bandwidth in db
Ok,
two years ago, I was into a design of UWB antennas, and in this case, the criterion was -10 dB (2:1). And in a narrow band communications I use 1.5:1.
May be In some cases (I think), for example in digital communication systems the spectral mask and time-delay performances are implied is involved (we can view a pair of antennas and free space as a filter), and in other cases the aboslute power of the system is directly related to criterion, for example, is not the same a 10mW transmission system with amount of 10% reflected power or a 10kw transmission system with same VSWR, in the first case the reflected power is 1mW in second 1kW that is disspated in heat or can cause high voltage breakdown in a cavity filter.