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.
Eagle CAD files for the preamplifier schematic and PCB can be downloaded from here.
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.
Suggested software to use to display a spectrogram of the real time VLF radio signal
iOS: SpectrumView by Oxford Wave Research.
Windows: Audacity by the Audacity Team
Camera tripod mounting
The preamplifier can be hand held or use a camera tripod for longer recording sessions.