THE WOBBLER
A Mystery Radio Signal
Investigations
This page last updated July 13, 2005
Wobbler
on 930 KHz
I have seen several posts online about a
strange signal most recently on 930
KHz.
Described variously, I liked the term Wobbler,
which is how it sounds to me. Also offered have been some
suggestions of what it might be, such as some sort of jammer. I have
heard
it on a table radio and thought it sounded more like some combination
of
atmospherics and mixing signals, but that was just my opinion. Tonight
it was very prominent so I fired up the big set (NRD-535D) to
hear what I could hear from my location in southeast Florida.
1100-0300 EST -
Dec. 16-17, 2004.
I made MP3 recordings for several hours playing all the
while with the receiver signal controls. I hit upon a combination of
USB,
narrow filter, and tuning down to 929.7 KHz, which emphasized the
signal.
With an adjustment of the passband control I was able to eliminate a
nasty
heterodyne. When I kicked in a DSP audio processor (GAP) and cranked
that
processor way down, I got the most amazing results. I added a clean
signal
- from a second receiver - on the other stereo channel of my recording
so I
could switch back and forth between the normal radio signal, and the
filtered and processed signal. This created an even more interesting
result
for evaluation purposes.
The four hours of recordings are too big to share, and besides, only
the
last half-hour incorporates all of the signal manipulations that I
discovered. Still that is nearly 9 MB in size. However, I picked off
some
quite demonstrative samples which are only 10-30 KB, MP3 files. Anyone
interested
in hearing these and drawing their own conclusions are invited to
access those files:
Note: To best evaluate these recordings you'll want to use the
Windows audio
mixer controls - or your equivalent - to
play the left and right channels independently. The Left channel is the processed
signal, the Right channel is
the signal as received.
Here is the JPG image of an FFT spectrogram of a strong,
unprocessed example of the Wobbler signal and its MP3 companion file
for comparison purposes with
other signals investigated as possible Wobbler sources.
Unprocessed Wobbler Sample MP3 file
(44K)
I have also included a JPG image of an FFT
spectrum analysis of a
portion of the Wobbler 7-5 processed signal recording. You
can read about how the signal is proccessed here.
This plot has the following characteristics:
- Sample resolution: 16 bit
- Sample type: Monophonic
- Sampling rate: 8000 Hz
- Frequency scale: Linear (vs logarithmic)
- FFT size (points): 1024
- Frequency resolution: 7.8 Hz
- Band: 100-4000 Hz
- Time scale: 6 msec
- Spectrum average: 1
From this analysis the
range of the tone appears to be from a few cycles to about 1 KHz.
The rate of change of the tones averages about 5 cps, producing the
warbling nature of the signal.
After looking at this analysis and seeing the symmetry of the signal, I
am less confident of my original opinion of its natural
origins. I would like hearing the opinions of others.
Investigations into this phenomenon continues. New
Wobbler results will be posted here as they are obtained.
©
W. Curt Deegan, 2005-2006
