>>I like to compare DX propagation on 160 to the tiny beams
>>of light
>>peeking through a very dark and stormy sky, moving as the
>>clouds
>>move across the sky. Each part of the earth may be
>>illuminated by
>>propagation for only 10-20 minutes, and it can be VERY
>>spotty.
> It has been interesting to hear all the activity, but as
> far as hearing
> any of
> the EU out here in KH6, they have been very few and only
> for a few
> minutes at most. Heard SM5EDX for a burst about 15
> minutes after
> my sunset, same for DF2PY, and a couple others, just the
> "gray line"
> burst from 30 seconds to a few minutes then gone, have not
> stayed up
> late (past 0800-0900) so not sure whats on after that past
> few days.
Visualizing propagation is difficult. Our local noise floors
are the lower limit below which there are no signals at all
as far as we are concerned.
The signal is always there below the MUF...it just decreases
in level until goes into the noise floor at the moment at
whatever location we are in.
That's a big reason if not the only reason we have "one way
propagation" when operating well below the MUF. As an
example people with almost no local noise floor can hear
distant signals coming from someone at or near their dark
time long before those stations closer to dark (or with
higher noise floors) hear them. We credit "one way
propagation" or narrow beam "search light effect" for what
might be only a few dB change that brings a signal up out of
dissimilar noise floors.
My noise floor changes 10-20dB on a quiet winter dark period
from daytime as long as I am looking into the dark area.
This is because the noise floor is established by
ionospheric propagated noise. During the daytime with
10-20dB less noise, while trying to communicate with
stations in their darkness, they have a 10-20dB advantage.
It is hopeless to try to work a European a few hours before
my sunset even if I hear them quite well, just like VK3ZL
can sit and copy USA stations who have no hope of hearing
him when he listens long before his sunset.
The 20dB attenuator they are transmitting through (daylight)
reduces their signal AND my noise almost equally because I
am surrounded by daylight. Unfortunately on the way back
there noise isn't reduced the same amount because it comes
strongest from the dark area while my signal comes from the
daylight path with the attenuation.
Local noise, if it dominates, throws another wrench into our
perspective.
A 3dB change in level of noise or signal can be like someone
throwing a light switch from our perspectives when signals
are near the noise floor.
73 Tom
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