The situation in which the internal conductor of a coaxial transmission line projects through a ground‐plane without any change in diameter is analyzed with reference to the infinite antenna and the parallel‐plate antenna. An experimentally meaningful value of the admittance which takes into account the geometry of the antenna‐to‐coaxial line junction is obtained. This is achieved by utilizing a mathematical model in which an annular aperture or “gap” is represented by a frill of of magnetic current. The field distribution in the aperture is assumed to be of the same form as the TEM mode alone in the coaxial line. It is shown that the effect on the admittance of neglecting the TM modes in the line at the aperture, is very small. In the case of the parallel‐plate antenna, the theoretical results are shown to agree remarkably well with experiment. It is also shown how the theory may be applied to the problem of finite length cylindrical antennas.

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The representation of the source of the electromagnetic fields for a thin-walled tubular trans· mitting antenna is examined. Application of a standard formula for the electromagnetic field leads to the concept of a magnetic current source. With this type of source, the correct integral equation for the tubular antenna is easily derived because the standard vector potential method (with an extra term for the potential due to the magnetic current) can be used. The method is easily applied to other excitation geometries.

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