Hello all,
after the recent messages by Geri and Peter G3LDO about the use of Spectran
or Spectrogram for QRS, I did some tests also.
I used the following setup: a selective voltmeter with high-impedance input
(Wandel&
Goltermann SPM-11 with PSE-11 tracking generator) is connected in parallel
between my receiver (Racal 1792 with 100 Hz IF filter) and the antenna.
The output of the tracking generator is radiated by a short wire via a step
attenuator.
This morning the noise was about -130 dBm in the 40 Hz bw of the voltmeter.
A CW signal to be decodable by (my!) ear from the RX had to be about 8 to 10 dB
louder (let assume -120 dBm for 10 dB S+N/N in 40 Hz).
Using Spectrogram the signal was easily decodable at -140 dBm using 0.3 Hz bw,
so I agree with the 20 dB improvement found by Geri; may be between 20 and15
dB,
it depends from the type of noise present and the skill of the operator.
Using Spectran I was able to read it at -150 dBm in 0.032 Hz. The setting was:
minimum speed, maximum resolution, default contrast (100), horizontal display,
600 Hz BFO frequency. To clearly see a signal needs about 10 dB S/N, if I go
down
to -157 dBm (approximative 3-5 dB over noise) the presence of the signal is
clear
but the report will be a "T" or "M".
Since we are used to 3 second dots with Spectrogram, it seems appropriate
to go to 30 sec with Spectran at maximum resolution.
Now, I want to arrange a test, if someone is interested, of course. I'm
planning to
beacon for some days at low power, sending a message with QRS in DFCW
mode on 137.790 / 137.792, at 30 sec/dot, with 20 sec interval.
I will reduce RF power to 10 Watt, giving an approximative ERP of 10 mW. With
this power setting my 900W TX will run unattended for days (I hope...)
I'm very curious to see the results, and to try skeds in this mode. I know that
this
is not the best way to go, PSK will be another affair, but QRS is cheap, easily
set up and used, does not require specialized hardware, and so on; so I think
this
is worth a try, if you have a stable RX with a stable BFO !
Any suggestion on frequency, times, dot lenght is welcome. For me 137.790 is
clear from weak carriers; but QSY is possible.
73 - Marco IK1ODO
[email protected]
Rivalta, ITALY JN35SA (N 45 01' 25.6", E 7 31' 09.4")
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