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Author Topic: Why can microwaves carry more information than radio waves?  (Read 6400 times)

Offline chris

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Microwaves are preferentially used over radio waves for mobile phone, audiovisual and satellite communications on the grounds that they can carry more information than a radio wave. But why can they carry more information?

« Last Edit: 08/12/2010 20:01:51 by chris »


 

Offline JP

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I strongly suspect it's simply because the wavelength is shorter.  If you're cramming the same amount of data into a single wave cycle, then you get more wave cycles per second passing you in the microwave band than in the radio wave band, hence more information.

You could go even shorter wavelengths to cram in more data, but as you do that you have less and less power to penetrate through or bend around obstacles.
 

Offline graham.d

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Microwaves are radio waves but the definition is to do with the wavelength range which is much shorter than what you get over your typical FM radio. As JP says you can get more information because of the shorter wavelength and higher frequency. If you Google "Nyquist limit" you will find websites that explain the maths behind this.
 

Offline Geezer

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Yes indeed. And, at the other end of the spectrum, literally and figuratively, there is ULF (Ultra Low Frequency), sometimes used by submarines. Unfortunately, the wavelengths are so great that the communication rates are very limited. I seem to remember that they get around this by using "canned" messages so they only need to send a very small number of characters.

For example, 37 might represent something like "We're running low on bully-beef." The possible bit rates would pretty much rule out any sort of Internet traffic.
 

Offline Bored chemist

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"You could go even shorter wavelengths to cram in more data"
as with optical links.


"The possible bit rates would pretty much rule out any sort of Internet traffic."
but my ISP would still label it as "up to 8 MB/S".

 

Offline techmind

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Various vague claims are commonly, even carelessly, made that you can get more information though in the microwaves, or "there's more space" or "more bandwidth" in the microwaves.

While it's kind-of true, it's really a mathematical trick somewhat predicated on the basis that we tend to think of radio frequencies on a logarithmic scale, while the "information" or data-rate we wish to transmit is largely fixed.

Take the example of a basic analog AM speech broadcast. Broadcast standards allocate 9kHz of radio bandwidth (and assume a signal-to-noise of perhaps 60dB). In the medium wave broadcast band (540-1600kHz) you can fit in (1600-540)/9 = 118 stations. (strictly you can't put them adjacent, but ignore that for now). Here the top of the band is roughly three times the frequency of the bottom of the band!

Now say we put the same voice transmissions in the television band, 460-860MHz (approx). The total band spans a factor of two in frequency, but (in kHz) that's now 860000-460000 = 400000kHz, /9 = space for 44444 AM-quality radio stations!
Similarly, between 10GHz and 10.4GHz (that's 400MHz of spectrum) you could have another whole TV band, or 44 thousand AM radio stations!

There's a lot of space "up there"!


Let's try a height/elevation analogy.

Imagine a telephone-quality voice call is a strip of paper (0.1mm thick)*
On that scale, a high quailty music radio station is a strip of cardboard 1mm thick.
A TV transmission is a plank of wood 1-3cm thick
A modern ADSL internet connection is a plank about 3-4cm thick.
A high-definition TV transmission is a piece of wood about 10cm thick.


In the first centimetre 'height' of airspace above your desk, you could have 100 voice calls, 10 music stations, or one lousy TV transmission (or some appropriate mix of those).
From 1 centimetre to 10cm, you could have nine times that,
From 10cm to 1metre, nine times again
From 1metre to 10metres...

In this analogy, saying "there's lots of space in the microwaves" is only as ingenious as saying "there's loads of space above 25 metres high".

(*equivalent data rates 15kbps, 150kbps, 1500-4500kbps, 4500-6000kbps, 15000kbps)
« Last Edit: 08/12/2010 22:51:55 by techmind »
 

Offline techmind

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A further consideration is practical aerials and transmission properties.

1GHz signals have a wavelenth of 30cm, making a half-wave dipole (efficient aerial) about 15cm long. It's practical to make reasonably effective antennas (a bit smaller than that) in sizes of things that fit in your hand. Similarly for 2GHz (half-wave is 7.5cm).

If you go to higher frequencies it's absorbed too much by walls and buildings, and becomes increasingly "line of sight" around hills/bumps in the land too.
Also at higher frequencies the aerials get so small that they have small cross-section and aren't very sensitive and/or become directional which is a problem for mobile use.

At lower frequencies, as well as there not being enough spectrum/space aerials get too big, or if forced to be small, hideously inefficient.


The limited transmission range of the 0.9-2.5GHz frequencies can be useful in a telecoms network for maintaining small manageable "cells", where you can re-use the frequencies again just a few miles away - especially good for built-up areas where you want lots of small cells for capacity.
Argueably the TV bands at 400~500MHz would be good for providing rural infill in areas with low population density as they travel further and will diffract around obstuctions to a greater extent.
 

Offline chris

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Why can microwaves carry more information than radio waves?
« Reply #7 on: 08/12/2010 20:08:51 »
Brilliant answer; thanks Techmind.

Chris
 

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Why can microwaves carry more information than radio waves?
« Reply #7 on: 08/12/2010 20:08:51 »

 

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