A three-axial tuned solenoid coil antenna with a diameter of
about 45 cm and 7000 turns, was set up about 2m high for the
horizontal magnetic flux and 30 cm above the ground for the
vertical flux. The observation frequency selected was 223Hz
in the ELF (extremely low frequency) band of 30–300Hz.
The reason for this was that unstable, misleading noise becomes
extremely low in the ELF band compared with the
ULF band of less than several tens of Hz. The principal noise
originated in the amplitude spectrum for geomagnetic variations
resulting from the magnetosphere and ionosphere. In
the ELF band, atmospherics are the principal noise with an
amplitude of the order of pT per root Hz, and for the ULF
band, pulsations are dominant at several tens of nT per root
Hz (Rokityanski, 1982). Furthermore, antenna moving noise
produced by cutting geomagnetism due to wind vibration of
the antenna coil becomes quite low in the ELF band compared
with the ULF band. The vibration frequency of the
antenna supported in the vertical direction is generally in the
range of 6–15Hz. For example, suppose the antenna at rest
causes a d-inch deflection of its supporting springs; the natural
oscillation frequency f o of the antenna, when damping
is neglected, is given by f o = 3.13/root d. For d = 0.25-
inch, then f o = 6.26 Hz, which is in the ULF band (Reference
Data 939, 1956). In addition, the receiver fluctuating
noise power of the top amplifier is inversely proportional
to the frequency (1/f noise) at the low frequency bands of
ELF and ULF. Therefore, the receiver noise is much more
decreased in the ELF band than in the ULF band (Reference
Data 496, 1956). On the other hand, at higher frequencies
exceeding 1000 Hz in the VLF (very low frequency) band
of 300 Hz to 3 kHz, lightning and manmade noise increase
severely, so much so that the source is more liable to radiate
at shorter wavelengths. Furthermore, in order to decrease the
manmade noise, the observation frequency was set at 223Hz
in the ELF band, which was the prime commercial power line
frequency of 50Hz and 60 Hz, so as to eliminate high-order
interaction noise produced by the nonlinearity of the power
systems. The selected bandwidth of the receiver was narrow
at about 1 Hz to remove the noise spectrum.
Since the wavelength of 223 Hz is about 1300 km, the
propagated noise components of that frequency hardly exist
inside the narrow parallel plate waveguide, consisting of the
conductive ground and the ionosphere to a height of 60 km
to 250 km, thus forming D and E layers.
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