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Diagram illustrating how isotropic gain is defined. The axes represent power density in watts per square meter. is the radiation pattern of a directive antenna, which radiates a maximum power density of watts per square meter at some given distance from the antenna. The green ball
The gain may be broken down into three factors; the antenna gain (the directional redistribution of the power), the radiation efficiency (accounting for ohmic losses in the antenna), and lastly the loss due to mismatch between the antenna and transmitter.
For a gain measured relative to a dipole, one says the antenna has a gain of " x dBd" (see Decibel). More often, gains are expressed relative to an isotropic radiator , making the gain seem higher. In consideration of the known gain of a half-wave dipole, 0 dBd is defined as 2.15 dBi; all gains in "dBi" are shifted 2.15 higher than gains in "dBd".
English: Diagram explaining how the directivity (directive gain) of an antenna is defined. R (grey) is the radiation pattern of a typical directive antenna. It radiates most of its power in a narrow lobe oriented along the z axis.
Patch antenna gain pattern. A directional antenna or beam antenna is an antenna which radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio waves in beams, when greater concentration of radiation in a certain direction is desired, or in receiving antennas receive radio waves from one specific direction only.
Diagram illustrating how directivity is defined. It shows the radiation pattern of a directional antenna (R, grey) that radiates maximum power along the z-axis, and the pattern of an isotropic antenna (R iso, green) with the same total radiated power.
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Diagram of antenna A and resistor R in thermal cavities, connected by filter F ν. If both cavities are at the same temperature , = The aperture of an isotropic antenna, the basis of the definition of gain above, can be derived on the basis of consistency with thermodynamics.