Hi Bill!
Here's an explanation of this year's rather poor 160M mid winter
propagation during a very unusual sunspot minimum year, with much higher
than normal solar flux.
At this time of year at sunspot minimum, we expect solar flux values in
the low 70s and high 60s, and the E layer MUF should be well below 1.8
MHz. As a result the E layer would have little or no effect on 1.8 MHz
signals, except for a few minutes at sunrise when the E layer has the very
beneficial effect of allowing excitation of efficient ionospheric "tilt
modes."
The realtively poor propagation we are experiencing this year is caused by
"deviative absorption," which takes place in the E layer when its MUF is
somewhat below 1.8 MHz (ie the E layer does not completely refract the
signal back to the earth). If the E layer MUF is sufficiently high to
partially refract a 1.8 MHz signal so it cannot immediately penetrate
the E layer, the resulting collisions while the signal is trapped in the E
layer cause excess signal absorption.
This year from late November (CQ WW CW) thru early December (ARRL 160M CW
Contest), the solar flux was very low, therefore the E layer MUF was well
below 1.8 MHz. As a result, 160M signals passed thru the E layer with
little or no refraction except at sunrise. These excellent conditions
should return in a few days when the quiet side of the Sun is again facing
the earth.
The illustration below shows typical 160M propagation, from late Nov thru
early February, during a normal sunspot minimum year when 1.8 MHz
signals penetrate the E layer with no refraction (high ray angle and
flat layers chosen only for ease of illustration!)
F Layer ------------------------------X----------------------------
X X
X X
E Layer - - - X - - - - - X - - -
X X
X X
Raypath X X
Same illustration with E layer refraction, caused by elevated E layer
MUF due to higher than expected solar flux
F layer ---------------------------------X--------------------------
X X
X X
E layer - - - - - - - X X X X - - - - - - - - - - - X X X X - - -
X X
X X
Raypath X X
73!
Frank
W3LPL
donovanf@sgate.com
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