A common comparison for antennas is how much gain it has referenced to a 1/2 wave dipole. There is a theoretical antenna that illuminates in all directions equally and it’s called an isotopic antenna which has 2.14dB less gain than a 1/2 wave dipole. So if someone told you the Sirio 5000 has 3.0dBi gain that would mean it has 3.0dB more gain than the theoretical isotopic antenna or .86dB more gain than a 1/2 wave dipole, which is impossible.
A 1/2 wave dipole is about 18ft tall and a 6 1/2ft tall mag mount antenna will probably have at least 3dB less gain than a 1/2 wave dipole putting it around -5dBi to -7dBi, which is gain compared to a theoretical isotopic antenna.
