Directivity
Understanding Antenna Directivity
Directivity quantifies how effectively an antenna concentrates radiation in its preferred direction. A highly directive antenna (like a parabolic dish) has a narrow main beam and low sidelobes, concentrating most radiated energy in a pencil beam. The directivity can be estimated from the half-power beamwidths in the principal planes.
Directivity vs Gain
Gain = Directivity x Efficiency. If an antenna has directivity of 30 dBi but only 50% radiation efficiency (-3 dB), its gain is 27 dBi. For well-designed antennas, efficiency is 80-95%, so gain and directivity are nearly equal. For small antennas and printed antennas, efficiency can be much lower.
Estimation from Beamwidth
For antennas with a single main beam, directivity can be approximated from the half-power beamwidths: D = 41,253 / (theta_E x theta_H) where the beamwidths are in degrees. This gives D in linear terms; convert to dBi with 10 log10(D).
D (dBi) = 10 log10(4 pi U_max / P_rad)
From beamwidths:
D = 41,253 / (theta_E x theta_H) (approximate)
Gain = eta x D
where eta = radiation efficiency (0 to 1)
Dipole: D = 2.15 dBi
10 lambda dish: D ~ 36 dBi
Frequently Asked Questions
What is directivity in an antenna?
Directivity measures how focused an antenna's radiation pattern is in a particular direction compared to a uniform radiator. Higher directivity means more energy is concentrated in the main beam. It is a purely geometric property independent of losses.
What is the difference between directivity and gain?
Directivity measures the focusing ability of the antenna pattern. Gain equals directivity multiplied by radiation efficiency. For lossless antennas, gain equals directivity. For real antennas with ohmic and dielectric losses, gain is always less than directivity.
Can directivity be less than 1?
No. The minimum possible directivity for any antenna is 1 (0 dBi), which corresponds to an isotropic radiator that radiates equally in all directions. All real antennas have directivity greater than 1 in at least one direction.