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Hi Spiros,
not really strange: the rising
sun is suddenly ionizing lower ionospheric layers and increasing the
absorption.
Here's a somewhat simplistic explanation of diurnal
propagation: At night, LF is mostly reflected by the lower boundary of the
E-layer, at a height of about 100 km. After sunrise, radiations starts to
penetrate deeper and ionize lower layers. So the conductivity of the
D-layer increases so much that it starts first absorb and later reflect
LF.
This is somewhat similar to a shunt
resistor inserted in a transmission line: During the night it's
a large resistor which will let most of the wave pass through, absorbing
only a small fraction of the power which depends on "conditions" (like DST
index, describing charges leaking down from a storage ring). At sunrise, the shunt suddenly conducts
more ("closer to 50 ohms") so most of the wave is being absorbed. At midday
when the sun is highest, the conductance is approaching a "short
circuit" which reflects the wave from a lower altitude (about 80
km).
After sunset, the electrons and ions in the lower
layers recombine quickly so the D-layer becomes transparent again. However at
high altitudes, the gas density is so low that charges take long
to find a partner, thus ionisation of the E and F layers persists
throughout the night.
Best 73,
Markus (DF6NM)
Sent: Friday, May 03, 2013 5:58 AM
Subject: LF: 136KHz propagation
GM to all.
The same strange propagation tonight as every
night.
At around 03.00z my signal disappear from DF6NM, DK7FC, YO/4X1RF &
G4WGT
grabbers.
I have better prop earlier and during the day with day
light.
73, Spiros
SV8CS
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