Re: Microhertzing at LF:

[Chris 4X1RF <chrisgomy@gmail.com>]

Markus, LF,

As a newcomer to LF I find this as an interesting experiment for real low power or long distance tests for those who can achieve the necessary frequency stability requirements. I found this to be quite a challenge...

I have followed your initiative and started similar windows on my grabbers in YO: http://dl.dropbox.com/u/10280270/YO/TA3600.jpg and 4X: http://dl.dropbox.com/u/10280270/4X/TA3600.jpg

The 4X grabber is currently using the Mercury HPSDR receiver (http://openhpsdr.org) which is GPSDO locked. However, the audio is routed to SpectrumLab using Virtual Audio Cable as a virtual sound card. I have no info regarding VAC sample rate accuracy or how I can automatically adjust it so this may be a limiting factor, further experimentation is needed. In the worst case, a real sound card could be used but then it also has to be locked to GPS.

The YO grabber is usually using GQD as a reference for the sound card sample rate calibration in SpectrumLab and the LF receiver is based on a laboratory grade VTCXO. GQD being off air, I was forced to use DHO which is not very reliable during the night for me as can be seen on the 137.775 Loran line.

Thanks for the challenge!


73s
Chris 4X1RF



 

On Wed, Aug 31, 2011 at 8:05 PM, Markus Vester <markusvester@aol.com> wrote:

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)