what is the GAIN ?in antennas

hai every one , can some one tell , what is the gain in antenna,for example if some is saying my antenna gain is 5dbi what does he mean?

thanks![/b]

One important thing to remember with antenna's is that they are most of the time passive elements, no additional power is injected apart from RF. This means that the gain we are talking about can only be realized by changing the radiation patern. The gain you refer to is dBi and this means that the gain of the antenna is compared with a perfect round object (like the sun) it is radiating the same energy in all directions.

In Antenna's people are focusing the RF energy into one or multiple direction(s). Gain is always refered to the increase of signal strength in that MAIN direction compared to the perfect round object.

You sometimes also see dBd and then it means it is compared not with a perfect round object but with the dipole antenna as reference. The difference between dBi and dBd is 2.16 dB, this is the gain of the dipole compared to a perfect round object radiating so we can say a dipole is 2.16 dBi !.

Your 5dBi does mean that your antenna is having a forward gain of 5dB compared to a perfect round object. If you antenna is giving a circular radiation patern it means that the radiation field is more like a donut shaped object with the antenna in the middle. 5 dBi does not sound a lot but this is al depending on the frequency used. HF and 5dBi is a large antenna. SHF (SAT) 5 dBi is nothing. a normal SAT Dish is doing 28dBi @ 12 GHz.

Hope you understand it now !.

Paul.

antenna's gain is ratio of maximum radiation intensity of an arbitrary measured antenna to maximum radiation intensity of a reference antenna.

reference antenna can be an isotropic antenna (it's only an imaginative antenna) or a half-wavelength dipole (which has gain 2.16dB to isotropic antenna, so we can say ?λ dipole's gain is 2.16dBi ).

isotropic antenna is an imaginative antenna which has uniform radiation intensity over a spherical area.

so, 5 dBi means that the maximum radiation intensity of this antenna is higher than radiation intensity of isotropic antenna ( i dont say its maximum radiation intensity, because the reason i stated above) or if we use ?λ dipole as a reference antenna (usually we use it), we get the gain of measured antenna, let say 2.84 dB?λ , then we can convert it into dBi gain : 2.84 + 2.16 (gain of ?λ dipole) = 5 dBi

regards

masadi