TopBand: Backscatter at home

[Dan Robbins]

Nobody bit on this before, so let me tweak your interest once more. Once 160
quiets down for the winter, you might try this: 

In the QSK mode with the amp on, make a single, fast (60 wpm) dit.  See if
you can hear your backscatter echo. If so, listen carefully to see if you
can hear more than one echo.  The echoes will be very closed spaced and
sound like brrrp.  If you lower the speed of the dit, eventually you will
reach a point where the individual echoes blur into a solid tone.  At this
point the duration of each dit is equalling the roundtrip time of your
signal.  A 60 wpm dit normally lasts for about 20 ms., but most rigs shorten
up the first dit in the QSK mode to less than that.  QST usually shows this
in their rig reviews. If you have a scope you can accurately measure the dit
length.  You may find a single dit at 40 or even 30 wpm is truncated to
around 20 ms. by your rig/amp.  Playing with the weight control may help
keep the speeds more reasonable. 

A full 20 ms. corresponds to round trip time of about 3700 miles, but this
counts the distance up to the ionosphere as well as down range, plus the
return. The ground distance is dependent upon the height of the ionosphere,
but for a 20 ms duration, the ground reflection point is about 1500 miles
out.  At 24 ms. (a dit at 50 wpm) the distance is around 1800 miles.

Many times I have been able to hear echoes, but not individual ones.  On
those rare times when I have been able to hear the individual backscatter
echoes from each hop, the hop length seemed to be close to 1500 miles and
the band seemed quite good.  (These are also the times the vertical hears
the echoes, too.  Normally they are more prevalent on the inverted vee.)
I've never heard any hop lengths longer than this, but I have clearly heard
at least 3 hops.  According to the ARRL Antenna Book, this distance probably
represents a takeoff angle of around 15-20 degrees.  When my ear can't
discern individual echoes, I assume the signal is leaving at a higher angle
and the hops are too short.  Or it might be that a larger range of angles
are being backscattered and they all blur together.

Anyway, maybe somebody with a scope and a little curiousity can try this
from a different area of the world and see if they hear their own
backscatter signals.  They might even be able to trigger off of the start of
the dit and measure true time rather than just "guesstimate" it. Maybe some
of the ionosphere experts can come up with a better time vs. ground distance
set of standards for the typical nighttime ionosphere.  Maybe you guys with
directional RX antennas can do a little probing to see where the signals
seem to peak. The backscattered signals are often amazingly loud at my
place. One trick to hear the echoes better is to silence the sidetone. (Or
use the RIT to give the echo a different pitch.)  Turning the AGC off might
help, too.  I understand some shortwave stations use backscattered signals
to determine the best freq or angle of radiation for targeted areas.  Maybe
we can, too.

                                        Dan KL7Y


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