Apropos the nighttime AM activity on 20 meter, this from the recent ARRL Propagation letter:
Lou, VK5EEE sent in a couple of interesting questions, which I
passed on to Carl, K9LA.
Here was Lou's first question.
"I have observed on several occasions over the past 6 months or so,
an unusual propagation, which I cannot explain. I have searched far
and wide on the Internet, and short of an atmospheric nuclear
explosion causing a strong ionization of E layer, which does not
appear to have occurred based on Geiger counter readings, I find no
explanation.
"Sporadic-E, we are told, occurs from around 12m (25MHz) upwards, is
that correct? Can it occur on 21MHz? CAN IT EVER OCCUR on 20m? It
appears to me not on 20m. Also, what I describe does not last a
short time, as would be expected, but for hours. Short skip of 600km
AT NIGHT on 20m, this should not normally be possible?
"At the same time as this short skip late evening propagation from
VK5 to VK3 (dipole facing broadside to VK3 and HS at my VK5 QTH) the
VK3 was using 5 element beam up 20m beaming to HS (same direction as
VK5) and HS was beaming to VK3 with a 4ele beam up 22m. The VK3-VK5
was exceptionally strong 599+20dB on my 8m high dipole. Not to be
expected, even more so with a 5ele beam during the middle of the
day, most of the energy should pass way overhead. The VK3-HS were
both 599 to each other, but HS-VK5 was only S4 from me, S7 from HS.
"Given it is taking place, and the phenomenal signal strengths, 20
or so dB above what would be normal via F layer propagation, would
that be E layer propagation, and why is the E layer there?"
Carl, K9LA responded:
"With respect to your observation number 1, I downloaded ionosonde
data from Canberra (the closest to your VK5 to VK3 path - we can get
a general idea of what happened in the ionosphere) for Jan 1, 2016
to May 31, 2016. That's 152 days of data, and data is taken every
hour - that gives 3648 possible data points. For an E mode (110 km
height), the elevation angle for the 600 km path is around 18
degrees. The value of foE must be greater than about 5 MHz to
support 14 MHz for this short path. For an F mode (300 km height),
the elevation angle is around 45 degrees. The value of foF2 must be
greater than about 9.3 MHz to support 14 MHz. The foE data had 847
data points, so there is a lot of data missing. Of the data
available, there's nothing above 4.5 MHz. The foEs data had 1759
data points. That's better - almost 50% of the possible data points.
"In your summer (January and February), there are many foEs values
above 5.0 MHz. Towards your winter (May), there are still quite a
few foEs values above 5.0 MHz. The foF2 data had 2887 data points
(about 80% of the possible total). There were not many foF2 values
above 9.3 MHz. Most foF2 values were 7 MHz and below. This cursory
investigation suggests that Es could have been the mode. But your
question asking if Es occurs on 20-Meters is very relevant. With Es
layers being thin (I've seen values from 1 to 5 km), there just
isn't enough vertical extent of the layer to have pure refraction
(bending) take place, and the ionization doesn't appear to be enough
for reflection.
"Thus in my mind the answer to your question appears to be 'no, Es
generally doesn't happen on 20-Meters.' If foEs was much higher,
then a thin layer might support reflection at 14 MHz. There is an
'above-the-MUF' mode with both the E region and F region when the
operating frequency is somewhat above the MUF. This mode is believed
to involve a scatter mechanism, which implies additional loss. Using
the 'above-the-MUF' estimates of additional loss on 14 MHz says your
observations could have been either E or F. And there's also the
possibility of a back-scatter mode, with the scatter region being
somewhere northwest of you along the path to HS.
"That's about as far as I can go with this. I don't have a definite
answer - that happens more than we'd like with some HF propagation
observations due to the lack of suitable ionospheric data."
