A portable VLF preamplifier

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. The portable system is useful for tracking down sources of VLF interference and for performing site surveys when installing a fixed VLF antenna.

Last update Oct 2018: Added link to Spectroid software by Carl Reinke

Software Spectrogram apps

Suggested software to use:

iOS: SpectrumView by Oxford Wave Research.

Android: Spectroid by Carl Reinke
Android: SpectralPro Analyzer by RadSoft

Windows: Audacity by the Audacity Team

01 Sep 2018

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.

06 Nov 2016

CAD files

Eagle CAD files for the preamplifier schematic and PCB can be downloaded from here.

Assembled PCB

05 Nov 2016


The PCB is designed to fit a Hammond 1455D601 enclosure.

Here is a drill guide for the enclosure end panels: mobileEzPanels.odg

05 Nov 2016

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

04 Nov 2016

Example configuration

Camera tripod mounting

The preamplifier can be hand held or use a camera tripod for longer recording sessions.

01 Nov 2016