> With QRSS3 6km was my limit some years ago.
Very impressive. I think that to substantially increase the
range, ultra narrow band signals would be the way to go, just as
with VLF radio. I haven't seen anyone try earth mode with very
narrow bandwidth using GPS disciplined signals and receivers.
I expect the path would be very stable and quite suitable for
long integration times.
I don't think these undisciplined EbNaut experiments would do
very well, the bandwidth is too wide - has to be to get the
message across before the phase drifts too far to decode.
My experiments and calculations use loop coupling - I don't
know how to calculate for earth dipoles. Using a spice model
to simulate performance of two 50cm loops, a 5W transmission
of prefixed EbNaut decodes at about 1500m without much trouble.
I'm struggling to improve on the 1.25dB Eb/N0 cost. It seems
to take very little energy to make a detectable prefix to draw
attention to the signal and to indicate the message length and
coding, and to lock onto the symbol timing. The expensive part
is getting enough signal to measure the frequency accurately
enough to get away with undisciplined oscillators.
At slightly stronger signals, eg Eb/N0 of 3 or 4dB it becomes
pretty solid and the squared signal begins to provide a good
enough phase and frequency reference.
The combination of a short prefix, say 0.5dB worth, and GPS
timed tx and rx, would work well for un-announced transmissions.