1,212
edits
Changes
no edit summary
As the name suggests, directivity is the measure of how directed the antenna radiation pattern is. That is how much an antenna transmits in a particular direction as opposed to transmitting uniformly in all directions. The radiation pattern can be depicted as a function of azimuth angle and elevation angle. The ratio of max power in radiation pattern function to the average power of radiation gives us the directivity of any antenna. Mathematically, if F(Θ,Φ) denotes the radiation function, directivity can be described by:
[[File:Directivity.JPG|center|frame]]
Based upon directivity, antennas can be divided in three groups:
[[File:Isotropic.jpg|thumb|frame|Isotropic]]
[[File:Omni.jpg|thumb|frame|Omnidirectional]]
[[File:Directional.png|thumb|frame|Directional]]
* '''Isotropic:''' An isotropic antenna is an ideal antenna which radiates uniform power in all directions. Hence it's directivity would be 1(0 dB).
* '''Omnidirectional:''' Omnidirectional antenna radiates uniform power in a single plane and its power diminishes as we go away from the plane. Hence it's radiation pattern resembles a “doughnut”.
* '''Directional:''' Directional antennas can radiate greater amount of power in a specific direction compared to any other direction. Their directivities are very high. They have complicated (and often interesting) radiation patterns.
{| class="wikitable"
|+Typical directivities of some common antennas
|-
|'''Antenna Type'''
|'''Typical Directivity'''
|'''Typical Directivity (dB)'''
|-
|Short Dipole
|1.5
|1.76
|-
|Half-wave dipole
|1.64
|2.15
|-
|Patch (Microstrip)
|3.2-6.3
|5-8
|-
|Horn
|10-100
|10-20
|-
|Dish
|10-10000
|10-40
|}
As is evident, directivity can vary greatly with type of antenna. Hence it's important to understand and know directivity before selecting antenna for your application. Like, if you are going to receive/transmit signal from/to any direction, it is better to use antenna with low directivity. Similarly in an application where the direction of signal is fixed, highly directed antennas work way better.