  We discuss physical constrains that observations of high brightness
temperature coherent radio emission, with brightness temperatures as high as
$T_b \sim 10^{35}$ K, impose on the plasma parameters at relativistically
moving astrophysical sources. High brightness temperatures imply a minimal
plasma energy density at the source. Additional important constraints come from
the fact that resonantly emitting particles lose most of their energy to
non-resonant inverse Compton and synchrotron processes.
  We also interpret recent observations of high-to-low frequency drifting
features in the spectra of repeating FRBs as analogues of type-III Solar radio
bursts produced by reconnection plasma beams within magnetospheres of highly
magnetized neutron stars.
