New radiation Q limits for spherical wire antennas

Considerable work has been published to define the minimum possible antenna radiation Q as a function of the antenna size and gain. Many of the derivations assume an enclosing sphere and deal only with the fields external to this sphere. As a result, the limits tend to be overly optimistic, and realizable antennas generally have Q's considerably higher than the minimum values predicted by these theories. This paper defines stricter limits that apply to a class of antennas (or scatterers) consisting of any arrangement of conductors on a spherical surface. Energy stored within the sphere is included in the analysis. The minimum Q values are as much as three times the values based only on external fields. Recently published independent experimental data on electrically small antennas in this class reported Q values only slightly higher than the new limits provided in this paper.