no stupid question, don’t worry 🙂
dBi is a “gain” from an antenna. It is not a power. Just a difference of power : what is out from the antenna = what is in + the gain.
For example, take one antenna with gain = 15dBi
if you input 40dBm in the antenna, the output will be 55dBm.
However, you understand that the antenna doesn’t “produce” energy. It just redirects the energy forward, in the front lobe, while it removes energy from side, top, bottom and behind 🙂
It is a “directional” antenna… and this “gain” comes from the comparison with a “non directional” antenna, also called “isotropic” antenna.
dB also follows the same principle, it is a “gain” or a “loss”, in other words dB is a difference of powers.
dBm is an actual power.
0dBm = 1mW
3dBm = 2mW
33dBm = 2W
36dBm = 4W
Attenuation of power from an antenna, it increases with the distance from the antenna.
Usually, the attenuation is function of log(d).
… what is r ?
and what about the magnetic sphere ? I never noticed that antennas acted as giant magnets… :))