The radiation efficiency improvement of an electrically small antenna is presented. We consider a balanced miniature inverted-F-antenna-inspired circular antenna whose radius is y28 at 433 MHz and radiation efficiency is 30%. By modifying the antenna strip width, we identify, by electromagnetic simulations, an antenna radiation efficiency improvement up to 53% while keeping its overall dimension and general structure identical. Then, we confirm the antenna performances improvement by radiation efficiency measurements performed inside a large anechoic chamber and using the gain pattern integration technique. An optical fiber link is used to avoid possible perturbation due to the feeding coaxial cable. The radiation efficiency improvement, confirmed by experimentation, is of major interest for ultranarrowband antennas for which the radiation efficiency is the most critical parameter. This efficiency improvement is discussed in light of the fundamental limits of miniature antennas. © 2002-2011 IEEE.