The decibel (symbol: dB) expresses two values of power to another on a logarithmic scale. Wikipedia
In telecommunications power is compared to a refrence that is normally 1 mW (in 50 ohm when talking about RF) the aditional letter at dB (e.g. dBm) points to the reference used for expressing the comparison number. In the case of 1 mW reference it is dBm.
When looking to antenna’s, antenna gain (a comparison between two antennas expressed on a logartihmic scale), gain can be compared to two references. These are the dipole- or the isotrope antenna. When the dipole is used as a reference this is noted as dBd when a Isotrope antenna is used as reference this is noted as Dbi.
The diploe antenna has a donut shape radiation pattern while the isotrope has a perfect sphere shaped radiation pattern. The isotrope is a mathematical model that only exists in theory and can never be reproduced.
The differences in gain between a isotrope and a dipole antenna in dB are:
- the gain of a diplole antenna is 2,15 dBi (see Wikipedia for quick reference)
- the gain of a isotrope antenna is -2,15 dBd
The isotrope gain is popular among manifacturers because the increases the gain of their antenna with 2,15 dB for free. It does look great for selling.From enigeering point of view it is common to use dBd because the engineer knows how the dipole behaves. Only trouble is that dBi and dBd are randomly used.
My advice is only compare antenna gain when i or d are the same in the dB. and preferrebly use dBd.
Why is dBi used in this case? That is because regulators prefer to use undisputed references like the (mathematical) isotrope antenna. Doing so they have a scientific reference and leave the calculation to the manufacturers, engineers and radioplanners.
In the case of gateways we have to fullfill the requirement that we emit no more power than the regulator allows. This is expressed in Eirp. “Effective radiated power while using an isotrope as a reference”.
Assume the limit at +14 dBm Eirp, a 5 dBd antenna and 1 dB cable loss.
- The 5 dBd antenna equals to 5 + 2,15 = 7,15 dBi gain.
The maximum power that can be applied to the antenna system is: - 14 - 7,15 + 1 = 8,85 dB.
I hope this information helps.