dbd

dBd stands for decibels relative to a half-wave dipole. It is a unit used to express the gain or directivity of an antenna relative to a standard reference antenna, namely a half-wave dipole, rather than an isotropic radiator.

In practice, a dBd value indicates how much more or less power an antenna concentrates in a particular direction compared with a half-wave dipole occupying the same location and orientation. The measurement assumes the same operating frequency and free-space conditions, and it can be affected by factors such as efficiency, mismatch, and surrounding structures. To compare with the more common dBi specification, the relation is G(dBi) = G(dBd) + 2.15 dB, because a half-wave dipole has about 2.15 dBi of gain relative to an isotropic radiator.

A typical half-wave dipole, used as the reference, has a directivity of approximately 1.64 (linear), corresponding to about 2.15 dBi. Many antenna designs—such as Yagi-Uda arrays, log-periodic antennas, or other directional antennas—are specified in dBd to indicate their gain relative to a dipole. For example, an antenna listed as 9 dBd has a gain about 11.15 dBi.

Notes on usage: dBd provides a convenient reference for engineers and amateurs when designing or selecting directional antennas. The actual realized gain depends on efficiency and installation conditions; environmental factors and impedance matching can cause deviations from the stated dBd value.