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
|
