. 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.
. 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.
. 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.
. How good are store bought antennas and if they are better than factory issue, which ones are most advantageous?
Ultimately the only things that matter about an antenna are (1) how much charge it moves in response to the correct radio transmission and (2) how little charge it moves in response to the wrong radio transmissions. Most store bought antennas probably just boost the amount of moving charge by attaching an amplifier to an otherwise undistinguished antenna. While that trick will increase the amount of charge moving in response to the correct transmission, it will also increase the amount moving due to undesired transmissions. Almost everything electrical transmits radio waves and these may well interfere with your reception. For example, your neighbor's lawn mower may send out radio waves and introduce noise into your music. Just amplifying the antenna signal does nothing to eliminate that problem. Your best bet is to find a directional antenna; an antenna that responds most strongly to radio waves coming from a particular direction. TV antennas are typically directional, with many separate antenna elements. Satellite dishes are highly directional.
. 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 timekeeper. With the help of its timekeeper, 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 wristwatch. 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 is the charge moving in the waves related to what is actually played on the radio?
First, there isn't any charge moving in the waves themselves. The waves contain only electric and magnetic fields. These fields will push on any electric charges or magnetic poles they encounter, but they are not themselves electrically charges or magnetically poled. The amount of fields in a radio used for audio transmission depend on the station's transmitting power and on the encoding format for the music. In AM (Amplitude Modulation) encoding, the music is encoded as the strength of the radio waves. Each time the radio wave's strength goes up and down once, the speaker cone in your receiver goes forward and backward once. In FM (Frequency Modulation) encoding, the radio wave's strength remains steady but its precise frequency changes slightly. Each time the radio wave's frequency goes up and down once, the speaker cone in your receiver goes forward and backward once.
. If electric and magnetic field are forever recreating one another - in radio waves - how do you change the sounds they produce?
Within each portion of the wave, the local electric and magnetic fields endlessly recreate one another. But this portion of the wave heads outward from the transmitting antenna at the speed of light and is soon far away from the earth. As the transmitter changes the amount of charge on the antenna or its frequency of motion up and down, it creates new portions of the wave that may differ from the portions sent out a minute ago, a second ago, or even a few millionths of a second ago. Thus the transmitter's changes very quickly pass outward to all of the receivers nearby. The farther you are from the transmitter, the longer it takes for the various patterns in the wave to reach you and your receiver. All of the music transmitted by radio stations in the 50's is still traveling outward because the patterns emitted back then continue to travel. They are now 40 or 50 light years away from the earth and are so widely dispersed across space that it would take a phenomenally sensitive receiver to detect them. But they are out there nonetheless. Many of the searches for extraterrestrial intelligence have focused on trying to detect this sort of radio transmission across the depths of space. If other peoples have invented radio, they are quite likely to have chosen AM or FM modulation as their encoding schemes, too.
. 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.
. What is the difference between an electric and a magnetic field?
An electric field exerts forces on electric charges while a magnetic field exerts forces on magnetic poles. If you place a positive electric charge in an upward-pointing electric field, that electric charge will accelerate upward (in the direction of the electric field). But if you place a stationary north magnetic pole (if you could find one) in that same electric field, nothing will happen. An electric field exerts no force on a stationary magnetic pole. On the other hand, if you place a north magnetic pole in an upward-pointing magnetic field, that pole will accelerate upward (in the direction of the magnetic field). But if you place a stationary positive electric charge in that same magnetic field, nothing will happen. So electric fields act on stationary electric charges and magnetic fields act on stationary magnetic poles.
. 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.