The gain G of antenna is a power ratio, function of directivity D and efficiency K :
G = K*(P max/Pin avg) = k*D (G is expressed in dB)
The directivity of an antenna is the maximum gain measured compared to its gain averaged in all directions. The directivity D of an antenna is given by the next relation : D = P (max)/ Pin (avg)
To compare performance of a 20 dB gain antenna over an isotropic, let's image that this antenna radiates its 20 dB gain (in fact 20 dBi) from the center of a sphere. This energy will travel through a surface 100 times smaller than the total surface of the sphere, or 41253squer° / 100 = 41squer° of beamwidth.
This calc. it's for 100% efficiency antenna
Dmax =4π/ΔΩ =4π/Δθrad1 Δθrad2 =41 253/Δθ°1 Δθ°2
Gmax =K/Δθ1 Δθ2
Practical
values of K fall in the range 25000–35000 (with the beamwidth angles in degrees.)
Let P(Θ,Φ) denote the far field radiation pattern with respect to power. You need to square if you have the field pattern. . Find P(Θ,Φ)max. Then find the average of P(Θ,Φ) by taking a grid of say 50 by 50 points or even finer. Find 10log(P(Θ,Φ)max/P(Θ,Φ)ave) That will give your gain.
Let P(Θ,Φ) denote the far field radiation pattern with respect to power. You need to square if you have the field pattern. . Find P(Θ,Φ)max. Then find the average of P(Θ,Φ) by taking a grid of say 50 by 50 points or even finer. Find 10log(P(Θ,Φ)max/P(Θ,Φ)ave) That will give your gain.
Hi:
Please clarify the difference between radiation pattern and field pattern. Because we measure the field pattern (e.g. azimuth) in anheonic chamber which is power measurement.
from the pattern you get the directivity, which differs from the gain in the loss of the antenna. To have a measure of the gain you need a standard gain antenna and make a power measurement (which is out of your means if I understand properly), or an estimate of the loss of your antenna. Assumed this is the case, you have the loss (estimate) and the directivity (pattern), so you have the gain...
btw field pattern, radiation pattern is all the same.
Hi
The definition given by plasma makes sense.
G = K*(P max/Pin avg) = k*D (G is expressed in dB)
Actually measuring the radiation pattern using an antenna of known gain (usually horn antenna) is an estimation of loss in a specific plane. I think we can measure the gain by subtracting the gain of known receiving antenna to one obtained from above definition.
please let me know if i am wrong