Directivity

Lore & Background

The directivity of an actual antenna can vary from 1.76 dBi for a short dipole to as much as 50 dBi for a large dish antenna. The term 'directive gain' is deprecated by IEEE. If an angle relative to the antenna is not specified, then directivity is presumed to refer to the axis of maximum radiation intensity. The directivity is defined as the ratio of the maximum signal strength radiated by the antenna to the signal strength radiated by an isotropic antenna with the same total radiated power. In an antenna array, the directivity is a complicated calculation in the general case. For a linear array, the directivity will always be less than or equal to the number of elements. For a uniformly weighted standard linear array, this reduces to simply N, the number of array elements. For a planar array, the computation is more complicated and requires consideration of the positions of each array element with respect to all the others and with respect to wavelength. The directivity of a planar array is the product of the array gain and the directivity of an element only in the limit as element spacing becomes much larger than lambda. The beam solid angle is defined as the solid angle which all power would flow through if the antenna radiation intensity were constant at its maximal value. If the beam solid angle is known, then maximum directivity can be calculated as 4π divided by the beam solid angle.

Reader's Guide

Directivity serves as a fundamental measure in antenna and optical system design, quantifying how effectively a system concentrates radiated power in a desired direction. Its significance lies in its relationship to gain, from which it differs by the radiation efficiency factor. The principle of reciprocity ensures that directivity is identical for transmitting and receiving operations. In antenna arrays, directivity calculations become complex, with linear arrays having a directivity at most equal to the number of elements, and planar arrays requiring detailed consideration of element spacing and positions. The relationship between directivity and beam solid angle provides a practical method for estimation, particularly for antennas with a single narrow major lobe. The concept of partial directive gain extends the measure when polarization is considered. Directivity values span from 1.76 dBi for a short dipole to 50 dBi for large dish antennas, illustrating the wide range of concentration achievable. The legacy of directivity as a parameter is its role in comparing antenna performance against the ideal isotropic radiator, with decibel expressions relative to isotropic (dBi) or half-wave dipole (dBd) references.

Did You Know?

• The directivity of a hypothetical isotropic radiator is 1, or 0 dBi.
• An antenna's directivity is greater than its gain by an efficiency factor called radiation efficiency.
• The directivity of a perfect half-wave dipole is 1.64.