Dear LF,

 

inspired by the very fruitful narrowband weak-signal detections on VLF, I have set up a temporary LF spectrogram, using Rubidium locked oscillators and a 0.438 mHz FFT bandwidth
 http://www.alice-dsl.net/df6nm/grabber/TAnarrow.jpg

(updated every two hours).

 

Frequency:
The receive band is currently 7 Hz wide around 137777.5 Hz, within which the visible display slots can be shifted around a posteriori on request. At the moment I'm showing two slots around European Loran lines
 137778.3705 Hz (GRI 7499 from Sylt and Lessay),
 137775.0000 (GRI 8000 from Slonim and Simferopol).
I chose the TA slot for the availability of other grabbers, but maybe a dedicated ultra-narrowband slot (eg. 10 MHz/73 = 136986.301 Hz) would be a better option - suggestions welcome!

 

Sensitivity:
Compared to my own 21 mHz TA grabber, the bandwidth reduction should provide a 17 dB reduction of noise, or 11 dB versus Argo-120 at 5.3 mHz. Of course you will need to transmit a much longer dash (eg. one hour instead of one minute) with good frequency stability. A GPS- or Rubidium-derived carrier would be the best option, but a good 1e-9 class OCXO would work just as well.

 

Groundwave:
In quiet daytime conditions, a radiated power of only 10 nanowatts should be able produce 10 dB SNR. This could be achieved by sending 5.5 mA up a whip at 1 m effective height! Or perhaps even from Lubos' ferrite antenna in TX mode?

 

Skywave:
During a few nights in Februrary 2006, John W1TAG transmitted GPS controlled straight and 15-minute phase-cycled carriers. Receptions in Europe by G3PLX and myself revealed long periods of stable skywaves, well suited for sub-millihertz analysis. Some details and spectrograms can be found in
http://df6nm.bplaced.net/LF/slow_psk/

 

Now I hope to see some really feeble signals soon...

 

Best 73,
Markus (DF6NM)