Antenna theory recognizes a point of reference called the isotropic radiator. The isotropic antenna - the imagined antenna which completely does not have directed properties, i.e. the spatial diagram looks like sphere
This device is a theoretical construct consisting of a spherical point source of RF radiation
in all directions . It is truly omnidirectional (“all directions”)
because it creates an ever-expanding sphere as the RF wavefront propagates outward.
Thus are related the concepts directivity and gain.
From definition follows, that the directivity factor is function of corners j, q and is designated D (j, q), and does not consider losses of power in the antenna, therefore is introduced parameter G, considering these losses and named antenna power gain(or gain).
G = D(j,q)h

Directivity and gain are specified in three dimensions.
The gain is a measure of how much of the input power is concentrated in a particular direction, or
antenna gain is the ratio of the power density of an antenna’s radiation pattern in the direction of strongest radiation to that of a reference antenna. This ratio can either be expressed linearly as a dimensionless number G, or logarithmically as a decibel figure (10 dB·log10 G). (It is expressed with respect to a hypothetical isotropic antenna, which radiates equally in all directions. )
The gain of an antenna is a passive phenomenon – power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has positive gain in some directions, it must have negative gain in other directions, as energy is conserved by the antenna.
For practical use, antenna gain usually represent graphically in polar coordinates from angles j, q for Е and Н planes. On radius of the diagram postpone absolute size of factor gain in дБ. The circle with value 0 дБ represents the diagram of an orientation of an isotropic radiator.

There are four ways of expressing antenna gain. These are:

• dBi Gain over an isotropic source (a theoretical antenna having no dimensions: a geometric point).
• dBd Gain over a dipole (0 dBd = 2.15 dBi).
• dBq Gain over a quarter wavelength whip (bigger numbers than dBi).
• dBadv LARGE RANDOM numbers generated by the advertizing and marketing departments

The high-gain antenna (HGA) is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows more precise targeting of the radio signal - also known as a directional antenna. High gain means it is hard to aim. In good-signal areas, avoid high gain antennas.
Examples of high-gain antennas may be 8-bay Channel Master CM4228 and Channel Master CM4248 Yagi/Corner-Reflector.