February 24, 1995
One Minute Papers - Questions and Answers
How is charge distributed to a tank circuit with the "correct" frequency?
The transmitting station has an electrical oscillator, an electronic system that experiences periodic reversals of current. This oscillator contains a tank circuit or some other clock-like system that acts as a time-keeper. With the help of its time-keeper, the oscillator causes the transmitting station to send current to the main antenna tank circuit at just the right moments to sustain and enhance the sloshing current there. The oscillator and the current sloshing in the tank circuit remain in perfect synchrony with one another. One of the best clock-like systems is a quartz crystal oscillator, like that in a typical wrist watch. In a quartz oscillator, a quartz crystal vibrates like the bar of a xylophone. In a watch, these vibrations are used to control a digital clock system so that it keeps accurate time. In a transmitter, these vibrations are used to control the distribution of current to the tank circuit at the antenna.
How does a dipole antenna understand what "top-bottom" is when it is arranged like the letter "T"?
A dipole antenna is essentially two wires that start together (the base of the "T") and then split to head directly apart (the two top wings of the "T"). For complicated reasons, those two wings are often connected by another wire that runs from the tip of one wing to the tip of the other. It the "T" is turned on its side, then one wing is clearly the top and the other is clearly the bottom. The antenna should work best in that arrangement. Sometimes the dipole is kept upright so that the two wings are horizontal. I don't know why this arrangement should work, given that normal radio transmissions are made from vertical antennas. I think that there are enough reflections that even a horizontal antenna such as the upright "T" can still receive something.
How does turning the dial on your radio allow your radio to distinguish between stations? How does the receiver only recognize one frequency at a time?
When you turn the dial on your radio, you are adjusting the resonant frequency of its tank circuit (or some electronic equivalent). The tank circuit only responds to charge sloshing on the antenna when that charge is moving back and forth at the tank circuit's resonant frequency. When you tune the tank so that its resonant frequency is the same as the broadcast frequency of your favorite radio station, it only responds to charge moving up and down at that frequency. As a result, your radio detects signals from your favorite station but no others.
When a station transmits a signal do all receiving antennae have the same reciprocal charge?
Yes. The transmitting antenna pushes huge amounts of charge up and down so that all of the receiving antennae respond primarily to it rather than to one another. However when many receiving antennae are very near one another, they can begin to cause trouble. In effect, each antenna draws a small amount of energy out of the radio wave. If there are too many nearby antennas, they will sap the radio wave's energy and each receiving antenna will get less than its normal amount. The other way to look at this effect is to realize that the receiving antennas actually retransmit the radio wave that they receive, but upside down. They weaken the wave as a result. If there are too many antennas around, they will reduce the wave to almost nothing.
What is the polarization of radio waves?
This topic will be covered on Monday.
How does the distance between the transmitting antenna and the receiving antenna affect the amount of current flowing between the two systems?
Actually, there is no current flowing between the two systems. Current flowing up and down the transmitting antenna causes current to flow up and down the receiving antenna, but there is no direct connection between the two and they do not share any current. That explains how an isolated radio can still receive music. But the amount of current flowing in the receiving antenna does depend on its distance from the transmitting antenna. When the two are very close, the charge in the receiving antenna responds directly to the charge moving on the transmitting antenna. As they move apart, this direct response quickly dwindles to virtually nothing. In its place, a new effect appears. The transmitting antenna creates radio waves that exist apart from the accelerating charges that created them. The strength of the radio wave diminishes in power roughly as the square of the distance from the transmitting antenna. The electric and magnetic fields diminish in power roughly in proportion to this distance. The current flowing in the receiving antenna also falls roughly in proportion to this distance.
Occasionally my receiver will pick up two stations at the same time, fading in and out and fighting to be heard. How is this possible?
In AM radio, the sound is encoded as the strength of the radio wave. If two transmitters are using the same frequency (or your receiver cannot distinguish between them due to its limited resolution), then it will responds to both of them at once. The sound that you hear will be the sum of them both, as though they were two musical instruments in the same room. In FM radio, the sound is encoded as the exact frequency of the radio wave. In this case, your receiver is likely to follow the strongest of the two stations and flip in between occasionally when their strengths change (due to weather or reflections from moving objects). Thus it is common for AM radio receivers to superpose two stations but not so common for FM radio receivers to do the same trick.
How do different antennas distinguish between incoming/available charges?
The charges aren't incoming but the electric fields they create do travel through space. These electric fields push and pull on the charges in your receiving antenna and may cause your radio to create sound. To distinguish between stations, your receiver has the ability to tune its response so that it only recognizes electric fields that vary at the frequency of your chosen station.
Where does the charge on the antenna come from?
In the transmitting station, the moving charge is pumped back and forth between the ground and the antenna. The net charge in the vicinity of the station remains zero, but it is constantly being redistributed. Sometimes the antenna is positively charged and the ground is negatively charged and sometimes its the reverse. In the receiving station, the same may be true. But there are also hand-held receivers that do not touch the ground. In that case, the receiver is still neutral, but charge is being pushed back and forth along the antenna and tank so that when the antenna is positively charged, the bottom of the tank circuit itself is negatively charged.
Why does the light bulb turn on when the receiving antenna is aligned with the transmitting antenna but not when the receiving antenna is perpendicular to the transmitting antenna.
When the two antennae are aligned, any movement of charge along the transmitting antenna causes an movement of the opposite charge along the receiving antenna. These charges are always present in a metal rod, but they only move when they experience forces. The charge moving in the transmitting antenna exerts those forces on the charges in the receiving antenna. As the charges in the receiving antenna flow back and forth, they deposit some of their energy in the light bulb's filament and it glows white hot. But when the receiving antenna is horizontal next to the vertical transmitting antenna, the bulb remains dark. The forces on charge in the receiving antenna are all vertical but those charges cannot move vertically in a horizontal rod. The charges in the horizontal antenna cannot move up and down so they remain in place. The bulb has no current through it so it remains dark.
How can an antenna be short and still work as well as a long one?
The length of an antenna is very important. If the antenna is too short, the charges will reach its end too soon and the charge will not flow very smoothly back and forth in it. If the antenna is too long, the charges will not reach its end before it is time for them to reverse directions and some of the antenna will not be used (it will actually cause more trouble than help). Thus there is an ideal length for the antenna and this length depends on the frequency of the radio wave it is trying to create. But it is also possible to shorten an antenna by delaying the flow of charge to its ends. Adding a coil to the antenna (an inductor) will slow the flow of current through the antenna and make a short antenna behave like a longer antenna. Most portable AM radios use a coiled antenna that behaves as though it were much longer than its physical length. FM radios work best with antennas that are about 1 meter long.
Why do radio waves travel better at night?
AM radio waves travel remarkably long distances near dusk because of the behavior of the earth's atmosphere. A layer in the upper atmosphere, the ionosphere, contains many electrically charged particles and it behaves like a poor electrical conductor. Its conductivity improves in the early evening. When low frequency radio waves encounter this conducting layer, it responds to them and reflects them just like a mirror reflects light. As a result, you can hear very distant radio stations as their waves bounce of the ionosphere. FM transmissions occur at high frequencies that are too fast for the ionosphere to reflect.