On Small Dipole Antennas in Lossy Half Space and Undersea Propagation

Based on vector wave functions, we systematically studied electric(and magnetic) dipole generated by an electric current line(and loop) source as the lowest radiation modes. While far-zone
$\vert \frac{E}{H}\vert$
of magnetic- and electric- dipole are identical, their near-zone behaviors are drastically different. For electrically small dipole of normalized half dipole-length
$\vert ka\vert \ll 1$
, analytic results show that the radiation efficiency
$\eta_{r}$
is proportional to
$\vert ka\vert ^{3}$
for electric dipole, and
$\vert ka\vert$
for magnetic dipole, respectively. Consequently,
$\eta_{r}$
of magnetic- can be two to three orders of magnitude better than that of electric- Hertz dipole for under seawater communication. The propagation of EM waves in shallow water is mainly in the up-and-over path, in which the Zenneck surface wave plays important role. In this paper, surface wave fields are provided; design and tuning of low-profile magnetic dipole antenna are outlined.