Why do we run out of radio frequencies?

General Questions
#1

I’ve read articles which bring up concerns about how crowded the radio frequences are and how we have to keep using new regions. But I don’t understand why the frequences get filled up. It seems like there should be infinite frequencies in any range.

For example, my FM radio has stations like 99.5 and 99.7 MHz. That’s 99,500,000 and 99,700,000 Hz, right? Why couldn’t they use any number of frequencies between that range like:

99,500,001
99,500,002
99,500,002.1
99,500,002.2

etc.

I mean, it’s not like there are only 200,000 frequencies between 99.5 and 99.7 MHz. There are infinite frequencies. So how do we run out of them?

#2

That would be true if each radio station emitted a pure sine wave. But that wouldn’t be very interesting, would it? To carry information you need to modulate that sine wave. A modulated signal is no longer a pure sine wave, and you need a finite bandwidth to transmit that information. FM (Frequency Modulation) works by shifting frequency up and down slightly to indicate sound (movement of the microphone/speaker), like up/down movement of the needle on a record turntable. Therefore the you need a certain range of frequencies in which to shift. AM (Amplitude Modulation) works by changing the amplitude of the radio wave, but if you take a Fourier transform of the amplitude modulated signal you’ll see that it takes up a certain width around the carrier frequency.

#3

But could they tighten up the frequency modulation so that it didn’t take quite such a large range? Like use the range 99,500,000.000000 to 99,500,000.999999. Even if they had discreet frequencies at .000001, that’s still a million frequencies. Is there a natural reason the frequency range used is so large?

#4

The capacity to transmit information is diectly related to bandwidth. An AM voice channel transmits less information and needs less bandwidth than a TV video channel.

#5

One way to increase the capacity is to digitize the transmissions. Each descreet frequency is tnen called a multiplex and , depending on the sample rate , you can get many more radio and TV stations on that frequency. This is already happenig here in the UK . The only drawback at the moment is the high cost of digital radios. The cheapest is £99 ( $150 ) which is ten times what you would pay for a simple am/fm radio. Prices shiuld come down though in time. On a digital TV frequency it is possible to get at least six seperate stations.

#6

Could you drive 60 mph on the freeway with cars packed into all four lanes just as tightly as they park in bumper-to-bumper traffic? If everybody’s going the same speed and controlling their vehicles, you should be able to pack a whole lot more cars on the freeway.

Lacking that control, you’d just be asking for an accident. In the case of radios, it means overlapping signals.

#7

Yes, It is noise. The bandwidth taken up by FM modulation can be anything you choose. You basically decide the maximum modulating frequency deviation. However if you choose to deviate by a very small amount your radio will be very susceptible to noise and will sound like crap.

Ethilrist I am not sure I really understand you analogy what does controlling vehicles have to do with anything?

#8

Ethilrist, it’s not a matter of control unless you mean not carrying an audio component. No matter how precisely you control the carrier singal it will take bandwidth to carry information. FM stereo has two channels of 15KHz bandwith and with common analog technolgy there is no way around the bandwidth requirement.

Analog signals have some non-intuitive behaviors. As said above a pure sine wave is one frequency. A square wave is one frequency if you’re on a digital TTL circuit but that doesn’t apply to analog. A Square wave is made of the orginal frequency plus harmonics that when added up turn the curvy sine wave into a square wave. IIRC it’s third order harmonics but I might be wrong. Start with a signal of 100Hz then superimpose 300Hz. Add them and the result is a little more square. Keep adding harmonics in that series and the wave gets more square. This is how old analog synthesyzers like the original Moog worked to make saw and square waves from sine wave harmonics. A perfect square wave would have an infininte bandwidth. I’m a bit rusty on this so someone please correct me if I’m way out of line.

#9

The reason it’s like that in America is because the American standard for broadcasting is 200 KHz of bandwidth. The standard modulation for music is 15 KHz and the standard deviation is 75 KHz. Added together, that’s 90 KHz of deviation on either side of the center frequency with 10 KHz buffer to avoid bleeding over into someone else’s frequency.

In English, this means that in order to keep frequencies from bumping into each other, an FM broadcast can only take up to 200 KHz of bandwidth. Otherwise, you’d have one station bleeding into another, and the interference would be unacceptable.

At least that what my husband tells me, and he does this for a living.

Robin, with help from Airman Doors, USAF

#10

I think Ehilrist was making the same point you were: noise. The lack of total control over a vehicle on the highway is roughly analogous to noise, if you think about it.

#11

A flash of inspiration, a highway analogy that works. It has some holes but try this for size.

Our highway is uncontrolled access so it can carry anything that rolls. Everything from ball bearings (it’s a smooth road, work with me) to tractor-trailer rigs. A ball bearing is the most pure rolling thing, a sine wave if you will. Elegant but you can’t carry shit. A unicycle can carry a human rider but we’re talking low quality voice communincation not musical quality. A pickup truck is the narrow bandwidth of AM radio. You can can load it with music but there is a distinct limit to the quality. A tractor-trailer can carry and orchestra and doesn’t limit its sound quality.

#12

We could certainly broadcast the same recorded music with a different bandwidth than 15KHz for FM. People would need to change their equipment but there is nothing particularly sacrosanct about 15KHz.

Expanding on the notion of square waves taking up infinite bandwidth. FM modulation also takes up infinite bandwidth we just filter out all but 15KHz or whatever the standard says around the carrier. This leads into another reason that we cannot get infinite number of channels. It is very expensive to make a filter that passes 98.999999 to 98.000001 MHz and at some narrow frequency range becomes impossible with current technology.

#13

Specific frequency ranges are allotted for specific uses. FM is line-of-sight, so you can have some duplication of frequencies without excessive interference, but you’re still limited to the same frequency ranges.

One of the problems with frequency allotment is that commercially available equipment can never be as precise as the OP thinks. Electronic equipment, like everything else, ages over time, and as it ages, falls out of “tolerance”, that is, it won’t work as well as it did when it was new. For most commercial (and non-commercial) radio stations to maintain their equipment, and for the average Joe to keep his car radio in that kind of condition is impractical.

Robin

#14

Not a particular magic number but a tradeoff between sound quality and bandwidth. Reduce the bandwith and you start losing high frequencies. A person with good hearing, listening on a good sound system can tell a distinct difference between an FM stereo broadcast and original source material. AM radio has less bandwith and nearly everyone can tell the difference between it an FM. I remember when we were being screened for hearing loss in the navy some guys claimed they could not tell the difference bewteen music on AM and FM radio. In almost all cases it pegged them as having severe high freqency hearing loss.

Back in the 60s pop music was often recoded and engineered to sound good with the limitations of AM radio. That same music may not sound much different on FM.

#15

A small nitpick if I may?

FM is a modulation method, and the FM Broadcast band in ITU region 2 is at a frequency that is “line of sight”, in most cases. But the fact that the modulation method is FM does not make it line of sight, it’s the frequency that does it. Line of sight propagation is determined by frequency, not modulation method.

Non-HDTV broadcast television signals consist of components that are both AM modulated and FM modulated. But they are generally line of sight because of the frequency used.

For example, a signal in the 20 meter band would propagate pretty much the same regardless of the modulation method used.

#16

MsRobyn, I think modern eqluipment could handle a much smaller buffer between channels but that’s not the issue. Would gaining a few more FM stations in a few large, but localized, markets be worth people having to junk nearly every FM reciever they already own? The FCC typically doesn’t work that way. Old systems are phased out while new ones take over in a new part of the spectrum.

#17

You are correct and I was wrong.

I was thinking that as you took the modulation index down very low the bandwidth of the significant side bands would approach 0. This is incorrect. The bandwidth of the signal as modulation index gets small is twice the frequency of what you are trying to transmit.

#18

I was wondering about this too. Do you have a cite? (just for my own curiosity)

#19

Yeah, I know. I thought about that this afternoon when I was listening to my nice, digitally-tuned car radio.

Robin

#20

Some nice plots of spectrum.
http://www.tpub.com/neets/book12/49c.htm
More stuff.
http://robotics.eecs.berkeley.edu/~sastry/ee20/modulation/node3.html