FM Radio waves have the longest wavelengths in the electromagnetic spectrum. In addition to transmitting music to your radio, they also carry signals for your television and cellular phones. These waves are emitted by objects in space such as planets and comets, giant clouds of gas and dust, and stars and galaxies. An interesting thing to note is while radio waves travel at 186,000 miles per second through air, sound waves travel at only 1/5 of a mile per second. ****************************************************** 06-28-02, 08:35 PM mahal "FM" stands for "frequency modulation". There are four common ways of modulating a synthetic radio wave:
AM -- amplitude modulation PM -- phase modulation FSK -- frequency shift key FM -- frequency modulation
AM is the easiest to understand, so I'll do that first. All you need is a continuous wave that never varies, and you can really make it any frequency that you want, say at 8 megahertz.
Then you need two radios tuned exactly the same way. One will add the 8 megahertz wave to whatever you want to send, so that the radio transmits a combination of the two, the other radio receives the signal and removes the 8 megahertz wave so that all you have coming over the speaker are the sounds you wanted to transmit.
Frequency modulation does the same thing, only it's the frequency of your sounds that varies on the carrier wave, and not the amplitude (voltage).
06-30-02, 09:27 PM mahal anguilla is correct. AM and FM really don't have anything to do with wavelength, it just turns out that AM is usually used for lower frequencies in the HF range (3-30 MHz) and FM is usually used for VHF and UHF signals (30-3000 MHz). Lower frequencies have longer wave lengths.
Actually, the longest radio waves ever made are those of around 50 hertz used by submarines. They require antennas more than a mile in length, and there are only a few places in the US where they can be transmitted from.
FM isn't used for cell phones. Newer cell phones use frequencies in the 900 MHz range or 1500 MHz range, which are in the high UHF band transmitted by FSK (preferred for digital transmission). Old cell phones (like the ones that were considered dinosaurs in the 90's) might have used FM.
FM is a man-made carrier wave, it is not naturally occuring as said above: "These waves are emitted by objects in space such as planets and comets, giant clouds of gas and dust, and stars and galaxies."
06-30-02, 09:43 PM gizmogram You know those endless questions kids ask? And hopefully, usually we know the answers? Like why is the sky blue, etc....
My daughter asked me one yesterday that I couldn't answer, and it's coincidentally related to this question! Go figure!
She asked me why, if radio waves move through the air, the AM dial only has 53-160 and the FM dial only has 88-108. She wondered why all the radio stations have to stay within those numbers.
The reason this came up is because where we are, there's so many stations on FM that some are so close they overlap...she figures that if the spectrum were wider, more stations could fit, I guess.
Any help?
07-01-02, 02:17 PM Walks On Water Everybody wants a frequency. Now that cellphone and pagers are popular, there is even more of a scramble for freq.
Do you have any idea just how many freq. are coming down just from satellites. Think how many TV and Radio Station just in your city.
The FCC, Federal Communication Commission, allocates the freq. spectrum. The need for freq. is such the we have gone from 82 TV broadcast channels to something like 69 with the top end going to cellphones.
I am a Ham, Amature Radio, operator. We are allocated certain freq. the we must use for Morse Code, for voice, for amature TV and other forms.
Most of these are set by International Convention.
While impossible to imagine is all of these freq. surrounding us everyday, and one push of a button brings in our favorite station, blanking out the rest. Millions of radio waves all around us.
07-01-02, 08:32 PM mahal There are two major differences between "then" (the time of only AM, FM and television) and "now".
Back then, most signals used frequencies from 3,000,000 to 300,000,000 hertz. There are only so many bandwidths that can be sold in that range before the sidetones of neighboring frequencies begin bleeding into one another. The only cities that really experienced problems were those with dozens of stations. Most of these signals are good for only up to 50 miles or so, and less where there are hills blocking the signals. So the FCC could conceivably sell 57 MHz (+/- 3000 Hz) to 200 different cities in the US without worrying about competing signals.
Today, the spectrum is a lot larger, from 3,000,000 to 50,000,000,000,000 hertz, so there's a lot more room. (A lot of the freqs are reserved for military use--radars, air traffic control, emergency channels, satellite channels, etc.)
There are also a lot of different strategies for broadcasting signals simultaneously:
multiplexing -- I've heard of up to 8 signals being streamed into one continuous transmission to be broken down at the receiving end. A company could theoretically broadcast dozens of signals on only a few frequencies, if it had that many to send.
CDMA -- (code demand multiple access) used by cell phone companies to divide up the use of a single frequency. The theory is that one cell phone user doesn't need his or her own dedicated frequency; hundreds of users can be stacked up on a single network, and networks can be stacked and run simultaneously. The way it works is your cell phone will access a portion of broadcast time at a booster site, say the 14th of a list of 256 codes. All 96 codes can broadcast a few bits of information every 15 milliseconds or so, and the rotation begins again. You won't hear it, but your own voice is being broken up to mesh it in digitally with 255 other voices. The error detection circuits and sensitivity of the receivers are sophisticated enough to make the stream of voices sound uninterupted.
TDMA -- (time demand, multiple access) This is the kind of system I worked with for ship to ship communications. It works the same way as CDMA, except for the division of time blocks instead of code blocks. We could "sell" time to over a thousand users on a single network, and we could stack up to 20 simultaneous networks before noticing any degrading signals. (We had the advantage several different kinds of encryption, one being the use of many different frequencies for the same signal).
07-01-02, 08:46 PM mahal Gizmogram: It's been awhile since I looked at an analog radio (one with a tuning dial). Doesn't it say "53-160 KHz" on the AM dial and "88-108 MHz" on the FM?
These were the only sections of the radio spectrum sold by the FCC. Television stations used VHF and UHF signals (in the FM range and higher).
I bet you didn't know that the FCC tried to own the air?
07-01-02, 10:04 PM gizmogram You know, I'm not sure...when I posted the question, I just went to the only radio in the house that showed the numbers on the AM/FM dials...I haven't paid attention to the numbers in years, of course until my daugher posed this question!
07-04-02, 12:39 AM mahal gizmogram:
Hey, it took me awhile, but I found one. I had to go to work and snoop around for a radio that some of the really old employees listen too. I was right, the AM is in the KHz and FM is MHz.
You can tell your daughter that the frequency ranges chosen were purely arbitrary. (I've seen AM used in transmissions as high as 1 gigahertz).
I think one of the reasons that lower frequencies were used for AM was that they bend easier. High frequencies travel in a perfectly straight line, meaning that with the curvature of the earth, after 20 or 30 miles, they just go out into space. Frequencies as low as the KHz range can travel hundreds of miles if there's enough wattage behind them because the signals are absorbed into the ground and this causes them to bend with the earth's curve.
07-04-02, 12:46 AM gizmogram Thanks for the expanations...
I'm not sure that she'll understand them, but it will give her an excuse to go out and find info to help her understand!
This message has been edited. Last edited by: DorianGreyed,
Posts: 9192 | Location: Atlanta, GA, USA | Registered: 06-03-02
