[Lf] why qrss?

[Andre' Kesteloot]

Johan Bodin wrote:

> Hi Mal,
>
> >Can anyone explain to me why qrss is necessary on 136 khz. All the
> >activity that I have heard on this frequency is workable on normal speed
> >cw at my qth.
>
> And, the activity you didn't *hear* is not ;-)
>
> Seriously, the received noise power is proportional to the receiver
> bandwidth (assuming random noise). Most of the information energy
> of a typical QRSS CW signal is contained within a fraction of a Hz of
> bandwidth. I have been told that the equivalent bandwidth of the
> human ear (trained CW operator) is about 50 Hz, which means that
> 22 dB (7 watts instead of 1kW, for example!) can be gained from
> using Spectrogram running a 16K point FFT (about 1/3 Hz BW per
> FFT bin at 5513 Hz sampling frequency). In addition, a few more
> dB's can be gained by averaging FFT's. Unfortunately, averaging is
> not as effective as reducing the BW since the S/N is proportional to
> the square root of the number of averaged samples.
>
> For example, I got a solid copy of IK1ODO's qrss transmission on
> the Spectrogram screen while the signal was *totally* inaudible to
> my ear. I even tried to move the VFO up and down (I think a slowly
> varying tone is easier to detect by ear) but all I heard was noise...
> Marcos's CW was solid on the screen!
>
> 73, Johan, SM6LKM