TopBand: Backlobes and Back Reflections

[Robert g. Flory]

I want to clarify my opinion that Jeff was indeed hearing NL7Z's signal
from a Southerly direction.  Since longpath is out of the question, we have
to come up with an explanation for a strong reflection back towards the
source.

Jeff observed that Kevin's signal came up significantly when he beamed SE. 
Kevin observed that Jeff's signal came up significantly when Jeff beamed
SE.  In order for this to be an illusion caused by a backlobe, there would
have to be a backlobe that is significantly larger than the front lobe at
some vertical angle of incidence.  The model of a 4-square I have (K6STI AO
6.5), which corresponds to what Jeff is using, shows 11dB F/B at 50 degrees
vertical angle and 5dB at 70 degrees.  The forward lobe is bigger no matter
what the angle is.  Yes, the backlobe is big enough to work people on, but
the critical piece of data is that Jeff switched the array and the signal
was significantly louder to the SE.

My real world experience with phased arrays on 160 is that while F/B may
wash out(be reduced) during high-angle conditons, I have yet to see a
stronger response off a backlobe.

A back reflection at significant distance could explain some of the
long-delayed echoes that people hear, and explain the reception of signals
at 180 degrees from short path in the absence of longpath conditons.  The
trick here is to get a back reflection that is very strong.  I am proposing
that Jeff was on the fringe of a skip zone on the direct signal so that the
signal was weak, the signal passed over him and was brought down to earth
at near vertical angle and reflected strongly.  Then as it propagated back
towards Alaska, Jeff was at a favorable distance to receive a strong
signal.

What say, propagation gurus?  Has this ever been observed with MF radars? 
If this is an unknown phenomenon, is it possible based on current
understanding of ionization at the terminator?

Rob K2WI  

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