Theoretical study of a cylindrical antenna with a hemispherical cap

The integral equation of a cylindrical monopole antenna with a hemispherical cap driven by a coaxial line is formulated and solved by two approximations. First, the current on the cap is approximated by a sinusoidal distribution. Secondly, the field across the driven gap is approximated by the fundamental transverse electromagnetic (TEM) coaxial-line mode. Results are compared with the tubular model. It is shown that the input admittance curve of the capped antenna has the same shape as that of the tubular antenna with open ends but is shifted. The radiation patterns of the two antennas are essentially the same. The capped antenna is approximately equivalent to the tubular antenna with length 0.25 times the radius shorter than the total axial length of the antenna with a cap.