Subject: limits for the EM spectrum?

are there any limitations for the existence of em radiations?
are there any upper limit for th frequency of a radiation, perhaps it could be
that measure  I saw on a magazine a while ago in which scientists measured the
smallest time period possible, or with the quantum mechanics there would be
possible to have frequencies which wouldn't be limited by this timestep

and are there any holes in the spectrum, ranges of frequencies that couldn't
exist or be mainteined after emitted due to some combinations of circunstances
common to just about the entire universe?

Re: limits for the EM spectrum?