The electrical and radiation characteristics of innovative high-efficient 254-nm wavelength ultraviolet (UV) radiation sources employing ferrite-free inductively-coupled low-pressure discharges excited in closed-loop quartz tubes are experimentally studied. The discharge was excited using a 3-turn induction coil at a frequency of 1.7 MHz and lamp power equal to 90-160 W in a mixture of mercury vapor at a pressure of around 0.01 mmHg and buffer gas (Ar, a mixture of 30%Ne+70%Ar) at pressures of 0.7 and 1.0 mmHg in a closed tube 16.6 mm in diameter and 815 mm long. The coil turns made of multiconductor wire (Litz wire) with a low per unit length resistivity of 0.00014 Ωm/cm and 1.5 mm in diameter were arranged over the discharge tube perimeter. It was found that, as the lamp power was increased, the power loss in the induction coil wire decreased from 7-9 to 3-4 W, and the coil efficiency increased from 92 to 98%. In lamps filled with buffer gas at a pressure of 1.0 mmHg, the maximal plasma UV radiation generation efficiencies equal to 68% and 66%, respectively, were achieved at plasma power levels of 105-155 W. The decrease of buffer gas pressure to 0.7 mmHg entails a drop of lamp UV radiation generation efficiency by 10-20% and a shift of its maximum values to the zone of lower plasma power values.