Electromagnetic sensors which provide high coupling gain between the transmitter and receiver of a dielectric tool include a conductive pad having a cavity filled with dielectric filler, and an electrical conductor that provides electrical current to a conductive feeder placed inside the dielectric filler. During operation, current is fed to the conductive feeder to thereby produce an electric monopole that feeds electromagnetic power into the cavity, which then radiates out into the formation as electromagnetic signals. Because of the design of the electromagnetic sensors, the transmitter/receiver coupling gain and SNR are optimized over the entire range of the dielectric constant (i.e., 5≤ϵr≤80).