. 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 it's 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 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.
. How do radio waves transport energy? — AD, Manaus City, Amazonia, Brazil
Radio waves consist of nothing more than electric and magnetic fields that are perpetually recreating one another as they travel through space at the speed of light. An electric field is a phenomenon that exerts forces on electric charges and a magnetic field is a phenomenon that exerts forces on magnetic poles. Both electric and magnetic fields contain energy because they are capable of doing work on and thus transferring energy to electric charges or magnetic poles that they encounter. In a radio wave, this energy or capacity to do work moves along with the fields at the speed of light. The radio transmitter uses electric power to create the radio wave and the radio wave delivers that power to the receiver. While most modern receivers use local electric power to amplify the information arriving in the radio wave, simple "crystal radios" are able to reproduce sound using on the power that is arriving in the radio wave itself.
. If I want to create a radio controlled device, how do I make sure it does not create interference with other devices or receive interference. How does digital RF work and does it stop interference problems? — KG, New York, NY
Radio interference occurs whenever two nearby radio transmitters are simultaneously emitting radio waves that overlap in space and frequency. The receivers for these two waves can't tell them apart and end up receiving both at once. This interference is familiar with AM radio, where you can sometime hear two broadcasts at the same time. With FM radio, the receivers are clever enough to distinguish one radio wave from another, but they can't determine which broadcast they're supposed to follow. Instead, they lock onto whichever wave is strongest and will often flip back and forth from one station to the other as their signal strengths fluctuate.
The only way to avoid interference completely is to choose a radio frequency that no one else nearby is using. That way your transmission is certain to be stronger than any other at the same frequency and your receiver will follow only your broadcast. If you have no choice but to share a particular frequency, then you must use some encoding scheme such as digital transmission so that your receiver can tell when it's receiving a broadcast from your transmitter and not from some other transmitter. Your receiver looks for your personal encoding scheme and won't respond to that of some other transmitter. However, if that other transmitter is strong enough, it will probably prevent your receiver from detecting your transmission. That trick of overwhelming a receiver with a second transmission is the principle behind jamming of a radio transmission.
. How do remote garage door openers work? — JD, Greenville, SC
The communication from the remote to the opener is done with radio waves. When you push the button on the remote, it produces a brief burst of radio waves at a specific frequency and with a selected pattern of pulses. A radio receiver in the opener is continuously looking for a transmission at that same frequency and with that same pattern of pulses. While other garage door openers may use radio waves of the same frequency, it's extremely unlikely that they will make use of the same pattern of pulses. This pattern of pulses is the security code that prevents unauthorized opening of your garage door. These security codes have grown longer and more sophisticated over the years. Early garage door openers had no security code at all and could be opened by almost any radio transmission at the right frequency. You could drive around neighborhoods with a remote and open garage doors right and left. But now the security codes are complicated enough that opening someone else's garage door is almost impossible.
. How does a crystal radio work?
A crystal radio uses a crystal diode to detect tiny fluctuating currents in its antenna system. When a radio wave passes across an antenna, the wave's electric field pushes electric charges up and down the antenna. The crystal diode acts as a one-way gate that allows some of this moving charge to flow onto another wire and then prevents it from returning to the antenna. Since the charge can't return to the antenna, it flows elsewhere—passing through a sensitive earphone and creating sound. An AM radio station encodes sound as changes in the intensity (or amplitude) of the radio wave. As the radio wave's intensity fluctuates, the amount of electric charge flowing through the earpiece of the crystal radio also fluctuates and you hear sound.
. Is there a homing device small enough to fit onto or inside a pc laptop? How does a homing device work?
There are homing devices small enough to fit on bugs, so there should be no problem fitting one on or into a laptop. A homing device is simply a radio transmitter and, while it has recently become possible to build a homing device that actually knows where it is and can tell you via its transmission, the techniques involved in locating most normal homing devices are those of trying to find the source of a radio transmission. Using directional receiving antennas and studying the transmission from several locations, you can figure out where the transmission is coming from.
. Is there a device that would provide a variable output of radiated energy in the infrared that would be obtainable to experiment with? — NAT, Marion, SC
You can produce a broad range of infrared lights with a heat lamp. A heat lamp looks very dim because most of the thermal radiation it emits is in the infrared portion of the electromagnetic spectrum. Just attach the heat lamp to a normal light dimmer and you'll be able to vary its infrared output over a wide range of intensities. Its frequency range will also shift farther away from the visible as you lower its temperature by turning down the dimmer. If it produces more visible light than you want, you can put a filter in front of it that absorbs visible light while permitting infrared light to pass. Such filters are certainly available from filter companies such as Hoya or Corning but cheaper versions (perhaps even plastic filters) may be found through scientific supply companies.
. I'm a poor student and can't afford the deposit for a telephone line. Is there any kind of telephone or radio that I can use to communicate with other people? — AG, Tulsa, OK
Yes, you can use a radio to communicate with your friends, but they will also have to have radios. Amateur radio has been popular almost since the invention of radio and the most accessible version of this hobby, citizen band or CB radio, was extremely popular in the 60's and 70's. You can still buy CB radios and communicate with friends directly through the air, but the general interest in CB radio has waned in recent years. Unfortunately, you can't make your friend's radio ring to alert them to begin listening. You'll have to anticipate your "call." Also, there is no privacy on conventional radio—any nearby person with a similar radio can listen in.
. What does the inside of a radio look like and what is the difference between AM and FM?
These days, radios just look like electronic circuit boards inside. You'd have some trouble telling the difference between a radio and a computer. AM and FM are both techniques whereby the radio station tells your radio which way to move the diaphragm of its speaker and by how much, in order to make sound. In the AM or Amplitude Modulation technique, the station raises or lowers the power of its radio wave to tell your radio to move its speaker diaphragm toward you or away from you, respectively. The higher the power of the radio wave, the more your radio pushes its diaphragm toward you. In the FM or Frequency Modulation technique, the station raises or lowers the frequency of its radio wave slightly to tell your radio to move its speaker diaphragm toward you or away from you, respectively. The more it raises the frequency of its radio wave, the more your radio pushes its diaphragm toward you.