  We propose a simple analytical jet model of magnetic jets, in which
radially-averaged profiles of main physical quantities are obtained based on
conservation laws and some results of published GRMHD jet simulations. We take
into account conversion of the magnetic energy flux to bulk acceleration in
jets formed around rotating black holes assuming the mass continuity equation
and constant jet power, which leads to the Bernoulli equation. For assumed
profiles of the bulk Lorentz factor and the radius, this gives us the profile
of the toroidal magnetic field component along the jet. We then consider the
case where the poloidal field component is connected to a rotating black hole
surrounded by an accretion disc. Our formalism then recovers the standard
formula for the power extracted from a rotating black hole. We find that the
poloidal field strength dominates over the toroidal one in the comoving frame
up to large distances, which means that jets should be more stable to
current-driven kink modes. The resulting magnetic field profiles can then be
used to calculate the jet synchrotron emission.
