Antenna Gain and Directivity

Antenna Gain and Directivity are two terms that are sometimes not that well understood. The Antenna Gain and Directivity are related through the following equation.

G(θ,φ)=E*D(θ,φ)

That is, the Antenna Gain in a particular direction is equal to the Directivity in that direction multiplied by the Antenna Efficiency. Antenna Directivity is the ratio of energy transmitted (or received) by the antenna in a particular direction to the energy transmitted (or received) in that direction by an isotropic source. This is also known as the Directive Gain.

The Antenna Gain (also known as the Power Gain) seems to be a better metric to quantify the performance of an antenna as it takes into account the efficiency in converting electrical energy supplied to the antenna into radiated energy.

The 3-dimensional plot of the Gain of an antenna is known as the radiation pattern. The Antenna Gain with reference to an isotropic source is given in dBi (decibel above isotropic source). Sometimes the Antenna Gain is given with reference to a Dipole Antenna and is labelled as dBd. The figure below shows the Directivity of a Patch Antenna embedded inside a human body [1].

Directivity of a Patch Antenna
Directivity of a Patch Antenna

Note:

1. An isotropic source (a source that radiates uniformly in all directions) is only a theoretical concept and does not exist in reality.

2. The sun can be considered an isotropic radiator since it radiates uniformly in all directions (almost).

3. When no direction is given the Gain refers to the maximum Gain.

Reference:

[1] http://www.hindawi.com/journals/ijap/2008/167980/

Author: Yasir Ahmed (aka John)

More than 20 years of experience in various organizations in Pakistan, USA and Europe. Worked as Research Assistant within Mobile and Portable Radio Group (MPRG) of Virginia Tech and was one of the first researchers to propose Space Time Block Codes for eight transmit antennas. The collaboration with MPRG continued even after graduating with an MSEE degree and has resulted in 12 research publications and a book on Wireless Communications. Worked for Qualcomm USA as an Engineer with the key role of performance and conformance testing of UMTS modems. Qualcomm is the inventor of CDMA technology and owns patents critical to the 5G and 4G standards.

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6 thoughts on “Antenna Gain and Directivity

  1. Does the ratio of gain to directivity really give the value of efficiency? if it is true then the value we get is n or %n?

  2. do you know how to obtain this formula dB=20 log E/50micro v/m and E= roof (30.Wt.D)/d where E is the fieldstrength electivity, Wt=transmitter power,D=constanta directivity,d=distance…
    and what is the relationship between directivity and gain.

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