Terrestrial antenna for high power VLF radiation into the magnetosphere

The radiated power limitations for VLF horizontal half-wave dipoles located above the Earth have been examined to determine the feasibility of using such a source to investigate wave-particle interactions in the magnetosphere. It is found that as the antenna approaches ground the maximum radiated power limits for the dipole are significantly reduced over the value for free space. The reduction is a strong function of the conductivity being less for lower conductivity. The primary reason for the reduction is lowered radiation resistance. A secondary reason is the reduction in resonant frequency from dielectric loading which increases the charge density on the antenna by the ratio of frequency reduction squared. The radiation efficiency is also greatly reduced by the presence of the Earth, however this does not affect the radiated power limit but does increases the transmitter power required. For a practical system it will be important to find a location with low ground conductivity.