VLF Beacon

From Smithnet

Contents 1 Overview 1.1 System Description 1.2 Purpose and Design Principles 1.3 Receiver 2 Exciter Hardware 3 Exciter Software 4 Transmitter 5 Aerial System

Overview

System Description

This aim of this project was to build a VLF/LF transmitter system that would repeatedly send a message in Morse code using OOK or FSK. The message would be constructed partly by a fixed, stored, message (programmed by an RS232 serial line) and partly by externally sampled analogue and digital data.

The system has the following features:

• Message programming and configuration by RS232 (9600 bps, 1 stop bit, no parity, hardware flow control)
• Stored message of 0 to 255 bytes
• 8 bit sampled analogue voltage (0-5 V)
• 4 bit sampled digital word
• 32 bit iteration counter
• User controllable speed
• User controllable message iteration delay
• Very slow speed morse mode
• Programmable frequency synthesiser (10.0 kHz to 99.9 kHz in 100 Hz step)
• Frequency stability at least 0.01 %

Purpose and Design Principles

The project had the aims:

• In order to experiment with PIC Microcontrollers
• In order to play with some real electronics
• In order to experiment with receiving transmissions under the noise floor using signal averaging techniques
• Design with as many parts as possible that I already had in my spare parts box
• To be sufficiently simple that I could design from scratch
• To be sufficiently complex to be interesting and perform well

Receiver

A receiver was not part of this project, instead a PC audio card was used, see

• PC Soundcard

Exciter Hardware

A midrange Microchip PIC microcontroller is used, the 16F819.

Inexpensive CMOS 4000 series logic is used for oscillators, counter/dividers and other glue logic.

For the main article see VLF Beacon Exciter Hardware

Exciter Software

Current Version: 0.8.2

The source code in PIC assembler, suitable for building in the Microchip MPLab can be obtained here. The resultant hex dump suitable for device programming can be obtained here. Many programmers can be used; I used a parallel port programmer with PICALLW software.

For the main article see VLF Beacon Exciter Software

Transmitter

This has yet to be designed, but I plan something with a dual MOSFET output (about 4 or 5 A at 16 V) into tuned output to drive the low impedance loop.

For the main article see VLF Beacon Transmitter

Aerial System

A smallweb design atlanta gawas chosen, due to its compact size. A small loop (diameter << wavelength) will have a transmit/receive pattern that is maximal perpendicular to the plane of the loop. A loop has a natural resonant frequency given its inductance L and stray capacitance C but typically is operated at a lower frequency by the addition of a parallel tuning capacitance, such that:

If the required operating frequency is greater than the resonant frequency, one can:

• Decrease the number of turns (decreases inductance)
• Separate the turns (decreases inter-loop capacitance)

The sensitivity of a receiving loop can be increased by increasing the number of turns, or increasing the diameter. However, the noise generated in the loop improves with the area and is not affected by the number of turns. To ensure a receiving loop's inherent noise < atmospheric noise, the diameter should be at least 1 metre.

The Q of the loop can be increased by using heave gauge wire (reducing resistance).

For the main article see VLF Beacon Aerial