[Lf] 20 dB Advantage for QRS

Sat Apr 15 16:14:55 CDT 2000

"'Geri' Kinzel, DK8KW" wrote:

> Hello friends,
>
> I made some laboratory tests this morning 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
> Signal  Signal
> Strength        Strength
>
> - 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
>
> Conclusions:
>
> 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 as
> well.
>
> Best 73
>
> Geri, DK8KW (W1KW)
>

Peter Dood replied: 

Hello Geri

> I made some laboratory tests this morning to get some indication about the
> ability to communicate with signals below noise level using Slow-CW.

A most interesting set of measurements. Most of us who use this mode 
felt that an it gave an extra 15 to 20dBs but this is the first time 
I have seen objective measurements.
While not giving the same noise improvement described in the 
pionering work of G3PLX and G4JNT described in the LF Experimenter's 
Book, Spectrogram and Spectran does provide a convenient compromise 
that can be managed by us non-techies.
I have made some improvements to the antenna and although this gives 
me an extra 0.5Amps on transmit, the downside is that it has 
increased the level of the Loran sidebands. While the strength of 
each of these sidebands is not that high the effect of several 
hundred of them in a passband of 2.7kHz holds the S-meter at S9! 
While the fix would be to use a directional receiver antenna, such as 
that used by Laurie or Derek, with QRSS it is not necessary because 
of the ability of this mode to read signals between the Loran lines. 

> 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 as well.

During the month of tests receiving the 20mW erp from I5TGC over an 
1800km path (plus Alps) I found Spectran had a slight advantage (even 
at its Beta stage) mainly because it has a higher frequency 
resolution. I used the 0.125Hz setting which placed the Loran 
sidebands (on average) about 15mm apart, which gives plenty of space 
to see a signal. The best compromise dot period for 0.125Hz apears to 
be around 5 seconds.

At this stage the Speed Control (integration time?) is not slow 
enough take advantage of the 0.064 and 0.032Hz settings of Spectran. 
It would be interesting test these narrower settings to find the 
limit of the soundcard.

Geri, I would be most interested in any tests you may make on 
Spectran - I take it you have a copy.

-- 
Regards, Peter, G3LDO

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