An experimental study of the insulated dipole antenna immersed in a conducting medium

The driving-point admittance and the amplitude and phase distributions of the current referred to the driving-point were measured for an insulated cylindrical antenna immersed in a conducting medium. The ratio
\sigma/\omega\epsilon_{r}\epsilon_{0}
of the conducting medium was varied from
\sigma/\omega\epsilon_{r}\epsilon_{0}= 0.036
to 8.8, a range which includes a variety of media such as poor insulators, the ionosphere, plasmas, dry earth, wet earth, lake water and sea water. The antenna height
\beta h
in radians was varied from
\beta h=0.1
through
\beta h=2\pi
at intervals of 0.1. The thickness of the insulator was varied from
b/a = 1.25
to
b/a = 12.0
where
a
is the radius of the antenna and
b
the radius of the insulator. Measurements have also been made of the admittance, current and phase distributions along an insulated antenna with a conductive top load, that is, one whose tip is in direct contact with the conducting medium. It is found that when the tip of the antenna is in direct contact with the conducting medium, the current increases almost linearly as the end of the antenna is approached. This is quite unlike the decaying sinusoidal distribution on the completely insulated antenna. In a general sense, the experimental results are in fair agreement with an approximate theoretical expression for the admittance of an insulated antenna immersed in a highly conducting solution. The approximate theory is based on the driving-point admittance of a coaxial line whose outer conductor is imperfectly conducting and infinite in extent.