Rik van Riel asked:
"One big question is, where does the path loss on top band come from?"
Per our current understanding of the lower ionosphere, the loss due to
absorption on 160m in the lower ionosphere at night is around 10 dB per
hop. Added to that is the loss due to multiple ground reflections.
If we assume 1 kW and 0 dBi antennas, multi-hop appears to be able to go to
about 10,000 km before it's below a quiet noise level. This is why ducting
is invoked for extremely long-distance QSOs - less transits through the
absorbing region and fewer ground reflections.
There's a natural electron density valley in the nighttime ionosphere right
above the nighttime E region peak that provides the ducting mechanism -
this is the boundary between the top of the E region and the lower F
region. Ducting on 160m can be seen when doing ray traces through the
nighttime ionosphere.
Carl K9LA
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