Quality Factors of ESA
In a 1947 paper, Harold Wheeler defined an electrically small antenna (ESA) as an antenna that could be enclosed within a radian sphere (Wheeler, 1947). The radian sphere was a sphere of radius equal to , where is the wavelength. The antennas used by Marconi and Fessenden in the early years of wireless telegraphy were electrically small antennas even though they were very large physical structures, often involving wires strung as an inverted fan or cone from masts several hundred feet tall. These antennas were electrically small antennas since in order to achieve long-distance transmission the wavelengths used, typically greater than 3000 m, were much longer than the antenna heights. These electrically small antennas were characterized by a very low radiation resistance and a large capacitive input reactance. The purpose of the large inverted fans and cones was to increase the antenna capacitance and thus reduce the capacitive reactance. Wheeler introduced the radiation power factor (RPF) as a figure of merit for these electrically small antennas. He considered two basic antenna types: the magnetic dipole or loop antenna consisting of a solenoid coil with N turns, length b, and radius a; and a short electric dipole antenna consisting ...