Monday, February 1, 2010

Behaviour of radio waves

There are a few simple rules of thumb that can prove extremely useful when making first plans for a wireless network:

The longer the wavelength, the further it goes
The longer the wavelength, the better it travels through and around things
The shorter the wavelength, the more data it can transport
All of these rules, simplified as they may be, are rather easy to understand by example.

Longer waves travel further
Assuming equal power levels, waves with longer wavelengths tend to travel further than waves with shorter wavelengths. This effect is often seen in FM radio, when comparing the range of an FM transmitter at 88MHz to the range at 108MHz. Lower frequency transmitters tend to reach much greater distances than high frequency transmitters at the same power.

Longer waves pass around obstacles
A wave on water which is 5 meters long will not be stopped by a 5 mm piece of wood sticking out of the water. If instead the piece of wood were 50 meters big (e.g. a ship), it would be well in the way of the wave. The distance a wave can travel depends on the relationship between the wavelength of the wave and the size of obstacles in its path of propagation.

It is harder to visualize waves moving “through” solid objects, but this is the case with electromagnetic waves. Longer wavelength (and therefore lower frequency) waves tend to penetrate objects better than shorter wavelength (and therefore higher frequency) waves. For example, FM radio (88 to 108MHz) can travel through buildings and other obstacles easily, while shorter waves (such as GSM phones operating at 900MHz or 1800MHz) have a harder time penetrating buildings. This effect is partly due to the difference in power levels used for FM radio and GSM, but is also partly due to the shorter wavelength of GSM signals.

Shorter waves can carry more data
The faster the wave swings or beats, the more information it can carry every beat or cycle could for example be used to transport a digital bit, a '0' or a '1', a 'yes' or a 'no'.

There is another principle that can be applied to all kinds of waves, and which is extremely useful for understanding radio wave propagation. This principle is known as the Huygens Principle, named after Christiaan Huy-gens, Dutch mathematician, physicist and astronomer 1629 - 1695.

Imagine you are taking a little stick and dipping it vertically into a still lake's surface, causing the water to swing and dance. Waves will leave the center of the stick -the place where you dip in -in circles. Now, wherever water particles are swinging and dancing, they will cause their neighbour particles to do the same: from every point of disturbance, a new circular wave will start. This is, in simple form, the Huygens principle. In the words of

“The Huygens' principle is a method of analysis applied to problems of wave propagation in the far field limit. It recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed. This view of wave propagation helps better understand a variety of wave phenomena, such as diffraction.”

This principle holds true for radio waves as well as waves on water, for sound as well as light -only for light the wavelength is far too short for human beings to actually see the effects directly.

This principle will help us to understand diffraction as well as Fresnel zones, the need for line of sight as well as the fact that sometimes we seem to be able to go around corners, with no line of sight.

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