I made some laboratory tests this morning (April 15, 2000) to get some indication about the ability to communicate with signals below noise level using Slow-CW.
I used a calibrated frequency synthesizer (Adret 2230), an 0-120 dB attenuator in 1 dB steps (Schlumberger BMD500) and my Praecitronic MV61 Selective Level Meter. With a BNC t-connector I fed the normal band noise including loran lines on 137.500 kHz (+/- 50 Hz) to one side of the t-connector, and the output of the frequency synthesizer to the other side.
With the attenuator I made sure that a 0 dBm (50 Ohm) signal with the synthesizer corresponds to a -80 dBu (75 Ohm) signal at the MV62 (plus/minus 1 dB).
The band was quite this morning, with a background noise around -110 dBu (approx. S 4) and Loran lines clearly visible.
Using the 100 Hz bandwidth of the MV62 and the cascaded 250 Hz/500 Hz CW filters of the IC-746 I checked the signal by ear as well as with the Spectrogram software with the normal parameters I use for "3-5 second-dot-length" Slow-CW (5.5k sample rate, 16bit mono, 16384 points FFT = 0.3 Hz resolution, 60 dB scale, 300 ms time scale, 10 x average) and obtained the following results:
Injected Received Comment
- 20 dBm - 100 dBu good audible CW signal (approx. S6)
- 30 dBm - 110 dBu CW signal approx. equal to background noise (S4), just can be copied
- 35 dBm - 115 dBu boundary for aural CW, signal just detectable by ear
- 45 dBm - 125 dBu good "O" signal in Slow-CW, signal same level as Loran-lines
- 50 dBm - 130 dBu still good readable Slow-CW signal "M"
- 55 dBm - 135 dBu Slow-CW just detectable "T"
- 60 dBm - 140 dBu Slow-CW signal not any more detectable with above listed parameters
Slow-CW has a 20 dB signal level advantage over normal (aural CW), which means that the minimum detectable and/or readable Slow-CW signal that might just allow communication lies 20 dB below the signal, that can just be detected and/or decoded by a trained CW-operator's ear (yes, I consider myself to be a trained CW operator ...). If I consider the "CW-operator's ear/brain bandwidth" to be 30 Hz, this roughly corresponds to the bandwidths used (0.3 vs 30 Hz).
I would be interested to get your comments or own measurements on this
subject. I do not yet have sufficient experience with Spectran to make
full advantage of this software, so I would like to hear about that software
Geri, DK8KW (W1KW)
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