Admittance correction factor for coaxially excited cylindrical antennas

A long standing problem in antenna theory has been the accurate determination of the input admittance of coaxially excited antennas. While reasonably accurate results have been obtained for a range of problems using the so called "TEM approximation" for the coaxial aperture electric field, these results nonetheless contain an error of small but unknown extent. By examining the infinite coaxially excited antenna problem this approximation error has been quantified and a simple approximation factor has been deduced. This correction factor is applicable to a range of antenna problems having a coaxial source geometry.