Listening in on Very
Low Frequencies
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Since I wrote this I've
bought an SDR which is better than the soundcard method described
below.
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Now that I've almost finished
refurbishing my R206 I remembered my low frequency adaptor, now
long gone, cannibalised for an HF transmitter. In those days
the bands were nice and quiet without deaded "digital revolution",
switching power supplies, broadband and mains-bourne networking
interference, and several very long wave transmissions could
be heard. Wondering what those low bands sounded like today I
thought about building a converter but when I looked on the Internet
for ideas I saw that many people use their computer's soundcard
as a VLF receiver.
Intrigued, I looked for the
requisite software and initially found some which after installing
left my brain numb. Not to be defeated so easily, I tried a package
called "SAQ Panoramic VLF Receiver" and installed that.
It's ever so much more user friendly and after a few minutes
I could hear some transmissions. My aerial is a long wire which
is threaded through various trees and connects to a phono plug
at the end of a lead plugged into the computers microphone jack
socket (in doing this I'm aware of the potential damage to the
sound chips from high levels of static).
The picture below shows a green
trace for the radio spectrum from zero to 96KHz and, inset on
the left, the audio spectrum in blue. This latter extends from
about 0.2 to 1.2 on the AF scale and represents the CW bandwidth.
Switching to SSB and AM stretches this out. The peak on the left
of the audio is the bleep from the timing signal selected by
the cursor on 60KHz. There isn't a tuning knob alas because tuning
is by cursor keys or mouse.
The hardware used for the VLF
receiver is the Realtek sound chip on my Asus motherboard. Although
this is intended for audio, the fact that it's a digital processing
circuit, making use of a sampling technique, means that one can
use it for processing RF signals. The first point to consider
is the sampling technique. A typical sound card or built-in sound
handling hardware will now have a selectable sampling rate. Mathematical
theory predicts that, given a fair wind, one can reconstruct
a complex waveform given a sampling rate of just double the maximum
frequency in the waveform. You can see below that the frequency
coverage provided by my sound chips is 96KHz because the sample
rate I selected was 192KHZ. In practice I guess there's a compromise
between fidelity (the reproduction of precisely what is fed in)
and the errors met in working close to the sampling limit, for
example in reducing processing time, but listening to the results
shows that my sound chips seem to make excellent RF chips. Of
course, all credit for this excellent VLF receiver must go to
the original software designer who I understand is Johan Bodin,
SM6LKM. |
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The strongest signal is
that at 16KHz but, tuning upwards in frequency, I found the 60KHz
signal used for radio clocks. With all the hash, burbling noises
and buzz saws you must listen out for clean, steady carriers
carrying identifiable modulation or having regular pulsing to
identify proper radio transmissions although some of the weird
noises may well be emanating from aerials because the ELF and
VLF bands are used for communicating with submariners and providing
direction-finding information.
Results can be very variable
and, to hear signals most cleanly, I set my microphone input
to maximum volume and used the maximum boost of +30dB then selected
the best sampling rate of 192,000Hz (Studio Quality) which lets
me tune to about 96KHz. At this setting and with the software
AF gain at -6dB, selecting L (I'm using the left channel for
my aerial), the amplitude of the transmission is indicated as
-40dB. The audio level baseline is -80dB with a 600Hz tone rising
to about -50dB. The mode setting is CW, the tuning cursor carrier
reads 60,390Hz and the VFO 59,640 Hz. There are loads of signals
across the band but I guess many are just squeals and groans
from local equipment, lighting, and other stuff. I'll investigate
later maybe... below is a list of what I could hear. I wonder
if there are any low frequency crystals in my computer, radio
clocks, telephones, printer etc etc ? For example most digital
watches and clocks use 32.768KHz.
Don't pay too much attention
to the frequency I've entered as my tuning was only approximate.
To identify some signals I might connect a small pickup coil
to the Mic input and investigate some items around here...
Later, around midnight rather
than daylight when propagation is different I checked and found
at least some of the wobbly carriers may have been teletype and
a weak carrier about 64.4KHz had 1 second pulses present. |
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FREQUENCY KHz |
SIGNAL |
SOURCE |
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95.5 |
S3 rapidly fading in and out |
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80.6 |
S2 carrier |
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79 |
S7 buzz |
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77.2 |
S5 1 second pulses |
DCF77 Germany |
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69.4 |
S2 carrier |
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66.1 |
S8 buzz |
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64 |
S2 carrier |
1 sec pulses |
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62.6 |
S8 buzz |
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60.1 |
S7 1 second pulses |
MSF UK |
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57.1 |
S2 carrier |
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52.2 |
S2 carrier |
pos TTY |
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37.1 |
S3 wobbly carrier |
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31.8 |
S5 buzz |
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24.1 |
S4 buzz |
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22.5 |
S3 wobbly carrier |
pos TTY |
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21.3 |
S3 wobbly carrier |
pos TTY |
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20 |
S2 carrier |
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16.4 |
S9 buzz |
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Since I experimented with
reception via the computer sound card, I've purchased several
"Software Defined Radio" receivers. One of these is
the Andrus SDRMk1.5 made in Estonia. This receiver covers the
frequency range from virtually zero to around 30MHz and is ideal
for listening to VLF because it can't be overloaded by local
VHF broadcasts. I've logged and identified a number of those
strange teletype transmissions listed above, although I'm not
too sure about the 24KHz station as it seems too far away to
hear during daylight hours. My location is in the New Forest
close to the south coast of the UK and the aerial I used is a
long wire of a few hundred feet. |
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I made a recording of
GZQ which sounds virtually identical to the others. click the
picture above to hear it. |
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Later, I'll add some pictures of reception across
other frequency ranges... |
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