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Table of Contents
Introduction
Although there are no ham bands for VLF, there are VLF transmissions that hams can receive. This page is about how to receive the transmissions from the Alexanderson transmitter at Grimeton in Sweden, sent on Alexanderson Day (2nd July) and Christmas Eve (24 December) every year.
Hardware
You will need
- an antenna
- a soundcard
- a computer.
Antenna
Unless you have a garden that is 8.7km (>5 miles) long, don't try to set up a dipole. You will have a short antenna, and tuning is not really needed. You won't be transmitting, and they'll be transmitting 200kW.
If you have a HF antenna, that will probably work best. I used my 160m skyloop.
If you don't have a HF antenna, string out a long wire – as long as you can.
The other end goes into the soundcard. I made up a patch wire to connect a 3.5mm jack to a UHF screw connector. The patch lead was ugly, but this is audio frequency.
Soundcard and Computer
The connection should probably go into the microphone input of the soundcard or, failing that, the line in. Soundcards with better resolution (e.g. 16 bits) will be more sensitive.
Software
I used GNURadio Companion to produce a SDR using soundcard input. The GRC file will be attached as soon as I can figure out how to do that.
The SDR is a simple direct conversion receiver. The sample rate is set to 48ksamples/second which should give a range of 0-24kHz. If your soundcard supports higher sample rates, by all means change the samp_rate value – but the 48ksamples/second rate is plenty for the Grimeton transmissions on 17.2kHz.
Because the receiver does not use IQ values or any kind of bandpass filter, there is potentially aliasing on the input. So the default VFO is at 18kHz which means it receives 17kHz to 18kHz, and Grimeton will produce an audible tone at 800Hz. But, because there is no discrimination between USB and LSB, it also receives 18kHz-19kHz. If you have nearby interference above 17kHz, tune down to below 17.2kHz and the interference above will be filtered out.
If, like me, your Morse is sketchy, the output of GNURadio can be piped to a Morse decoder like FLDigi.