dBi and dBd
Gain in dBi is measured against an ideal isotropic radiator that sends the same power in every direction. Gain in dBd is measured against a half-wave dipole, which already has 2.15 dBi. So dBi = dBd + 2.15. A beam advertised as 10 dBd is 12.15 dBi.
ERP and EIRP
EIRP is the power an isotropic antenna would need to give the same field in the strongest direction: transmit power plus antenna gain in dBi, minus feeder loss. ERP uses the dipole as a reference, so ERP = EIRP - 2.15 dB. Example: 100 W with a 6 dBd antenna gives an ERP of about 398 W and an EIRP of about 653 W. Regulations quote one or the other, so always check which.
What gain does and does not do
Gain does not create power: it concentrates it. A higher-gain antenna narrows the beam, so it must point at the other station and in the vertical plane it can miss a nearby hill station. On HF the pattern also depends on the height above ground, which creates the take-off angle of the signal. In the coverage map choose the antenna type, the gain and the height, and switch on beam aiming to see the lobe on the map.
