Dipole Antenna
The dipole is the basic antenna structure consisting of two conductive arms extending symmetrically on either side of the feed point. It needs no ground plane, because the two arms form each other's counterpoise. It is used as the reference structure in antenna gain measurements, and the unit dBd is defined relative to this antenna.
The radiation pattern of a dipole is doughnut-shaped in the plane perpendicular to the arms: there is no radiation along the axis in which the arms extend. A vertically mounted dipole radiates equally in all directions in the horizontal plane while leaving a null directly above and directly below.
- The feed impedance is about 73 Ω in free space and in practice matches 50 Ω systems directly.
- The symmetrical structure usually requires a balun to prevent the outer braid of the coaxial cable from radiating.
- Independence from a ground plane makes the dipole advantageous in applications where it is to be mounted on a metal body.
Its practical importance is twofold. First, the dBd value in an antenna catalogue is given relative to this structure. Second, the null at the top shows up in the field: an operator standing directly beneath the receiver antenna may get a weaker signal than one thirty metres away.
Context in which this term is used: 433 MHz or 2.4 GHz?