Radio Frequency range?

Question

Why do AM and FM radios only transmit on a set

broadcast spectrum? I read that some frequencies

are not used because they are either harmful or

they do not have the ability to be transmitted a

great distance, however, in some instances and for

some uses a wave that travels a short distance may

be more preferred because of the reason it is being

used and because it will not be likelly to interfer with

or jam other radio products.

Answer 1

the radio spectrum is divided by the International Telecommunications Union. The FCC, Industry Canada, and CE are members of the ITU.

The spectrum is divided into bands, each group of users or service is assigned a band and there is a guard channel between each service so one will not interfer with the other.

AM and FM are only two methods of conveying inteligence.

Depending on the band, some signals are worldwide while some only go feet.

Some bands can be transmitted on cable while some bands are transfered in a metal pipe. Microwaves travel in pipes while lower freqs travel in coaxial cable.

Microwaves are very line of sight, point to point, and do not propagate far, while HF radio bends around the earths curve and will be heard around the world.

All radio frequencies heat, microwaves heat best, and this is why the microwave oven exists. The oven itself is a reasonant cavity and contains the microwave radiation.

Answer 2

The spectrum used is set by the Federal Communications Commissionn. This is so that different products don't interfere with each other (ie cell phones, pagers, wireless routers)

The frequencies and power levels selected are used based on their characteristics while traveling through the air at certain power levels.

Typically, lower frequencies carry further than higher frequencies, so they don't need as much power.

Because lower frequencies carry so well, there ends up being a lot of noise and interference below the 86Mhz used by FM radios, especially below 30Mhz

And frequencies themselves are not harmful until you get into microwave type spectrums(generate heat at a molecular level) or if you used crazy power levels.

Answer 3

In the USA, the Federal Communications Commission (FCC) is responsible for allocating different radio bandwidth for different uses. There are all sorts of different frequencies, and tradeoffs between them.

In general, lower frequencies travel better than higher frequencies. For example, AM radio is pretty easy to pick up.. most receivers use a small internal antenna, and it can even travel "on the skip"... bouncing through the atmosphere to travel hundreds or thousands of miles. Short wave radio does this even better, with the intent of traveling around the world.

But there's only so much room there... the commerical AM band covers the range of 0.540MHz to 1.600MHz.. that's barely over 1MHz for the whole band, 10kHz for each channel. For FM radio, channels are spaced 100kHz apart (and actually only assigned 200kHz apart in the USA...), considerably more room, though FM (in the VHF TV band between channels 6 and 7) doesn't travel quite as well. Compare that to broadcast television, in which each channel occupies 6MHz of space... they had to use more and higher frequencies just to clear out that much room.

Sometimes you want to limit range, or trade off range for number-of-channels. Most US cellphones can run at either 850MHz or 1900MHz... the latter offers more channels but shorter range; the former fewer channels but better range, particularly in rural areas, as 850MHz penetrates foliage much better than 1900MHz.

Some radio bands are set aside for random short-range use. For example, in the USA, 915MHz and 2400MHz are set aside as "ISM", or "Industrial, Scientific, and Medial" bands.. but really available for all different kinds of unlicensed use, as long as power is kept low. That's why microwave ovens operate at these frequencies (industrial at 900MHz, home units at 2400MHz), and also why wireless networks run in the 2400-2500MHz band.

You may find really short range is an advantage. For example, there's Bluetooth, which runs on 1MHz channels in the 2400MHz band. The usual Bluetooth headphone is only good for about 10-30ft, but you really don't even need that much range. And keeping it that short allows lots of people to use their wireless headphones in the same area without significant interference.

There's room there.. but not unilimited. 802.11b or g channels each use 22MHz, so while you have 11 channels available to your Wifi hub, there are actually only three non-overlapping channels available. 802.11a and n have the option of running in the 5800Hz band, where there's even more space available.. and they need it, these can run 40MHz wide channels.

And that's not the end.. there are several efforts on the way to create a very short range radio link (10feet or so) to hook together systems like media centers (TV/monitor, DVD/Blu-Ray, etc) in a super high speed (5Gb/s or better) network that will allow high-def channels to run around, uncompressed. This may wind up at 60,000 MHz or so.

So basically, long range things are well regulated to avoid interference.. these channels are licensed by the FCC, and they dictate which kinds of radio can go where in the spectrum, based on the various needs of the different users of such radio. A few bands exist with no licensing, but regulations that only allow fairly short range radio.