[amsat-bb] Fresnel zone blockage of Moon Reception

[Robert Bruninga]

Lesson learned about Fresnel Zone blockage.

Students had one opportunity this semester to hear the 216.98
MHz SPACE RADAR reflection from the moon during their lab period
and it was only the 2nd day of class.  So on the first day of
class we had to get them up to speed to do the calculation as to
when, and I only had one day in advance to test it.  My pre-test
worked fine.  Heard a very weak tone for less than a minute.

Next day, same setup, same place with class in tow, standing on
the plaza.... NOTHING.... Grrr...  Guessed that moon must have
been blocked by building.... (it was near new moon, so you
cannot see it optically in the hazy summer day.  Although the
time of passage through the fence was within a few minutes
day-to-day, GRRR, I forgot that the moon rises 56 minutes later
each day and so would be 15 degrees lower... For the same radar
fence crossing.

Came back out later with protrator and measured the angles to
the surrounding buildings and went back inside an plotted it on
the blackboard.  Discovered that moon was close to being blocked
but not quite.  It was actually clearly Line-of-sight visible.
Past the corner of the building...

My excuse then, was maybe it was in the first fresnel zone.
Looked it up on http://en.wikipedia.org/wiki/Fresnel_zone and
did the calculations and YES!  The moon was clearly visibile
optical line-of-sight, but if it was within 20 feet or so of the
corner of the building, then that would be in the first fresnel
zone and cause signficant reduction in signal strength.

The first time I calculated it, I came up with 7 feet and it was
a stretch to say that the moon's position was that close to the
corner of the building over 100 feet away.  But then I realized
that was in meters, and so there was no question that our
optical line of sight  was well within 20 feet of the corner of
the building.  Hence, we now had our explaination as to why we
didn’t hear the moon's reflection.  

Our set-up consists of about a 10 dB corner reflector into a
stock AOR receiver set for Upper sideband (to avoid a birdie on
LSB).  See photo:
http://www.ew.usna.edu/~bruninga/craft/NSSSant.jpg.  See a plot
of the reflection using DIGIPAN:
http://www.ew.usna.edu/~bruninga/craft/moon1234AOS.gif

This also explains why I don’t get the greatest horizon coverage
for our satellite station.  With our OSCAR array  in the center
of a flat roof (up about 12 feet) surrounded on all sides out
about 100 feet by a 4 foot metal façade, then the horizon is in
our first fresnel zone both at 2m and 70 cm.

Bob, WB4APR