Like a Yagi antenna, a quad is directive. That is, it focuses your RF power in a particular direction. In terms of how they are put together, quads are different animals. They consist of two or more loops of wire, each supported by a bamboo or Fiberglass cross-arm assembly. The loops are a quarter wavelength per side (one full wavelength overall). One loop is driven and the other serves as a parasitic element—usually a reflector. A variation on the quad is called the delta loop. The electrical properties of both antennas are the same. Both antennas are shown in Figure 2. They differ mainly in their physical properties, one being of plumber’s delight construction, while the other uses insulating support members. One or more directors can be added to either antenna to obtain additional gain and directivity.
From QST August 2000