Wireles Networking is a practical guide to planning and building low-cost telecommunications infrastructure. See the editorial for more information....


Just like visible light, radio waves are reflected when they come in contact with materials that are suited for that: for radio waves, the main sources of reflection are metal and water surfaces. The rules for reflection are quite simple: the angle at which a wave hits a surface is the same angle at which it gets deflected. Note that in the eyes of a radio wave, a dense grid of bars acts just the same as a solid surface, as long as the distance between bars is small compared to the wavelength. At 2.4GHz, a one cm metal grid will act much the same as a metal plate.
Figure 2.5: Reflection of radio waves. The angle of incidence is always equal to the angle of reflection. A parabolic uses this effect to concentrate radio waves spread out over its surface in a common direction.

Although the rules of reflection are quite simple, things can become very complicated when you imagine an office interior with many many small metal objects of various complicated shapes. The same goes for urban situations: look around you in city environment and try to spot all of the metal objects. This explains why multipath effects (i.e. signal reaching their target along different paths, and therefore at different times) play such an important role in wireless networking. Water surfaces, with waves and ripples changing all the time, effectively make for a very complicated reflection object which is more or less impossible to calculate and predict precisely.

We should also add that polarization has an impact: waves of different polarization in general will be reflected differently.

We use reflection to our advantage in antenna building: e.g. we put huge parabolas behind our radio transmitter/receiver to collect and bundle the radio signal into a fine point.

Last Update: 2007-01-24