Biquad WiFi antenna

The biquad is a directional antenna popular with hobbyists for extending 2.4 GHz wireless LAN links. Its radiating element is a single length of stiff wire bent into two adjoining squares, forming a figure that resembles a bow tie or a pair of diamonds sharing a central point. This double-quad loop is mounted a short distance in front of a flat metal reflector, usually a square plate, which directs the radiation forward and increases gain compared with the loop alone.

The appeal of the biquad is that it can be built cheaply from common materials while delivering moderate directional gain, typically enough to noticeably improve range on an 802.11b or 802.11g link. The dimensions follow from the operating wavelength: each side of the squares is about a quarter wavelength, and the spacing between the loop and the reflector is chosen to set the impedance and shape the forward beam. A short piece of coaxial cable feeds the centre of the element, with the shield and centre conductor connected to the two halves of the loop.

Because it is directional, a biquad must be aimed along the wanted path, and both ends of a link should share the same polarization. Construction accuracy matters: at these frequencies small errors in loop size, reflector spacing or the feed connection shift the resonance and reduce performance. The biquad is often used as the driven element behind a parabolic or grid reflector to obtain still higher gain for longer point-to-point links.

Frequently asked questions

Why is it called a biquad?

The radiating element is formed from two adjoining square (quad) loops of wire, so the design is a double quad, shortened to biquad.

What is the reflector for?

The flat metal plate behind the loop reflects radiation forward, turning the antenna into a directional one and raising its gain along the aimed direction.

Why does build accuracy matter so much?

At 2.4 GHz the wavelength is short, so small errors in the loop dimensions or the spacing to the reflector shift the antenna off resonance and cut its performance.