I wrote:
> A challenging experiment would be to measure the actual path
> length. This requires transmitting two fairly close frequencies
> either simultaneously or alternating with FSK.
No sooner said than done!
Markus DF6NM just happens to have a servo controlled variometer
which samples the radiated field and adjusts the tuning to
maintain the phase of the transmission - even when toggling
between two frequencies. This, in conjunction with Spectrum
Lab's ability to generate a carrier with absolute phase, enabled
the experiment to proceed.
We used two frequencies 60Hz apart, alternating between them
at 5 minute intervals.
Some arithmetic shows that for the 60Hz separation, the group delay
corresponds to 13.88 km of path per degree of phase difference
between the two carriers.
Markus made a long transmission (after some mechanical troubles)
with 10uW ERP, beginning just before 2015-01-06 11:30 UT and the
received signal was analysed at Todmorden in a 4 hour window
(sufficient to get good S/N for accurate phase), sliding the window
in 1 hour steps.
Table below lists the S/N of the two carriers, their phase (lead)
and phase difference, and the resulting path length:
Period S/N dB Phases Diff Path
11:30 to 15:30 19.0/17.9 -7.4/-84.3 76.9 1068 km
12:30 to 16:30 18.5/17.2 -11.1/-85.2 74.1 1029 km
13:30 to 17:30 17.8/16.9 -13.5/-85.6 72.0 1000 km
14:30 to 18:30 16.3/16.5 -20.6/-94.7 74.1 1029 km
15:30 to 19:30 15.9/17.3 -21.2/-102.9 81.7 1135 km
16:30 to 20:30 16.8/17.3 -15.4/-111.2 95.8 1330 km
17:30 to 21:30 16.5/17.0 -17.2/-113.3 96.1 1335 km
18:30 to 22:30 16.9/15.4 -23.7/-111.0 87.3 1212 km
19:30 to 23:30 15.9/13.8 -28.7/-101.9 73.3 1017 km
20:30 to 00:30 15.8/13.2 -30.6/-95.5 65.0 902 km
21:30 to 01:30 16.2/13.7 -27.9/-82.2 54.3 754 km
22:30 to 02:30 16.3/15.0 -18.5/-84.8 66.2 920 km
23:30 to 03:30 18.8/15.8 -15.2/-88.6 73.4 1019 km
The great circle distance to Todmorden is 1028 km.
Phase measurement error with the stronger signals, around +/- 60km.
For the weaker signals, about +/- 180km.
Looks like the results are upset at night due to multi-path
interference. At VLF the ground wave is still significant at 1 Mm
and there will be several sky wave components in the mix.
Not a bad result for 10uW ERP. A stronger signal would probably
not have improved the result, given the multi-path propagation.
A longer range test would be less affected by multiple paths.
Would require smaller frequency shift: eg about 12Hz for
6000 km. 12Hz shift is difficult though, without a strong signal.
17 dB S/N would give an error of about 500km on a 6 Mm path.
--
Paul Nicholson
http://abelian.org/
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