Tag: radio

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 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 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.

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 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.