This portable VLF preamplifier will plug into a phone, tablet or laptop. Software can display a live spectrogram of the signal or record it for later analysis. A portable system is useful for tracking down sources of VLF interference and for performing site surveys when installing a fixed VLF antenna.
VLF amplifier schematic
The amplifier consists of a VLF bandpass filter followed by a single stage JFET common source amplifier. Power comes from the microphone bias voltage output of the phone or tablet device.
CAD files
Eagle CAD files for the preamplifier schematic and PCB can be downloaded from here.
Enclosure
The PCB is designed to fit a Hammond 1455D601 enclosure.
Here is a drill guide for the enclosure end panels:
Antenna and Cabling
For the antenna element use a BK Precision AT 21 Right Angle Telescoping Antenna with BNC Connector.
Cables are needed to connect the preamp to the headset audio jack. The easiest way to do this is with a 4 pin headset splitter cable and a stereo or mono 3.5 mm audio patch cable.
Startech 3.5mm to 2x 3 pin 3.5 mm headset splitter
Startech 6 ft 3.5mm Stereo Audio Cable
Software
Suggested software to use to display a spectrogram of the real time VLF radio signal
iOS: SpectrumView by Oxford Wave Research.
Android: Spectroid by Carl Reinke (Recommended for Android users!)
Android: SpectralPro Analyzer by RadSoft
Windows: Audacity by the Audacity Team
Example configurations
Camera tripod mounting
The preamplifier can be hand held or use a camera tripod for longer recording sessions. The 4 mm banana socket and cable can be used to earth the preamplifier to the nearby ground for possible improved signal reception.
