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Antennas do not emit radiation uniformly in all directions. Antenna directivity is a function of zenith and azimuth which quantifies this. It is
Antennas do not emit radiation uniformly in all directions. Antenna directivity is a function of zenith and azimuth which quantifies this. It is
( dP/dA ) / (P / 4 pi r^2) r large
( dP/dA ) / (P / 4 pi r^2) r large
where dP is the differential of radial power flux through dA at the zenith and azimuth and range and P is the total power flux through the sphere tangent to dA. Directivity is unitless and independent of radiator power level. Of practical interest is the maximum directivity, for these tuned radiators
where dP is the differential of radial power flux through dA at the zenith and azimuth and range and P is the total power flux through the sphere tangent to dA. Directivity is unitless and independent of radiator power level. Of practical interest is the maximum directivity, for these tuned radiators
Short Dipole 1.5
Short Dipole 1.5
Half Wave Dipole 1.63
Half Wave Dipole 1.63
Half Wave Quadrupole 3.2
Half Wave Quadrupole 3.2
Half Wave Yagi 5.6
Half Wave Yagi 5.6
Antenna directivity can also be defined in terms of solid angle as
Antenna directivity can also be defined in terms of solid angle as
( dP/dO ) / ( P / 4 pi ) r large
( dP/dO ) / ( P / 4 pi ) r large
where the differential of solid angle is used instead of dA. Since in the limit of r large dA=r^2 dO, the definitions are equivalent.
where the differential of solid angle is used instead of dA. Since in the limit of r large dA=r^2 dO, the definitions are equivalent.
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