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.