Directivity is a measure of how much the energy radiated by an antenna is concentrated in any single direction. Mathematically, It is the ratio of the radiation intensity in a given direction to the radiation intensity averaged over all directions.
Directivity is calculated as a ratio of the antenna gain to its efficiency
D = G/η
Antenna Efficiency (η) is the ratio of power radiated (PR) by the antenna to the power delivered (PD) to it. The theoretical maximum value is 1 when it radiates all the power delivered to it
η = PR/PD
The theoretical maximum value is 1 when the antenna radiates all the power delivered to it. This is an ideal situation and in practice a well-designed and manufactured antenna will only radiate up to a maximum of 60% of the input power.
Note that this power loss is in addition to the mismatch loss and should be accounted for when making a range calculation. You can simply add it to the Cable and other losses.
Frequently Asked Questions
What is the directivity of an isotropic Antenna?
The directivity of an isotropic antenna is 1. An isotropic antenna radiates equally in all directions. Its Efficiency is 1 or 100% and its Gain is 0 dBi.