Antenna Gain, Directivity & EIRP Explained

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Discussion Overview

The discussion revolves around the concepts of antenna gain, directivity, and Effective Isotropic Radiated Power (EIRP). Participants explore the definitions and differences between these terms, as well as the relationship between transmitted power and radiated power, particularly in the context of antenna design and performance.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks for clarification on the definitions of antenna gain and directivity, and how EIRP relates to radiated power.
  • Another participant explains that gain refers to the concentration of transmit power in a specific direction, with a yagi antenna serving as an example, and notes that gain is typically expressed in dBd.
  • It is mentioned that the radiated power can appear greater than the transmitted power due to the directional concentration of the signal, although it cannot exceed the power supplied by the transmitter.
  • A participant distinguishes between directivity as the theoretical gain of an antenna and gain as the realizable gain, which accounts for efficiency losses due to material properties.
  • There is a reference to isotropic radiators as a common basis for measuring gain and directivity, with values expressed in dBi.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between gain, directivity, and efficiency losses, indicating that there is no consensus on these definitions and their implications.

Contextual Notes

Some assumptions regarding the efficiency of materials and the definitions of gain and directivity may not be fully explored, leading to potential ambiguity in the discussion.

muh_j18
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hi all,
what exactly is the gain of an antenna, and what is the difference between the gain and the directivity?
also, what is the EIRP, is it the same as the radiated power? and how can the radiated power be greater than the transmitted power? (EIRP= transmitted power+gain-losses in dB)

any help to explain these concepts is appreciated.
 
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what exactly is the gain of an antenna, and what is the difference between the gain and the directivity?

An antenna, such as a yagi, concentrates the available transmit power mostly in one direction. In that direction only, the signal transmitted will be stronger than if it was coming from a dipole in the same position ( and radiating at right angles to the dipole wire). This difference is called the gain of the antenna and is always given as gain over a dipole, in dBd.

In all other directions, the signal will be weaker than it would be from a dipole.

The more gain an antenna has, the more concentrated the radiation pattern is. This is measured by moving a little off to the sides of the main beam of energy and noting the directions where the power is reduced by 3 dB on each side. The angle between these directions is the beamwidth of the antenna, or the directivity.


also, what is the EIRP, is it the same as the radiated power? and how can the radiated power be greater than the transmitted power? (EIRP= transmitted power+gain-losses in dB)

The radiated power can never be more than the power supplied by the transmitter. However as above, it can be concentrated into one direction in ways that make it appear more powerful than the same power from a diplole would be.
Sometime this extra strength is achieved by taking power that would radiate upwards from a dipole and concentrating it horizontally. This is done with TV transmitting antennas where 360 degree coverage is required horizontally, but there is no need for power to radiate upwards.
 
The difference between directivity and gain is that directivity is the theoretical gain of the antenna. Gain is the realizable gain of the antenna when you take into account the fact that not all power sent to the antenna is radiated. You have efficiency losses due to finite conductivity of the metals in the antenna and due to lossy behavior of the antenna's dielectrics. So if the antenna had perfectly lossless materials, it would have 100% efficiency for the gain and the gain and directivity would be the same.

I would point out though that we often choose an isotropic radiator as our reference for the gain/directivity. In this case the numbers are given in dBi.
 

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