VLF Radio Receiver Hardware

This section contains information about VLF radio receiver hardware designs suitable for use with the UltraMSK software.

16 May 2015

System Overview

A typical UltraMSK hardware setup will consist of 1, a VLF antenna and preamplfier, and 2, a power supply for the preamplifier and a GPS receiver with a 1PPS signal output. A computer & sound card is needed to run the UltraMSK software.

1 A VLF Electric Field Antenna System

The VLF electric field antenna system consists of the preamplifier unit, an aerial element and antenna hardware. The design details for the entire system are outlined below.

E-field VLF Preamplifier

The preamplifier contains filters, amplification and a balanced signal output suitable for driving long cable runs.

The complete schematic features:

Preamp CAD files

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

Complete preamplifier unit

The preamplifier unit must be mounted outside at the base of the antenna. A waterproof enclosure is required. The enclosure needs to have a temperature rating that matches the expected environmental range. Polycarbonate plastic enclosures are typical rated for service from -40 degC to +120 degC.

Pin out for the Amphenol Ecomate connector:

The center PE pin on the ecomate connector engages first. Use this for the 0 V GND connection. The VLF output signal is a balanced output requiring a 2 wire connection. Pins 3 & 4 can be left unconnected or used for additional ground connections. Alternatively they could be used for a secondary VLF signal output.

Pin number Signal description
1 +12 V DC input power
2 + VLF balanced signal output
3
GND or NC
4 GND or NC
5 - VLF balanced signal output
6 -12 V DC input power
PE GND

The E-field antenna hardware

The following hardware is required to construct the complete antenna system:

The aerial element for the electric field signal

The aerial element can be constructed from RG-58 coax and a BNC connector. The length is not critical. Match the length of the upper section to the length of the PVC pipe. Add on an additional length to reach the preamp. Keep the length of coax, from the BNC to the point where the outer braid is cut, as short as possible.

An alternative aerial option would be to use a commercial CB whip style antenna. Locate the VLF pre-amplifier at the base of the antenna. Connect the preamp to the antenna via the shortest length of coax practical.

Antenna installation

The antenna needs to be installed in high position such as the roof of a building or on a mast. On roof tops, the VLF electric field is usually much larger at an edge or near a corner than in the centre of a flat roof. Keep away from other electrical equipment such as heating, ventilation and air conditioning units as these can generate a lot of noise at VLF. The antenna must not be under or even close to other conductors, not even trees, which are grounded at VLF and so tend to short out the VLF electric field.

2 Preamp Power Supply and GPS receiver

This unit receives the balanced VLF radio signals from up to 2 preamplifiers via 1:1 isolation transformers. A DC/DC converter generates an isolated +/- 12 V DC power supply from the USB input power. A GPS receiver generates a 1PPS signal.

30 Jan 2015

Power supply and GPS schematic

Two modules from SparkFun electronics are used:

04 Jan 2015

Power supply CAD files

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

Cat 5 power and signal cable

For the connection from the outdoor preamplifier to the inside power supply unit, Cat 5 networking cable can be used. Use STP, outdoor, direct burial etc rated cables as appropriate for a given installation. The following wiring convention is suggested.

Brown/White: +12 V, 0V; Blue/White: -12 V, 0V; Orange/White: Primary balanced VLF signal; Green/White: Secondary balanced VLF signal

02 Jan 2015

Computer sound card connection and signal check

Connect the 1PPS signal to the first input channel and the VLF signal to the second input channel of the computer sound card.

Finally, use a sound recorder such as Audacity to check the connections and signal quality are OK.