ETS-Lindgren 3148B Log Periodic Dipole Array - Manual (Page 25)

Application | 25
For IEC/EN 31000-4-3 type testing, the antenna tip can be placed at any
distance between one and three meters from the EUT as long as the front face
plane is illuminated according to the -0, +6 dB uniform field specifications.
It is usually necessary to place RF absorbing material between the EUT and
antenna to suppress ground plane reflection to ensure the field uniformly, or to
conduct the immunity test in a fully-lined anechoic room. In general, closer
distances require less power to create a given field strength.