We can see a lot of terms like dB dBm or dBi when dealing with RF remote control transmitters and receivers, so we thought it would be helpful if we do a short explanation here about what these unit means.
First, although they look similar with each other, dB dBm and dBi actually means very different things, so they are not interchangeable, dB is a relative term measuring two different power or voltage levels, and dB usually measures relative power when used in RF remote control transmitters, dB value can be calculated by 10 log P1/P2, where P1 and P2 are two different power levels, so 10dB means P1 is ten times P2’s power level.
Since dB is all about relative power, so it’s about relative power levels comparison, or power gain and loss, as a relative value, dB doesn’t mean absolute power output level in any circumstances.
dBm is usually used as power level measurement unit in RF remote control transmitters, m in dBm means mW, so dBm measures how strong the transmitters output power is compared to 1 mW, thus 0dBm equals to 1 mW output power, and 1 Watt output power equals to 30dBm power output, there is also a large scale unit called dBW, and 0dBW equals to 1 Watt output power, but we usually don’t use that unit in ISM transmitters, since the maximum allowed power is usually only 1 W or 30dBm.
dBi is usually as unit to describe antenna gain, which means gain of an antenna as referenced to an isotropic source, a isotropic source is perfect omnidirectional radiator, a true point source, which doesn’t exist in real world, dBi is useful for comparing antennas, also there is another unit dBd that compares antenna gain to dipole antenna instead of isotropic source, value of dBi equals to dBd plus 2.41dB, which means simple dipole antenna has 2.41dBi and 0dBd.
Many antennas sold on market tend to be rated in dBi, as the people buying them understand it, and amateur radio antennas tend to be dBd, as hams are more familiar with dipoles, anyway each value can be converted to the other easily.
In conclusion, we usually use dBm to measure RF transmitters power as compared to 1mW power output level and use dBi to measure antenna gain as compared to perfect isotropic source.