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Two orthogonal horizontal components (north-south and east-west) of the VLF/ELF wave
magnetic field are received by two vertical single-turn 56 m2 loop
antennae, and amplified by twin low-noise preamplifiers. The frequency range
is 0.1-25 kHz. The aerials and preamplifiers are located sufficiently
distant (1.8 km) from Halley station (see photo), that locally generated electromagnetic
noise is negligible over the whole frequency range. Sensitivity
(typically 10-31 T2 Hz-1 at 5 kHz,
equivalent to a wave power flux of 2.4 × 10-17 W m-2
Hz-1) is normally limited by global thunderstorm noise (radio
atmospherics) rather than receiver system noise. Calibration is achieved by
simulating a signal of known intensity, frequency, and arrival direction,
through control of the amplitude and phase of a.c. currents flowing in two small
orthogonal coils mounted at the centre of the loop aerials. Short (1 s) 5-frequency
calibration tones are injected once a minute, with longer tones on the 10 minutes past
the hour (3 s) and on the hour (10 s). The broadband signals from the preamplifiers
are inputs to the specialised receivers described below. For more information
see Smith (1995).
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