THE United States Navy operates antennas on the Very Low
and Low Frequency (LF) bands (10 to 300 kHz) for long range
communication. These antennas operate at voltages above 200
kV and in a variety of locations ranging from tropical to frigid
climates. Consequently, there is interest in their performance
under a wide variety of environmental conditions. The interest
stems from the fact that insulation equipment that was designed
and tested at power frequency (50/60 Hz) did not operate as
expected when used at VLF/LF [1, 2]. In several cases
catastrophic failure of insulated support structures due to the arc
produced by the external breakdown of the device have occurred
[1]. To improve equipment design and reliability, there is a need
to understand the breakdown processes involved.
There is a great amount of work available in the literature for
breakdown and corona in uniform field gaps at power frequency
(50/60 Hz) and dc. The general consensus is that the breakdown
voltage increases with humidity [3, 4]. With an increase in the
humidity, electron attachment increases. It also causes reduction
on secondary emissions due to the absorption of photons by
water molecules. Both of these factors increase the breakdown
voltage with humidity [5]. In contrast, there is very little work
reported on the breakdown characteristics at VLF/LF [1, 2].
Corona is a serious problem for high power VLF/LF antennas
for several reasons. Power losses originating from corona