SAQ Transmission Reception Reports at the W2NAF-KC3EEY VLF Observatory Site

(12/24/2021) Christmas Eve 2021:

Merry Christmas All!

SAQ at Grimeton, Sweden transmitted their annual Christmas Eve message
on 17.2 kHz CW using the Alexanderson alternator.

Recently, Nathaniel and I finished installing conduit and cable for a
VLF SDR and the PSWS ground magnetometer. We installed the VLF SDR and
active VLF antenna as well as the ground magnetometer and got both up
and running.

For many years, I wanted to be able to receive the SAQ Christmas Eve
transmission, but here in the US, it can be difficult. The
transmission was scheduled to occur at around 3AM EST or 8 UT. My VLF
SDR was recording the VLF band this snowy morning and captured the
spectrum. Unfortunately, the propagation conditions weren't the best,
but I was able to hear it, but just barely, and I mean just barely!
Here is a link to the mp3 file:
https://drive.google.com/file/d/1DhiKg9azAJzBZkv6rqFCXZQh1VZnhiSy/view?usp=sharing

Only at 14:40 into the recording can you just barely make out the 500
Hz downmixed carrier. I can maybe copy a few characters. If anyone is
interested how I created the file, here is a long pipe using
vlfrx-tools to read data from the data store, bandpass filter at 17.2
kHz center with 1 kHz bandwidth. Comments are added for description:

vtread -T 2021-12-24_07:41:30,+24m /data/vlf_96k | # read from data store starting at 12/24/2021 7:41:30 UT for a duration of 24 minutes.
   vtfilter -h bp,f=17200,w=1000 -g50 | # Insert a bandpass filter with fc=17200 Hz, 1000 Hz wide with a gain of 50.
   vtmult -f 16700 | # downmix carrier to 500 Hz using 16700 Hz local oscillator
   vtmix -c 1,-j | # select the upper sideband
   vtraw -ow | # convert to wav
   lame -b64 -m m - SAQ122421.mp3 # encode pipe output to mp3

vtfilter’s bandpass filter is a brick wall filter and when I tried to filter the spectrum further, I didn’t get a good enough result. The bandpass filter that sox uses is a two-pole Butterworth filter and worked much better! I used the following:

sox -v 20 SAQ122421.wav SAQ122421_20Hz.wav bandpass 493 20

and encoded to mp3. The file can be downloaded at https://drive.google.com/file/d/1KgLsOCMnkfhC5yBDN0bz8N0WOLn5uXZa/view

vlfrx-tools has a a few spectrum analyzing utilities and I was able to look at the non-coherent spectrum integrated over the duration of the transmission. It generated a plot with a little peak about 3 magnitudes above the noise floor. Plot below: