Hello group,
yesterday night, the qrn seemed to be fairly moderate, so I decided to start
spectrogram at 22:00 UT (5513 Hz, 512 pts, 30 s dwell, 128 averages,
resolution bandwidth with windowing appr. 14 Hz). In the morning, I was quite
surprised to see a fat trace of CFH 137.0 kHz beginning at 00:10. A first
maximum occured around 00:55 with about 11 dBuV/m/14Hz, then the signal dived
down for 15 minutes before it came back with similar strength. After staying
fairly constant for about 1.5 h, it peaked to 13 dBuV/m/14Hz from 02:55 to
03:10, then decayed and became invisible around 04:10.
SVX from Greece was strong all night until 02:40, with a pronounced
double-minimum at 03:00 and 03:14 around sunrise. Starting at 23:45, there
was another unidentified narrowband signal with dx-like fading on 136.3 kHz,
up to 10 dBuV/m. The noise between the statics was around -10 dBuV/m/14Hz
during the night, decreasing by 5 dB with daylight.
For the calculation of signal strength, I have found a ratio of 7 dB between
CFH's observed power density in 14 Hz and its total power, corresponding to
an effective modulation bandwidth of 70 Hz. So the actual field-strength
would be 20 dBuV/m in the maximum. Reducing the noise density to 1 Hz, I
would see -10-11.5 = -21.5 dB/Hz, resultimg in an SNR of 41.5dB/Hz. The ERP
of CFH is said to be around 10 kW, 40 dB above an amateur station's limit.
That would leave us with 1.5 dB SNR in 1 Hz, or 11.5 dB with 10 s dots in 0.1
Hz, even here in the middle of Europe. In other words:
Had we tried last night, we would have made it across!
BTW: Comparable observations in winter and early spring had shown a quite
similar field strength, it was only the time window of common darkness which
was much longer.
Cheers
Markus, DF6NM
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