Spectrum Lab for VLF reception
Spectrum Lab is a powerful software tool for analyzing, charting and logging data of radio
signals. Here, we have described the use of this software for processing very low frequency
(VLF: 3-30 kHz) signals by connecting a VLF receiver to soundcard of desktop. A loop antenna
is connected with the amplifier. VLF signals can also be received by directly connecting antenna
to soundcard. VLF has both natural and manmade origins e.g. natural origins of signals are radio
atmospherics like whistler, chorus, tweaks and the background Schumann resonances. Whereas
manmade signals are propagated by VLF transmitters install worldwide by different nations for
communication and navigation purposes. The figure below shows VLF signals using Spectrum
Lab at Sonmiani (25.20 °N, 66.75 °E). The Spectrum Graph window shows signal amplitude vs.
frequency. Frequency span is adjustable and different VLF transmitters are positioned on
frequency scale. If there is any signal it would have a peak above noise floor. The Waterfall
Display shows time development of received signal in different color schemes. Since VLF
signals have bandwidth of about 100 Hz from center frequency therefore appears as vertical strip
directly below indicated VLF station. Four stations named DHO (23.4 kHz), NWC (19.8 kHz),
VTX1 (18.0 kHz) and UNID (26.0 kHz) can be seen clearly. Sharp vertical or horizontal lines
are because of surrounding electrical noise.
Spectrum
Graph
Electrical
interference
Waterfall
Display
NWC signal
Fig. 1: Spectrum Lab view as VLF receiver connected with antenna
is plugged into soundcard of Desktop.