Dear LF Group,
Very interested to see Hartmut's report of VO1NA's 5wpm signal. I have been
doing some measurements on VO1NA's signal strength, and the band noise
levels here over the past week or so, by getting a reasonably accurate
calibration of FS vs. output on the receiving loop antenna, and plotting the
signal strengths using DL4YHF's Spectrum Lab.
Measuring the noise in a meaningful way depends on the context in which the
measurement will be used - bandwidth of receiver, type of modulation, type
of noise. I find that the noise measurement algorithm suggested by G4JNT,
which Wolf has built into Spec Lab, gives a pretty good picture of the
effect of band noise on CW or QRSS signals. I find that, if the noise power
measured by this algorithm is scaled for 100Hz bandwidth, a 0dB signal to
noise ratio (i.e. signal power equal to noise power) gives a "just about
readable" CW signal under different band noise conditions, so this seems a
convenient figure to use as an "audible signal threshold" - of course,
anything like this that depends on the operator's senses will be a bit
subjective. Using this measure, the daytime band noise level is
about -20dBuV/m under quiet conditions, but rising up to about -10dBuV/m
when there is thunderstorm activity. During darkness, the noise level rises
to about -10dBuV/m on a quiet night, and several dB above that when there is
a lot of QRN. So these are also the signal strengths required for an audible
CW signal. VO1NA has typically been reaching -20dBuV/m between about 0000z
and 0500z, which I guess corresponds to twighlight in Newfoundland to dawn
in the UK. The best SNR usually occurs at dawn, when the noise starts to
fade before the signal, and there is a narrow window where the SNR is
boosted by a few dB.
Over the last week or so, VO1NA has mostly been about -10dB to -15dB below
the audible threshold. This gives good "O" copy at QRSS30, and would mostly
be OK at QRSS3. But, at best, the signal has been about 7dB below the
audible threshold. The 5wpm signal is visible as a "smudge" on the
spectrogram, but so far has not been audible. So Hartmut's reception is
remarkable - the SNR at his QTH must have been 10dB better than they were
here, which shows the degree of variation that exists between different
locations. I would be interested to know if the 5wpm signals were readable
over just a few minutes, or an extended period of time?
Cheers, Jim Moritz
73 de M0BMU
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