Parabolic WLAN antennas

A parabolic antenna uses a curved reflecting surface to focus radio energy, and at 2.4 GHz it gives the highest gain of the antennas commonly used for wireless LAN. The reflector has the cross-section of a parabola, a shape with the geometric property that rays arriving parallel to its axis all converge on a single focal point. A small feed antenna placed at that focus therefore collects energy from a wide aperture on reception, and on transmission the reflector turns the feed's radiation into a narrow, nearly parallel beam.

For WiFi use the reflector is often not a solid dish but an open grid of metal rods or a mesh. Because the wavelength is around twelve centimetres, gaps much smaller than that still behave like a continuous surface to the radio wave, so a grid reflects almost as well as a solid one while being lighter and far less affected by wind. The feed at the focus may be a dipole with a small reflector, a biquad, or a short waveguide, matched to the coaxial cable from the radio.

The reward for this design is long range: a parabolic antenna can concentrate energy enough to link buildings kilometres apart given a clear line of sight. The cost is a very narrow beam, so the antenna must be aimed precisely and mounted rigidly, since even slight movement can swing the beam off the distant station. Both ends of such a link must also use matching polarization to receive the full signal.

Frequently asked questions

Why does a parabolic shape focus radio waves?

A parabola reflects all rays arriving parallel to its axis to one focal point, so a feed placed there gathers energy from the whole aperture and forms a narrow beam.

Why do many WiFi dishes use a grid instead of a solid surface?

At 2.4 GHz the wavelength is large compared with the grid gaps, so a mesh reflects the wave almost as well as a solid dish while weighing less and resisting wind.

What is the drawback of a high-gain parabolic antenna?

Its beam is very narrow, so it must be aimed accurately and mounted firmly, because small movements can point the beam away from the target.