Magnetic field gradients are nowadays indispensable to most nuclear magnetic resonance experiments and are at the basis of magnetic resonance imaging (MRI). Most of time, gradients of the static magnetic field are employed. Gradients of the radio-frequency (rf) field may constitute an interesting alternative. Until now, they were produced by a single loop. We demonstrate in this paper how two unsymmetrical series loops can be optimized to produce rf gradients of much better performances. This optimization is based on a thorough theoretical approach and the gradient uniformity is studied through accurate simulations. Two prototypes were devised: one for a 2.34 T horizontal magnet (used in MRI), the other for a 4.7 T vertical magnet (used for pure spectroscopic applications). These two-loop systems were designed for proton resonance frequencies (100 MHz and 200 MHz, respectively). Performances of both