why: The new person's phone responds to changes in the frequency
of the radio wave. By varying that frequency slightly back and forth, you get
the person's phone making noise.
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?
Answer: When your scheme is working well, the electric fields of the various pieces of the original wave are opposing one another and reducing the forces exerted on electric charges in the talker's antenna.
Why: This phenomenon is known as destructive interference: several
wave pieces are coming together in such as way that their electric fields
cancel one another almost completely. The field of one wave piece might point
upward while the field of another wave piece might point downward. The
resulting response in the antenna is very weak and the reception is terrible.