Antenna Designer's Notebook

In his classic 1948 paper, Lan J. Chu presented the equivalent circuit for an electrically small antenna. This circuit quantified the relationship between the lower bound for the radiation Q of an electrically small antenna and its physical size relative to the wavelength. A recent article has highlighted the fact that Chu defined a lower bound for a hypothetical antenna. A key issue is how close can the Chu limit be approached, in practice? Chu's lower bound assumes that there is no stored energy inside a sphere that circumscribes the largest antenna dimension. Another recent article has extended Chu's work to include the internal stored energy, and provides new lower bounds for electrically small antennas. This paper presents a summary of the recent findings regarding this issue.