Re: VLF: EbNaut on 2 frequencies

[Markus Vester <markusvester@aol.com>]

Hi Paul,

yes, quite the opposite!

The decode attempts for Stefan's nighttime 16-character transmission showed that the signal was very weak. An optimally weighted average of six evenings (Oct 10 to 15, 8.9 hours each) did not produce a decode. Using the known message content (the pop song) to reconstruct the carrier, a small peak with 8.4 dB SNR in 5.2 uHz became visible, corresponding to -11.2 dB Eb/N0 for 16 times 5.7 bits.

Comparing this result to +16.3 dB Eb/N0 from stacking three days of the 1-character message, we find that the SNR at night was around
 16.3 + 11.2 + 3 - 12 = 18.5 dB
weaker than during the day. The spectrograms show that the noise level (after blanking) on the EW-antenna was approximately 10 dB higher at night, indicating that the signal must have been around 8.5 dB weaker.

As a crude first Approximation to the model, we might look only at groundwave and single-hop skywave. For 230 km range, the geometrical path difference would be 63.3 km (1.37 lambda) for 90 km reflection height (night), and 40.3 km (0.87 lambda) for 70 km (day). Assuming that the ionosphere is acting like a conducting surface, a vertically polarized field would be bounced with zero  phase shift (this may seem counterintuitive at first, as horizontal electric fields at normal incidence would be flipped. But the vertical electric and horizontal magnetic components are not). Using equal amplitudes for both waves, the phasing would then predict a 7 dB weaker signal at night.

BTW I think that at our frequencies, ground wave losses are still negligible up to a few 100 km range for all realistic types of ground, and a simple 1/r calculation for monopole-over-perfect-groundplane will be quite adequate.
  
Best 73,
Markus (DF6NM)



-----Ursprüngliche Mitteilung-----
Von: Paul Nicholson <vlf0403@abelian.org>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Di, 17. Okt 2017 21:01
Betreff: Re: VLF: EbNaut on 2 frequencies

Here's the nighttime signal along the 88 degree bearing to Amberg,

http://abelian.org/vlf/tmp/171017d.gif

Quite the opposite to what signal reports suggest?

The daytime signal on 88 deg isn't significantly different to
307 deg (not much anisotropy apparent at these short ranges).

I don't put much faith in this model but it serves to illustrate
the complicated way that signals can vary as a function of
range. A major difficulty at short ranges is the ground wave
calculation, knowing how much power couples into the ground wave,
and the average ground conductivity and permittivity.

The precise ups and downs depend critically on the amplitude
and phase of ground and several sky waves. At best it is in
the right ballpark in terms of typical average field strengths.

(I take some comfort that LWPC doesn't do any better.)

But at least we can see that it's plausible to blame propagation for
the different performance of signals at Stefan's two frequencies.

--
Paul Nicholson
--