Antenna Gain

Value to Enter:

Enter the dBi gain value for the both antennas. Even a standard antenna has some gain, that's just the nature of the underlying physics. Many 802.11 access points come with an antenna roughly 3- to 5-inches long as standard equipment. This type of antenna has a gain of 2.15 dB, which is the default value for the calculator.

Significance of This Value:

The antenna gain is used by the calculator as part of the link suitability assessment. Power level and antenna gain are attenuated by cable loss, rain fade, link fade, and free space path loss.

Background and Technical Perspective:

A theoretical point source that radiates an electromagnetic field does so in a perfect sphere, with energy propagating outwards equally in all directions. This theoretical antenna is called an isotropic radiator. The simplest real-world antenna is a length of wire, called a dipole radiator. Signal energy from a dipole radiates outward to the sides (perpendicular to the length of the antenna) but not out the ends. This is analogous to the way light shines out of a fluorescent light tube. The transmission volume has a torus shape (doughnut shape) around the radiating element. Instead of radiating out in a spherical shape the energy is "flattened" into a torus. Relative to a sphere, the torus has less volume for the same radius. The density of energy in the torus is greater than that of the sphere (in the direction perpendicular to the antenna.) This is a basic antenna gain factor that is the natural result of the way a dipole radiator creates the electromagnetic field.

Gain measured relative to a theoretical isotropic radiator is assigned the measurement unit "dBi" (decibels relative to isotropic.) Unless specified otherwise, if you see a gain expressed simply as "dB" it probably implies "dBi". The inherent gain of a dipole antenna is 2.15 dBi. The dipole, by the nature of the toroidal field volume, introduces this gain. An alternative unit of measurement is to specify gain in terms of "dBd" (decibels relative to a standard dipole). Although less common, dBd is sometimes given in antenna specifications. An antenna with 0 dBd gain has a gain of 2.15 dBi.