Admittance Dependence of the Infinite Cylindrical Antenna Upon Exciting Gap Thickness

The admittance of an infinite cylindrical antenna is calculated as a function of exciting frequency and exciting gap thickness. The results obtained show that an asymptotic analytical expression derived for the susceptance by Chen and Keller and also later by Fante, is strictly valid only in the limit of "Vanishing gap thickness. An altenate analYtical. form is presented-for die susceptance, which removes this restriction and which gives good agreement with the correct susceptance values obtained in this study. 1. lntrodudion The admittance of a cylindrical antenna of finite or infinite length, fed by a circumferential voltage gap, is a subject which has received a great deal of atten· tion. A solution for the antenna current can be exactly obtained only for the infinite antenna, the finite antenna current being determined by an integral equation. The reason for this is the reflected current waves. which are set up by the ends of the finite antenna, but which are absent from the infinite antenna. While the current, and thus the admittance, of the infinite and finite antennas may be very different, they do have in common the feedpoint singularity associated with an infinitesimal (delta function) exciting gap in the ou~ward traveling current wave. As a result, the change m antenna susceptance that results from. changing the gap thickness might be expected to be similar for both antennas. Whether or not the· depend­ ence of the susceptance upon gap thickness is sig­ nificant will depend upon the relative importance of other determining parameters such as antenna radius, and in addition for the finite antenna, the antenna length. The purpose of the work, which is discussed here, is to investigate, using a numerical approach, the admittance variation of the infinite antenna as a function of the gap thickness, as well as the excitation frequency. It will be demonstrated from some numerical results to be presented that a condition for the asymptotic form derived for the antenna susceptance by Chen and Keller (1962) and also subsequently, by Fante (l966a), to be valid is that a~ 0 rather than k8 ~ l,. where k is the wave number and 8 the gap thickness. In addition, the numerical analysis here is not re­ stricted to finding an average gap current in the