Stefan,
Your message included:
“… thinking about the old VLF propagation graphs https://dl.dropboxusercontent.com/u/19882028/VLF/fig_02_25a.png (what was the original paper where it comes from?) which make more and more sense to me…”
The figure (Fig 2-25) comes from: “Naval Shore Electronics Criteria: VLF, LF, and MF Communication Systems” - NAVELEX 0101,113 - August 1972
http://www.navy-radio.com/manuals/shore-vlf.htm
The above link downloads as 9 separate chapters totaling 52.5MB
Beware a 24.2MB copy of the same manual elsewhere on the web; it has some blur which by Murphy’s Law will interfere only with reading of the plots important to you.
The manual states that the data from which Figure 2-25 is generated comes from the first formula in Section 2.3 of the same manual (paragraph beginning with “Wait’s formula for the vertical electric field strength… is given by:”); and the attenuation factor chosen for Figure 2-25 comes generally from the attenuation-factor plot in Figure 2-20 (see the two paragraphs above Table 2-1 describing the derivation).
Comments:
a) Modeling has come a long way since the manual was published in 1972, but I very much agree with and appreciate your point that Figure 2-25 tells a vital story (and I think that the Figure 2-25 story has been almost lost in all of the modern modeling). I don’t think that any of the Figure 2-25 story is negated by modern modeling; the modern models generally add low-accuracy +/- variations for time/date/lat/long etc.
b) It seems to me that your experiments are importantly refining and adding to the Figure 2-25 story; including new chapters. Very exciting.
Other comments:
A statement in the manual above Table 2-1, referring in part to Figure 2-25, may be misleading in one particular (but important) context. The statement says: “… noise energy at VLF/LF propagates in the same manner (i.e. same manner as in Figure 2-25, which is for a 1kW ERP from a vertical antenna, 80km ionosphere reflection height) and is subject to the same attenuations as signal energy in these spectra…”.
That statement, if extrapolated to SNR (which the manual does not do, but which most readers would like to do), would need to be carefully applied to assessment of SNR in any particular experiment, except perhaps in wintertime. In wintertime at +50 degrees latitude one might not need to be too careful in using Figure 2-25 to determine which frequency will give best SNR for a particular distance, but in regional thunderstorm season the application of Figure 2-25 to best frequency for best SNR at a given TX-RX separation might be dependent on where thunderstorms tend to concentrate regionally and how much those thunderstorms affect the height of the ionosphere over the chosen TX-RX path, especially for TX-RX separations between 300 km and 1000 km (changes in ionospheric height can cause particularly large changes in signal propagation at TX-RX separations 300km-1000km). So estimating best frequency for best SNR from Figure 2-25 is particularly affected by thunderstorms in the region in two ways: (1) rather flat VLF emission spectrum of lightning and clustering of lightning at various ranges during regional thunderstorm season (this matters even if lightning is not in the TX-RX path); and (2) elevation of the ionospheric reflection height by the lightning (this matters only if the lightning is alongside or in the TX-RX path, and the effect would be enhanced by modal-interference effects if the TX-RX path is 300km-1000km)
Last comment:
Lulu Publishing advertises a bound copy of this manual (“Naval Shore Electronics Criteria: VLF, LF, and MF Communication Systems”) with a small note:
“… The chapters on VLF are omitted.”
I thought about writing a letter of indignation but instead requested that they add the full manual to their inventory.
73,
Jim AA5BW
Thanks Paul (and Renato and Wolf!), very well!
The carrier on 5170.001250 Hz is still on the air and will run until 18 UTC.
Since it appears that you and Jacek are the only ones trying to receive my EbNaut, i'll stay at 16K25A, just to use the better code gain.
And since the last ~ 24 hour experiment was running so well, let's try 48 hours! Maybe it leads to a 30 0 30 0 phase pattern:
f = 5170.000000 Hz
Start time: 07.Jan.2017 20:00:00 UTC
Symbol length: 64 s
Characters: 20
CRC 16
Coding 16K25A
Duration: 45h, 30m, 40s
Antenna current: ~ 225 mA
The first time i used your calculator (http://abelian.org/ebnaut/calc.php?sndb=-63&snbws=2500&snmps=&code=16K25&sp=64&crc=16&nc=20&submit=Calculate ) to chosse the number of characters and the symbol length BEFORE the transmission :-)
With your given RAM, how many characters can you decode in 16K25A? And how long does the decode process take then?
These 2 day long transmissions mostly failed on 6.47 kHz, or gave poor results. Stacked single day transmissions were a better choice. For a 50 or 75 character message on 5170 Hz we may have to use the same technique.
I'm often thinking about the old VLF propagation graphs https://dl.dropboxusercontent.com/u/19882028/VLF/fig_02_25a.png (what was the original paper where it comes from?) which make more and more sense to me! On 5170 Hz we already see a real advantage of lower QRN relative to 8270 Hz or 6470 Hz. According to the graphs, the optimum frequency should be arround 4 kHz because the QRN from far away is attenuated much more whereas the poor propagation on that frequency is not so much expressed for 'short' (1000 km) distances. And BTW, 4 is a very nice number, isn't it!? Sooner or later someone has to do something near 4 kHz! I would be curious to see how this band (e.g. 4270 Hz or 70 km!) behaves. I can imagine that it is the best choice, even in summer or especially in summer!
When looking on the todays 'wideband' window (the upper one on http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html) we can see that we are already diving below the QRN :-)
73, Stefan
Am 07.01.2017 05:09, schrieb Paul Nicholson:
Decoded '73 DK7FC' from Cumiana (Renato Romero, vlf15, 504.6km)
with constant ref phase, Eb/N0 = 0.6, S/N 16.16 dB in 11.8 uHz,
-67dB in 2.5kHz.
Very strong at Bielefeld (Wolf Buescher, vlf6, 303.8km)
Eb/N0 11.6dB, 27.17 dB in 11.8 uHz, -56.1dB in 2.5kHz,
constant reference phase.
Here, improved my decode to 3.9dB when I remembered to use the
-a option which normalises the amplitude by the average noise.
I am not seeing much day/night phase shift at any site. Some
measurements on the carrier will be the next job.
--
Paul Nicholson
--
