When using a 0-3dB gain antenna, such as on a vehicle, the antenna distance from persons is 20cm. When using a 9dB gain or high gain outside antenna on an amplifier, it should be 6m horizontally away from persons or 1m vertically away from persons. The lobe pattern of the 9dB gain antenna is flat and does not radiate down which allows persons to be located closer vertically to the antenna. This safety factor should be followed due to the power level that is being broadcast from the antenna.
These specifications hold true for ALL amplifiers, as they are FCC regulations.