VLF: Opds-4H detections in Todmorden

["Markus Vester" <markusvester@aol.com>]

Most who are interested in VLF will be aware of the excellent work of Paul Nicholson, who is running a very sensitive dual-loop receiver for VLF and natural radio in Todmorden UK. He is also actively interested in monitoring man-made VLF signals, and has provided outstanding reception reports on several low-power transmit experiments.

 

With the recent upsurge of trying to put Opera mode to work at VLF, Paul has kindly provided spectrum data during announced Opera-4H transmissions from DJ8WX and myself. In Todmorden, signals from the two loops are continuously being digitized along with GPS time stamps. The loops were combined in software for maximum SNR from the expected direction, and noise blanking was applied. Then relevant five-hour chunks were Fourier transformed (0.12 Hz around 8970 Hz, 54.936 µHz bin width) and emailed to me. Here the files were fed to unmodified Opds correlation software, using a search list containing nine callsigns from potential VLF transmit stations http://dl.dropboxusercontent.com/u/26404526/callsloc_vlf.txt. This process has resulted in four unambiguous hits:

 

Receiver: Paul Nicholson  IO83XQ   Opds-4H   8969.94 - 8972.06 Hz
   date      time   call   distance  frequency bandwidth  snr    correlation
2014-01-02 06:14:01 DJ8WX    760km   8969.997Hz   0mHz -54.1dBOp 100% 17.2dB
2014-01-02 01:50:49 DJ8WX    760km   8969.997Hz   0mHz -55.0dBOp 100% 18.0dB
2014-01-01 16:36:38 DJ8WX    760km   8969.990Hz   0mHz -53.6dBOp 100% 16.4dB
2014-01-01 14:22:32 DF6NM   1026km   8970.000Hz   0mHz -59.1dBOp 100% 16.3dB

 

For comparison, my first two Op-4H transmissions on Dec 31st had been successfully detected by Stefan in Heidelberg, but have not produced sufficient SNR in Todmorden at the time.

 

Receiver: DK7FC JN49IK VLF OPDS-4H correlation  Range: 8969.87 ... 8970.23 Hz
   date      time   call   distance  frequency bandwidth  snr    correlation
2013-12-31 22:30:04 DF6NM    175km   8970.000Hz   0mHz -60.3dBOp  82% 14.2dB
2013-12-31 18:07:58 DF6NM    175km   8970.000Hz   0mHz -58.3dBOp 100% 17.0dB

 

Today we were able to also extract transmissions on Uwe's new frequency 8270 Hz, although the SNR seemed not as good as before:

 

Receiver: Paul Nicholson  IO83XQ   Opds-4H   8269.94 - 8272.06 Hz
   date      time   call   distance  frequency bandwidth  snr    correlation
2014-01-05 07:18:54 DJ8WX    760km   8270.000Hz   0mHz -57.2dBOp  97% 15.1dB
2014-01-05 02:55:38 DJ8WX    760km   8270.000Hz   0mHz -56.1dBOp 100% 14.4dB
2014-01-04 22:32:14 DJ8WX    760km   8270.000Hz   0mHz -57.6dBOp 100% 12.8dB

 

Currently Uwe has another transmission ongoing, and the carrier is visible on Paul's spectrogram http://abelian.org/vlf/fbins2.shtml#b=080&s=sp
 
Many thanks to Paul, Uwe, Stefan, and all who have contributed to this fine success!

 

Best 73,
Markus (DF6NM)