If a 3 dBd gain antenna is replaced with a 9 dBd gain antenna, with no other changes, the Effective Radiated Power (ERP) will increase by:

The first step is to determine the net difference in gain between the new antenna and the old one. Replacing a 3 dBd antenna with a 9 dBd antenna yields a net increase of +6 dB (9 dBd minus 3 dBd equals 6 dB). Next, convert this decibel increase into a power multiplier. Remember the standard rule of thumb: every 3 dB increase doubles the power. An increase of 6 dB is equivalent to two consecutive 3 dB increases. Doubling the power twice (2 x 2) results in a total power multiplier of 4. Therefore, regardless of the transmitter's starting wattage, the Effective Radiated Power (ERP) will increase by a factor of 4.