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Propagation of Radio Waves

In order to be able to install a wireless network, and in particular to place the access points so as to get the best range possible, a little background should be given on how radio waves propagate.

Radio waves (shortened to RF for Radio Frequency) propagate in a straight line in several directions at once. In a vacuum, radio waves propagate ar 3.10⁸ m/s.

In any other medium, the signal gets weaker due to

• Reflection
• Refraction
• Diffraction
• Absorption

Absorption of radio waves

When a radio wave reaches an obstacle, some of its energy is absorbed and converted into another kind of energy, while another part is attenuated and continues to propagate, and another part may be reflected.

Attenuation is when a signal's power is reduced as it is being transmitted. Attenuation is measured in bels (symbol: B) and is equal to the logarithm base 10 of the output intensity of the transmission, divided by the input intensity. Decibels (symbol: dB) are generally preferred as a unit of measure; each decibel is one-tenth of a Bel. With a Bel representing 10 decibels, the formula becomes:

R (dB) = (10) * log (P2/P1)

When R is positive, this is called amplification, and when negative it is called attenuation. In the case of wireless transmissions, attenuation is more common.

Attenuation increases with a rise in frequency or in distance. Also, when a signal collides with an obstacle, the level of attenuation depends strongly on which type of material the obstacle is made of. Metal obstacles tend to reflect a signal, while water absorbs it.

Reflection of radio waves

When a radio wave hits an obstacle, some or all of the wave is reflected, with a loss of intensity. Reflection is such that the angle of incidence is equal to the angle of reflection.

By its nature, a radio wave may propagate in several directions. After being reflected several times, a source signal may end up reaching a station or an access point after having taken several different paths (called multipath).

The time difference in propagation (called propagation delay) between two signals which had taken different paths may interfere with reception, since the data streams that are received overlap with one another.

This interference becomes greater as the speed of the transmission increases, since the intervals between receiving the data streams become shorter and shorter. Therefore, multipath limits transmission speed in wireless networks.

To overcome this issue, Wi-Fi cards and access points use two antennas per emitter. With an AGC (Automated Gain Controller), which immediately switches from one antenna to the other depending on signal strength, the access point is able to distinguish two signals coming from the same station. Signals received by these two antennas are said to be decorrelated (independent) if they are spearated by Lambda/2 (6.25 cm at 2.4GHz).

Properties of media

The weakening of signal strength is largely due to the properties of the medium that the wave is passing through. Here is a table showing attenuation levels for different materials: