  We design and implement an efficient high-current radio-frequency (RF)
circuit, enabling fast and coherent coupling between magnetic levels in cold
alkali atomic samples. It is based on a compact shape-optimized coil that
maximizes the RF field coupling with the atomic magnetic dipole, and on coaxial
transmission-line transformers that step up the field-generating current
flowing in the coil to about 8 A for 100 W of RF power. The system is robust
and versatile, as it generates a large RF field without compromising on the
available optical access, and its central resonant frequency can be adjusted in
situ. Our approach provides a cost-effective, reliable solution, featuring a
low level of interference with surrounding electronic equipment thanks to its
symmetric layout. We test the circuit performance using a maximum RF power of
80 W at a frequency around 82 MHz, which corresponds to a measured Rabi
frequency $\Omega_R/2\pi \simeq 18.5$ kHz, i.e. a $\pi$-pulse duration of about
27 $\mu$s, between two of the lowest states of ${}^6$Li at an offset magnetic
field of 770 G. Our solution can be readily adapted to other atomic species and
vacuum chamber designs, in view of increasing modularity of ultracold atom
experiments.
